A 50 year review of basic and applied research in compound parabolic concentrating solar thermal collector for domestic and industrial applications

Solar Energy 187 (2019) 293–340

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Review

A 50 year review of basic and applied research in compound parabolic concentrating solar thermal collector for domestic and industrial applications

T

V. Pranesh , R. Velraj, S. Christopher, V. Kumaresan ⁎

Institute for Energy Studies, CEG, Anna University, Chennai 600025, India

ARTICLE INFO

ABSTRACT

Keywords: Non-imaging solar collector Concentrating solar collector Compound parabolic concentrating collector Simulation tool analysis Solar collector standards CPC applications

In the recent years, compound parabolic concentrating (CPC) collector draws the attention of researchers and industrial developers towards meeting the downstream requirement of about 60–240 °C because of its unique features of capturing the solar rays including diffuse rays, no tracking mechanism at low to moderate concentrations, minimal heat loss and higher collector efficiency. The objective of this review is to identify the research drift towards CPC collectors for domestic and industrial applications. In this study, a comprehensive review for the CPC solar collector was carried out for the past 50 years in terms of types, historical growth in milestones, concept, design strategies, heat transfer fluids, experimental studies, theoretical studies (Numerical studies and simulation studies), applications, standards, certifications, market players and recent developments. The review, exhibited that the CPC solar collector has been continuously developed, modified and improved to achieve better collector efficiency and there is a greater potential to increase its utilization in various applications in the near future.

1. Introduction Presently, the foremost crises found in the major portion of the developing nations are: i) a severe shortage in meeting the basic energy demand and ii) perpetual rise in pollution levels. Hence there is an urge to find an alternative and sustainable way to meet the energy demand and to reduce the pollution levels mandatory worldwide. In India, the total electricity consumption in building sector is about 33% with 8% and 25% for the commercial and residential sectors respectively (ECBC, 2009) which reflects the contribution in pollution. This will further increase in the future due to substantial growth in the country’s economy and the improvement in life style of the people. Rogers et al. (2008) presented preliminary results and methodology for the electricity utilization in residential sector in their study report on ‘India low carbon growth’. The report highlights the distribution of power consumed by appliances (Lightening, kitchen appliances etc.,) and heating/ cooling appliances as shown in Figs. 1 and 2 respectively. Indian government is also interested in reducing the impact caused by the depletion of fossil fuels and environmental pollution. India has moved to fifth position due to increase in policy support and an improved investment on climate change aspects (Sen and Ganguly, 2017). In India, during the last decade an annual growth rate of about 22% for

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renewable energy has been observed. Among the various renewable energy sources, solar energy complies both the requirements. Also, because of its unique features like abundant energy availability and eco-friendly, it draws the attention of the scientists as a suitable replacement. In India, there is an average of 250–300 clear, sunny days in a year with an average annual solar irradiation which varies between 4 and 7 kWh/m2/day in most regions of the country, thus it receives about 5000 trillion kWh of solar energy in a year, that shows the abundant availability of solar energy (Bhaskar, 2013; Chennai Climate, 2018; NASA, 2018; Radiation, 2016). The two basic types of solar collectors are non-concentrating and concentrating collectors. A non-concentrating collector has the same area for intercepting and for absorbing solar radiation, whereas concentrating solar collector consists of reflecting surfaces to intercept and focus the sun radiations to a smaller receiver or absorber area, resulting in increase of radiation flux. The concentrating solar collectors can be further categorized based on the tracking mechanism such as single axis, dual axis and non-tracking (stationary). Movable collectors have higher maintenance requirements (Kalogirou, 2003). In non-concentrating collectors, for very high temperatures (above 1000 K) spectral selectivity become less beneficial because of material

Corresponding author. E-mail address: [email protected] (V. Pranesh).

https://doi.org/10.1016/j.solener.2019.04.056 Received 6 April 2018; Received in revised form 17 December 2018; Accepted 17 April 2019 Available online 28 May 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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Nomenclature

MaReCo Maximum Reflector Collector Max. Maximum Min. Minimum MWCNT Multi-wall carbon nanotube NA Not available N-S North south NS AL CF N-S with counter-flow tube with Alanod reflectors NS AL UT N-S with U-tube with Alanod reflectors NS RT UTN-S with U-tube with Reflectech reflectors Nu Nusselt number (Dimensionless) Ammonia NH3 ORC Organic Rankine cycle PDR Parabolic dish reflector PHP Pulsating heat pipe PTC Parabolic trough collector PV Photovoltaic Pvt. Private RefleC Segmented booster reflector RO Reverse osmosis SACH Solar absorption cooling and heating SAI Surface areal irradiance SCPC Serpentine compound parabolic concentrator SDHW Solar domestic hot-water SEIA Solar Energy Industries Association SHAMCI Solar Heating Arab Mark and Certification Initiative SI International System of Units SRCC Solar Rating and Certification Corporation SSACPC Single-sided absorber CPC SWCNT Single wall carbon nanotube SWH Solar water heater TEWI Total equivalent warming impact TLCC Total life cycle cost TRNSYS Transient Systems Simulation TiNOx Titanium nitride oxide U.A.E. United Arab Emirates UEG Useful energy gain U.S. United States of America VARS Vapor absorption refrigeration system VCC Vapor compression cycle w.r.t. with respect to XCPC External Compound parabolic concentrating/concentrator 1D, 2D and 3D One dimensional, two dimentional and three dimentional respectively Aap Aperture area of CPC solar collector (m2) Surface area of the absorber or receiver of CPC solar colAab lector (m2) Surface area of the virtual receiver enclosing the absorber Av and the gap C Simulation concentration ratio or flux concentration ratio (Dimensionless) CR Concentration ratio (Dimensionless) Cconc Comparative concentration ratio (Dimensionless) Concentration ratio for the two-dimensional, truncated Ctrunc CPC solar collector (Dimensionless) Cfull Concentration ratio for the two-dimensional, full CPC solar collector (Dimensionless) Iin(θT) Average total input heat flux (sum of the values of top and bottom heat flux) at θT (W/m2) Total energy flux (W/m2) It Iirra Energy flux for the solar irradiance (W/m2) n Index of refraction of medium surrounding the absorber (Dimensionless) Tsat Saturation temperature of the working fluid (°C) UL Overall heat transfer coefficient θa Half acceptance angle of the CPC solar collector (degree)

AFPC AIDMO AMEC ANOVA ANSI ASHAE

Advance flat plate collector Arabian Industrial Development and Mining Organization Arab Ministerial Council of Electricity Analysis of variance American National Standards Institute American Society of Heating and Air-Conditioning Engineers ASHRAE American Society of Heating, Refrigerating and AirConditioning Engineers ASRE American Society of Refrigerating Engineers BCCCT Black chrome-coated copper tube BS British Standard CEN European Committee for Standardization CENELEC European Committee for Electrotechnical Standardization CHIT Chitosan CNP Carbon nano particles COP Coefficient of performance CPC Compound parabolic concentrating/concentrator CPC-TSS CPC-assisted tubular solar still CPC-CTSS CPC-concentric tubular solar still CSA Canadian Standards Association CTC Cylindrical trough collector COPcooling Coefficient of performance for cooling applications COPheating Coefficient of performance for heating applications CO2 Carbon dioxide CuO Copper oxide DASC Direct absorption solar collector DSACPC Double-sided absorber CPC EES Engineering Equation Solver EST Evacuated solar tubes ESTIF European Solar Thermal Industry Federation ETC Evacuated tube collector exp. Experimental E-W East west EW AL CF E-W with counter-flow tube with Alanod reflectors EW AL UT E-W with U-tube with Alanod reflectors EW AL XT E-W with X-tube with Alanod reflectors EW RT UT E-W with U-tube with Reflectech reflectors FLC Fresnel lens collector FPC Flat plate collector GSECL Gujarat state electricity corporation limited Gr Grashof number (Dimensionless) HC Hydrocarbon HCF Hydrofluorocarbon HCFC Hydrochlorofluorocarbons HFC Heliostat field collector HTC Heat transfer coefficient HTF Heat transfer fluid HVAR Heating, ventilation, air conditioning and refrigeration HX Heat exchanger H20 Water ICS Integrated collector storage IREC Interstate Renewable Energy Council ISO International Organization for Standardization IAM Incident angle modifier IEA International Energy Agency INTEC Institute for Sustainable Technologies ISS In Situ scientific software LD Limiting diameter of the tubular receiver lg. Length LCS Life cycle saving LiBr Lithium bromide Ltd. Limited 294

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θT φ ω τcpc

Transversal angle of the solar collector (degree) Stream function (Dimensionless) Vorticity (Dimensionless) Fraction of radiation transmitted or effective

ρ

transmittance of CPC (Dimensionless) Reflectivity of reflector (Dimensionless) Average number of reflections (Dimensionless)

Fig. 1. Distribution of power consumed by appliances.

Fig. 2. Distribution of power consumed by heating/cooling appliances.

degradation and spectral properties (Rabl, 1976b). Moreover, the radiation losses play an important role in limiting its application. Hence the only option is to reduce the absorber area by concentrating the collector (Duffie and Beckman, 2013). The concentrators can be

classified as non-imaging and imaging (Kalogirou, 2004; Pitz-Paal et al., 2010). In imaging concentrators, irrespective of the ray’s path through the system, the rays from the source enter the aperture zone get reflected at the reflector and imaged on one single point at the exit

Table 1 Selection of solar collector types based on the working temperatures. Source: Goswami (2007) and POSHIP (2001). Temperature range

Process

40 °C 40–70 °C 70–100 °C Above 100 °C

Unglazed selective collectors or low cost standard flat plate collectors. Highly selective flat plate collectors or CPC collectors. CPC collectors, evacuated tubes or other high efficient stationary collectors; concentrating collectors for medium and large systems. Concentrating collectors; evacuated tubes with CPC collectors.

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aperture, whereas in non-imaging systems, all rays entering the entrance aperture leaves the exit aperture somewhere. Non-imaging concentrators do not provide defined images of the sun on the absorber instead radiation is distributed from entire parts of the reflectors onto entire absorber surface. Based on the studies, the heat demand in the industrial sector has been confirmed with more than 50% of industrial heat demand, which can be categorized as low (< 60 °C), medium (60–150 °C) and mediumhigh (150–250 °C) (Goswami, 2007), especially in high demand in the food industry, pulp and paper industry and textile industry. The heat demand for a large number of industries in Spain and Portugal were analyzed under POSHIP project (European research project, funded by the European Commission within the 5th Framework Program). Based on POSHIP (2001) data, the selection of solar collector types w.r.t. the working temperatures are represented in Table 1. For industrial process heating, the two major types of collectors are non-tracking (stationary) collectors and single-axis tracking parabolic trough collectors (Kalogirou, 2003). Kalogirou (2003) discussed about the solar industrial process heat application requirements based on temperature range from 60 °C to 260 °C. He also presented an overview of efficiency, cost of existing technologies and characteristics of medium to medium-high temperature solar collectors. The author suggested that the stationary CPC solar collector of tubular/flat absorber type with a concentration ratio (CR) ranging from 1 to 5 is capable of producing a temperature range of 60–240 °C, which can be utilized for various process heat applications. Table 2 shows different fields of solar thermal energy applications (Industrial sectors, domestic heating applications and sorption systems) based on different temperature levels from medium to low-high temperatures. The solar operated absorption cycle air conditioning system (Single effect, H20 – LiBr pair) needs a minimum useful temperature range of 75–80 °C, as the driving potential for operation of vapor absorption refrigeration system (VARS) and the energy below the mentioned range may not be helpful (Duffie and Beckman, 2013; Hsieh, 1986; Wang and Ge, 2016). A summary of the typical heating medium temperature required for the operation of sorption cycles for a heat rejection temperature of 30 °C and 50 °C for cooling and heating applications respectively, with an evaporating temperature of 2 °C is shown in Table 2 (Wang and Ge, 2016). Also, the sorption cooling/heating sectors given in Table 2 inferred that the cooling applications require a heating medium in the temperature range of 70–140 °C, whereas the heating applications require higher temperature of 110–140 °C. Therefore, the thermal storage of high temperature requires storage in the range from 70 °C to 145 °C. Rabl et al. (1980) stated that the performance of lower concentration of 3X (CPC solar collector) was significantly better than double glazed flat plate collector of ≥∼70 °C and competitive below, with a requirement of only semi-annual adjustment for its annual operation. Carvalho et al. (1995) computed the average yearly performance for the developed CPC solar collector with an inverted “V” shaped receiver and compared it with other collector types (FPC, ETC) and found that up to 100 °C, the proposed collector outperformed the others and also the cost was comparable. Kalogirou (2003) analyzed five collector types such as simple stationary FPC with a slope of 40°, advanced flat plate collector, stationary CPC collector orientated with its long axis in the East-West (E-W) direction tilted at 35° (local latitude), ETC sloped at 40° and PTC with EW tracking. FPC can obtain up to 100 °C with good efficiencies. In advance flat plate collector (AFPC), the heat conduction gets improved by using ultrasonic-welding machines. ETC efficiencies are higher at low incidence angles that make ETC advantageous over FPC in day-long performance. PTC can effectively generate heat between 50 °C and 400 °C. No more energy can be collected by FPC for the demand temperature of about 90 °C and above. However, the author concluded that FPC is more suitable for low temperature applications and the concentrating collectors for the higher ones.

Table 2 Temperature ranges (35–260 °C) for different applications. Source: Kalogirou (2003), Pouyfaucon and García-Rodríguez Quaschning (2005) and Wang and Ge (2016). Sl. No.

Sector

A. 1

Industrial Sectors Dairy

2

Tinned food

3

Textile

4

Paper

5

Chemical

6

Meat

7

Beverages

8

Flours and byproducts Timber by-products

9

10 11

Bricks and blocks Plastics

B.

Domestic hot water applications

1

3 4 5 6 C. 1 2 3 4 5

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Process

Temperature range (o C)

Pressurization Sterilization Drying Concentrates Boiler feed water Sterilization Pasteurization Cooking Bleaching Bleaching, dyeing Drying, degreasing Dyeing Fixing Pressing Cooking, drying Boiler feed water Bleaching Soaps Synthetic rubber Processing heat Pre-heating water Washing, sterilization Cooking Washing, sterilization Pasteurization Sterilization

60–80 100–120 120–180 60–80 60–90 110–120 60–80 60–90 60–90 60–90 100–130 70–90 160–180 80–100 60–80 60–90 130–150 200–260 150–200 120–180 60–90 60–90

Thermo diffusion beams Drying Pre-heating water Preparation pulp Curing Preparation Distillation Separation Extension Drying Blending

80–100

Dishwashing (per day per person) Hand washing (per day per person) Bath tub (per day per person) Shower (per day per person) Hair wash (per day per person) Desalination

2

Sorption cooling/ heating applications

(2018),

Half effect H2O-LiBr Single effect H2OLiBr Double effect H2OLiBr Single effect NH3H2O Adsorption H2Osilica gel

90–100 60–80 60–70 60–80

60–100 60–90 120–170 60–140 120–140 140–150 200–220 140–160 180–200 120–140 Temperature (°C) /Consumption (l)/ Useful heat (Wh) 50/12–15/550–700 37/3–5/95–160 40/150/5,200 37/30–45/940–1400 37/10–15/310–470 60 to 90 °C/- /Temperature (°C) (Cooling/Heating) and (COPcooling/COPheating) 67/108 and 0.35/1.33 92/- and 0.72/142/- and 1.29/86/140 and 0.6/1.52 90/120 and 0.41/1.26

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At low operating temperatures, the storage tank size for superior collector (PTC) requires less storage, whereas, at higher operating temperatures, PTC requires more storage. Comparatively, poor performance collectors (FPC and AFPC) are insensitive to storage volume. Based on the solar contribution at low temperatures, AFP provides best result, whereas at higher temperatures, it is PTC and CPC is found superior to ETC at all temperature levels. Based on life cycle savings (LCSs) for non-subsidized fuel price, the following conclusions were obtained: PTC cannot be used effectively for low temperature applications, ETC is very expensive, CPC shows a much better behavior, AFPC offers good performance compared to their cost and FPC obtains maximum LCS at an operating temperature of 90 °C and no more extra benefit at higher temperatures. The cheaper the collector, lower the heat price value obtained. When the subsidized and non-subsidized values of fuel (fight fuel oil – LFO) are considered for the solar heat price analysis, the following results were observed: in the first case, only FPC and AFPC collector at 60 °C demand temperature provide heat price lower than that of the fuel, whereas in the second case, all collectors provide lower heat prices than that of the fuel. The initial cost of the solar system and fuel cost play an important role in deciding the economic feasibility of each system. Pei et al. (2012) concluded that for the low temperature applications such as residential hot water and floor heating, evacuated tube solar water heater (SWH) system without a mini-CPC reflector is suitable because of its higher thermal and exergy efficiencies, whereas for the high temperature applications such as air conditioning, refrigeration, sea water desalination, and industrial heating, solar low-temperature heat power generation, evacuated tube SWH system with a

mini-CPC reflector is suitable because of its higher thermal and exergy efficiencies. Kim et al. (2013) showed that the test results for the proposed model of CPC solar collector (Counter-flow type) outperformed other non-tracking solar thermal systems in both simulation and in experiment (Configurations were counter-flow evacuated tube CPC, evacuated tube CPC with heat pipe and evacuated tube heat pipe without concentrator). The proposed system of above 100 °C with high performance was superior to other non-tracking thermal collector systems. Widyolar et al. (2014) and Winston et al. (2014) experimentally proved that the XCPC solar collector is capable of providing thermal power for space cooling applications. It is inferred from the above information concerning the various temperature requirements in different sectors, such as solar cooling, domestic heating and industrial process heating applications can be achieved by using stationary CPC solar collector. Moreover, the advantages of CPC are listed below: (a) It can attain extensive achievable view area for a given geometric concentration, thereby, obtaining a useful concentration without tracking mechanism for low to moderate concentration CPC collectors (O’Gallagher, 2008). (b) A significant fraction of diffuse insolation gets collected by CPC collector, whereas in conventional focusing concentrators there will be ample losses. The efficiency of CPC collectors in accepting diffuse light is much larger than focusing collectors (Winston, 1974). (c) Concentrating the diffuse radiation is possible in CPC collector, whereas in an imaging collector it is impossible (Wang et al., 2014). (d) There is no observable loss in performance due to truncation of

Fig. 3. Types of CPC solar thermal collector. 297

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(e) (f)

(g)

(h) (i)

(j) (k)

CPC. Truncation can be significant when it is viewed from optical gains, explicitly based on following criteria: (i) Acceptance of both beam and diffuse radiations get increased and (ii) Average number of reflections gets reduced (Carvalho et al., 1985; Ustaoglu et al., 2016). Among FPC, ETC and XCPC solar collector, the XCPC solar collector requires a lesser area for the generation of the same amount of useful energy gain (UEG) (Sobhansarbandi and Atikol, 2015). A combination approach with the CPC as secondary stage or terminal concentrator for conventional parabolic or Fresnel mirrors results in the requirement of an ideal concentrator with smaller reflector and also offers considerable benefit in high temperature solar systems (Rabl, 1976b). Several intermediate angle CPCs are combined to form a “fly eye” second stage or terminal concentrator which helps to meet the requirement of a tower with a central receiver without sacrificing concentration (Rabl, 1976b). It is capable to collect diffuse light even during cloudy or hazy days, which is especially important in regions with high diffuse percentage (Widyolar et al., 2014; Winston et al., 2014). The CPC solar collector may be cheaper than PTC for achieving temperature up to 300 °C, due to non-requirement of tracking system and also collects a part of diffuse radiation (Karwa et al., 2015). Compared to traditional solar collector, CPC solar collector has higher efficiency (Lu et al., 2013). XCPC solar collectors can accommodate installation misalignment (Widyolar et al., 2018).

Fig. 5. Cross section of ICS solar heater with CPC reflectors (Kessentini and Bouden, 2016).

2.1. Based on construction 2.1.1. Serpentine compound parabolic concentrator (SCPC) solar collector Fig. 4 shows the schematic of the SCPC solar collector which combines the structure of a flat plate collector with the CPC solar collector. This solar collector combines the advantages of concentrating collector and flat plate collector. This combination improved its thermal efficiency, reduced the heat loss and achieved high freezing resistance (Zheng et al., 2016). 2.1.2. Integrated CPC solar collector Fig. 5 shows the integrated CPC solar collector with inbuilt storage tanks, T1 and T2, which are used to store the hot water generated during the sunny hours of the day and utilize it when there is a demand.

Based on the above technical information, a comprehensive review for the CPC solar collector is carried out in terms of types, historical growth in milestones, concept, design strategies, heat transfer fluids, studies performed with major parameters of CPC solar collector (Experimental studies, Theoretical studies – Numerical and Simulation studies), applications, standards and certifications for solar collector, research publication status, market players, cost analysis, recent development and installation. The review article is mainly focused on the study of two-dimensional (2D) CPC solar thermal collector.

