thermal system using parabolic trough collector

Journal Pre-proofs Experimental and theoretical analysis of hybrid concentrated photovoltaic/ thermal system using parabolic trough collector Nikhil Gakkhar, Manoj S. Soni, Sanjeev Jakhar PII: DOI: Reference:

S1359-4311(19)34920-8 https://doi.org/10.1016/j.applthermaleng.2020.115069 ATE 115069

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Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

17 July 2019 10 January 2020 10 February 2020

Please cite this article as: N. Gakkhar, M.S. Soni, S. Jakhar, Experimental and theoretical analysis of hybrid concentrated photovoltaic/thermal system using parabolic trough collector, Applied Thermal Engineering (2020), doi: https://doi.org/10.1016/j.applthermaleng.2020.115069

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Experimental and theoretical analysis of hybrid concentrated photovoltaic/thermal system using parabolic trough collector Nikhil Gakkhar1, Manoj S. Soni2, Sanjeev Jakhar3* B, Sardar Swaran Singh National Institute of Bio Energy, Kapurthala, Punjab, India 144601 2Associate Professor, Department of Mechanical Engineering, Birla Institute of Technology & Science, Pilani, Rajasthan, India 333031 3Assistsant Professor, Mechanical Engineering Department, Mody University of Science and Technology, Lakshmangarh, Sikar, Rajasthan, India 1Scientist

Corresponding author E mail address; [email protected] (S Jakhar); [email protected] (N Gakkhar)

Abstract In the current work, a hybrid concentrated photovoltaic thermal system was designed and coupled with a parabolic trough collector and investigated theoretically and experimentally for combined heat and power output. In the design, a photovoltaic module was mounted on a flat surface of parabolic trough absorber tube having semi cylindrical shape. A provision was made to cool photovoltaic panel from both the surfaces by flowing water through the absorber tube as well as the annulus of between absorber tube and glass cover. The model was developed using first law of the thermodynamics and then validated using experimental data generated through the fabricated setup. During the experimentation, the annulus flow rate was varied from 0.008 kg/s, 0.017 kg/s and 0.025 kg/s and inner flow rate was varied from 0.075 kg/s, 0.083 kg/s and 0.091 kg/s. The field testing results showed the mean overall efficiency of system obtained as 61.42%, 64.61% and 66.36% for inner tube flow rate of 0.075 kg/s, 0.083 kg/s and 0.091 kg/s respectively for annulus flow rate of 0.008 kg/s. The theoretical results of hybrid system obtained from the simulation are in good agreement with the experimental data. In the end environmental cost analysis was also carried out for the proposed system. Keywords: Solar energy, Parabolic trough collector, Concentrated Photovoltaic/thermal, Hybrid system, Renewable energy. Nomenclature A area (m2) CR concentration ratio D diameter (m) FF fill factor Gz Graetz number h convective heat transfer coefficient (W/m2K) I current(A) Ir irradiance (W/m2) J Radiosity K incident angle modifier k thermal conductivity (W/mK) N penalty factor Nu Nusselt number ηo cell efficiency at STC (%) P power (W) Page 1 of 41

Pr Q 𝑞′ 𝑅𝑎56 Re STC T U Uaw Uct Ug,ted V W

Prandlt number instantaneous heat transfer (W) heat transfer per unit length (W/m) Rayleigh number for glass outer cover Reynolds number standard test conditions temperature (K) heat transfer coefficient (W/m2K) overall heat transfer coefficient from absorber to water flow (W/m2k) conductive heat transfer coefficient from solar cell to flowing water through tedlar (W/m2K) overall heat transfer coefficient from glass to tedlar through solar cell (W/m2k) voltage (V) width of PV cell (m)

Subscripts a b ba c cond conv cp el f fl g i irr m max mi mod o oc rad s sc t ted th tot w 1 2 3 4 5 6 7

aperture back surface back surface of absorber cell conductive heat transfer convective heat transfer specific heat capacity electrical front surface fluid glass inlet solar radiation mass flow rate maximum mirror module outlet open current radiative heat transfer sun short circuit tube tedlar thermal total water bulk surface of inner fluid inner surface of semi circular tube outer surface of semi circular tube inner surface of glass outer surface of glass surrounding/ambient conditions sky Page 2 of 41