2.1.3. Number of stages in CPC solar collector Based on the stages, CPC is categorized into one stage and two stages. In one stage CPC, there is only one stage, whereas in two stages, there are two portions as shown in Fig. 6 and it is suitable for photovoltaic (PV) applications (Rabl, 1976c). A two stage CPC trough is preferred over single stage system for concentrations above ten, as it can reach closer to an ideal limit without excessive reflector/aperture ratio and transmission loss (Rabl, 1976b).

2. Types of CPC solar collectors

2.1.4. Dimension (shape) of CPC solar collector Based on the dimension (shape), CPC is categorized as two-dimensional (2D) concentrator (Trough) and three-dimensional (3D) concentrator (3D crossed CPC) as shown in Fig. 7. Based on the truncation of the concentrators (reflectors), the collectors are further classified as truncated and full CPC solar collectors, which are discussed in the

Based on the literature survey, the authors classified the CPC solar thermal collectors (mainly focusing the 2D (trough type)) as illustrated in Fig. 3 and their descriptions are described below.

Fig. 4. Schematic of the SCPC solar collector (Zheng et al., 2016). 298

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flat, one-sided absorber CPC solar collector for computing the thermal performance of an air heater (Fig. 9), whereas Gu et al. (2014) conducted experiments for non-tracking CPC solar collector with flat two-sided, horizontal position absorber (Fig. 10). The flat, one-sided absorber configuration of CPC solar collector is of first generation collector (Allen et al., 1975; Giugler et al., 1975; Rabl et al., 1975; Rabl et al., 1980) and a cavity is used as the absorber. Due to high absorptance of the absorber, it is supposed to outweigh the heat losses. After analysis of this configuration, the industry preferred the absorber of this configuration should have a selective coating because of high heat losses from the cavity and becomes undesirable. 2.1.5.3. Inverted vee shaped (wedge type) and fin type absorber. Rabl et al. (1979) discussed about various types of CPC solar collector based on the absorber surface such as flat, fin, inverted vee shaped (wedge) and tube absorber surface. Inverted vee shaped and fin absorber surface with CPC are represented in the Fig. 11 (a) and (b) respectively. Rabl et al. (1980) stated that heat losses through the back of flat, one-sided absorber, wedge like absorber and through the reflector could be serious. Hence, they suggested that for “backless” configurations, such as flat two-sided absorber and tubular absorber as shown in Fig. 11 (b) and 8 respectively, conduction heat transfer through the back is minimal. The heat loss reduction more than compensates for the circumstance that the average number of reflections is ∼50% higher than for flat one-sided absorber, CPC configuration. “Backless” configurations are preferable due to the following economic reasons: i) They require only half of the relatively expensive absorber material, ii) They are less deep and iii) They have less reflector surface. Fig. 12 shows the cross-sectional view of the CPC solar collector with inverted “V” shaped absorber of the CPC solar collector which was described and tested by Carvalho et al. (1995). This type differs from the previously explained wedge type absorber (Fig. 11 (a)), although the names are similar (for differentiation this type is named with letter “V” instead of “Vee”). In this type, the absorbers are not in contact with the concentrator (reflector) and the copper tube located at the center has two wings (absorber) of thin aluminum plates through which HTF passes.

Fig. 6. Two stage CPC solar collector (Rabl, 1976c).

‘concept and design strategies’ section. 3D crossed CPC type collectors are used in PV applications (Mammo et al., 2013). 2.1.5. Absorber surface The CPC (2D type) is sub-classified based on the absorber surface types: such as (i) external type CPC solar collector (Tube type absorber), (ii) flat, one-sided and two-sided absorber types, (iii) inverted vee shaped (wedge type) and fin type absorber and (iv) heat extraction by the flat sheet (roll bond) and individual pipes. 2.1.5.1. External type CPC solar collector (Tube type absorber). Fig. 8 shows a schematic cross-sectional view of the different types of CPC solar collector based on the number of envelopes which are provided over the absorber for reducing the convective heat loss. For the evacuated tube CPC solar collector, the portion between the envelope and absorber is made vacuum to reduce the heat loss and to increase the UEG. As the tube is placed above the CPC with a gap between the reflector and tube, it is called as external type CPC (XCPC) solar collector. Lambert (2007) categorized the evacuated tube based on the vacuum levels as soft vacuum (down to ∼10−3 atm) and hard vacuum (down to < 10−6 atm) tubes. The soft vacuum eliminates convection, but not conduction which requires hard vacuum. Rabl et al. (1980) stated that the design problems among the non-evacuated and evacuated receiver CPC solar collectors are entirely different, such as the heat loss through the reflector, which plays a vital role. Gu et al. (2014) experimentally showed that the achieved stagnation temperature for the proposed CPC collector was 118 °C at 1000 W/m2 without vacuum and under vacuum the temperature was increased to 338 °C.

2.1.5.4. Heat extraction by flat sheet (Roll bond) and individual pipes. Fig. 13 (a) shows several CPC troughs combined in a single collector panel with the heat extraction by flat sheet such as roll bond (Rabl, 1976c). This type is desired for heat extraction rather than individual pipes as shown in Fig. 13(b). With CR above five, individual absorbers will probably provide better performance. The CPC with absorber on all sides (such as XCPC) has practically no back losses because it has no back. It is attractive because they require only half the absorber material needed for the ordinary CPC, thereby, it reduces the cost and thermal inertia.

2.1.5.2. Flat one-sided and two-sided absorber type. Tchinda (2008) used

Fig. 7. (a) Trough shaped concentrator (Winston, 1974) and (b) 3D Crossed CPC collector (Mammo et al., 2013). 299

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Fig. 8. Schematic cross section of the CPC solar collector with (a) Two envelopes (b) Single envelope and (c) no envelope (Non-evacuated tube) (Prapas et al., 1987).

Fig. 11. Schematic cross section of the CPC solar collector with (a) inverted vee shaped (wedge) and (b) Fin absorbers (absorber perimeter ‘a’ and acceptance half angle θa) (Rabl et al., 1979).

Fig. 9. Schematic cross section of CPC solar collector with flat, one-sided absorber (Tchinda, 2008).

Fig. 12. Schematic cross section of the CPC solar collector with inverted “V” shaped (Carvalho et al., 1995).

2.1.6. Reflector (Asymmetric and Symmetric CPC solar collector) In an asymmetric CPC solar collector, the reflectors are not symmetric to the axis of CPC ie., each parabolic profile varies from other. Harmim et al. (2012) used asymmetric CPC for the study of solar cooker (as shown in Fig. 14), whereas in the symmetric CPC solar collector, the reflectors are symmetric to the axis of CPC ie., each parabolic profile is identical (as shown in Fig. 12). Adsten et al. (2005) stated that the annual solar radiation at high latitudes will be asymmetric over the year, thereby suggesting the collector should be asymmetrically truncated for maximizing the yield. This leads to the development of a design concept consisting of an asymmetric truncated CPC collector with a bi-facial absorber and without any tracking mechanism. As the main objective of their work was to maximize the reflector to the absorber area for a given ground

Fig. 10. Schematic cross section of CPC solar collector with flat, two-sided, horizontal position absorber (Gu et al., 2014).

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Fig. 13. CPC solar collector panel with heat extraction by (a) flat sheet (Roll bond) and (b) individual pipes (Rabl, 1976c).

area, this collector is called as a Maximum Reflector Collector (MaReCo). Knowledge about the angular distribution of the annual solar radiation is required for designing the solar concentrator. The advantage of an asymmetric CPC solar thermal collector is due to a narrow acceptance half angle covering the summer solstice peak, which results in a high concentration ratio, thereby accepting a high fraction of the available radiation.

Fig. 15. Evacuated tube collectors using heat pipe (Kim et al., 2013).

difference between inlet and outlet, highest outlet temperature of the fluid can be obtained for the inlet fluid temperature and heat transfer rate throughout the heat exchanger is more uniform because of uniformity in temperature difference in the tube (Kim et al., 2013). Duong and Diaz (2014) used coaxial copper pipe in the numerical simulation study of XCPC solar collector and the cross- sectional view of the evacuated tube collector portion is shown in Fig. 18.

2.2. Based on the flow path of HTF Based on the flow path of HTF, the CPC solar collector can be subcategorized as below and these types can also be considered under construction sub-classification. 2.2.1. Evacuated tube collectors using heat pipe Heat pipe connected to the cylindrical shaped metal absorber fin coated with a selective material (low emittance of < 0.1 and high absorptivity of > 0.9) is depicted in Fig. 15 (Kim et al., 2013). Fig. 16 also shows the evacuated tube collector using heat pipe. The main advantage is the plumbing connection between the absorber and the manifold is not required. Due to its simplicity, it is widely used for low temperature applications. The two factors that govern the heat pipe performance are its size and effectiveness of the condenser.

2.2.3. Number of passes and flow through evacuated glass tube Fig. 19 represents two passes CPC solar collector with evacuated tube and U-shaped pipe. Usually, copper pipe is used, which is held by the conducting strip and HTF passes inside the pipe. Kim et al., (2008) carried out numerical and experimental studies using CPC solar collector with evacuated tube and single pass absorber tube (as shown in Fig. 20). Also, Fig. 21 shows the evacuated tube with single pass absorber tube where one end of the inner tube has a spiral coil portion and the other end is plain. The spiral coil portion takes care of the thermal stress, which occurs during heating of the glass tubes. The outer surface of the inner tube is selectively coated with copper oxide for the effective rise in temperature of the absorbing surface.

2.2.2. Evacuated tube collector with coaxial pipe Fig. 17 corresponds to a counter-flow tube which consists of a coaxial pipe attached to the absorber fin which makes the directions of the working fluids opposite. The counter-flow tubes have several advantages over heat pipes such as it minimizes the thermal stresses throughout the tubes by maintaining the constant temperature

Fig. 14. Asymmetric CPC solar collector integrated to the box type solar cooker (Harmim et al., 2012). 301

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Fig. 16. Schematic sketch of Evacuated tube collectors (Kalogirou, 2004; Suman et al., 2015).

angle adjustment, which are discussed below. 2.3.1. Collector outlet temperature Srivastva et al. (2015) described different types of collector based on the operating temperature as follows: (i) Low temperature collectors (Operating range from above ambient to about 80 °C, used in solar water heating application, solar based space heating and cooling applications, solar ice making etc.,); (ii) Medium temperature collectors (Operating temperature range of 85–270 °C, used for process heating applications such as power, textiles, processing of raw materials etc.,); (iii) High temperature collectors (Operating temperature exceeding 300 °C, used for industrial process heat and electricity generation). Prapas et al. (1987) presented the achievable absorber temperature for various CPC solar collector configurations.

Fig. 17. Evacuated tube collector with counter flow absorber tube (Kim et al., 2013).

2.3. Based on the concentration ratio Kothdiwala et al. (1995) concluded their study by the following statements: Under conditions of high beam component insolation, the highest CR will be the most efficient CPC whereas with low beam insolation and high diffuse insolation, the most efficient CPC will be the lowest CR. Also, the authors suggested that a moderate to high insolation intensity (> 800 W/m2) would be having high beam insolation and therefore, in general higher CR CPC would be used. Concentration ratio is further classified based on collector outlet temperature and tilt

2.3.2. Tilt angle adjustment Rabl (1976b) arrived the collector tilt angle adjustment based on the collector normal pointing at an angle θa above the solar noon elevation

Fig. 18. Evacuated tube collector with coaxial pipe (Duong and Diaz, 2014). 302

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Fig. 19. Two passes CPC Solar collector with evacuated tube and U-shaped pipe (Pei et al., 2012).

CR ≤ 2 (Adjustment not required) CR = 3 (Biannual adjustment is required) CR ≈ 10 (Daily adjustment is required) and these systems are also called as quasi-static. 2.4. Based on operating pressure In pressurized system, the flow in the pipe is occurred by an external source (pump) or maintaining sufficient static height. In this type, the solar collector is generally designed to withstand a maximum pressure of 10 bar (LR, 2018; Taylormade, 2018). In non- pressurized system, flow in the pipe occurs through thermosyphon effect (Okoronkwo et al., 2014; Liu et al., 2013). 2.5. Based on tracking system Based on the concentration ratio, the requirement of tracking system will be decided for CPC solar collector. The collector with tracking mechanism (Kim et al., 2008) tracked the movement of the sun that cause the incident solar radiations always to fall perpendicular to the collector, whereas the non-tracking (Hu et al., 2011; Pei et al., 2012; Sagade et al., 2014; Umair et al., 2014; Ustaoglu et al., 2016) collector had fixed position and also called as stationary collector.

Fig. 20. CPC solar collector with evacuated tube and single pass absorber tube (Kim et al., 2008).

of 23o27′ during the summer solstice and their details are represented in the Table 3. Rabl et al. (1980) stated that the required frequency of tilt adjustments for CPC concentrations of 6X and 3X are 12–20 times per year and semi-annual adjustments per year respectively. Gudekar et al. (2013) proposed CPC with an acceptance angle of 6°, requiring once a day tilt adjustment for a daily operation of 6 h was designed to achieve steam temperature up to 150 °C. Also, Kalogirou (2013) and Wang and Ge (2016) provided the general guidelines for the required frequency of collector tilt adjustment based on its CR, which are listed below:

3. Historical growth in milestones The evolution of CPC collector started during 1960’s and only very recently, this technology attained the stage of commercialization for various applications. The various milestones occurred during the evolution period are briefly presented in this section. Tabor (1958) explained the method of using an E-W cylindrical

Fig. 21. Evacuated tube collector where the HTF passes through a glass tube with one end spiral coils and the other end is plain. 303

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Table 3 Collector tilt adjustments. Require minimum collection time 7 h/day (except for θa = 5.5° min. collection time = 6.78 h/day). Source: Rabl (1976b). Acceptance half angle (θa – degree)

Concentration of full CPC

Collection Time average over year (hour/day)

No. of Adjustment/annum

Average collection time if tilt adjusted every day (hour/day)

19.5 14 11 9 8 7 6.5 6 5.5

3.00 4.13 5.24 6.39 7.19 8.21 8.83 9.57 10.43

9.22 8.76 8.60 8.38 8.22 8.04 7.96 7.78 7.60

2 4 6 10 14 20 26 80 84

10.72 10.04 9.52 9.08 8.82 8.54 8.36 8.18 8.00

parabolic mirror (concentrator) without diurnal movement and its tilt was adjusted at certain periods of the year. Tabor (1966) presented various configurations of boosting the radiation with the help of side mirrors. The fundamental concept of the CPC was initially developed by Roland Winston during 1966 for the Cherenkov radiation study while working at Argonne National Laboratory (Hinterberger and Winston, 1966; Welford and Winston, 1989; Winston, 2016). Almost simultaneously described 3D CPC as ‘FOCON’ and suggested its utilization for solar energy collection (Baranov, 1965; Baranov and Melnikov, 1966; Welford and Winston, 1989). Certain illumination properties of 2D CPC were described. ‘FOCLIN’ term was used to define 2D CPC. For several CPC configurations Baranov obtained Soviet patents (Baranov 1967; Welford and Winston, 1989). In 1967, Ploke (Ploke, 1967; Ploke, 1969; Welford and Winston, 1989) explained the axially symmetric CPC with generalization to design incorporating refracting elements in addition to the light guiding reflecting wall and in 1969, he obtained a German patent for various photometric applications. Winston (1970) derived an appropriate generalization of Abbe’s sine law (sine inequality) from phase space considerations. The constructed 2D and 3D non-imaging systems reduced the f number (ratio of focal length/aperture diameter) to minimum allowable value by the sine inequality. Winston (1974) discussed the 2D CPC configurations such as trough shaped concentrator with energy receiver and concentrating flat plate configurations. The use of CPC for solar energy collection for various applications like central power, cooling and heating of buildings was recognized. The fraction of total sky light (isotropic radiation) collected when compared to an FPC is precisely the reciprocal of the concentration factor. Rabl (1976b), Rabl (1976c), Winston (1970) and Winston and Hinterberger (1975) proved that highest concentration factor (Ideal case) achievable for 2D CPC is n/Sin θa and 3D CPC is n2/Sin2 θa. The 2D CPC design principles are provided in United States of America (U.S.) patents (Winston, 1976; Winston, 1977a; Winston, 1977b). Rabl (1976b) proposed the use of CPC as second stage concentrators for conventional parabolic or Fresnel mirrors. Rabl (1976c) presented the formulas for evaluating the performance of solar collectors based on CPC principle. Rabl (1976a) derived the differential equation which describes the reflector of an ideal 2D radiation concentrator with an absorber of arbitrary convex shape. He also presented the formulas for determining the attenuation of radiation from aperture to absorber by considering the effect of absorption at the reflector. Rabl (1977) discussed the relationship between the fraction of radiation transmitted (τcpc), reflectivity of reflector (ρ) and average number of reflections (〈n〉) is expressed as τcpc ≈ ρ〈n〉. He developed a new technique for approximating exchange factors for specular radiation passages which shows the computation of average number of reflections that can be found by a simple analytic formula without ray tracing. McIntire (1979) presented the reflector shapes for truncated non-imaging cusp concentrators with cylindrical absorber having various acceptance angles. Similarly, curves for height/aperture and mirror arc length/aperture ratios versus CR were presented and these curves have special

significance for thermoformed plastic reflector substrates. McIntire (1980a) developed a reflector that removes the loss of solar radiation through the gap between the tubular absorber and the reflector by considering a reflector of ‘W’ faceted shape at the bottom. Higher optical efficiencies were obtained for the new design by the elimination of gap losses and enhancement of net absorptance of the receiver tubes. McIntire (1980b) carried out the detailed study optimization of stationary non-imaging reflectors for tubular evacuated receivers aligned in N-S direction by considering the effects of reflection losses, reflector-receiver alignment errors, variation of selective surface absorptance with incidence angle on the receiver and gap losses between receiver and reflector. Rabl et al. (1980) designed and discussed about four different absorber configurations such as flat one-sided, fin (flat, two-sided), wedge (inverted-vee) like and tubular types CPC, which leads to the development of a range of different reflector design. Winston (1980) designed V-grooved cavity of the lower (facet) portion of the reflector for augmentation of absorption by the receiver with a gap between them. McIntire (1984) presented the design parameters for faceted concentrator such as largest gaps allowed for given numbers of facets. Carvalho et al. (1985) derived analytical expressions for the average number of reflections and angular acceptance function of 2D CPC solar collectors with the tubular absorber of arbitrary degree of truncation and represented graphically for various acceptance angles and different truncations. The authors suggested that for optimal orientation at large acceptance angles of CPC, N-S orientation is preferable rather than E-W orientation. Chakraverty et al. (1987) developed an analysis in order to study the performance of a CPC collector for time source input functions such as solar intensity and ambient temperature. Fasulo et al. (1987) developed a CPC collector with low thermal losses by analyzing cases with different tilt angle w.r.t. thermal losses and efficiency. Prapas et al. (1987) carried out theoretical and experimental studies on CPC solar collector with different absorber configurations. El-Assy (1988) studied CPCs in two-phase flows and their thermal performance were analyzed. Chew et al. (1988) performed the free convective heat transfer analysis between cylindrical absorber and flat aperture surface of CPC for various configurations (absorber temperatures and truncations). Chew et al. (1989) analyzed the numerical and experimental results of laminar free convective heat transfer between tubular absorber and flat aperture surface of the CPC solar collector. Eames and Norton (1991) analyzed numerically the effects of ambient temperature and radiation level on the efficiency of the CPC solar collector. Norton et al. (1994) and Kothdiwala et al. (1995) developed the theoretical (numerical) model of thermal transfer in a line-axis, E-W oriented, symmetric, CPC solar collector, which described its steady state thermal behavior. Also, they investigated the effect of the tilt angle of CPC system ie., the latitudinal and tracking configuration on performance. Khonkar and Sayigh (1994) applied AutoCAD as one of accurate ray tracing techniques for the investigation of hot spots location on the tubular absorber of CPC solar collector. They calculated the profile of CPC and absorber, analyzed the intensity distribution around 304

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the circumference of the absorber and analyzed the phenomena of rays inside CPC at different incidence angles. Kothdiwala et al. (1995) suggested the validity of the model is only for low concentration CPCs of CR less than 5, although the model can predict the CPC performance of high concentration. Eames and Norton (1995) investigated theoretically and experimentally the introduction of a transparent baffle into the cavity of non-evacuated CPC solar collector on the basics of modifications in optical and thermal performance. Carvalho et al. (1995) developed a novel CPC solar collector with an inverted “V” shaped receiver, which was described and tested optically and thermally in different configurations. Ronnelid and Karlsson (1996) performed heat loss measurements on a V-trough collector model. They also discussed about the conversion of experimentally measured heat losses into heat losses for actual collector and also the practical material considerations. Tchinda et al. (1998) presented the analysis of heat transfers in the CPC solar collector, including axial heat transfer in the receiver. They also studied the dynamic behavior of collector on the basis of the influence of different parameters like inlet HTF temperature and mass flow rate. Two new technologies of integrated CPC reflector evacuated solar collector and the solar operated double effect absorption chiller were demonstrated first time in a commercial building in the year 1998 (Duff et al., 2004). Fraidenraich et al. (1999) used non-evacuated CPC solar collectors with a cylindrical absorber to describe a mathematical model for the optical and thermal performance, where the heat loss coefficient is temperature-dependent. They developed a new way of plotting solar thermal collector efficiency, which enabled the representation of the performance of a large class of solar collectors, such as flat plate, CPC and parabolic troughs, such that measurements for a broad range of solar radiation levels can be unified into a single curve, where the heat loss functions are represented by second degree polynomials. TamainotTelto and Critoph (1999) developed a prototype of CPC solar collector (for a sorption refrigeration) and one dimensional (1D) numerical model of isobaric absorber resulting to predict the maximum bed (granular or monolithic carbon) temperature up to 172 °C. Kothdiwala et al. (2000) proposed the correlation between Nusselt and Grashof numbers for convective heat transfer in CPC solar collector aligned in E-W orientation. They also discussed the discrepancies between these correlations. Various configurations are considered ranging from a tubular absorber with or without an envelope, with different truncation levels, with one or two concentric envelopes, with different eccentric envelopes and inclination angles. Brogren et al. (2001) discussed a water-cooled PV thermal hybrid system with low concentrating aluminum CPC. Oommen and Jayaraman (2001) designed and fabricated CPC solar collector with tubular absorber, oversized reflector and thereby reduced the gap losses. Florides et al. (2002a) discussed about the modelling, simulation of a domestic size solar based absorption cooling system using Transient Systems Simulation (TRNSYS) for Nicosia, Cyprus weather condition and evaluated the total equivalent warming impact (TEWI) of cooling unit. Also, they conducted experiments with the solar cooling system. Florides et al. (2002b) investigated the performance and economic viability of different solar collectors (FPC, CPC and ETC) for the operation of absorption cooling system using TRNSYS. Adsten et al. (2005) developed an asymmetrical truncated non-tracking CPC solar collector and it is called as Maximum Reflector Collector (MaReCo). Pramuang and Exell (2005) carried out a transient test of solar air heater using truncated CPC with flat absorber painted with non-selective matt black (Chungpaibulpatana and Exell, 1990). Tchinda and Ngos (2006) and Tchinda (2008) theoretically (mathematical model) studied the thermal process in CPC collector with a flat one-sided absorber. Lambert (2007) designed and performed solar power adsorption heat pump for residential cooling and heating by using CPC and evacuated flat panel. Kaiyan et al. (2007) developed an imaging 3D CPC with prominent characteristics. Kim et al. (2008) compared the conventional stationary and single axis tracking CPC solar collector.

Rodríguez et al. (2010) studied the performance of the solar water disinfection using TiO2 and Ru (II) complex as fixed catalysts located in a CPC collector photoreactor, in the laboratory as well as at a greenfield site. Buttinger et al. (2010) developed and analyzed a new non-tracking CPC with different configuration of reflectors (symmetrical/asymmetrical), tilt angles, CRs and thin gases (Krypton/air). Gang et al. (2010) used small CR CPCs for the operation of innovative configuration of low temperature solar thermal electric generation with regenerative Organic Rankine Cycle (ORC). Sharma and Diaz (2011) numerically investigated the performance of a novel minichannel based solar collector for the configurations with and without CPC. Hang and Qu (2011) optimized an integrated solar absorption cooling and heating system consisting of non-tracking XCPC solar collector and accessories by using Energy plus and TRNSYS. Kaiyan et al. (2011) developed a novel multiple curved surfaces compound concentrator which was composed of a parabolic and a flat contour and a comparison between the traditional paraboloid, CPC and proposed concentrator were carried out. Nkwetta and Smyth (2012) used computer based simulation software ‘Eazea’ and Tecplot for the generation of innovative single-sided absorber CPC and double-sided absorber CPC solar collectors (in both full and truncated forms). The generated coordinates for these solar collectors were moved to ‘AUTOCAD’ formats for the following purposes (i) to represent the profiles in a ‘dxf’ format, (ii) to enable the construction of reflector support and profiles. Harmim et al. (2012) introduced a novel solar cooker of boxtype equipped with an asymmetric CPC as booster-reflector with a vertical double-glazing cover on one side and a vertical absorber plate behind the transparent cover. They also studied the dynamic behavior of the cooker by mathematical model. Horta et al. (2012) adopted internal convection control strategies for the thermal assessment of nonevacuated CPC concentrators for applications at constant operating temperature. Gudekar et al. (2013) presented the fabricated CPC solar collector for steam generation, which showed better performance and reduction in cost when compared to the conventional CPC solar collector. Kim et al. (2013) modeled a stationary solar thermal collector system working for medium temperature (100–300 °C). Lu et al. (2013) used new medium CPC solar collector for the experimental investigation of solar adsorption and LiBr-H2O absorption chiller. Sagade et al. (2014) conducted experimentation on the prototype compound parabolic trough for its performance study with different combinations of receivers and its surface coating. Widyolar et al. (2014) and Winston et al. (2014) experimentally studied the non-tracking XCPC solar collector performance in driving the absorption chilling system and this was the first system with the combination of XCPC and double effect absorption chilling technologies. Gu et al. (2014) analyzed experimentally and optically an innovative portable solar collector with a non-tracking CPC solar collector with flat plate absorber. Okoronkwo et al. (2014) studied experimentally the performance of a thermos-syphon water heating system with CPC solar collector. Wang et al. (2014) carried out experimental and numerical studies for simplified CPC solar air heater. Lu and Wang (2014) investigated the performance and economic analysis for three solar cooling systems with different configurations in collectors and sorption chillers, one among the configuration was high efficient evacuated tube CPC solar collector with single effect LiBr absorption chillers. Antonelli et al. (2014) studied the Organic Rankine Cycle with CPC solar collectors as the heat source collector and by using simulation tool AMESim v. 12, a numerical model was developed. Santos-González et al. (2014) used 1D numerical model for designing CPC solar collector and Mexican standard was applied for the experimentation. Also, ray tracing analysis was performed. Duong and Diaz (2014) simulated numerically the XCPCs combined with ETCs and PTCs with carbon dioxide (CO2) as HTF for medium and high temperatures. Almeida et al. (2014) carried out a simulation study using TRNSYS and GENOPT for 305

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themosyphon and forced circulation systems with different collectors (including CPC solar collector under thermosyphon system) and compared it with results using ISS (In Situ scientific software), v2.7. Chamsa-ard et al. (2014) used ISO standard for testing the heat pipe evacuated tube with CPC solar collector. Jiang et al. (2015) presented the performance analysis of two novel mid temperature CPC solar thermal collector (XCPC and Integrated CPC). Wang et al. (2015) studied the performance improvement of the simplified CPC solar collector (using resilient stainless steel mesh) from their previous study (Wang et al., 2014). Sobhansarbandi and Atikol (2015) used TRNSYS for showing the possibility of using CPC solar collectors instead of FPC for under floor heating system. Benrejeb et al. (2015) designed a new integrated collector storage solar water heater with two concentrating stages and the optical and thermal performances were analyzed. Karwa et al. (2015) conducted thermohydraulic performance of a bifacially irradiated receiver (a combination of flat and U-tube receiver) in a vacuum enclosure, CPC solar collector. Kessentini and Bouden (2016) analyzed the fabricated double-glazed ICS with CPC reflectors and compared it with other configurations. Zheng et al. (2016) studied numerically and experimentally the designed SCPC collector. Waghmare and Gulhane (2016) designed and performed ray tracing for CPC solar collector. Also, determined the reflector’s LD, maximum diameter, its location and the relationship between LD and maximum diameter. Arunkumar et al. (2016) investigated experimentally a novel approach on technology integration of CPCs, pyramid and single slope solar stills, which focused for the process efficient augmentation. Nashine and Kishore (2017) compared CPC performance for different locations. Shrivastava et al. (2017) critically reviewed TRNSYS of solar water heating systems. Li et al. (2017) analyzed techno-economic performance of the novel, low concentrated solar collector (consisted of prism arrays, Fresnel lens and CPC). Waghmare and Gulhane (2017) determined solar flux concentration of CPC collector by surface areal irradiance (SAI) for designing solar collectors on the basis of utilization ratios and heat equations which provides an optimal location for the receiver. Waghmare et al. (2017) refined SAI method (geometric cosine factors) using the reflector area for the determination of concentrated flux which helps in the design of solar collectors with desired requirements. Zheng et al. (2017) conducted simulation and experimental studies related to semi-passive beam steering prism array. Pouyfaucon and García-Rodríguez (2018) assessed solar thermal-powered desalination technologies and found solar thermal-driven reverse osmosis for seawater desalination advantageous as it requires less energy. Xu et al. (2017) integrated a closed-end pulsating heat pipe (PHP) and CPC and obtained a high thermal efficiency. Expósito et al. (2018) used CPC solar collector to study the intensification of solar photo-Fenton degradation of carbamazepine with ferrioxalate complexes and ultrasound. Gutiérrez-Alfaro et al. (2018) studied the efficiency of the solar disinfection process for the inactivation of three of the microorganisms (Escherichia coli, Enterococcus spp. and Clostridium perfringens) using a CPC solar collector for five months under various conditions of irradiance and temperature. Maddigpu et al. (2018) presented a novel approach to use a chitosancarbon nano particles (CHIT-CNP) composite membrane against Escherichia coli under solar irradiation that showed high efficient antimicrobial activity when used in a recirculating CPC reactor compared to solar disinfection alone. Aguilar-Jiménez et al. (2018) compared two CPC’s in both N-S and E-W directions. Li et al. (2018) showed the desalination system with CPC had a superior performance without power consumption and showed good freshwater production performance. Widyolar et al. (2018) designed and developed XCPC of a medium temperature nontracking solar collector, where the absorber shape was optimized through simulation tool and a prototype was built using low-cost materials. Experimental results of the prototype were compared with the simulation results and also an economic analysis was presented.

Francesconi and Antonelli (2018) numerically analyzed heat transfer in a panel containing several CPC collectors and suggested consideration of the panel (configuration) in the design of the solar field for increased efficiency of the system. Hadjiat et al. (2018) presented a new design of integrated collector storage solar water heater combined with CPC for predicting the thermal behavior of the system under Saharan climate. Hassanzadeh et al. (2018) simulated and experimented a novel medium-temperature CPC solar collector with optimized (Pentagon) absorber geometry for medium temperature applications to challenge natural gas as California’s primary heat source in the near future. Mahbubul et al. (2018) analyzed the effect of Single Walled Carbon Nano tube-water nanofluid on the collector performance and the nanofluids proved to enhance the efficiencies of (CPC) solar collectors. Saini et al. (2018) integrated PV thermal with CPC and evaluated the annual electrical gain, overall thermal energy and exergy gain with five different solar cell materials. Ajdad et al. (2019) proposed the application of the particle swarm optimization method for the optical geometric optimization of linear Fresnel reflector solar concentrators with CPCs as secondary reflectors. The evolution of the 2D CPC solar collector based on the literature survey is depicted in Fig. 22. 4. Concept and design strategies Moderate levels of concentration with complete stationary concentrators are possible with the application of the technique of nonimaging optics (Kalogirou, 2004). All rays get collected by the CPC collector and those rays that are nearer to the axis are not brought to a focus. Even while designing the optical surfaces of the non-imaging optics, extreme angular rays are considered rather than for axial rays (O’Gallagher, 2008). Most of the literature surveys observed CPC collector as a nonimaging concentrator, as the sun rays get focused at two points instead of one, which comprises of two parabolic mirror segments (reflectors) with two focal points (Fig. 23). A parabola on each side extends until its surface is parallel with the CPC axis. However, in imaging CPC, the focus of concentrator will be located outside the concave of CPC ie., the absorber will be located backside of the reflector. Kaiyan et al. (2007)

Fig. 22. Evolution of CPC Solar collector. 306

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heat should spread quickly. The distance along a tangential line from the receiver to the curve can be divided into the following two sections: involution and reflection as shown in Fig. 26. Wang et al. (2014) designed the truncated simplified CPC with a flat curve shaped bottom instead of the involute curve shaped bottom for reducing the processing difficulty and cost. Fig. 27 (a) and (b) shows the distribution of rays along the circumferential direction on the cover glass tube at incidence angles of 0° and 15° respectively using ray tracing (TRACE PRO) software. The observation showed that the majority of incident light on the bottom of CPC was not captured by the absorber. Hence, this model performed lesser than CPC with involute. When the incident angle changes from 0° and 15°, the simplified CPC decreases its performance by 12–16%. Based on the designed calculation and test result, the concentrating efficiency of the simplified CPC with a flat curve bottom was about 15% lesser than the involute bottom. The theoretical incident energy is dependent on the transversal angle following cosine law, therefore the simulation concentration ratio or flux concentration ratio is expressed as follows (Gu et al. 2014):

C=

Iin ( T ) Iin ( T ) = It Iirra cos( T )

(1)

The comparative concentration ratio is analyzed for the reduction on concentration ratio and it is expressed as (Ustaoglu et al., 2016)

Cconc = Ctrunc / Cfull Fig. 23. Schematic drawing of a symmetric CPC with flat receiver (Hess, 2014).

(2)

The concentration ratio for 2D, full CPC solar collector is calculated (Ustaoglu et al., 2016) by

and Kaiyan et al. (2011) discussed about imaging CPC. Kaiyan et al. (2011) developed a new type of imaging concentrator consisting of parabolic and a flat contour which was named as imaging CPC. It has the following features: (i) It provides forward reflective light rays, (ii) Focus of the concentrator is totally located in the backside of the concentrator. Comparing with the parabolic dish, it has a larger surface area and there will be two reflections for every light ray before reaching the focus, which will increase the reflection loss. Fig. 24 shows the light transmission pattern in an imaging concentrator. In non-imaging CPC, focus of the concentrator will be located on the concave of CPC for flat one-sided absorber (Fig. 9) and above the concave of CPC for tubular absorber type CPC solar collector (XCPC) (Fig. 8). Fig. 23 shows 2D CPC solar collector accepting incoming radiation entering the aperture over a wide range of acceptance angles ( ± θa) and with multiple internal reflections, the rays will be focused on the absorber surface not concentrated at a point or line and does not produce an image of the light source (Chamsa-ard et al., 2014). Fig. 25 shows the compound parabolic reflectors assembled in the solar collector module. The HTF takes away the captured solar energy in the absorber and can be either stored in the thermal storage tank or directly utilized for applications. Chew et al. (1988) stated that by applying thin aluminum foil on the cavity wall surface for producing a smooth reflector surface, the radiative contribution was reduced to about 10% for the convective heat exchange rate in combination with nickel chromeplating for the surfaces of heater tube and cold plate. Balkoski (2011) and Winston (2012) discussed about the types of reflector such as: i) Polished aluminium: Alanod MIRO-SUN 90 was used for polished aluminium designs. It is most commercially available for outdoor solar use. After 2–3 years of outdoor use, it loses significant specular reflectance. ii) Metalized (silver) polymer film: Reflectech was used for film-based designs. It was developed by NREL. Both hemispherical and specular reflectances (using the Devices and Services portable specular reflectometer, Model 15R) of ReflecTech film exceed 94%. Generally, in the optical design of solar thermal systems, hot spots are undesirable because it may generate high local temperatures, which may damage materials and cause flow instability (Gu et al., 2014). Therefore, the selection of material has to be done in such a way that

Cfull = 1/Sin

a

(3)

The concentration ratio for 2D, truncated CPC solar collector is calculated (Ustaoglu et al., 2016) by

Ctrunc = Aap / Aab

(4)

Liu et al. (2013) conducted experiments with simplified CPC, which had a flat curve shaped bottom (as shown in Fig. 28) instead of the involute curve shaped bottom when compared to truncated CPC. Due to this flat profile, there was a reduction of about 10–15% in concentrating heat efficiency of simplified CPC, when compared to that of the truncated CPC with the change of acceptance half angle. Based on the benefit of significance in cost reduction in producing CPC, this marginal efficiency depreciation is fairly acceptable. Ustaoglu et al. (2016) carried out simulation work using ray tracing technique for the CPC solar collector and found a significant improvement on the uniformity of the absorber, which can be achieved by truncating the reflector without substantial loss on the performance. Lower part of the reflector is designed as a cavity which reflects all radiation onto the absorber. W- or V-shape are the simple forms of the cavity, in which V-shape is the mostly seen shape for non-imaging edge ray collectors while considering practical constraints (Buttinger et al., 2010). Buttinger et al. (2010) analyzed the CPC with V-shape. McIntire

Fig. 24. Light transmission pattern in an imaging concentrator (Kaiyan et al., 2011). 307

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Fig. 28. Profile of simplified CPC solar collector with a flat curve shaped bottom instead of the involute curve shaped bottom (Liu et al., 2013). Fig. 25. Compound parabolic concentrators (Aluminum sheet reflectors) mounted in the frame.

Fig. 29. Overall efficiencies of CPCs w.r.t. incident angle (Wang et al., 2016a).

north–south (N-S) or the east–west (E-W) direction where the aperture is tilted at an angle equal to the latitude directly towards the equator (Li et al., 2013; Kalogirou, 2003). With N-S direction, the collector must track so as to face the sun continuously by turning its axis (Kalogirou, 2003). For stationary CPC collectors oriented with its long axis along the E-W direction, the minimum acceptance angle is equal to 47° (Kalogirou, 2004). Large acceptance angle of the CPC solar collector will be chosen when it has to operate fully stationary in both E-W and N-S orientations (Carvalho et al., 1995). Kim et al. (2013) experimented and showed that at higher working temperature, the radiation loss in EW collectors becomes lesser than N-S collectors because of higher CR for E-W collectors. For temperatures above 169.9 °C, the E-W oriented XCPC solar collector outperforms the N-S oriented XCPC solar collector because of its high CR. For N-S collectors, the measured optical efficiencies at zero normalized temperature were 68.3% and 66.9% for the flow rates of 80 g/s and 40 g/s respectively and for E-W collectors, it was 64.7% with the flow rate of 80 g/s. Due to the large amount of diffused radiations captured by the collectors, the optical efficiency of the N-S collectors (based on the direct normal irradiance) was higher than that of considering an effective irradiance. Gu et al. (2014) found

Fig. 26. Two sections of XCPC (Involution and Reflection) solar collector (Kim et al., 2013).