8 9

bulk surface of annulus fluid on flat side bulk surface of annulus fluid on curved side

Greek letters α α56 β γ ε ν η ρ σ τ μ

absorptivity or absortance thermal diffusivity at temperature T56 (m2/s) temperature coefficient on power of the PV cells (%/K) packing factor emissivity kinematic viscosity of air (m2/s) efficiency reflectivity Stefan-Boltzmann constant (5.67 x 10-7 W/m2K4) transmissivity refractive index of the fluid

1. Introduction The photovoltaic (PV) technology works on the principle of direct conversion of sunlight into electricity using semiconductor materials. The PV technology is commercially proven technology with achievable cell efficiency between 15-20% [1]. Although widely acceptable, a major challenge with PV systems is high cell/panel temperature due to incessant solar radiation which reduces the efficiency of the cell and its lifespan [2]. To utilize maximum solar energy, the systems are designed to utilize the excess waste heat into a useful form. One of the technology, Photo-Voltaic Thermal (PVT) increases the PV efficiency along with recovery of excess heat as reported in various studies [3–5]. Under high radiation, concentrated photovoltaic/thermal (CPVT) system not only provides higher electrical output but substantial thermal energy due to their inbuilt cooling systems. While a variety of cooling approaches have been used to the maintain the PV panel temperature, most of them are based upon removal of heat from the back of the cell (opposite surface of the incident flux exposed surface) [6]. Although the PV installation has grown many folds to more than 400 GW across the world and many commercial plants are installed across various countries, the installed capacity of PVT or CPVT is too low to be reported [7]. The major work on CPVT is still in research phase with lab scale testing of prototypes. The literature discusses the work related to design, modelling, prototype testing and various cooling technologies of CPVT. In one of the work, Ju et al. [8] carried out an extensive review on wasted heat recovery CPVT systems and classified it into low, medium and high concentration CPVT systems. In their article, the authors discussed different system configurations, their characteristics, and current research trends in CPVT technology. They recommended that focus should be on dynamic simulations to optimize the design of CPVT. They also suggested standardization of the technology to make it commercially viable. The author extended their review on spectral beam splitting technology [9], in which they carried out an extensive literature review on spectral beam splitting CPVT systems. The author discussed pro and cons of various beam splitting technologies like planar holographic, liquid absorptive, diffractive etc, which are being used for CPVT. The author pointed out that the Page 3 of 41