(1984) stated that straight, symmetric facets are the easiest to fabricate accurately and provide slightly larger gaps than asymmetric designs. Practically, for the required gap, least number of facets has to be considered, including the manufacturing tolerances such as tube bow and absorber reflector alignment. Similarly, least number of facets are desirable while considering rounding of corners. Wang et al. (2016a) proposed a model based on the edge ray principle, with a new V-shaped profile at the bottom of the reflector and analyzed its parameters such as geometrical optic efficiency, transmittance, reflectance and absorption ratio, with methods of ray tracing (TracePro) and geometrical optics. They concluded the selection of a model based on the gap size and proved that the CPC with new Vshaped profile had a good prospect (as shown in Fig. 29). The overall efficiency is the product of gap efficiency and relative concentration ratio. The orientation of CPC collector w.r.t. long axis can be either along

Fig. 27. TRACE PRO simulation results for the simplified CPC solar collector at incident angles of (a) 0° and (b) 15° (Wang et al., 2014). 308

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that the proposed CPC solar collector with E-W and N-S orientations provided ∼2300 and ∼1275 collection hours respectively throughout the year in Sydney, which shows the longer operating duration for the collector with E-W orientation than N-S orientation. Rabl (1976b) discussed CPC collector with an acceptance angle of around 30° requiring an average one reflection which causes ∼10 percent loss, even for good reflectors and may necessitate cooling (extracted heat may be useable). CR is one of the vital parameters of a solar concentrator (Dang et al., 1983; Rabl, 1976b). Low to moderate concentrations of 1.1 X – 2 X and up to 4 X can be achieved by stationary CPC collector and for slightly higher levels from ∼3 X to 10 X, seasonal adjustment is necessary (O’Gallagher, 2008). The angle of acceptance determines the requirement of tracking (Rabl, 1976b). For practical interest, smaller concentrations are mostly preferred. Among the line focus solar collectors for the hot water generation between the range of 80 °C and 120 °C, low-concentration CPC collectors are desirable when compared with high concentrated collectors. CPC solar collector partially collects diffuse radiation depending on the acceptance angle and tilt angle, therefore the expense of an accurate tracking system gets avoided (Kalogirou, 2004; Li et al., 2013; O’Gallagher, 2008). High CR requires larger tilt angle and vice versa (Antonelli et al., 2014). Smaller values of CR have benefits in terms of compactness of the concentrators, the annual number of operating hours and sensitivity to cloudy weather i.e., capable of capturing the diffuse radiation. For N-S direction, the collector with higher CR has to track the sun so as to face the sun continuously and for lower CR, the seasonal tilt adjustment is not necessary. Rabl (1976b) suggested that for stationary collector, the limitation of concentration is almost two and he had also explained the adjustment of collector tilt for various CRs. Rabl et al. (1979) discussed about the selection of receiver type, the optimum method for providing a gap between the receiver and reflector in order to minimize optical and thermal losses, the effect of a glass envelope around the receiver, effect of mirror errors, effect of receiver misalignment and the effect of the temperature difference between the fluid and absorber plate. The receiver should not touch the reflector in order to minimize the conduction losses (Gu et al., 2014). Prapas et al. (1987) predicted that the optimal annular gap for the non-evacuated arrangement of approximately 5 mm, resulted in best overall collector efficiency. Liu et al. (2013) experimented CPC solar collector with a distance between CPC cusp and absorber of 6 mm. The general performance of solar collector is not affected by providing small gap (3 mm) between the absorber and tip of the reflector (Jiang et al., 2015). Buttinger et al. (2010) measured thermal losses of the CPC solar collector for three different collector fillings, such as air at 1 bar, air at 0.01 bar and krypton at 0.01 bar. A prototype with an aperture area of 2 m2 showed an efficiency of about 50% for krypton and 40% for air at 0.01 bar and process heat of 150 °C with a radiation of 1000 W/m2. The use of thin inert gases in the collector show heat loss reduction and is as important as radiation concentration. The collector parameter stagnation temperature plays a vital role in describing the collector’s performance with regard to collector-ambient heat loss (Bejan et al., 1981). The operating temperature of the working fluid at a specific flow rate in the solar thermal systems is governed by two key factors such as: (i) The total solar flux into the collector and (ii) The overall heat loss in all forms of heat transfer: conduction, convection and radiation (Gu et al., 2014). The heat transfer coefficient plays a vital role in the overall CPC efficiency, as it represents nearly 76% of the overall heat transfer coefficient (Kothdiwala et al., 1995). Lambert (2007) stated that, in CPC solar collector the emitting area is only a fraction of aperture area and so the thermal efficiency of the CPC solar collector will be high. Compared to standard flat panels with solar selective coatings, the evacuated flat panel and CPC collectors are at least three times more efficient (55–58%). Gudekar et al. (2013) stated that 71% of energy can be made available to the HTF with major improvement in convective loss,

radiative loss, receiver absorption and glass tube transmittivity. In most cases, the effective thermal capacitance is low, which is explained by the low collector weight because of low material content (Adsten et al. 2005). Okoronkwo et al. (2014) stated that the thermal capacitance of absorber components has a significant impact on the performance of the system and should be maximized. By providing transparent baffle in the CPC solar collector, the internal convective heat transfer from the absorber to cover will get reduced by reduction of total fluid movement, thereby resulting in the reduction of heat losses and increase in total collector efficiency (Eames and Norton, 1995). The associated reduction in optical efficiency is small due to this configuration. A transparent cover provided on top safeguards the reflectors from dirt and other foreign substances and also decreases the heat loss from the solar collector, which improves the system performance significantly (Okoronkwo et al., 2014). Also, the life of the reflector and receiver coating get increased by using transparent cover (Sagade et al., 2014). Gudekar et al. (2013) compared the thermal efficiencies of the newly modified CPC system without and with glass tube cover on receiver pipe and found the overall increase in thermal efficiency due to glass tube cover as 7.4% in absolute terms (46.8% relative rise). Although double glazing for the integrated collector storage (ICS) integrated with CPC reflectors decreases the total optical efficiency, the numerical simulation study carried out by Kessentini and Bouden (2016) showed that the double glazing performed better than single glass cover with respect to heat retention of the stored water during night operation and also operation during day time. The use of transparent insulation (Teflon film), in low-concentrating solar collectors can reduce the heat losses significantly (Ronnelid and Karlsson 1996). Also, Carvalho et al. (1995) discussed about the heat loss reduction by using Teflon film between the glass cover and inverted “V” shaped absorber of CPC solar collector. The authors suggested that a simple way to reduce the heat loss of CPC collector for higher temperature applications of around or above 100 °C (Solar cooling applications) is achieved by providing Teflon film in the collector system. The top portion of fully developed or untruncated CPC is nearly parallel to the optical axis, which does not contribute much radiation. So, the depth of concentrator and its corresponding mirror area is reduced by cutting off the top portion without much loss in the concentration (O’Gallagher, 2008; Rabl, 1976b). Rabl extensively discussed about the issues regarding truncation of CPC collector. In practical applications, for economic reasons and compactness, most of the CPCs will be truncated (Antonelli et al., 2014; Rabl, 1976c). Santos-González et al. (2014) showed that, due to truncating full CPC, the percentage of energy loss was 6.7% with 42.4% of material saving. Generally, the troughs are truncated to approximately one-third of full concentrator height (Prapas et al., 1987). When the cavity height is decreased, the convective heat transfer rate from the cylinder to the aperture portion (flat top) increased for the same temperature difference between them (Chew et al., 1988). For similar conditions of operating temperature, the heat loss to free air from the cylinder is about 50% higher when it is surrounded by full CPC cavity and about 30% higher when the cavity truncated to 1/3rd of full height. Deciding the absorber tube diameter is very important, as it determines the quantum of reflected rays that can be captured. Waghmare and Gulhane (2016) used a term called ‘Limiting diameter’ (LD), which is defined as the diameter of a tubular receiver below which no reflected rays will be cut. The reflected rays that are tangent to the receiver will be considered as LD of the receiver at the focus. Also, they determine the relationship between LD and maximum diameter of the tubular receiver and its location. The relationship between major and minor axis of the ellipse is dependent on the acceptance angle and CPC height. The collector effective thermal performance for large receiver can be improved up to 3% with proper selection of receiver tube and fin dimensions (Karwa et al., 2015). The performance gain will be higher at lower collector mass flux. It was experimentally found that there was no significant difference between the performance of horizontal and 309

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vertical absorber fin orientated integrated CPC solar collector (Duff et al., 2001, 2004). However later it was reported under substantial degradation of the reflector, the horizontal fin integrated CPC retains better optical performance than the vertical fin integrated CPC (Duff and Daosukho, 2014). Kim et al. (2013) stated that higher mass flow rate with fixed collector inlet temperature causes lower outlet temperature resulting in better thermal heat exchange and less temperature dependent thermal losses. By choosing the optimum mass flow rate, the thermal efficiency can be increased by a maximum of 2% for CPC solar collectors (Karwa et al., 2015). The coating is classified into non-selective and selective (Fig. 30). The term ‘solar selectivity’ is used to define the optical behavior of the coating and it is defined as the ratio of solar absorptivity to emissivity at a given temperature (Suman et al., 2015). Therefore, the overall efficiency of solar collector can be improved by enhancing the optical characteristic of coating and its thermal stability at high temperatures. Tchinda and Ngos (2006) showed that the selective coating and nature of the reflector material had considerable effect in the performance of the CPC collector. Gu et al. (2014) chose titanium nitride oxide (TiNOx) as the receiver coating of CPC solar collector even though at short wavelength the absorptance of black chrome is slightly higher than that of TiNOx but at long wavelengths (> 2.5 μm), the thermal emissivity of TiNOx is 0.08 at 100 °C is lower than that of black chrome (0.29). The total power which is absorbed decreases as the reflectivity of the CPC reflector is lower. Therefore, for higher power, the CPC surface performance gets increasingly important. Emittance (in infrared) of the reflector has an effect on the heat losses (Ronnelid and Karlsson, 1996). The overall heat losses increased about 5–8%, while using high emitting reflectors instead of low-emitting reflectors. For the effective utilization of captured solar energy, the emitted radiation from the absorber should not be permitted outside its acceptance angle (Rabl, 1976b). By meeting the following three conditions, the placement of the parabolas is defined (Hess, 2014) for the flat faced receiver:

(a) Compared to a simple parabola, a fully developed or untruncated CPC is very deep and requires a reasonably large reflector area for a given aperture field. This issue is sorted by removing the top portion with almost no loss in performance (Rabl, 1976c). (b) Frequency of adjustment goes up with the increase in CR, resulting in requirement of tracking system to enable the collector to follow the sun. For example, CR of 3 requires only biannual adjustment whereas CR of 10 requires nearly daily updates (Winston et al., 2005). (c) For capturing diffuse radiations larger acceptance angles are practically used at the expense of a smaller CR (Kalogirou, 2004). (d) Reflectance of the solar reflecting surfaces may depreciate and thus require periodic cleaning and renovating (Kalogirou, 2004). (e) The CPC height increases rapidly with aperture, making the structure clumsy to handle and a substantial percentage of radiation incident within the acceptance angle suffers multiple reflections before reaching the receiver, resulting a drop in its optical efficiency (Gudekar et al., 2013). 5. Heat transfer fluids (HTFs) This section describes about the various HTFs that are used in the solar collector. Sun energy is captured by a receiver/absorber of the solar collector and transferred to a thermo fluid, also known as heat transfer fluid (HTF). The indirect loop system uses a heat exchanger (HX) which separates the portable HTF from the intermediate HTF that circulates between HX and solar collector, whereas in the direct loop system, HTF will be circulated between the utility and the solar collector. Indirect system offers overheat protection and freeze protection (Srivastva et al., 2015). Generally, solar thermal energy collectors are categorized into low, medium and high temperature collectors. Low temperature collectors operating range is from above ambient to about 80 °C and use water and refrigerants (Hydrocarbons like propane, pentane and butane) as HTFs (Srivastva et al., 2015). Also, waterglycol mixtures and water based nano fluids are used in low temperature applications where high freezing point is hampering and degrading water quality. This type is commonly used for the applications such as solar water heating, solar based space heating and cooling, solar ice making etc. The refrigerants have a low boiling point and high heat capacity, and used in applications like solar space heating and cooling,

(i) The endpoints of the receiver are the focal points. (ii) Their center lines are sloped by θa, towards the receiver. (iii) The two parabolas intersect the endpoints of the receiver. Disadvantages of CPC solar collector are listed below:

Fig. 30. Classification of Absorber coating (for solar application) (Kaushal, 1997; Suman et al., 2015). 310

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refrigerator and air conditioning. The traditional refrigerants such as R717 (anhydrous ammonia) and R-764 (sulphur dioxides) are harmful for domestic usage due to their leakage. Similarly, chlorofluorocarbon (Freon R-12) causes global warming; hence it is replaced by hydrochlorofluorocarbons (HCFC, R-134a) (Antonelli et al., 2014). But both CFCs and HCFCs are harmful to the ozone layer. Bolaji and Huan (2013) suggested that natural refrigerants (Hydrocarbons-HC) are the most suitable long time alternative solution for the refrigeration and air conditioning applications. HCs are more environmental friendly, nontoxic, chemically stable and highly soluble in conventional mineral oil. Bu et al. (2013) evaluated four working fluids (R123, R245fa, R600a and R600) of the organic Rankine cycle/vapor compression cycle (ORC/VCC) ice maker driven by solar energy (Using PTC solar collector) for identifying suitable working fluids which provide high system efficiencies. The highlighted results showed that for a heat source temperature range of 60–160 °C, based on ORC efficiency, ratio of net work output to mass flow rate for ORC, volumetric flow ratio and expander size parameter, R600 (Butane, HC) and R600a (Isobutane, HC) are more suitable working fluids for ORC. R600a is the most suitable working fluid for VCC in terms of security risk caused by air suction due to system leakage. R123 (HCFC) is the most suitable working fluid for ORC/VCC based on COPs, overall efficiency of solar ice maker and ice production per square meter of collector per day. Water is preferred as HTF for the applications such as domestic usage, swimming pool heating, solar heating and cooling (Hu et al., 2011; Lambert, 2007; Li et al., 2017; Lu et al., 2013). Water as HTF has following advantage (Srivastva et al., 2015; Lambert, 2007) such as: (i) Abundantly available, (ii) Inexpensive and naturally nontoxic, (iii) Highest specific heat of any liquid, (iv) Higher thermal conductivity than all except liquid metals, (iv) Easy to pump because of very low viscosity. The disadvantages are (i) Easily loses its neutrality in the pH value by picking up contamination resulting in corrosion hazards, (ii) Mineral deposits on heat transfer surfaces cause a depreciation in heat transfer capability and also lead to blockages in the piping systems, (iii) Not suitable for use in extreme conditions due to its low boiling point and high freezing point. Pure water will cause corrosion and therefore the life of a heat pump

will be ∼20 years. The relationship between the thermal conductivities of oil and ethylene glycol w.r.t. water are represented as koil ≈ 0.20 × kwater and kEG ≈ 0.40 × kwater. Oils are non-corrosive and ethylene glycol is commonly used in lower temperature applications. In solar water heaters, normally, the water quality of less than 200 ppm of hardness is suggested (Srivastva et al., 2015). Karwa et al. (2015) stated that, at a mean temperature of 100 °C, HTFs (water and thermal oil) had no effective performance even with the reduction in tube diameter of the CPC solar collector. For temperatures up to 150 °C, the recommended HTF is water. Below 150 °C, water performed better than thermal oil at all mass flow rates with no substantial improvement in collector performance achieved by reducing the tube diameter. Antonelli et al. (2014) stated that the saturation temperature of the working fluid (Tsat) influences the efficiency of both solar field and thermal cycle, which is considered as the main operating parameter in the system. Increase in Tsat provided a reduction in the captured energy because of increase in thermal losses. So, it has to be decided correctly according to the downstream requirement. El-Assy (1988) found that under the same working condition, the results of CPCs in two-phase flows had greater thermal efficiency than similar CPCs in single-phase flows. Also, he stated that as CR is increased, the working pressure can be increased so that the thermal conversion efficiency gets improved. Otanicar et al. (2010) performed experimentation on solar collectors using nanofluids made from a variety of nanoparticles (carbon nanotube graphite and silver) and observed that the solar collector efficiency improved up to 5% by utilizing nanofluids. Additionally, the experimental data of a solar collector with direct absorption nanofluids were compared with a numerical model. The maximum temperature obtained can be closer to the center of the fluid and also minimized the heat loss with the optimum profile of volumetric absorption, resulting in further enhancement in efficiency. The unique advantages of using nanofluid as a direct absorption solar collector (DASC) over conventional collectors are as follows:

• Limiting the need for hot surface as the heating is within the fluid

Fig. 31. Elements of nanofluid and their commonly used examples along with thermal conductivity value (W/m K) (Suman et al., 2015). 311

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volume and only heat transfers to a small area of fluid resulting in the location of peak temperature obtained away from the surfaces losing heat to the surroundings, Modification to maximize spectral absorption of solar energy through the fluid volume is possible by the changeability of size, shape, material and volume fraction of the nanoparticles, Efficiency improvements are obtained by an enhancement in the thermal conductivity and Nanofluid based solar systems are attractive for thermochemical and photocatalytic processes due to vast enhancements in surface area of the extremely small particle size.

bulk temperatures of above 120 °C, as the HTF degradation increases appreciably resulting in acidic behavior and corrosion. The essential properties of the glycol-inhibitor mixture of solar thermal application are non-toxic, optimal reserve alkalinity, corrosion protection nature and thermal stability. High temperature collectors use water, air, synthetic hydrocarbon oils, nanofluid compositions, molten salts, molten metals, dense suspension of solid silicon carbide particles etc., as HTFs (Srivastva et al., 2015).

Liu et al. (2013) designed a novel evacuated tubular solar air collector integrated with simplified CPC and special open thermosyphon using working fluid as water based copper oxide (CuO) nano fluid to provide air with high and moderate temperature. CuO nanoparticles were made by the gas-condensation method with a mean average diameter as 50 nm. Experimental results of solar air collector using nanofluid as open thermosyphon’s working fluid for the air outlet temperature and system collecting efficiency was higher than that of using deionized water. Different mass concentration of CuO nanoparticles was used for experimentation which varies from 0.8 to 1.5 wt%. The results highlighted that the heat transfer coefficient (HTC) of nanofluids moves upwards significantly when compared with that of deionized water. The HTC slightly worsens the behavior when the mass concentration was above 1.2 wt%. Compared with mean HTC of water for the operating temperature of 100 °C, the mean HTC of nanofluid (with optimal concentration of 1.2 wt%) was about 15% higher. By adding right amount of CuO into the water with the open thermosyphon, the maximum value and mean value of the collector efficiency was 6.6% and 12.4% respectively more than that of water. During winter, even though the experimental system consists of only two collecting panels, the maximum air outlet temperature was of 155 °C and 174 °C by using deionized water and nanofluid respectively, at an air volume rate of 7.6 m3/h. Comparative study based on the performance of solar collector integrated with open thermosyphon and with the common concentric tube was conducted. A much better collecting performance of mean collecting efficiency as 12.74% was found experimentally for the solar collector integrated with open thermosyphon. The authors had used a heat conduction medium paste like mixture of high temperature oil and graphite powder between the inner glass tube and evaporator tube of open thermosyphon which was a core technology. It has properties such as good thermal endurance, high thermal conductivity and considerable expansion capacity. Fig. 31 illustrates the elements of nanofluid and their commonly used examples along with thermal conductivity values. Based on the dispersion of nanoparticles in base fluids, there are three major types of nanofluids: metallic, ceramic and carbon nanotube nanofluids. Also, based on the concentration level of the nanoparticles in base fluid, it gets further classified as dilute, semi-dilute, semi-concentrated and concentrated nanofluids. Sometimes, surfactant is used during dispersion, which is a stabilizing agent. Nanofluids have the following unique properties such as minimal clogging in flow passages, long-term stability and homogeneity (Chen and Ding, 2009; Das et al., 2006; Li et al., 2009; Suman et al., 2015; Wang and Mujumdar, 2007; Yu and Xie, 2015). Duong and Diaz (2014) stated that the thermal efficiencies comparable to thermal oils can be achieved using CO2 as working fluid. The main drawback is the requirement of high operating pressure. Medium temperature collectors use water, water-glycol mixtures – emerging green glycol i.e., trimethylene glycol, naturally occurring HC oils (aromatic oils, naphthenic and paraffinic oils) and semi-synthetic oils as HTFs (Hess, 2014; Lambert, 2007; Srivastva et al., 2015). The three most commonly used glycols in heat transfer applications are ethylene glycol, propylene glycol and trimethylene glycol. Corrosion inhibitor is added to the glycol based fluid for the system operated at

Table 4 illustrates the summary of researches on CPC solar thermal collectors globally.