current trend of the research was shifting towards experimental work on prototypes, especially interference and liquid absorptive splitting technology based CPVT system. The authors concluded these two types of technology would be commercialized in the near future. In terms of experimental investigations on CPVT, Coventry [10] investigated the performance of a CPVT system having optical concentration of 37 Suns using aluminium tubes internal fins. The author reported electrical and thermal efficiencies of 11% and 58% respectively, with overall efficiency of the system as 69%. In other research work of CPVT based combined heat and power, Coventry et al. [11] reported electrical and thermal efficiency of the system as 10% and 50% respectively. Darwish and Boehm [12] experimentally analyzed optical and thermal performance of high concentration CPV cooling system and reported the performance of CPV system in terms of cell efficiency. Their research showed that there is a need of a novel receiver system especially for PTC which can provides combined heat and electrical power, as their work was in preliminary stage for development of CPVT system. Col et al. [13] carried out the experimental and numerical investigation of CPVT system based on their novel design. They used two axes concentrator with triple junction linear solar cell at focus. The author reported the overall efficiency of 70%. The work was extended in [14], in which author used flat aluminium absorber mounted on asymmetrical parabolic trough concentrator having concentration ratio of 42. The authors reported overall thermal efficiency of 64%. Bernardo et al. [15] investigated the ‘Solar8 CPVT’ collector to obtain both thermal and electrical energy. The authors designed the prototype and tested out to compare it with conventional CPVT system. The authors reported 65% thermal optical efficiency and 8% electrical efficiency of the system. They concluded that the newly designed Solar8 could replace traditional PVT collectors to achieve higher overall efficiency in lesser area. Bernardo et al. [16] also tested carried out the experimental investigation of PTC based CPVT system having triangular absorber tube. They reported hybrid electrical efficiency of 6.4% and 45% thermal optical efficiency. Similar experimental works on newly designed prototypes of CPVT system are reported by Smeltink and Blakers [17], Xu et al. [18] and Cappelletti et al. [19,20]. The work on system modeling of CPVT was carried out by Mittleman et al. [21] where the authors carried out the performance investigation of CPVT polygeneration system using different models. They reported higher overall efficiency of the system. Later on the authors investigated single effect absorption cooling system coupled with CPVT system [22]. The model developed by them indicated that CPVT technology could be at par with conventional cooling technologies in terms of cost and higher overall efficiency. The authors concluded that experimental work was required for validation of the model. Burhan et al. [23] carried out optimization to design CPVT cooling system. In their design, the optimized CPVT system operated a hybrid vapour compression chiller which effectively utilized heat and power obtained from proposed system. They also utilized the energy for hydrogen production through electrolysis. The author reported the overall efficiency of solar system as 71%. Chaabane et al. [24] carried out three dimensional CFD analysis of water cooled CPVT system and observed increased thermal and electrical efficiencies of the system with increase in number of water cooling pipes behind CPV. They reported that the panel temperature could be reduced by 17 °C when two water rectangular Page 4 of 41

shaped cooling system was used, which leads to increase in thermal efficiency by 28.3% and electrical efficiency by 0.92% in compared to normal CPV system. Similar to the work of Bernardo et al. [15], Smeltink and Blakers [17] and other researchers, the present work is an attempt to design a new prototype of CPVT system having hybrid receiver. The current design, which is termed as hybrid concentrating PV thermal (HCPVT) system, has a provision of both side cooling of PV panels (front and rear) to maintain the panel temperature. Such a hybrid system could be used for combined heat and power as the thermal energy could be utilized for low-grade thermal applications like distillation in arid and semi arid regions of India. For the designing of prototype, a mathematical model was developed and validated with the experimental data obtained from the new designed setup, which was installed in Birla Institute of Technology and Science (BITS) Pilani, Rajasthan, India. The performance of hybrid system is evaluated in terms of respective efficiencies. The environmental cost analysis of the fabricated set up was also carried out in the current work. 2. Description of Experimental setup The main purpose of developing the experimental test setup was to carry out the outdoor testing of the designed novel HCPVT system by varying operating parameters and validate the simulated results with the data generated from the experiments. The novel HCPVT system consists of two parts; a reflective parabolic trough and a novel designed HCPVT receiver, which is termed as D-shape receiver. The trough includes reflective anodized aluminium sheet, support structure and tracking mechanism (east-west) while D-shape receiver was designed to accommodate PV cell on the semi circular absorber tube. The fabricated PTC coupled with D-shape receiver was installed, in the BITS Pilani, Rajasthan (28°21'58" N 75°35'07" E) and tests were conducted during the period of May and June 2017, for local conditions of Pilani, Rajasthan, India. The region receives high solar radiation for more than 300 days in a year. During the testing period, the meteorological data for the location are taken from Sangwan et al. [25], which represents the average monthly irradiation and average ambient temperature and is shown in Fig 1.

Page 5 of 41

5.5 5 4.5 4 3.5 3 2.5 2

Average Ambient temperature (°C)

38 36 34 32 30 28 26 24 22 20 18

Ja

nu a Fe ry br ua ry M ar ch A pr il M ay Ju ne Ju ly A ug Se ust pt em be r O ct ob N o v er em D ber ec em be r

Average Solar Radiation (kWh/m2/day)

6

Months

Average Ambient temperature

Average Solar Radiation

Fig. 1.Solar radiation and ambient temperature for the Pilani, India[25].

The HCPVT receiver was designed to obtain thermal as well as electrical energy from it. The novelty of the system is its design in which a customized PV module was mounted on the flat surface of D-shape absorber, facing the concentration. The high concentration falling on the PV module generates higher electrical output. The copper was selected as absorber tube material because of its higher thermal conductivity and it was used without any coating. The borosilicate glass tube was selected for the outer cover. The header to join glass with absorber tube which allowed annulus water flow was also fabricated. The receivers were designed in such a way that the heat transfer fluid (HTF), which is water in this case, flows through the absorber tube as well as the annulus between absorber tube and glass cover. As the water flows through the front and rear of the PV panel, it absorbs the excess heat (from front and rear side of PV panel) and maintains the panel temperature. Such a novel system is quite useful for the arid and semi-arid regions like Pilani due to high solar radiation in that region. A model indicating the design of Dshape receiver is shown in Fig. 2.

Fig. 2 Model of designed hybrid HCPVT receiver tube

Page 6 of 41

For the fabrication of HCPVT receiver, before mounting with the PV, a layer of epoxy adhesive was applied on the flat surface of receiver tube. Three K-type thermocouples were mounted on the centre of PV panel at a distance of 400, 800 and 1200 mm from the inlet of absorber tube between PV panel and copper tube. Three additional K-type thermocouples were also mounted on the front surface centre of PV cell at the same distance. Thus a total of six thermocouples were used to measure the surface temperature of front and back side of the PV panel. The front surface thermocouple wires are brought to edges of the panel to minimize the shading effect on PV cells. All the thermocouple wires were brought out from the hole provided in the header which was then sealed with anti-leakage sealant to avoid any leakage and were connected to temperature scanning logger. The fabricated HCPVT receiver is shown in Fig. 3. The technical specifications of the PV panel and absorber tube are shown in Table 1.

Fig. 3 Fabricated HCPVT receiver tube

Table 1: Technical specifications of the Hybrid CPV/T system Absorber tube (dimensions in mm)

PV panel (dimensions in mm)

Length of glass

1500

Type

Mono crystalline silicon PV panel

Glass OD (borosilicate)

115

Model

HTMO 18-16

Glass thickness

5

Make

Shanghai

Wintrans

International

Trade Co. Ltd) Copper tube diameter

75

Length

1500

Copper tube thickness

2

Width

72

Length of copper tube

2000

Thickness

4.2

Rated Power

16 W

The PTC structure was installed in the north-south direction with east to west tracking. A high reflective anodized aluminium sheets having area of 1.48 m2 was mounted on the existing parabolic trough. With the current dimension of receiver and high reflective anodized aluminium sheets, the CR of the system comes out to be 6 Suns. By adjusting focal length with the help of screw mechanism, the fabricated HCPVT receiver was mounted on the same. The minor Page 7 of 41

adjustments were carried out to achieve perfect alignment of falling solar radiation. The thermocouples wires coming out of header hole were connected to temperature scanning logger for continuous reading during the test days. In the experimental setup, two tanks were used, one connected to the inlet and another to the outlet of HCPVT system. The inlet tank (100 L) was mounted at the height of 3 meter to provide sufficient head for water to circulate through HCPVT receiver. The water from the gravity potential flows into the receiver through two rotameters and flow control valves (ball valves). The first rotameter (range 0 – 0.33 kg/s) was fitted in between the higher tank outlet and HCPVT receiver. The second one (range 0 – 0.08 kg/s) was fitted in between outlet of ‘T joint’ and inlet of annulus of novel HCPVT receiver. The mass flow rate was controlled by using two ball valves. Two RTDs and pressure gauge were installed at inlet and outlet of the HCPVT receiver to measure the temperature and pressure respectively. The water coming out from HCPVT receiver was then collected into the outlet tank (350 L) which was placed at ground level. A monobloc pump was used for circulation of water from lower tank to higher tank, at the end of the day. All the pipes and junctions were properly insulated with glass wool to minimize the thermal losses. The schematic diagram of the entire experimental setup is shown in Fig. 4. The technical details of the equipments/consumables used during the experiment are given in Table 2.