• • •

6. Studies performed with major parameters of CPC solar collector

6.1. Experimental studies Chew et al. (1988) presented experimentally the free convection heat transfer between the cylindrical absorber and the flat aperture surface of CPC. The maximum input power to the heater was about 55 W. The experimental curve occurred within about 8% of heat loss calculated using the heat transfer correlation of Rice. The trend wise agreement between the numerical and experimental results was very good for a cavity truncated to 1/3rd of full height. Using an equivalent length and cavity height as the characteristic length of the CPC cavity, correlation of Nusselt number (Nu) and Grashof number (Gr) were obtained. The authors recommended for the correlation based on the cavity height and length ratio because of the computational convenience. The authors stated that for tall cavities, i.e., full height and 2/ 3rd of full height by tilting the axis of about 60° to vertical, Nu was about 12% larger than that of the untilted cavity, whereas for short cavity (1/3rd of full height), Nu was reduced by 15%. Florides et al. (2002a) experimentally determined the heat and mass transfer coefficients which were applied in designing and estimation of a 11 kW cooling capacity solar cooling machine. Using TRNSYS software, the optimum system consisted of 15 m2 CPC solar collector with tilt angle of 30°, 600 l hot water storage tank, a boiler, H20 – LiBr pair absorption cooling system and a typical house load throughout the year. Pramuang and Exell (2005) conducted transient test of a solar air heater with a truncated CPC and a flat absorber whose surface was painted with non-selective matt black (Chungpaibulpatana and Exell, 1990). In a tropical climate to obtain high air temperatures, this type of collector will be chosen where the proportion of diffuse solar radiation is high. Four collector parameters were determined for evaluating the performance of the solar collector such as effective heat capacity, optical efficiency, first and second order heat loss coefficients. The solar collector testing method of Chungpaibulpatana and Exell had been successfully applied to a CPC solar air heater. The authors concluded that their method can be used at any time of the year in a variable tropical climate. Adsten et al., (2005) developed the design concept of an asymmetrically truncated non-tracking CPC type collector, which is optimized for the northern latitudes. The main objective was to maximize the reflector to the absorber area for a given ground area and this collector is called as a Maximum Reflector Collector (MaReCo). This design concept is also applicable for PV modules concentrators to reduce the cost of electricity generated with solar cells. The article described about collectors for the stand-alone, roof and wall mountings. Six different collector prototypes were built and tested outdoor. The tested prototypes are listed as follows: stand-alone collector ground mounting, stand- alone collector with Teflon convection suppression film around the absorber, standard roof MaReCo design for roof facing south with tilt angle of 30°, roof facing east or west of 25° tilt angle, roof model with load adapted and the active area reduced to 1/3 during summer and Integrated vertical MaReCo for south facing walls (Figs. 32 and 33). 312

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The overall performance of the Reflectech version was better than Alanod versions. The performance of X-tube over the counterflow tube appears to improve at lower temperatures and at higher temperatures, the effect is small. NS collector may prove superior for the sites where clouds are more frequent and hazy locations. U-tube performs the best among other collectors analysed. U-tube collector may be the best candidate for commercial use till its costs are not significantly more than the other tube types. Horta et al. (2012) adopted internal convection control strategies such as the use of low CRs, use of air or Argon as enclosure filling media and the adoption of physical barriers within the CPC cavity. Strategies were assessed in terms of the optical and thermal characteristic parameters of the collector and annual collector yield. Integrated design, optical and thermal assessment of non-evacuated CPC concentrators, simulated incidence angle modifier (IAM) and efficiency curve parameters were calculated for different CPC collector configurations and the results provided guidelines to implement the strategies to a new collector design. Results also illustrated the impact of climatic conditions in optical and thermal effects of each strategy, for applications at constant operating temperature. Nkwetta and Smyth (2012) designed, fabricated, experimented, analyzed and compared the innovative concentrated evacuated tube heat pipe (single-sided absorber and double-sided absorber) CPC solar collectors as shown in Fig. 34. The truncation of CPC reduces the size of reflector and reflector losses. An overall improvement of 42.4% in average outlet and inlet fluid temperature differential was found by truncated double-sided absorber CPC compared to truncated singlesided absorber CPC solar collector (at 0° transverse angle). The singlesided absorber CPC (SSACPC) offers greater possibility in the collection of direct incident solar radiation due to higher utilization area, whereas in double-sided absorber CPC (DSACPC), the incident solar radiation gets collected on both surfaces of the absorber, thereby eliminating the back losses. Due to its higher outlet temperature with greater rise in differential temperature and improved thermal performance, DSACPC solar collector was considered better when compared to SSACPC solar collector. Gudekar et al. (2013) presented a working model of CPC system. The proposed design of CPC solar collector overcame the disadvantages of conventional CPC solar collector with the following modifications in design: (i) The two foci were very close to the plane of the aperture, (ii) The segments of two parabolic curves above the focal point were removed and those below focal point were selected, (iii) The receiver pipe was located near the aperture, and (iv) Receiver pipe size was selected such that all rays incident within the angle of acceptance were captured by it after reflection. The proposed collector when compared to the conventional CPC system was easy to fabricate, helped in the reduction of overall system cost and reduced the mirror area requirements per unit aperture area substantially and also the proposed collector worked reasonably well for steam generation at atmospheric pressure. Heat loss analysis was carried out and found that with additional modification in the design and scale up would further enhance the system performance and also a significant fraction of the generated steam at various temperatures can be utilized for the process heat. In the present system, the final available energy to water was 175 W/m2 which was 25% of incident radiation of 700 W/m2. Kim et al. (2013) modeled a stationary XCPC collector system using an evacuated glass and a counter flow absorber tube for medium which was analyzed, fabricated and tested. Efficiencies for both N-S and E-W orientations were modeled and measured at various working temperatures. The schematic representation of test facility is shown in Fig. 35. The proposed model achieved more than 40% efficiency above 200 °C. The counter flow tube consists of a coaxial pipe attached to the absorber fin which makes the directions of the working fluids opposite. Based on the simulation results, the total optical efficiency for E-W and N-S orientations were 65.31% and 69.01% respectively. At zero normalized temperature, the experimental data indicated that N-S

Fig. 32. Photo of a MaReCo designed for east/west-facing roofs. The white arrow indicates the south direction (Adsten et al., 2005).

Compared to flat plate collector, standard roof and stand-alone MaReCos are more or equally cost effective. However, these collectors can compete with standard collectors in the same areas. For the evaluation of collector, statistical method (dynamic testing model using multiple linear regression) and MINSUN simulation program were performed. Bifacial absorber (high fin efficiency sunstrip absorbers) with the combination of lower and/or upper reflector (anodized aluminum reflector with solar reflectance = 0.85) was used to minimize the absorber area. Lambert (2007) carried out design and performance of a solar (and/ or natural gas) powered adsorption (desiccant-vapor) heat pump for residential cooling (and heating). The adsorbent-refrigerant pair was carbon-ammonia. At the selected operating temperature of 170 °C, two types of solar collector are determined satisfactorily, (i) CPC with high CR (10+) and automatic tilt adjustment and (ii) evacuated flat panel (0.001 atm), similar to atmospheric pressure versions employed for domestic water heating. Low quality heat at 150–250 °C is required for the adsorption heat pumps. Kim et al. (2008) investigated numerically and experimentally the thermal performance of the evacuated CPC solar collector with a cylindrical absorber. Comparison of conventional stationary CPC and single axis tracking CPC solar collectors was carried out based on outlet temperature, net heat flux onto the absorber and thermal efficiency. Using commercial FVM code, the outlet temperatures of each CPC solar collector were calculated. The thermal efficiency of the tracking CPC solar collector was 59.56% and its average value was 14.94% higher than that of the stationary CPC solar collector. Balkoski (2011) carried out the performance analysis of seven different types of XCPC solar collectors and are listed below:

• N-S with counter-flow tube with Alanod reflectors (NS AL CF) • N-S with U-tube with Alanod reflectors (NS AL UT) • N-S with U-tube with Reflectech reflectors (NS RT UT) • E-W with counter-flow tube with Alanod reflectors (EW AL CF) • E-W with X-tube with Alanod reflectors (EW AL XT) • E-W with U-tube with Alanod reflectors (EW AL UT) • E-W with U-tube with Reflectech reflectors (EW RT UT) The experiments were carried out and found that the NS AL CF version had higher optical efficiency and also outperformed the EW AL CF collector upto 150 °C. 313

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Fig. 33. Sectional diagrams of Maximum Reflector Collectors (MaReCo) for different (a) Stand-alone MaReCo, Aperture tilt 30°; (b) Roof integrated MaReCo design for a roof angle of 30°; (c) East/west roof MaReCo design for a roof facing west; (d) Spring/fall MaReCo designed for a roof tilted 30°; (e) Wall MaReCo designed for a south facing wall (Adsten et al., 2005).

collectors performed 4% higher than E-W collectors. Lu et al. (2013) experimentally investigated solar adsorption chiller and LiBr-H2O absorption chiller with new medium CPC solar collectors. Experimentally, it was found that when the adsorption chiller was powered by 55 °C of hot water, the obtained average solar COP of the system was 0.16. In the absorption cooling system, the achieved average solar COP was 0.19 with the efficiency of the evacuated tube CPC solar collector as 0.5 and the hot water temperature obtained was 125 °C. The solar absorption chiller system operated in two working modes, one was heating mode in winter and the other was cooling mode in summer. Figs. 36 and 37 are the installed units of solar adsorption and absorption chiller systems respectively in China. The absorption chiller was powered by relatively high temperature hot water (usually ≥80 °C). Sagade et al. (2014) explored the experimental results of the prototype of compound parabolic trough made of mild steel and silver coated selective surface. Evaluation of collector performance has been made with three kinds of receiver’s coated with two kinds of receiver

coatings black copper, black zinc and top cover. Two different types of receivers are utilized (i) copper receiver and (ii) mild steel receiver. Using regression analysis, simple relationship between the parameters such as (i) receiver temperatures with heat losses, (ii) receiver temperatures with collector efficiencies, (iii) receiver temperatures with outlet water temperatures and (iv) receiver temperatures with temperature gradients were worked out. Positive relations were found for the cases (i), (iii) and (iv), whereas negative relation was found for the case (ii). Widyolar et al. (2014) and Winston et al. (2014) analyzed the performance of 53.3 m2 XCPC trough collectors for operating a 23 kW double effect (Water and LiBr pair) absorption chiller. For XCPC collector, an average daily efficiency of 36.7% and instantaneous efficiencies up to 40% were obtained with collector operating temperature ranging from 160 °C to 180 °C. The solar cooling system was designed, installed and tested for different conditions such as cloudy day performance, typical performance and performance test with dirty collectors and after cleaning by UC solar group at the UC Merced castle 314

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Fig. 34. Cross sectional view of the full and truncated (a) SSACPC solar collector and (b) DSACPC solar collector (Nkwetta and Smyth, 2012).

research facility, University of California. Calorimetric technique is used for the measurement of collector performance. Winston et al. (2014) suggested that by eliminating the intermediate water-glycol loop and associated heat exchanger, makes the heat transfer oil flow directly to the chiller, with improved system efficiency and simplified operation of the system. Okoronkwo et al. (2014) presented the experimental study on performance evaluation of a thermo-syphon water heating system using a CPC solar collector at Owerri, Imo state Nigeria. The experimental results were highlighted with the following observations: (i) Using this system with insulated water tank containing 200 L of water can be heated to 99.1 °C from an initial temperature of 25.8 °C. (ii) The absorber surface temperature can go above 100 °C. (iii) The system efficiency ranges from 43 to 69 %. The lower efficiency value may be attributed due to high relative humidity value for Owerri region, (iv) Prevailing weather condition was considered as a highly dependent factor for the performance of the CPC collector thermo-syphon system, (v) By incorporating a transparent cover over the collector and also providing proper insulation over the water tank, the system performance can be improved significantly, and (vi) Economically, this study showed that this system will compete favorably with conventional electric powered heating system. Gu et al. (2014) analyzed an innovative portable solar collector with a non-tracking CPC solar collector as shown in Figs. 10 and 38. The solar flux distribution on the receiver was determined using ray tracing optical analysis and experimentally measured properties as a function of incidence angle. At reasonable flow rates, the stagnation temperature of collector allows efficient methanol reforming.

Fig. 36. Installed solar adsorption chiller system in Dezhou city, China (Lu et al. 2013).

Wang et al. (2014) conducted an experimental study and numerical simulation on a new type all-glass evacuated tubular solar air heater with simplified CPC with the setup shown in Fig. 39. Fig. 40 shows the sectional view of the glass evacuated tubular solar air heater. The

Fig. 35. Schematic of the test facility (Kim et al., 2013). 315

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Fig. 37. Installed solar absorption chiller system in Jinan city, China (Lu et al., 2013).

system consisted of 10 linked collecting panels and each panel comprised of a simplified CPC with a U-shaped copper tube located inside the evacuated tube and a heat conduction medium between the inner glass tube and U-shaped tube. The results showed that the system outlet air temperature exceeds 200 °C for relatively large air flow rate even in winter. The comparative results of experimental and calculated data showed that the proposed model meets the general requirements of engineering calculations. Lu and Wang (2014) presented experimental performance

investigation and economic analysis of three small solar cooling systems with different kinds of collectors and sorption chillers. The integrated combinations were traditional ETC with silica gel-water adsorption chiller, high efficient evacuated tube CPC solar collector with single effect LiBr absorption chillers and PTC solar collector with double effect LiBr absorption chillers. The obtained hot water temperature from the solar collectors were 60–85 °C, 85–125 °C and 125–150 °C respectively. The auxiliary heat sources for the solar cooling systems were natural gas boilers. Silica gel water adsorption chiller can be driven by hot

Fig. 38. Schematic of the micro solar concentrator (a) close view of the copper receiver and (b) cross section of the combined tube (Gu et al., 2014). 316

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Fig. 39. Solar air heating system using CPC solar collector (Wang et al., 2014).

when compared to BCCCT absorber, due to higher emissivity of glass tube (0.84) than BCCCT (0.2–0.4). A vacuum pressure of below 10−3 mbar was maintained between the outer glass tube and the absorber with the help of Turbo pumping station. Arunkumar et al. (2016) experimentally studied the performance of CPC-assisted tubular solar still (CPC-TSS) and CPC-concentric tubular solar still (CPC-CTSS) with different augmentation systems. The productivity of the un-augmented CPC-TSS and CPC-CTSS were 3.71 l/day AND 4.96 l/day, respectively. Considering the heat extraction technique, the productivity of CPC-CTSS (with a single slope solar still) and CPC-CTSS (with a pyramid solar still) were 6.46 l/day and 7.77 l/day, respectively. Milczarek et al. (2017) designed to produce 40 kW of heating power using 98.3 m2 XCPC solar collector integrated with small scale double drum dryer and also demonstrated the potential for solar thermal energy to cater the heat for drum drying fruit and vegetable pomaces. Optimization of the system was performed via a split-plot design. To determine the significance of four independent variables such as added water, added maltodextrin carrier, dwell time, and drum surface temperature analysis of variance (ANOVA) was applied. To assess the drying performance, the response variables such as moisture content and overall color change were chosen. Pouyfaucon and García-Rodríguez (2018) assessed solar thermalpowered desalination technologies to identify key issues for development in market. He considered different scenes: (i) Rural communities with limited fresh water demand; (ii) Regions with high demands of both, water and electricity and (iii) Intermediate water demands; and

Fig. 40. Cross sectional view of the glass evacuated tubular solar air heater (Wang et al., 2014).

water of 55 °C. Wang et al. (2015) designed and conducted experiments using simplified CPCs consisting of a set of evacuated tube with high temperature air heaters and concentric tube heat exchanger for the air flow for industrial applications. Between the evacuated tube and concentric copper tube was filled with a resilient stainless steel mesh layer as shown in Fig. 41, instead paste mixture of high temperature oil and graphite powder of the previous work as shown in Fig. 40 (Wang et al., 2014). The system unit comprised of 30 linked all-glass evacuated tube collecting units. The article focused on the further improvement in performance of the solar collector from the authors’ previous study (Wang et al., 2014). Investigation of its thermal performance was conducted and the results demonstrated that even in the winter the designed solar air collector had an excellent high temperature collecting performance with maximum temperature that can reach up to 230 °C on sunny days. Zheng et al. (2016) designed a SCPC collector (Fig. 42) which provides low temperature hot water for space heating in cold regions. The maximum experimental thermal efficiency of the collector was obtained as 60.5%. Li et al. (2016) experimentally and numerically analyzed the vacuum-packaged volumetric receiver and vacuum-packaged black chrome-coated receiver CPC solar collector. The optical efficiency of MWCNTs nanofluid receiver with/without vacuum glass packaging was found to be lesser than the black chrome-coated copper tube (BCCCT) absorber with/without vacuum packaging. This was due to the optical reflective losses from outer glass surface of the nanofluid receiver. Also, particularly at higher temperatures the overall heat loss was approximately doubling for the glass tube (nanofluid receiver)

Fig. 41. Provision of stainless steel mesh between the concentric copper tube and evacuated tube (Wang et al., 2015). 317

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flow, the inlet temperature at the collectors and changes in the ambient temperature and solar radiation were studied. The results showed that on average, CPC with an E-W orientation had an increased optical efficiency (of 57.5%) than the N-S orientation (which reached 51.3%). The coefficient of heat losses was also higher for CPC in E-W position than the N-S position. CPC orientation will depend on the range of temperature and working hour as the N-S position will be limited to a concentration time due to its acceptance angle. 6.2. Theoretical studies 6.2.1. Numerical studies The mathematical model was much helpful in designing and optimizing the solar collector (Zheng et al., 2016). Prapas et al. (1987) presented a theoretical analysis of all the heat exchanges in the proposed CPC solar collector and their predictions were compared with experimental results and adequate corroboration was obtained. The absorber configuration considered for the analysis is a tube with or without a spectrally-selective surface, either directly exposed or enclosed within one or two glass envelopes. The following results are observed and tabulated in Table 5 for an acceptable lower limit of 40% instantaneous collector efficiency. Chew et al. (1989) numerically simulated the laminar free convection in a CPC solar collector cavity using the finite element method with first order triangular elements and the φ-ω formulation of the governing equation. The results obtained numerically are compared with those from a parallel experimental study and found to be very satisfactory. The present analysis mainly focused the convective heat transfer between the tubular absorber and the flat top of the concentrator cavity. As the truncation of CPC cavity was from full height to one-third full height, the convective heat transfer rate between the absorber and top got increased. Eames and Norton (1991) formulated the heat transfer coefficient in terms of fluid inlet, ambient temperature and insolation level. Kim et al. (2008) carried out numerical work for evacuated CPC solar collector. Thermal processes in a CPC collector with flat one-sided absorber was studied theoretically through mathematical equations by Tchinda and Ngos (2006) and also validated the proposed mathematical model with experimental study done by Rabl. An expression for the temperature of the HTF was developed as a function of the space co-ordinate in the flow direction and time dependent solar intensity. Also, the effect of some of the design parameters like rate of the fluid flow, inlet temperature, CPC length, selective coating and mirror reflectance was analyzed. The results showed that the shorter CPC length was more efficient than long length. Chaves and Pereira (2007) presented the practical designs for photocatalysis. To treat contaminated water chemically or biologically, the solar radiation spectrum could be used in the presence of a catalyst (Photo catalysis) or in Photo Fenton techniques. To collect and deliver solar radiation for the treatment of the contaminated water, an efficient optics is required. The authors used non-imaging optics (CPC collector) for the water treatment. It was found that concentration falls short by ∼1% of the maximum ideal concentration because of caustic formation inside the dielectric. Sharma and Diaz (2011) numerically investigated the thermal

Fig. 42. Structure of the SCPC solar collector (Zheng et al., 2016).

also presented a detailed analysis of solar thermal-driven desalination in comparison with solar PV/RO (Reverse osmosis). His highpoints were, fully developed membrane distillation systems, will have the marketplace opportunities for very small-capacity seawater desalination. Solar thermal-driven reverse osmosis for seawater desalination is of major advantage as it requires less energy. Also, the possibility of using thermal storage instead of batteries makes it much more advantageous. Xu et al. (2017) reported an experimental investigation of a newly proposed solar collector that integrates a closed-end PHP, used as an absorber and CPC. Prototype of the solar collector was built, with a PHP absorber bent by 4 mm diameter copper tube and with the collection area of 300 × 427.6 mm2. The operating characteristics and thermal efficiency of the solar collector were studied and the collector apparently shows start-up, operational and shutdown stages at the starting and ending temperatures of 75 ⁰C. The experimental results suggested that the heat flux of the PHP absorber’s evaporation section concentrated by CPC was appropriate and the use of CPC was reasonable. Collector’s instantaneous thermal efficiency can reach up to 50%, when the direct normal irradiance is 800 W/m2. Li et al. (2018) designed and tested a small-sized solar seawater desalination system with multi-effect heat recovery processes using CPC solar collector. The designed system was made up of 7 heat collecting integration units divided into 7 temperature/pressure states. Standard CPC was modified by truncating the reflector in order to increase collecting time. The system satisfied primary demand and showed good freshwater production performance. The study also provided useful guidelines for the development of passive type of high effective solar desalination system. The experimental tests were carried out by Aguilar-Jiménez et al. (2018) simultaneously and the analysis of the effects of varying mass Table 5 Consolidated results obtained for various CPC solar collector configurations. Source: Prapas et al. (1987). Sl. No.

CPC Solar collector configuration

Achievable Absorber Temperature

1. 2.

Collector with either an evacuated cavity or selective absorber with low CR less than 1.55 Collector with non-evacuated, a non-selective absorber

3. 4.

Collector with an evacuated cavity, a selective absorber and a CR of 3.85. Collector with high CR greater than 3.85.

Below 70 °C Below 115 °C (Actual value depends on the CR) up to 290 °C Above 290 °C

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performance of solar collector consisted of a novel minichannel with Ushaped flat tube absorber of selective coating on its external surface and was enclosed in an evacuated glass envelope (Fig. 43). Operation at higher temperatures was benefited by the reduction in the heat path from the absorber surface to the working fluids. The entire assembly of minichannel was enclosed in an evacuated glass tube and glass to metal seals were used to maintain the vacuum. The performance of the minichannel based solar collector with and without a concentrator was compared against the results of an evacuated tube collector without minichannels from the literature. In minichannel based solar collector without concentrator, the following observations were made:

cover, double glazing with 12 mm air layer, double glazing with low-e coated lower glass with 12 mm air layer and double glazing with low-e coated lower glass with 12 mm argon layer. Among all double glazing with low-e coated, lower glass with argon layer performed better. Nashine and Kishore (2017) analyzed the performance of a CPC collector and compared its performance for two different locations such as Visakhapatnam and Mumbai by considering the solar radiation data from the Indian Meteorological Department. The evaluated results showed the potential of improving the thermal efficiency up to 75%. The collector was designed for the steam temperature of 150 °C. The authors concluded with the following statements: (i) The variation of solar intensity was highly fluctuating for Mumbai during the seven months from December to June, while for Visakhapatnam it was almost the same except during February, (ii) The efficiency for both the cities were almost similar for most of the months except May and June, for which the solar collector placed at Mumbai had 14% higher efficiency than that of Visakhapatnam, (iii) In both the cities, the heat transfer coefficient increases with mass flow rate due to increase in temperature difference. In Mumbai, it was found that the heat transfer was 5% higher than that of Visakhapatnam, (iv) The useful heat rate for Visakhapatnam was 9.4% more than that of Mumbai and (v) The heat removal factor increases linearly with the mass flow rate and they found that for both cities the values are almost the same.