Inlet Tank

CPVT system

PTC Receiver (with annulus flow)

K type thermocouples

To, Po Outlet

Rotameter Ti, Pi

Tank Pump

Fig. 4 Schematic diagram of the experimental setup

Page 8 of 41

Hot water Water

PV module

Thermal Output Water

Concentrated Solar Radiation

Electrical output

Domestic Use

Distillation

Low grade thermal application

CPVT system

Fig. 5 Schematic diagram indicating flow of energy and its applications

Table 2: Details of equipments used during experimentation

Name of the instrument/ Make of the equipment consumables Pump KD Industries Rotameter Epoxy adhesive Thermocouples RTD Data logger Multimeter Temperature scanners Pressure guage

Specification

Head: 15 m Power: 50 W JPM Accuracy: ±1.14% Omega, OB-200 Alpha engineering Accuracy: ±2.71% company Temperature range: 150 ºC to 1350 ºC Pt100 Least count: 0.1 ˚C Campbell Scientific, V : 9.6 – 16 V DC model CR1000) RISH max10 DC Voltage accuracy: ±0.005 DC Current accuracy: ±0.015 Countronics, model: Accuracy: ±1°C±1 (for CT716 thermocouples), ±0.1°C ±1 (for Pt100) AMS instruments & Range: 0 - 7 kg/cm2 controls Accuracy: ±2%.

Page 9 of 41

In the previous research done by the authors [26], modelling and analysis of initial system showed that the optimum performance of the system can be achieved with total mass flow rate ranging from 5-7 LPM (0.083kg/s – 0.117 kg/s). In the present study, the HCPVT system was analysed by (a) varying the annulus flow rate and keeping inner tube flow rate as constant and (b) varying the inner tube flow rate and keeping annulus flow rate as constant. The experiments were conducted during clear sky days for 6 hours i.e. from 10 AM to 4 PM for three consecutive days (26-28 May, 9-11 June and 14-16 June) in the month of May and June 2017. The experiment setup started at 9 AM to achieve steady state condition and avoid any thermal shocks due to sudden temperature variation. By keeping the parallel flow arrangement, the mass flow rate was varied from 0.083 kg/s to 0.117 kg/s through HCPVT receiver. During 26-28 May, it was achieved by keeping mass flow rate through the inner tube as 0.075 kg/s and varying the annulus flow rate as 0.008 kg/s, 0.017 kg/s and 0.025 kg/s to get combined flow rate ranging from 0.083 kg/s to 0.10 kg/s. During 9-11 June, the mass flow rate through the inner tube was kept constant at 0.083 kg/s and during 14-16 June, the same was maintained at 0.092 kg/s, with varying annulus flow rate of 0.008 kg/s, 0.017 kg/s and 0.025 kg/s, so as to get that the combined flow rate ranges from 0.083 kg/s to 0.117 kg/s during these days. Measurement and recording of hourly data for the experimental setup was carried out using various instruments. The parameters measured were intensity of solar insolation, ambient air temperature, temperature at surface and rear of the PV panel mounted on absorber tube, temperature at the inlet and outlet of HCPVT system, pressure at the inlet and outlet of HCPVT system, flow rate at both pipe inlets, short circuit current and open circuit voltage. The on-site experimental setup during the testing is shown in Fig. 6.

Page 10 of 41

Inlet tank Inlet of HCPVT receiver

PTC with reflective sheet

Outlet tank

Outlet of HCPVT receiver

Fig. 6 On-site experimental setup during testing

3. Thermal modelling of HCPVT system This section presents numerical modeling of proposed HCPVT system for the climatic conditions of Pilani (Rajasthan). The model was designed using MATLAB (v2015a), which includes one dimensional model of novel HCPVT receiver system. The purpose of the model is to carry out the first law analysis of thermodynamics for the proposed system. 3.1 PTC system For the first law analysis of thermodynamics for PTC systems, followings assumptions are taken:    

The one-dimensional steady state condition is considered for the analysis. The ambient temperature across the system is uniform at a particular instant. Uniform circumferential heat flux across the absorber tube is taken Thermal properties of the fluid and surrounding air are uniform across the circumference.

The first law analysis of the proposed HCPVT receiver is carried out using equations given in the literature [27,28]. Balancing of the incoming solar radiation and optical losses to the surroundings were carried out using the similar approach carried in author’s previous work [29]. In the current model, six surfaces and two bulk surfaces are identified for the heat transfer analysis across the receiver. The incoming solar radiation falls on the surface 5 (𝑞′5𝑠), which is glass envelope outer surface, as well as on the surface 3 (𝑞′3𝑠) which is outer surface of semi Page 11 of 41

circular part. Across the glass tube, the absorbed energy is transferred through conduction ( 𝑞′45𝑐𝑜𝑛𝑑) and then by convection (𝑞′49𝑐𝑜𝑛𝑣) from surface 4 to bulk surface 9. It also absorbers the long wavelength radiation emitted by surface 3. The concentrated radiation falls on the front surface of PV cell (𝑞′𝑓) passing through the glass and fluid in the annulus. The flat part of the tube transfers heat mainly through convection to bulk surface 8 (between front of PV cell and inner glass surface) and surface 4. The surface 3 transfers the heat mainly through convection ( 𝑞′39𝑐𝑜𝑛𝑣) and conduction (𝑞′32𝑐𝑜𝑛𝑑) to the HTF and inner surface of tube respectively. From surface 2, the heat transfer occurs by convection (𝑞′21𝑐𝑜𝑛𝑣) to the HTF flowing through the absorber tube. The ambient and sky conditions are taken with subscript 6 and 7 respectively. One dimensional steady state energy balance of HCPVT absorber tube is shown in Fig. 7. The solar absorptance on surface 5 (𝑞′5𝑎) and on surface 3 (𝑞′3𝑠) are considered as heat flux term to simplify the analysis. As discussed by Forristall [30], this assumption simplifies and makes the conduction equation linear across the glass and absorber tube. Though this assumption introduces minimal error, but for small absorbtance coefficient of the glass, this error can be neglected. The energy balance equations of the system, as shown in Fig. 7, is represented from Eq. (1) to (5).

Fig. 7 One dimensional steady state energy balance of HCPVT absorber tube

𝑞′21𝑐𝑜𝑛𝑣 + 𝑞′𝑏1𝑐𝑜𝑛𝑣 = 𝑞 ′23𝑐𝑜𝑛𝑑

(1)

𝑞′3𝑠 + 𝑞′𝑓 + 𝑞′𝑓𝑏𝑐𝑜𝑛𝑑 = 𝑞′39 𝑐𝑜𝑛𝑣 + 𝑞′23𝑐𝑜𝑛𝑑 + 𝑞′34𝑟𝑎𝑑 + 𝑞′𝑓8𝑐𝑜𝑛𝑣 + 𝑞′4𝑓 𝑟𝑎𝑑

(2)

𝑞′49𝑐𝑜𝑛𝑣 + 𝑞′4𝑓 𝑟𝑎𝑑 + 𝑞′48𝑐𝑜𝑛𝑣 = 𝑞′45𝑐𝑜𝑛𝑑 + 𝑞′34𝑟𝑎𝑑

(3)

𝑞′5𝑠 + 𝑞′45𝑐𝑜𝑛𝑑 = 𝑞′56𝑐𝑜𝑛𝑣 + 𝑞′57𝑟𝑎𝑑

(4)

Page 12 of 41

𝑞′39 𝑐𝑜𝑛𝑣 + 𝑞′48 𝑐𝑜𝑛𝑣 = 𝑞′49𝑐𝑜𝑛𝑣

(5)

In Eqs. (2) and (5) 𝑞′34𝑟𝑎𝑑 and 𝑞′4𝑓 𝑟𝑎𝑑 are the radiation emitted by the semi circular surface and PV cell of the absorber tube. It is long wavelength radiations and got absorbed between the HTF present in between the annulus. 3.2

Heat Transfer Analysis

The incident solar radiation falling on the outer surface of the glass is treated as heat flux to simplify the thermal analysis. The optical efficiency of the glass is given by radiation absorbed by the glass envelop by the incident normal irradiation. The solar absorption in glass is defined as [30] (6)