• The effect of mass flow rate beyond 10

−3

• • • •

kg/s had no significant effect on fluid outlet temperature and collector efficiency of the respective fluid inlet temperatures, The collector efficiency was reduced by increasing the fluid inlet temperature due to radiation losses, The minichannel based solar collector shows a significant advantage, particularly at higher operating temperatures, The pressure drop level inside the minichannel based collector can offset any gain in heat transfer and High flow rates and low temperatures were not recommended for the operation of the collector.

In minichannel based solar collector with a concentrator, the following observations were made (Sharma and Diaz, 2011): i) The efficiency at an angle of incidence 0° is lower than the efficiency at 35° mainly due to higher gap losses at 0°. Therefore, the configuration was sensitive to the gap loss and ii) This arrangement had a distinct advantage in terms of efficiency at high operating temperature when compared to the literature. Antonelli et al. (2014) presented a study on a thermal power plant (ORC) that uses an expansion device driven with pressurized vapor generated with the heat collected by a CPC collector solar field. An analytical model of the collector was presented for the evaluation of collected heat w.r.t. sun incidence angle, external temperature, inlet carrier fluid temperature and mass flow rate. Numerical model was developed for the evaluation of delivered power, isentropic efficiency and specific working fluid consumption. The steady state analysis of global electrical energy delivered was presented as a function of both the thermodynamic parameters of the plant and the geometrical parameters of the collector. The considered saturation temperature (Tsat) range for the analysis purpose was 80–130 °C. Using different combinations of CR and solar collector tilt angle, same amount of collected energy was obtained. Higher value of solar collector tilt angle provides a more constant energy collection along the year. The maximum yearly amount of energy is collected, when Tsat = 120 °C with the CR and tilt angle in the range of 1.75–2.25 and 35–50° respectively. The optimal tilt angle was found to be 40° and CR = 1.75, these values were arrived by assuming that there was no tilt adjustment throughout the year. Zheng et al. (2016) studied the effects of key parameters including the structural and operating parameters of the thermal performance of SCPC collector. A detailed mathematical model was developed based on the analysis of heat transfer and further solved using software tool Matlab. The key parameters are length, diameter and emissivity of the tube receiver, CR, ambient temperature, solar radiation intensity, water flow velocity and water inlet temperature. Kessentini and Bouden (2016) presented the numerical simulation, design and construction of a double-glazed ICS with CPC reflectors (Fig. 44). The use of double glazing with argon layer and low-e coated lower glass instead of one glass cover shows significant improvement in thermal behavior of the ICS heater and reduction in the thermal losses during the night through simulation results. The results were presented as variation of the water temperature during stagnation test, daily and diurnal efficiencies and thermal loss coefficient during the night. The different investigation models were studied such as ICS with single glass

6.2.2. Simulation studies Table 6 illustrates the simulation tools used to study the CPC solar collectors. It covers three different sets of simulation tool: Programming simulators, Programmed simulators and Ray tracing simulators. Florides et al. (2002b) discussed about the modeling and simulation of solar operated single effect, H20 – LiBr pair absorption cooling system using TRNSYS software for Nicosia, Cyprus, covering a typical house load during the whole year. They also investigated the application of different collectors such as FPC, CPC and ETC for the solar based absorption cooling system using TRNSYS software. The article had shown the economic viability of the proposed system for both cooling as well as hot water generation for home needs. Additionally, the article presented the performance and evaluation of long term integrated system and behavior of dynamic system. Kim et al. (2008) investigated the heat flux with the incidence angle using ray-tracing code TRACE PRO. The amount of absorbed heat flux analyzed with the incident angle and the beam intensity with a ray-trace projection of 1,00,000 rays apart from fixed positions was carried out in the 2D models for

Fig. 43. Minichannel based solar collector (Sharma and Diaz, 2011). 319

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and the system factors according to the data provided by experiments. The system mainly consists of the non-tracking XCPC solar collectors, a buffer storage tank, an absorption chiller, a hot storage tank and an auxiliary heater in the space heating loop. The collector can achieve temperature up to 180 °C with 40% of the solar collector efficiency. No extra auxiliary heater is required in the cooling loop as the dual fired double effect absorption chiller was driven either by hot water or natural gas. The energy system simulation consists of two major parts: i) building simulation and ii) energy supply system simulation. The output variables such as fuel demand and consumption were predicted and used to calculate the life cycle primary energy savings. Gu et al. (2014) conducted experiments on non-tracking CPC solar collector using Ray tracing simulation. Some of the key factors for the optical performance is the quality of the selective absorber, absorption, transmission and reflection by the glass cover and absorption by CPC reflector. Simulation results were observed as follows:

Fig. 44. Photo of constructed double glazed ICS solar heater with CPC reflector (Kessentini and Bouden, 2016).

both stationary and tracking CPC solar collectors. Tchinda (2008) computed the thermal performance of an air heater with a truncated CPC through mathematical model for a flat one-sided absorber painted with non-selective matte black. Also, the author investigated the influence of air mass flow rate, wind speed and the collector length of the air heater thermal performance. Using a constructed computer code, a theoretical solution procedure of the energy equations for the solar air heater was discussed in predicting its thermal performance. Buttinger et al. (2010) developed and analyzed a new stationary, low-concentrating collector for the economical supply of solar process heat of the temperature between 120 °C and 150 °C. Asymmetrical reflectors with a concentration of 0.6X located below the headers provided extra radiation and prevented longitudinal radiation losses. They discussed about the merits and demerits of both asymmetric and symmetric reflectors. The heat losses were suppressed by using air or inert gas like krypton at a pressure below 10 mbar. The collector heat losses with air could be minimized as 50% and with krypton as 75%, compared to air at 1 bar. Optimal troughs were developed with the aid of FEM simulations. Balkoski (2011) performed optical and thermal models by applying optical efficiency data as inputs. Using LightTools, the optical characteristics of systems were analyzed. The software performed Monte Carlo simulations for ray-tracing. The thermal models highlighted the results of previous works and also compared with the experimental results. Hang and Qu (2011) presented the development of a method to optimize an integrated solar absorption cooling and heating (SACH) system. The building and secondary systems were analyzed using Energy plus and by using TRNSYS, the experimental trials (plant analysis) were conducted. A case study was conducted to apply the optimization method for the design of an integrated SACH system installed in a medium-sized office at Los Angeles. Regression analysis was used to identify the relationship between the life cycle primary energy savings

• The expected overall optical efficiency fluctuates between 0.75 and • •

• • •

0.80, over all operational transversal angles which shows that the collector optical performance is insensitive with time, For the transversal angle less than 20°, the optical losses were negligible and when increased to 25° considerable amount of rays pass through the gap between the receiver and the CPC reflector, When the transversal angle is 0°, then the heat flux along the receiver is symmetric and highest heat flux was observed at the edges of the receiver due to the reflection by the partial circle, and for 10°, the power on the top and bottom of the receiver were almost the same and for 10° to 27.4°, the peak heat flux moved from the left side towards the right side of the receiver respectively, The optical efficiency generally decreases with increasing longitudinal angle, When the longitudinal angle is less than 20°, the heat flux is larger than 2.2 kW/m2, for which working temperature of more than 250 °C is achieved. For the transversal angle of 27.4°, almost all the rays are concentrated around the parabola focal point for both top and bottom surfaces.

Santos-González et al. (2014) designed the CPC with copper cylindrical receiver using a detailed 1D numerical model. In this article, two CPC geometries of acceptance angles 30° and 40° were proposed based on numerical simulation and commercial availability of the material; one of them with 30° was tested experimentally at different inlet temperatures and mass flow rates. Experimental results obtained show that this technology could provide stable temperatures for industrial processes. The parameters that were analyzed in the article are thermal efficiency, increment of water temperature, UEG of the working fluid and pressure drop in the collector. Ray tracing analysis using Tonatiuh software was conducted for truncated and non-truncated models to quantify optical energy losses. Fig. 45 shows the visual outcome of Tonatiuh software. The highest UEG collected was 1.47 kW at a mass flow rate of 12 kg/min, with solar irradiance on the collector plane of 1026 W/m2 and ambient temperature of 36 °C. The maximum increase in temperature with water as the working fluid was 7 °C for mass flow rate of 2 kg/min, solar irradiance in the collector plane of 1054 W/m2, collector inlet water and ambient temperatures were 42.5 °C and 35.6 °C respectively. The highest pressure drop found was 1.2 kPa for the mass flow rate of 12 kg/min. Hess (2014) developed a stationary, double-covered process heat flat-plate collector with a one-sided, segmented booster reflector (RefleC), which approximates one branch of a CPC. The work included contribution of the instantaneous and the annual output of low-concentrating solar thermal collectors, which was investigated by ray tracing and monitoring of a pilot plant. External reflectors can highly increase the gain of stationary solar thermal collectors. From the simulation results, it is revealed that the RefleC booster increases the

Fig. 45. Visual ray tracing numerical simulation by using Tonatiuh software (Santos-González et al., 2014). 320

321

Das (2013); Math (2018)

Baloi et al. (2015); Math (2018)

Synopsys (2018)

Zemax (2018)

5

6

7

8

4

3

Sørensen et al. (2009); Gravagne and Treuren (2008); Riederer et al. (2009); TRNSYS (2018); Zhou (2013) Vela (2018)

f-chart (2018); Löf (1993) f-chart (2018)

1

2

Reference

S.NO

Ray tracing simulator

OpticStudio

Light tools

SIMULINK

Programmed Tool

Ray tracing simulator

MATLAB (Matrix Laboratory)

Polysun

TRNSYS

EES

F-CHART

Software Name

Programming Tool

Programmed/ Programming Tool

Programmed/ Programming Tool

Programming Tool Programming Tool

Type of Tool

Table 6 Particulars of the simulation tools for CPC solar collectors.

Mountain View, California, U.S.A. Kirkland, Washington, U.S.A.

Natick, Massachusetts, U.S.A.

Natick, Massachusetts, U.S.A.

Switzerland

University of Wisconsin

Madison, WI, United States Madison, WI, United States

Origin

Works by ray tracing—modelling the ray's path through an optical system. Contrast optimization by using Moore Elliott method – Zemax.

–

1990

1986

1984

–

18.1

8.5

R2017b

MATLAB 9.3 (R2017b)

Version: 10.1

1992

1984

Version 18

Version 10.342 2018

Version 10

Current version

1975

–

1975

Year

Various methods are followed based on the applications such as Approximate calculation method for solar fraction calculation; Rainflow Cycle counting method for battery lifetime estimation; Automatic, Catalog & From file methods used for cold water supply system calcuation; Number of transfer units – Effectiveness for air/ water heat exchanger calcuation. –

Successive substitition method

MARTIN hou (1955) equation of state and fundamental equation of state (Tillner-Roth (1998)) approach.

F-Chart Method

Methodology Used

Dr. Ken Moore; ZEMAX

MathWorksdesigned by Cleve Moler, Chief Mathematician, chairman and cofounder MathWorksdesigned by Cleve Moler, Chief Mathematician, chairman and cofounder Synopsys

Northern lights solar solutions and partnership with Vela Solaris.

S.A. Klein and W.A. Beckman Developed by F-Chart Software (by Professor Sanford A Klein from Department of Mechanical Engineering University of Wisconsin-Madison). University of Wisconsin by the members of Solar Energy Laboratory

Developers

(continued on next page)

Design, simulation, optimization and analysis solution Optical design and analysis

Simulation and model-based design

Object-Oriented Programming

Simulation software for renewable energy sector (such as performance of solar heating system)

Modular system simulation program

Analysis and design program Equation Solving Program

Type

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Hardy and Steeb (2008); Radiance (2018); Ward (1994)

Abraham et al. (2018); Photopia (2018) Ansys (2018)

10

11

322

Highlights

Life-cycle economics analysis; Variation in monthly parameter; Two dimensional IAM; BS and SI units.

Solves simultaneous non-linear equations up to 6000 (Commercial version), 12,000 (32-bit Professional version) and 24,000 (64-bit version). Helpful in solving problems related to thermodynamics, fluid mechanics and heat transfer with the support of inbuilt data bank of

1

2

Ray tracing simulator Ray tracing simulator

Ray tracing simulator

Ray tracing simulator

Ray tracing simulator

Type of Tool

S.NO

13

LMS (2018); Neto et al. (2007)

COMSOL (2018)

9

12

Reference

S.NO

Table 6 (continued)

$ 600 (Single)

One-time charge of $600 (including one year of instant update service).

Price

ANSYS CFX,CFD AMESim (Advanced Modeling Environment for Simulation of Engineering System) (or LMS Imagine.Lab AMESim)

Photopia

Radiance and RADIANCE Photon Map (P Map)

COMSOL Multiphysics

Software Name

All version of MS Windows.

Mechanical engineering.

–

All version of MS Windows.

Parameter range is limited.

Weather data (over 300 locations and can be added based on requirement); Various types of collectors (FPC, ETC, CPC, one and two axis tracking) can be analyzed. Thermodynamic and transport property database consisting of high accuracy. Very fast computational speed. Uncertainty analysis, optimization of single & multivariable and regression capability. Consistency checking of unit conversion and automatic unit. Spreadsheet-like table for parametric studies. Difficult to incorporate specific diagnostic capability.

Sectors

LMS Imagine.Lab Amesim 15

ANSYS 18.2

Photopia 2017.3

RADIANCE 5.0

5.3a

Current version

Operating Platform

1995

1970

1996

–

1986

Year

Demerits

Hybrid finite-element/ finite-volume approach Based on the Bond graph theory

Photon mapping – forward raytracing technique which supplements Radiance's standard backward raytracer. Monte Carlo sampling method for integration of contributions (diffuse skylight, interreflection). Monte Carlo Raytracing method

Finite Element Method

Methodology Used

Merits

U.S.A.

U.S.A.

U.S.A.

Gemany

Stockholm, Sweden

Origin

Engineering applications; Propagate the uncertainty of experimental data.

Educational and commercial applications.

Applications

Siemens PLM Software

John Swanson

LTI Optics

Svante Littmarck and Farhad Saeidi, COMSOL group Fraunhofer Institute for Solar Energy Systems ISE

Developers

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Interated tp Fortran, C/C++, Python, Excel and MATLAB.

–

Programming language

Engineering simulation software Modeling and simulation software

Non-imaging optical design software.

Simulation software

Modelling and simulation software

Type

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6

5

4

3

S.NO

Table 6 (continued)

Support provided for MATLAB commands with automation, contextual hints for arguments, property values and alternative syntaxes; Analyzing and modeling text data of proposed design; Problem based optimization using simple methods; Without SQL, database explorer app visualizes relational databases. Simulation Manager (Monitor, inspect and visualize simulation progress and outcomes); Advanced Simulation Data Inspector UI and API; Reduced memory usage for large models.

Time-saving and professional system design; Integration of worldwide weather data for reliable yield prediction.

thermodynamic and transport properties. It is made of two parts: i) Engine (Kernel) and ii) Extensive library of components. The engine reads and processes the input file, iteratively solves the system, determines thermophysical properties, inverts matrices, performs linear regression and interpolates external data files.

Highlights

The results are centralized in a list which is accessible using powergui button; Possibility to export the results in other useful files format (xls spreadsheets).

Allows users to accurately solve problems, easily produce graphics and produce code efficiently.

MATLAB INR 1,45,000 Individual and USD 23 (MATLAB Student – unbundled)

USD 45 (MATLAB and Simulink student suite)

Effectively simulate solar thermal, PV and geothermal systems; Easy optimization of existing and proposed systems; User-friendly in report generation; Continuous updation in the product databases; Automization of multiple simulaitons.

Behavior of transient systems can be simulated by extremely flexible graphical environmental software (TRNSYS). Dynamic systems such as traffic flow or biological processes can be modelled.

Merits

Annual Subscription: € 949 (Polysun Professional Solar thermal Simulation)

$ 5060 (Single user license – Commercial Price) and $ 2530 (10 user licenseEducational Price).

Price

Restrictions on code generated from Simscape models; Some tools and features do not work with Simscape

Only with considerable amount of uncertainty the long-term predictions of future inflation rate of an economic area are possible. Requires skilled programmer.

Mixed quality of the documentation, TRNSYS almost always reports unit mismatches, error propagation.

Demerits

Windows, Linux, and Macintosh.

Engineers and Scientists Worldwide Rely on Simulink (Automotive, aerospace, communications,

Scientists, researchers, and engineers (Automotive, aerospace, communications, electronics, industrial automation industries)

Best suited for: engineering offices, designing engineers, energy consultancies, producers.

Windows 7/8.1/ 10 and MAC OS X 10.11.3 or above

Windows, Linux, and Macintosh.

Researchers to consultants, engineers to building simulation experts, and students to architects.

Sectors

All version of MS Windows.

Operating Platform

Automotive systems, aerospace flight control and avionics, telecommunications and other electronics equipment, industrial machinery and medical devices. Automotive systems, aerospace flight control and avionics, telecommunications and other electronics

Solar systems (solar thermal and PV systems), low energy buildings and HVAC systems, renewable energy systems, cogeneration, fuel cells. Design of solar thermal, heat pump, PV systems and combined systems

Applications

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C, C++, C#, Java, Fortran and Python

C, C++, C#, Java, Fortran and Python

Java, Python, Matlab

Fortran, C, C++, language having compiler capable of creating a DLL.

Programming language

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User defined surfaces and objects; Design complex, custom CAD objects to create in a non-sequential optical system.

Sophisticated solid modeling with accuracy; Ray tracing speed with user control accuracy and resolution requirements; Build a light source for any geometric model; Application-specific utilities; Robust data exchange support.

7

8

Highlights

S.NO

Table 6 (continued)

$ 5600 per user (Standard)

$ 48,000 (full equipped, including a permanent license)

Price

Improvement in optical performance; Short duration to market; Reduction in cost.

Offers many timesaving solutions; Increase Engineering Productivity.

Merits

–

May require significantly disk space for large models and complex analyses.

blocks and its software.

Demerits

Windows, Mac OS, Linux, Unix

Windows 7

Operating Platform

Imaging optics, Lighting and illumination, lasers and fibers.

LEDs, displays, general lighting, solar, automotive, stray light simulation, projectors, photorealistic

electronics, industrial automation industries)

Sectors equipment, industrial machinery and medical devices. LED speedometer, LED flashlight, LED lamp, Cellphone applications, LED street lamp, luminescent solar concentrator, solar power tower, soar fresnel concentrator, solar compound concentrators utility, solar trough collector, CPC skinned solid etc., Thermal imagers.

Applications

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Zemax programming language macro programming, C#, MATLAB.

–

Programming language

V. Pranesh, et al.

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It is to control the deployment and distribution of running simulation apps. User developed apps can be run through web browsers/a Windowsinstalled client. Can handle arbitrarily complex objects; Can integrate with existing ray tracing techniques.

9

LTI Optics are not relied on third party data source, as over 700 fully measured materials are available; Output is relevant and standard compliant; Inbuilt optical design tools (AutoCAD, SolidEdge, SOLIDWORKS, Pro/ Engineer CREO and Inventor).

CFX has outstanding robustness and speed with rotating machineries.

Provides a unique integrated platform, realistic component and system models for every stage of cycle. It also enables engineers to initiate evaluation and validation phases initially in the design cycle.

11

12

13

10

Highlights

S.NO

Table 6 (continued)

325 –

$440

$800–$75,000

–

$ 1695 (CPU locked license)

Price

Provides a great scalability in main physical domains and application libraries. Generation of complex models are easy and simulation analysis is performed quickly.

Amazing flexibility, accurate, takes simulation to another level, provides highmemory efficiency, reliable and speed.

Accurate, The analysis is fast, as it can trace millions of rays in minutes.

Benefits of photon map are: efficient and globally allows other illumination phenomena to be rendered as it is not patent on the method.

Possibility to export geometries, meshes and surface plots in the STL format for 3D printing.

Merits

Do not support remote display of certain graphics, such as CFX_Pre and CFD_Post, to computer using Exceed3D Interpreting the reuslts are difficult and lack in accuracy of each parameter's effect.

Raysets often do not have accurate 3D emanation points, as the data complied from a set of 2D images.

A combination of different materials remains a significant challenge task. –

Demerits

Windows, UNIX (particularly under Linux)

UNIX and Windows

Windows 7

UNIX platforms, including SGI, Sun, HP, DEC, Apple (A/UX) and IBM

Windows, MacOS, Linux

Operating Platform

Engineering and research

–

Industrial applications.

Researchers, educators, architecturers and engineers.

Engineers and scientists

Sectors

Academic and research

Lighting system in architectural, engineering and manufacturing firms. Performance evaluation of nonimaging optical systems; Architectural lighting; Signaling devices; UV curing and disinfection; daylighting; Solar Concentrator. Multiphysics applications.

Engineering field.