𝑞′5𝑠 = 𝑞′𝑖𝑟𝑟𝜂𝑔𝛼𝑔 Here 𝜂𝑔 = 𝜖𝑔𝜌𝑚𝑖𝐾

(7)

In Eq. (7), 𝜖𝑔 is the estimate of effective optical efficiency as [27] The solar radiation falling on the absorber tube after passing through the glass envelop and heat transfer fluid can also be treated as heat flux. The equation of solar absorption at surface 3 would becomes 𝑞′3𝑠 = 𝑞′𝑖𝑟𝑟𝜂𝑡𝛼𝑡

(8)

𝜂𝑡 = 𝜂𝑔𝜏𝑔𝜏𝑓

(9)

The radiation falling on the PV cell surface f, is actually the concentrated radiation and is given as (10)

𝑞′𝑓 = 𝐶𝑅.𝑞′5𝑠

The rest of the receiver part consists of glass tube, absorber tube and cooling water. Within them, the heat transfer would occur by all three modes, i.e. Conduction, Convection and Radiation. 3.2.1

Conduction heat transfer

In the proposed model, the conductive heat transfer occurs within glass, absorber tube and PV cell. The correlation for conduction within the glass and tube are given as: 𝑞′45𝑐𝑜𝑛𝑑

=

2𝜋𝑘𝑔(𝑇4 ― 𝑇5) ln

( ) 𝐷4

(11)

𝐷3

Page 13 of 41

𝑞′23𝑐𝑜𝑛𝑑 =

2𝜋𝑘𝑡(𝑇2 ― 𝑇3) ln

(12)

( ) 𝐷2 𝐷1

It is assumed that the thermal conductivity of glass envelope and absorber tube is uniform throughout the surface and are taken from the literature[31]. The Heat transfer within the PV cell (𝑞′𝑓𝑏 𝑐𝑜𝑛𝑑) is taken from Jakhar et al. [32] and is discussed in Sec 3.3. 3.2.2

Convection heat transfer

In the current model, the convective heat transfer occurs at five different surfaces. As shown in Fig. 4, from outer glass surface, the convection (q′56conv) occurs from surface 5 to the surroundings (surface 6). The temperature difference is assumed to be linear here. The convection term is given by [31] 𝑞′56𝑐𝑜𝑛𝑣 = ℎ56𝐷4𝜋(𝑇5 ― 𝑇6)

(13)

with 𝑘𝑔 ℎ56 = 𝑁𝑢56 𝐷4

(14)

The value of Nusselt number for Eq. (14) can be calculated using correlations given by Churchill and Chu [33] by taking no-wind case and natural convection from the glass surface to the surrounding and are given as

{

𝑁𝑢56 = 0.60 + 0.387

𝑅𝑎56 =

𝛽=

𝑅𝑎0.166 56

}

2

0.296 0.559 0.562 𝑃𝑟56

[1 + ( ) ]

(15)

𝑔𝛽(𝑇5 ― 𝑇6)𝐷34

(16)

𝛼56𝜈56

1 𝑇56

𝑃𝑟56 =

(17)

𝜈56

(18)

𝛼56

The convection within the semi circular absorber tube can be evaluated by taking the correlations for Nusselt number given by Manglik and Bergles [34] 𝑞′21𝑐𝑜𝑛𝑣 = ℎ21𝐷ℎ𝜋(𝑇2 ― 𝑇1)

(19)

In this relation, Nusselt Number and hydraulic diameter (Dh) is given by [32] Page 14 of 41

𝑁𝑢 = 5.626[1 + 0.0533(𝐺𝑧)0.914] 𝐷ℎ =

0.476

(20)

𝜋𝐷 𝜋+2

(21)