Applications

Matlab, Python and Scilab or Microsoft Excel and Visual basic application

CFX Expression Language (CEL)

–

C

Java

Programming language

V. Pranesh, et al.

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a significant function of mass flow rate, Case 2: The effect of operating pressure on the thermal efficiency was negligible. The additional decrement of pressure will cause a significant reduction in working fluid density, which creates high flow velocities requirement for the fixed mass flow rate and Case 3: Difference in temperatures (inlet and outlet fluid conditions) varies linearly with solar irradiance. A reduction in operating pressure resulted in a larger temperature difference, particularly at high values of solar irradiance. Almeida et al. (2014) presented the simulation of the solar thermal system by TRNSYS for the identification of parameters with a different approach, optimized by GENOPT. A good agreement of differences lesser than ± 5% was obtained as a result for both thermosyphon and forced circulation systems, when compared with the results using ISS, v2.7. The objective of the article is to demonstrate the use of TRNSYS and GENOPT as an alternative to ISS, 2.7. Sobhansarbandi and Atikol (2015) made the following observations from the simulation results (i) the TRNSYS results suggested that a 2 m2 CPC solar collector can perform satisfactorily same as that of an 8 m2 flat plate collector array, achieving the same required circulating water temperature in the radiant slab. Hence lesser space requirement for system installation of CPC solar collector when compared to FPC; (ii) FPC and CPC solar collector outlet fluid temperature were between 25−70 °C and 25−95 °C respectively and (iii) higher total energy absorption by CPC solar collector than FPC. Benrejeb et al. (2015) presented a new design of an integrated collector storage solar water heater of capacity 100 l, combined with a full CPC in order to increase the quantity of absorbed energy and improve the optical and thermal performances. The model consisted of two concentrating stages where the upper part contains two symmetrical parabolic sections and the lower part is constituted by three involute reflectors. The lower stage ensures the reception of the reflected solar rays on absorber surface. The article studied the new and old designs (as shown in Fig. 47a and b), their optical and thermal performances through the following approaches (i) by providing the mathematical equations describing the geometric design of the ICS system, (ii) optical study with ray tracing technique, results and energy flux distribution on the absorber surface and (iii) heat balance on the absorber. The old design consisted of full CPC reflectors of three parabolic sections. Using analytical geometry and vector calculation, Matlab code was developed for data generation, plotting and simulating the reflected rays on the CPC reflectors at any instant. The maximum obtained temperature for the new design is about 65 °C.

Fig. 46. Photo of the RefleC prototype (Stationary, double covered process heat flat plate collector with a one-sided, segmented booster reflector) (Hess, 2014).

annual output of the double-covered flat-plate in Wurzburg by 20% at a constant inlet temperature of 40 °C and 87% at an inlet of 120 °C. The pilot plant as shown in Fig. 46 is demonstrated for the process heat generation with RefleC at working temperatures up to 130 °C. Considering all inlet temperatures, the total annual gains of RefleC was 39% above the flat-plates without reflectors. Duong and Diaz (2014) presented numerical simulations of XCPCs combined with ETCs and PTCs operating with CO2 as a working fluid for a range of temperatures which covers medium and high range. They also explored the benefits and drawbacks of using CO2 in a solar thermal system at medium and high operating temperatures. A mathematical model (XCPC collector) was generated for the implementation in Engineering Equation Solver (EES) to simulate the behavior of working fluid (CO2). Three different cases were simulated for the XCPC collector which are Case 1: Analysis of the thermal efficiency of the collector as a function of working fluid inlet temperature between the range 50 °C and 220 °C at 0.01, 0.02 and 0.03 kg/s mass flow rates, Case 2: Analysis of the thermal efficiency based on inlet temperature of CO2 at 9, 10 and 12 MPa operating pressures and Case 3: Analysis of the effect of solar irradiance for the range between 300 and 1000 W/m2 on the difference between outlet and inlet fluid temperature for the pressures 8, 9 and 10 MPa. The results for the different cases based on the consideration are highlighted as Case 1: Thermal efficiency was not

Fig. 47. Profile of the new and old ICS systems respectively (Benrejeb et al., 2015). 326

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Fig. 48. Cross-section view of CPC designed for (a) a U-tube receiver without fins and (b) a hybrid of bifacially irradiated flat receiver and a U-tube receiver (Karwa et al., 2015).

Karwa et al. (2015) modeled and evaluated the thermohydraulic performance of a bifacially irradiated receiver which was a hybrid of a flat and U-tube receiver in a vacuum enclosure with CPC and collector fluid (Therminol 66) with temperature in the range of 100–300 °C. Fig. 48 shows the cross-sectional view of CPC designed for (a) a U-tube receiver without fins and (b) a hybrid of bifacially irradiated flat receiver and a U-tube receiver. Modeling’s were carried out in the optical performance, useful heat gain in the collector and pumping losses. Maximizing the collector effective thermal efficiency was obtained by optimizing the receiver shape ie., tube and fin dimensions, which was dependent on the receiver area, gap size, CR, mass flux, fluid temperature, selective surface emissivity, etc. Normally, in the laminar – turbulent transitional regime, maximum effective thermal efficiency will be obtained. Due to the higher gap loss for the hybrid receiver, the optical efficiency will be lower than the tubular receiver. However, the hybrid receiver had similar or better thermal efficiency than tubular receiver. The author suggested that by appropriate selection of tube diameter using the proposed thermal model, improvement in the effective thermal efficiency can be obtained for the receiver perimeter with larger value, low to moderate flow rate (≤0.15 kg/s m2) and the fluid with less viscosity. The optimum outer tube diameter was between 5 and 10 mm with thickness of 1 mm with the dependency of receiver perimeter and the optimum copper fin thickness up to 1 mm for a mass flux of ≤0.15 kg/s m2. Waghmare and Gulhane (2016) designed CPC solar collector and performed ray tracing analysis without receiver using 2D sketcher software to distinguish the region of maximum collection and no

collection of reflected rays. Also, they determined the LD and maximum diameter of the tubular receiver at the focus of CPC by the geometrical method of ray tracing and their geometrical relationship. Through ray tracing analysis, it was observed that the maximum concentration of reflected rays occurred below the focus and with no reflected rays around the common focus of CPC. The portion with no reflected rays from reflector’s half follow an elliptical shape and will be tangent internally to LD of the receiver at the focus. Shrivastava et al. (2017) reviewed simulation of solar water heating system consisting of early works and simulation tools comparison in relation to TRNSYS. Also, they highlighted that the maximum error range of 5 to 10% could be provided by TRNSYS. Moreover, assumptions, modeling of different components, merits and limitations of simulation were reviewed. A low profile concentrated solar thermal collector was examined by Li et al. (2017) which provides medium temperature heat for heating and cooling applications in commercial buildings. Also, they carried out the viability of the collector design for solar heating and cooling using a TRNSYS model. The system consisted of the proposed solar thermal collectors, a double effect absorption chiller and an auxiliary heater. Among design options, yearly solar fraction and economic metrics are used as the selection criteria. The simulation outcomes showed that 2.4 m2 per kW cooling with 40 l/m2 (optimal storage tank specific volume) was sufficient to achieve 50% of load requirement of the building. Zheng et al. (2017) designed, analyzed and tested a compact, semipassive beam steering prism array for solar applications, thereby 327

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Table 7 Details of the application of CPC solar collector in various fields. Sl. No.

Application (Sector Wise)

Sub-category

Authors/Researchers

1. 2. 3. 4. 5. 6.

Solar Thermal

Process steam generation Thermal power plant (ORC) Adsorption heat pump for residential cooling (and heating). Two stage lithium bromide (Li-Br) absorption chiller for cooling and heating system. Solar absorption cooling and heating system Building heating and cooling applications (Solar based single effect and double effect absorption cooling systems) Solar adsorption chiller (one two-phase thermo-syphon silica gel-water) and LiBrH2O absorption chiller. Single effect LiBr absorption chillers Double effect absorption chiller

Gudekar et al. (2013) Antonelli et al. (2014) Lambert (2007) Hu et al. (2011) Hang and Qu (2011) Nkwetta and Smyth (2012)

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

Day lighting Others

31. 32. 33. 34. 35. 36.

Double effect absorption chiller, Heating and cooling applications in commercial buildings. Rural applications, such as water heating, steam cooking (Pressure cooker), sterilization, low power steam generator. Box type solar cooker Domestic water heating (of about 90 °C) in buildings in the tropical region Integrated collector storage solar water heater Integrated water storage Industrial heating application at lower flow rate Methanol reforming process at 250 °C to produce hydrogen for fuel cell Under floor heating systems Space heating Solar air heater Air heating (150–230 °C) for industrial application. Process heat application between 120 °C and 150 °C Process heat in the range of 100–300 °C Industrial and commercial heating applications Solar thermal drum drying system (Prune pomace and tomato pomace) Passive solar seawater desalination system Organic Rankine cycle, desalination, and air conditioning Solar day lighting system Optical transceiver Building integrated photovoltaic Solar distillation system To treat chemically or biologically contaminated water Solar water disinfection Solar disinfection of urban wastewater Solar disinfection of wastewater Water treatment

Lu et al. (2013) Lu and Wang (2014) Balkoski (2011); Widyolar et al. (2014); Winston et al. (2014) Li et al. (2017) Oommen and Jayaraman (2001) Harmim et al. (2012) Okoronkwo et al. (2014) Benrejeb et al. (2015) Kessentini and Bouden (2016) Sagade et al. (2014) Gu et al., (2014) Sobhansarbandi and Atikol (2015) Zheng et al. (2016) Pramuang and Exell (2005) and Tchinda (2008) Wang et al. (2015) Buttinger et al. (2010) Karwa et al. (2015) Li et al. (2016) Milczarek et al. (2017) Li et al. (2018) Aguilar-Jiménez et al. (2018) Kaiyan et al., (2007) Yoshinori Matsumoto et al., (2001) Guiqiang et al. (2012) Mammo et al. (2013) Prasad and Tiwari (1996) Chaves and Pereira (2007) Rodríguez et al., (2010) Gutiérrez-Alfaro et al., (2018) Maddigpu et al., (2018) Expósito et al., (2018)

evaluated the performance of a hybrid solar window which consists of 2D CPC that provides heating as well as daylighting (as shown in Fig. 49). Li et al. (2016) stated that by overcoming the loss mechanisms in volumetric receiver type CPC solar collector, results in providing an effective and low-cost approach, paving the nanotechnology into industrial heating and air conditioning applications. The proposed collector with lower profile (< 15 cm height) is suitable for supplying thermal energy in the range of 100–250 °C. Widyolar et al. (2018) stated that providing heat efficiently up to 200 °C, the XCPC collector can be used for a number of industrial process heat applications including boiler pre-heating, steam generation, absorption cooling, desalination, drying, and food processing.

providing an alternative solution in tracking the sun within limited space (Rooftops). The proposed design allows the linear concentrator to remain stationary and also it can be integrated with various solar concentrators. The proposed system demonstrated that it can increase the average daily optical efficiency of the collector by 32.7% and also increase the effective working hours from 6 h to 7.33 h. 7. Applications Table 7 illustrates the researchers who work in the field of CPC solar collector on application basis. The information is categorized based on the literature survey. Balkoski (2011) concluded that the XCPC technology would be ideal for applications using a double effect absorption chiller because of high performance at 140 °C and above. And also, this leads the researchers at UC Merced to develop and carry out a demonstration project using the first non-tracking XCPC solar collector to run the double effect absorption chiller. Kim et al. (2013) stated that their proposed model of non-tracking system consists of XCPC with an evacuated counter flow type absorber is suitable for applications such as solar heating, cooling, desalination, oil extraction, electricity generation and food processing. Widyolar et al. (2014) and Winston et al. (2014) mentioned the usability of XCPC solar collector in solar heating, cooling, food processing, electricity generation, desalination and oil extraction. Ulavi et al. (2014)

8. Standards and certifications for solar collector Table 8 illustrates the various standards and certifications for the solar thermal collectors issued by the technical committees of various countries and are also described below: 8.1. Standards

• CEN-CENELEC stated the European Standard as, “It carries with it

the obligation to be implemented at national level by being given the status of a national standard and by the withdrawal of any

328

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Fig. 49. Schematic diagram of hybrid solar window with typical cross-section view (Ulavi et al., 2014).

•

•

•

•

conflicting national standard“. EN standards are valid for 34 CENCENELEC member countries (CEN, 2018). Mexican standard (NMX-ES-004-NORMEX-2010) applies to domestic solar water heating systems that work by: Natural or thermosyphonic circulation and Forced circulation (Technologies using FPC, ETC and CPC collectors) (NMX, 2018). This standard cannot be applied to solar collectors with tracking systems and systems with more than one TES tank. ISO standard was developed by CEN Technical Committee, Thermal solar systems and components, in collaboration with ISO Technical Committee, Solar energy in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement) (ISO, 2018). ISO is a worldwide federation of national standard of bodies. – In ISO 9459-5:2007, by means of whole system tests, the system performance is characterized using a ‘black box’ approach (ISO, 2018). – ISO 9806:2017 is valid to all types of fluid heating solar collectors, hybrid solar collectors (co-generating heat and electric power), for laboratory testing and for in situ testing. It is not applicable for collectors with an integral thermal storage unit to the extent of collection process, not separated from the storage process for measuring purposes (collector thermal performance) (ISO, 2018). Canadian Standard (CAN/CSA-F378 SERIES-11) (CSA, 2018) comprises of F378.1-11 and F378.1-11 standards. F378.1-11 applies to liquid heating solar collectors such as glazed/unglazed FPC, glazed ETC, ICS systems with time constant < 30 minimum and concentrating collectors with θa > 30°. F378.2-11 applies to air heating solar collectors with glazed/unglazed and closed/open loop configurations. ASHRAE (ASHRAE, 2010; ASHRAE, 2018) is a global professional association seeking to advance HVAR systems in design and construction. It focuses on building systems, energy efficiency, indoor air quality, refrigeration and sustainability within the industry.

8.2. Certifications Solar Keymark is a CEN/CENELEC European mark scheme (ESTIF, 2018). This scheme has a network, which acts together with CEN. After two Solar Keymark schemes I and II, more than two thirds of the collectors sold had Solar Keymark. SEIA (SEIA, 2018) is a non-profit trade association of the solar energy industry that develops education and outreach programs for the development of solar energy in the U.S. DIN CERTCO (DIN, 2018) demonstrates the quality, safety, efficiency and reliability of the products. It concerns the following activities such as: develops objective assessment criteria, thus ensuring the high quality services and expert advice; certifies service providers; awards quality marks; and keeps the level of quality of companies and their employees. SHAMCI (SHAMCI, 2018) is an Arab certification scheme which promotes adopting standard quality measures, accreditation systems and quality labels across the Arab states. ICC-SRCC is an ISO 17065-accredited third-party certification body with programs for the certification and performance rating of solar thermal and small wind turbines (SRCC, 2018). The performance of the solar collector is analyzed based on time constant. The time required for the collector outlet temperature to rise by 63.2% of the total increase with the proceeding step rise in solar irradiance at time zero is defined as the time constant of collector (Kim et al., 2013). Buttinger et al. (2010) carried out tests according to DIN 12975. Nkwetta and Smyth (2012) evaluated the experimental results and analysis of the SSACPC and DSACPC solar collectors using ANSI/ ASHRAE standard 93-2003. Santos-González et al. (2014) used Mexican standard NMX-ES-001-NORMEX-2005 for the experimental study of the CPC solar collector system. Almeida et al. (2014) generated test sequences according to ISO 9459-5:2007 for the simulation study. Chamsa-ard et al. (2014) tested the heat pipe evacuated tube with CPC solar collector by ISO 9806-1 and found the thermal efficiency as 78%. They also developed mathematical model to compute energy production for Phitsanulok province. Test standard (ISO 9806-1) was satisfied regarding the accuracy of the experiment by Li et al. (2016). According 329

CEN (2018); Kalogirou (2013); QAIST (2018) Fischer et al. (2004); Kalogirou (2013); QUAIST (2018) ESTIF (2018); Kalogirou (2013); Weiss (2003)

ESTIF (2018); Kalogirou (2013); Weiss (2003) CSN (2012)

NMX (2018)

ISO (2018)

ISO (2018)

CSA (2018)

ASHRAE (2010); ASHRAE (2018)

ESTIF (2018); Nielsen (2009)

SEIA (2018)

DIN (2018)

1

4

6

7

8

9

10

11

12

13

5

3

2

Reference

S. NO.

330

DIN CERTCO (Certification)

SEIA (Certification)

The Solar Keymark (Certification)

ASHRAE 93-2010 (RA 2014) (Withdrawn Standard)

CAN/CSA-F378 SERIES-11 (R2016)

ISO-9806:2017 (Second edition)

NMX-ES-004NORMEX-2010 (Mexican standard) ISO 9459-5:2007

EN 12977-2 (European Standard)

EN 12976-2 (European Standard)

EN 12976-1 (European Standard)

EN 12975-2 (European Standard)

EN 12975-1 (European Standard)

Standard/certification

To promote, develop and implement the use of solar energy in US. Conformity assessment – Requirements for bodies certifying products, processes and services (DIN EN ISO/IEC 17065)

Dedicated to Solar thermal collectors(based on EN 12,975 and EN/ISO 9806) and Solar thermal systems, storages and controllers (based on EN12976 and EN12977).

Solar energy – Thermal evaluation of solar systems for water heating – Method of testing. Solar heating – Domestic water heating systems – Part 5: System performance characterization by means of whole-system tests and computer simulation. Solar energy- Solar thermal collector – Test methods. Replaced ISO 9806:2013, first edition. Comprised of F378.1-11 (Glazed and unglazed liquid heating solar collectors – Test methods) and F378.2-11 (Air heating solar collectors – Test methods) Methods of Testing to determine the Thermal Performance of Solar Collectors (ANSI Approved)

Solar thermal systems and components – Factory made systems, Part 2: Test methods. Test methods for solar water heaters and combisystems

Thermal solar systems and components – Factory made systems, Part 1: General requirements.

Thermal solar systems and components – Solar collectors – Part 1: General Requirements Thermal solar systems and components – Solar collectors – Part 2: Test methods.

Title

Table 8 List of various standards and certifications for the solar thermal collectors.

Driving force behind solar energy and building a strong solar industry to power America. Deutsche Akkreditierungsstelle GmbH (DAkkS), the national accreditation body for the Federal Republic of Germany for the certification of products according to the European Standard DIN EN ISO/IEC 17065.

Determines the thermal performance of both nonconcentrating and concentrating collectors that use single-phase fluds and having no significant internal energy storage. This standard is applicable to fluid entering and leaving the collector through a single inlet and outlet respectively. The Solar Keymark netwok members are Industrial representatives, Solar Keymark certification bodies, Solar Keymark test labs and inspectors.

Specifies test methods for assessing the durability, reliability, safety and thermal performance of fluid heating solar collectors. Replaces the previous edition of CSA F378, Solar Collectors, published in 1987.

Specifies a method for outdoor laboratory testing of SDHW systems. Also, this standard applies for insitu tests and for indoor tests at specified conditions

Objective is to ensure that systems operate reliably even under extreme conditions (Heavy snow/wind loads/extended stagnation periods during the summer) Purpose is to evaluate hydraulic pressure rating of all components and interconnections of a solar water heating system. Applies to small and large custom built solar heating systems with liquid heat transfer medium for residential buildings and similar applications. Establishes the test method and to evaluate the thermal behavior of solar water heating systems.

Allows the test institute all over Europe to perform the collector test according to their very own weather conditions.

Specifies requirements (for liquid-heating solar collectors) on reliability, durability and safety.

Description

–

Since 1974

Washington, DC, USA Berlin, Germany, Europe

Europe

USA

2014

Founded in 2000

Canada

Switzerland.

2017 2016

Switzerland

Mexica

Europe

Europe

Europe

Europe

Europe

Country

Last reviewed and confirmed in 2015.

2012 (Supersedes CEN/TS 129772:2010) 2010

2017

2017

2000

2006 (Amendment made in 2010)

Year

DAkkS

(continued on next page)

Developed by the European Solar Thermal Industry Federation (ESTIF) and CEN in close co-operation with leading European test labs and the European Commission. IREC

ASHRAE was formed by the merger in 1959 of ASHAE founded in 1894 and the ASRE founded in 1904.

Independent, non-governmental organization. Coordinated by Central Secretariat in Geneva. US Department of Energy authorizes CSA International for energy efficiency verification testing.

Independent, non-governmental organization. Coordinated by Central Secretariat in Geneva.

United States of Mexico

Standard prepared by Technical Committee CEN/TC 312.

CEN

CEN

CEN

CEN

Organization

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Washington, DC, USA Founded in 1980

ISO

Citation analysis is carried out based on the literature (finding) titled ‘CPC solar collector’ and the source of information is from Scopus. Fig. 50 (Scopus, 2017) represents the number of documents published based on country/territory in relation to the CPC solar collector from the year 1966 to 2018, where the size of the circle is proportional to the number of documents published. It illustrates that CPC solar collectors are widely focused by the European countries, Asian countries, American countries, Australia and some African countries. Fig. 51 (Scopus, 2017) represents the number of worldwide documents published on CPC solar collector from the year 1966 to 2017, which shows the increase in development of CPC solar collector in various applications. 10. Market players and recent developments Table 9 illustrates the particulars of manufacturers/suppliers for CPC solar thermal collectors. The survey among Indian suppliers of concentrating solar collectors has shown that an increasing number of firms are entering the market through this technology. Apart from manufacturers/suppliers list mentioned in Table 9, following are the list of manufacturers/suppliers empaneled by Ministry of New and Renewable Energy (MNRE), Government of India (as on 01 February 2017) (CST, 2017) for the installation of non-imaging concentrating system: (i) Thermax limited, Pune; (ii) VSM solar Pvt. Ltd., Bangalore and (iii) Vcare Engineering Pvt. Ltd., Vadodara. Concentrator solar collector manufacturers and systems supplier have welcomed the order published by MNRE, Government of India on 26 February 2018 about the extended financial support (MNRE, 2018). MNRE, Government of India made a benchmark cost and eligible subsidy for various technologies, in that the benchmark cost for CPCs is kept as ₹ 12000/m2. Manufacturers follow materials standards thereby assuring the quality for the end-user. In general, CPC manufacturer’s vision is to be innovative and competitive, providing end-to-end solar energy solutions to customers. With the most competitive sales network all over the world, CPC manufacturers will penetrate in the future markets with new technologies. Continuous development in the design and manufacturing of CPC solar systems by the pioneers focus on the quality and efficiency of their products with well-known recognitions. Some of the recent CPC solar collector information (STW, 2018) pertaining to the developments and installations are listed below:

Certification provided to solar consumers, solar thermal industry, local, state and federal regulatory bodies.

Cairo, Egypt Founded in 2013

The first Arab certification scheme for solar thermal products and services in the Arab states in the Middle East and North Africa. It is inspired by Solar Keymark. SRCC benefits include: State-of-the-art rating system; Develop consumer confidence; Solar incentive schemes. Provide certification for the solar collectors and Solar water heaters by SHAMCI bodies complying ISO/IEC 17,065 and ISO 9001.

• In 2015, Ritter Group was awarded by East-Bavarian Institute for

SRCC (2018) 15

ICC-SRCC (Certification)

SHAMCI (2018) 14

SHAMCI (Certification)

Country Year Reference

Standard/certification

Description

SHAMCI was initiated by the Regional Center for Renewable Energy (AIDMO – AMEC of the League of Arab States).

9. Research publication status

S. NO.

Table 8 (continued)

to EN 12,975 and ISO 9806-1, performance data of solar thermal collector was used for finding the coefficient of the nonlinear thermal efficiency.

Title

Organization

V. Pranesh, et al.

•

331

Technology Transfer, OTTI for their innovative demonstration plant of producing steam (above 150 °C) by CPC (plasma coating) vacuum tube collector combined with a steam-jet chiller. Oorja Energy Engineering Services Pvt. Ltd., India had installed 110 m2 ETC CPC field at axle and transmission manufacturer TML Drivelines for solar-heated metal degreasing with a payback time below four years. Also, they installed 500 m2 ETC CPC space heating system for 200 people shelter of the Indo-Tibetan border police, Ladakh. The data shows that this technology is far better than for a solar electricity heating system with space requirement of 1/3rd than that of a solar PV unit. A.T.E Enterprises Pvt. Ltd., Pune, India set up a 44 m2 roof-mounted ETC CPC field (@ 95 °C) at garment manufacturer (Frontier Knitters), Tamil Nadu, India with a payback

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Fig. 50. Number of documents published based on country/territory in related to CPC solar collector.

• Gujarat state electricity

•

corporation limited (GSECL) installed a capacity of 150 tons solar based vapour absorption machine as part of clean energy initiative in August 2017, to cool the Gandhinagar Thermal Power Station’s office building in Gujarat, India. Based on the data provided by GSECL, the installation saved 250 MWh of electricity per year. The installation was executed by German-Indian venture VSM solar, Bengaluru, India. The solar energy was captured by vacuum tube collectors with CPC of a total area of 1,575 m2 and hot water supply to the application is at 90 °C. In India, concentrating solar thermal technologies continue to flourish. CPC with vacuum tube collectors dominate the solar heat for industrial processes market share by nearly 80%.

11. Inferences and recent research trends Fig. 51. Number of documents published on CPC solar collector.

•

•

Among the surveyed literatures for the CPC solar collector, the most interesting behavior that can be observed is its significant progress in the performance and utilization in various fields because of the large amount of research works carried out. Its domestic usage has been increased now tremendously with its minimal cost and weight. Many industries have installed CPC as it tends to serve their purpose e.g. dairy, textiles, desalination, solar cooling, solar heating etc. Globally, water and energy are the most vital resources (STW, 2018). After agricultural businesses, the second biggest water consumer is the industrial division. In 2017, International Energy Agency (IEA) Solar Heating and Cooling Programme launched a new research platform (Solar energy in industrial water and waste water management) to analyze the link between water and energy across industrial division. Christoph Brunner (a researcher at Austrian-based AEE Institute for Sustainable Technologies (AEE INTEC) and chair of new research platform) stated that in industry, they envisioned to investigate and improve solar powered water separation and water purification. That shows the important of rise in demand of water and focus of research towards the technology of solar powered water separation. Since CPC solar collector meets the requirement of desalination, they will play a

period of less than three years when including subsidy from MNRE. German Agency for International Cooperation initiated the project ‘Solar Cooling in Industry and Commerce’ in Jordan, due to rapid increase in demand for air-conditioning. German Technical University, Berlin played a key role in this technology transfer, planning, installation and monitoring of four solar-driven air-conditioning systems in public and private buildings in Jordan. The solar fields comprise of vacuum tube solar collectors with CPC from German Ritter Group. Usage of hot water of up to 90 °C by vacuum tube collectors with CPCs has been on the rise in commercial application and by several industries (e.g. automotive, chemical, dairy, food processing and pharmaceutical). Solar thermal federation of India emphasized that unless the government intervenes for the check on quality for solar components and systems by establishing stringent and mandatory standards, India may soon become a haven for sub-standard items, resulting in deterioration of customer confidence. 332

333

Ultra- Conserve (2018)

Bayer (2018)

IVT (2018)

Ritter (2018)

Artic (2018)

10

11

12

13

Sunbest (2018)

5

9

Taylormade (2018)

4

Oorja (2018) and STW (2018)

BNS (2017)

3

8

LR (2018)

2b

Emsol (2018)

LR (2018)

2a

7

Suntask (2018)

1b

ATE (2018)

Suntask (2018)

1a

6

Reference

S. No

Artic Solar, Inc.

Ritter XL Solar GmbH

IVT GmbH & Co. KG

Solarbayer GmbH

Ultra conserve Pvt. Ltd.

Oorja Energy Engineering Services Pvt. Ltd.

Emsol Innovations Pvt. Ltd.

A.T.E Enterprises Pvt. Ltd.

Sun best

Taylormade Solar Solutions Pvt. Ltd.

BNS

Linuo Ritter International Co., Ltd.,

Linuo Ritter International Co., Ltd.,

Zhejiang Shentai Solar Energy Co., Ltd. Zhejiang Shentai Solar Energy Co., Ltd.

Company

Table 9 Particulars of the manufacturers for CPC solar thermal collectors.

CPC solar collector (Evacuated double glass tube) Non-imaging concentrating system Vacuum tube collector CPC Solar Collector LATENTO CPC Solar Collector Evacuated Tube Collectors XL Series and XL P Series (With CPC reflector) XCPC Solar Collector

XCPC solar collector (Evacuated tube)

CPC solar collector (Evacuated tube)

CPC Solar Collector (Pressurized system)

Fast assembly CPC Solar Collector Split non-pressurized solar water heaters (360°absorber) Forced circulation systems – Pressurized – Evacuated tube collector with CPC-reflector Forced circulation systems – Pressurized – CPC OEM XL Evacuated tube collector CPC Solar Collector (360 degree absorber) Evacuated tube collector with CPC-reflector

Type of CPC

Florida, U.S.A.

Germany

Manufacturer

Manufacturer

Rohr, Germany

Germany

Mumbai, India

Hyderabad, India

Chennai, India

Pune, India

Theni, Tamil Nadu, India

Gujarat, India.

Vidin, Bulgaria

Shandong Province, China

Shandong Province, China

China

China

Country

Supplier

Manufacturer

Manufacturer/Supplier

Key components (Coating materials and Evacuated tubes) imported from Linuo-Ritter, China. Manufacturing, Marketing and distribution of Linuo solar thermal products. Fabricator and Supplier (Product designed from Germany) Supplier (CPC supplied is based on technology from Ritter Gruppe, Germany) Supplier (Developed in collaboration with the University of California, Merced, CA, USA) Manufacturer

Manufacturer

Manufacturer

Manufacturer

Manufacturer

Manufacturer

Type of Company

–

–

–

15–65°

–

–

–

–

–

Minimum 15°

15–90°

–

Minimum 15°

–

–

Tilt angle

Roof top

(continued on next page)

–

On roof (Pitched/ flat roof) –

Rooftop/Ground

Rooftop/Ground

Rooftop/Ground

Rooftop/Ground

Arbitrary placement

On roof, Flat roof/ wall mounted (Except CPC OEM XL ETC = Pitched roof, flat, roof, onwall).

–

Pitched roof, flat, roof, on-wall

On roof, Flat roof/ wall mounted

–

–

Type of mounting

V. Pranesh, et al.

Solar Energy 187 (2019) 293–340

334

6–9 bar

10 bar/272 °C

3

4

10 bar/272 °C

2a

10 bar/272 °C

–

1b

2b

6 bar

1a

Solarus (2018)

15

Maximum pressure and Stagnation temperature

Baltic (2018)

14

S. No

Reference

S. No

Table 9 (continued)

2010

–

2001

2001

2000

2000

Establised Year

Solarus

Power collector

DIN EN12975-2 and thermal shock test.

German quality standard

DIN 4757-4 or EN12975

DIN 4757–4 or EN12975

Section 8 of S/NZS 3500 4:2003; AS/NZS 3500 5:2000

ISO 9001 : 2008 CERTIFIED

Solar Keymark, DIN CERTCO (Certification organization of TUV Rheinland Group and DIN, the German Institute for Standardization). Solar Keymark, DIN CERTCO (Certification organization of TUV Rheinland Group and DIN, the German Institute for Standardization). –

ISO9001:2008, SGS, Solar Keymark, SRCC certified

Same as Linuo Ritter Product.

Evacuated tube (Heat pipe [Φ58mm × (L) 1800 mm × (Thk.) 1.8 mm (ALN/AINSS/CU)]) Same as Linuo Ritter Product.

Evacuated Tube (Borosilicate glass 3.3 [Φ 47 × (L) 1500 mm])

Hot water generation, space and process heating and solar cooling.

Solar thermal

Evacuated Tube (Borosilicate glass 3.3 [Φ 47 mm × (L) 1500 mm])

Vacuum tube with heat pipe configuration [Φ 58 mm × (L) 1800 mm] (SS-C/ CU) Vacuum tube [Φ 47 mm × (L) 1500 mm]

Tube (Coating)

(continued on next page)

India

In and around Bulgaria.

–

Same as Linuo Ritter Product.

40 countries worldwide

Germany, South Africa, South Korea and Southeast Asia. 40 countries worldwide

Germany, South Africa, South Korea and Southeast Asia.

Marketing

Rooftop,wall and ground

On roof, flat roof and wall mounted

Type of mounting

64.2% (Aperture)

64.2% (Aperture)

–

–

Optical efficiency

–

–

Lithuania, Europe

Venio, Netherlands

Tilt angle

Country

Hot water generation, space and process heating and solar cooling.

Solar hot water heater

Solar hot water

Application

Supplier (Solfex Energy systems vacuum tube collectors are manufactured by OEM partner and EU market leader – Ritter Solar GmbH, Germany) Manufacturer

Type of Company

ISO9001:2008, SGS, Solar Keymark, SRCC certified

Certifications

SOLFEX evacuated-tube collector CPC INOX

Type of CPC

Section 8 of S/NZS 3500 4:2003; AS/NZS 3500 5:2000

Standards

BSP (Baltic Solar Projects)

Company

V. Pranesh, et al.

Solar Energy 187 (2019) 293–340

335

–

10 bar/301 °C (XL Series) and 10 bar/338 °C (XL P Series)

Exceed 200 °C

11

12

13

150 °C

8

6 bar and 259 °C

Peak temperature: greater than 250 °C;Stagnation tested upto: 350 °C

7

10

up to 120 °C

6

–

6 bar/160 °C

5

9

Maximum pressure and Stagnation temperature

S. No

Table 9 (continued)

–

2010

1994

2003

2009

2010

2013

1939

1989

Establised Year

–

DIN 4757–4 or EN 12,975

–

DIN EN 12,975

–

–

–

–

–

Standards

Solar Rating Certification (SRCC)

DIN ISO 9001, WRAS, DVGW, ATG 13/2284, DIN CERTCO, Solar Keymark. Solar Keymark, DIN CERTO, SRCC

Empaneled by MNRE Solar Keymark, DIN CERTCO

–

Empaneled by MNRE

Empaneled by MNRE

Empaneled by MNRE

Certifications

A/C RefrigerationDehumidification, Process heat -Hot water-Steam, Solar thermal desalination, Electric power generation.

Designed for Very Large Systems and Industrial Applications

–

Solar heating application

Boiler feed water, textile processing, distillation, degreasing/parts washing Industrial application (Process heating, drying, desalination, effluent evaporator, steam generation, food processing) and Consumer applications (Hospital, hotel, solar HVAR cooling) Solar drying, solar laundry, solar desalination, solar autoclaving/ sterlization, Industrial process heat, boiler feedwater preheating Domestic application

Industrial application.

Application

Single U-bend flow tube

Evacuated Tube

Vacuum tube [Φ58mm] (Selective AL-N/AL) Vacuum tube

–

–

–

Glass tubes with highly selective absorber layer

Vacuum tube

Tube (Coating)

68.7% (XL P series, except XL 19/49P = 68.8%) and 64.4% (XL series) –

64.20%

71.80%

–

–

–

–

Above 60%.

Optical efficiency

(continued on next page)

–

Worldwide

Germany, Russia, China, Australia

Germany

India

India

India, U.A.E.

Europe, Japan and other countries where during winter solar radiation levesl are very low. India

Marketing

V. Pranesh, et al.

Solar Energy 187 (2019) 293–340

Solar Energy 187 (2019) 293–340

Europe, South Africa, India Peak efficiency up to 70%.

Lithuania –

major role in desalination plants in the near future. Indian government has taken a great initiation towards the field of renewable energy. Incentive programs may help in stimulating further development and application of CPC with improved performance and reduced barriers. CPC collector modification based on the geometric parameters can be perceived right from the beginning till now (Buttinger et al., 2010; Prapas et al., 1987; Rabl et al., 1980; Santos-González et al. (2014); Waghmare and Gulhane, 2016; Winston, 1970; Winston, 1974). At the initial development of high CR CPC collectors, for the effective utilization of sun energy ie., to increase the working hours of the solar collector, tracking mechanism will be integrated to the whole collector for the external rotation. This results in a complex system and increases the cost of system. So, the researches focused in improving the selective coating, anti-reflective coating on glass and improving absorber to reduce the thermal resistance, thereby providing solution of using lesser CR CPCs with smaller collector area instead of using traditional high CR CPCs with larger collector area and also complex tracking mechanism. Recent research studies include usage of nanotube with nano fluid for improved collector performance along with the consideration of optical properties (Li et al., 2016; Mahbubul et al., 2018). Arrangement of collectors with high gap values (between top glass cover and tip of the concentrator) and higher truncation values serve to decrease thermal heat losses (Francesconi and Antonelli, 2018). Integration of CPC solar collector with other technologies such as PV, other collectors etc. continues to be under research (Ajdad et al., 2019; Duong and Diaz, 2014; Saini et al., 2018). In recent years, the technological development is of concern for utilizing the concentrated collectors in limited space where there is restriction of traditional tracking methods. So, researchers are focusing to non-rotational tracking mechanism like semi-passive beam steering prism array where the linear concentrator system remains stationary. This technology is capable of achieving high optical performance, compact, simple and suitable for rooftop solar applications. Kothdiwala et al. (2000) discussed the discrepancies of Nusselt and Grashof correlations by various authors. In earlier stages of research by Duff et al. (2001, 2004), experimentally observed that the performance of horizontal and vertical absorber fin absorber orientated integrated CPC collector with no significant difference, but later Duff and Daosukho (2014) found that horizontal fin integrated CPC performed better than the other type when there was considerable amount of reflector degradation. Some of the authors explained their future plan for researches: Li et al. (2016) planned to demonstrate commercial viability of the volumetric receiver type CPC solar collector through long-term stability of the nanofluid in comparison to a surface absorber. Further planned for determining the limits of the proposed technology to reach 400 °C. Widyolar et al. (2018) stated that in future, the installed XCPC collectors of 20 kW capacity at UC Merced Castle Test Facility will be used to power a solar wastewater evaporator for the reduction in waste stream volume.

International standards Max working pressure : 10 barStagnation temperature : 180 °C 15

2006

Certified BCorp

Hospitality sector, food industry, large residential buildings and institutions

Evacuated tube (Borosilicate glass tube (Aluminium nitrite sputter coating)) – Solar water heating, solar backup heating and space heating applications Solar Keymark Hailstone test according to DIN EN 12975–2 and thermal shock test. 2009 – 14

Standards Establised Year Maximum pressure and Stagnation temperature S. No

Table 9 (continued)

Certifications

Application

Tube (Coating)

Optical efficiency

Marketing

V. Pranesh, et al.

12. Conclusion A comprehensive review of the CPC solar collectors mainly focusing its two-dimensional profile type is presented and the state of art on solar thermal applications of these collectors in both domestic and industrial aspects by surveying the evolutional progress in the past five decades. Most of the researches were focused on introducing new design concepts and some on the modifications of the previously available designs. It is observed that the focus of recent studies is gradually shifting towards commercialization. Though this technology attained commercialization stage, it still needs considerable time and efforts of the scientists, designers and manufacturers to improve several aspects such as absorber temperature, material, selective coating, annular gap, overall heat transfer coefficient, glazing, etc., to reduce the cost of 336

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manufacturing. The major conclusion from the extensive survey made are summarized below:

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Advantages of using matlab simulink in laboratory lessons on operating conditions of overhead power lines. Procedia – Soc. Behav. Sci. 179–184. https://doi.org/10.1016/j.sbspro.2015.04.367. Baltic, 2018. Baltic Solar Projects [WWW Document]. URL http://www.bsp.lt/ (accessed 1.28.18). Baranov, V.K., 1965. Properties of parabolic focons. Opt. Mekh. Prom. 6, 1–5. Baranov, V.K., Melnikov, G.K., 1966. Study of the illumination characteristics of hollow focons. J. Opt. Technol. 33, 408–411. Baranov, V.K., 1967. Device for restricting in one plane the angular aperture of a pencil of rays from a light source. In: Russ. Certif. Authorsh. pp. 200530. Bayer, 2018. Solarbayer GmbH [WWW Document]. URL www.solarbayer.de (accessed 1. 28.18). Bejan, A., Kearney, D.W., Kreith, F., 1981. Second law analysis and synthesis of solar collector systems. J. Sol. Energy Eng. 103, 23. https://doi.org/10.1115/1.3266200. Benrejeb, R., Helal, O., Chaouachi, B., 2015. 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• The elimination of tracking mechanism in the CPC solar collectors •

•

• • •

for low to moderate concentration and higher efficiency attracts the end users. Hence there is a great scope of commercialization in a large scale for various applications in the near future. Modification of absorber shapes has been in progress right from its evolution. Till now different types of absorbers such as flat, tubular, bifacial, pentagon, volumetric absorber etc., are evolved. CPC solar collector with different truncation ratios are probed, the results show that the performance of CPC collector is not affected much due to truncation. Also, it has the advantage of material saving and can be significant when it is viewed from optical gains. Three different sets of simulation methods have been discussed: Programming simulators, Programmed simulators and Ray tracing simulators. Climatic data from the meteorological department will be of great importance for practical applications. The increase in the simulation studies in this field will be useful to optimize the system geometry and operational parameters to achieve higher efficiency. The use of simulation software will drive the market for CPC solar collector at a faster rate. Usage of HTFs for operation at various temperature ranges and different applications are discussed. Nano tubes with nano fluids as HTF have gained momentum in the recent years. The standards and certifications for collectors, their testing has been discussed briefly and they would be more helpful for the commercialization of the collectors for various applications. Research publication status showed that there is sustainable growth in development of CPC solar collector and it is concluded from the consolidation of this surveyed information that the CPC solar collector is continuously progressing towards the betterment in its efficiency and find applications in various sector worldwide.

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