The other three heat transfer due to convection occurring within the annulus are (i) from the outer surface 3 to the bulk surface 9 (𝑞′39 𝑐𝑜𝑛𝑣), (ii) from the inner surface 4 to bulk surface 9 ( 𝑞′49𝑐𝑜𝑛𝑣) and (iii) from inner surface 4 to bulk surface 8 (𝑞′48𝑐𝑜𝑛𝑣). The convective heat transfer for surface 3 and surface 4 can be calculated by using modified Petukhov correlation by adding the effect of characteristics length during the convection [35]

()

𝑓 𝑅𝑒𝑃𝑟 8

𝑁𝑢 =

0.66

[ () ]

. 1+

𝑓 1 + 12.7 (𝑃𝑟0.66 ― 1) 8

𝑑ℎ 𝐿

Where 𝑓 = (1.8log 𝑅𝑒 ― 1.5) ―2

(22)

(23)

The convection from the PV cell front surface ‘f’ to the bulk surface 8 (𝑞′𝑓8𝑐𝑜𝑛𝑣) as well as from the back surface ‘b’ to bulk surface 1 (𝑞′𝑏1𝑐𝑜𝑛𝑣) can be calculated using the following Nusselt number 𝑁𝑢𝑓8 = 0.678 𝑅𝑒0.5 𝑃𝑟0.3 3.2.3

(24)

Radiation heat transfer

The radiative heat transfer from the absorber tube occurs in two ways. The first one from the absorber tube to the glass (q′34rad) and the other from glass surface to the sky (q′57rad). The former one occurs due to high temperature of the absorber tube and is of long wavelength. Some part of it will get absorbed within the liquid and remaining will reach to glass tube surface. The absorber tube radiation (𝑞′34𝑟𝑎𝑑) can be treated as the radiation between two concentric cylinders with participating media in between. Here, for the simplicity the scattering and attenuation effect has been neglected between the fluid molecules. Similarly radiation from surface 4 to cell front surface (𝑞′4𝑓 𝑟𝑎𝑑) can be evaluated by considering the radiosity between flat plate and semi circular tube. The radiation equation for these cases are as given by Modest [36]. 𝑞′34𝑟𝑎𝑑 =

𝜇2σ(T43 ― T44)

( )(( ) )

D2 1 1 + ―1+ Ψ ϵt D3

1 ―1 ϵg

―1

(25)

Here Ψ is a non-dimensional radiative heat flux depends upon the ratio of two cylinders diameter (Di/Do) and given in terms of surface radiosities as [36]

Page 15 of 41

Ψ=

qh

(26)

Ji ― Jo

The radiation terms for 𝑞′57𝑟𝑎𝑑 is calculated using Stefan-Boltzmann law and is given by 𝑞′57𝑟𝑎𝑑 = 𝜎𝜖𝑔𝜋𝐷4(𝑇45 ― 𝑇4𝑠𝑘𝑦)

(27)

3.3 HCPVT system For the first law analysis of HCPVT system, one dimensional steady state heat transfer is assumed across the absorber tube with negligible heat capacity. It is also assumed that heat transfer to support structure is negligible. The epoxy thickness within the absorber tube and PV cell is considered to be negligible and have same thermal conductivity as that of the absorber tube. Within the PV cell, temperature gradient and Ohmic losses are neglected. Using the first law energy balance on HCPVT system, the electrical and thermal efficiencies of the hybrid system are evaluated. The energy balance for the PV module would become [37] 𝜏𝑔,𝑐𝛼𝑐𝛾𝑐𝐼𝑟𝑊𝑑𝑥 + 𝜏𝑔(1 ― 𝛾𝑐)𝛼𝑡𝑒𝑑𝐼𝑟𝑊𝑑𝑥 = [𝑈𝑐,𝑓𝑙(𝑇𝑐 ― 𝑇𝑓𝑙) + 𝑈𝑐𝑡(𝑇𝑐 ― 𝑇𝑏𝑎)]𝑊𝑑𝑥 + 𝜂𝑐,𝑒𝑙𝛾𝑐𝜏𝑔 (28) 𝐼𝑟𝑊𝑑𝑥 Where 𝑇𝑐, 𝑇𝑏𝑎, 𝑇𝑓𝑙, 𝑊, 𝐼𝑟, 𝜂𝑐,𝑒𝑙 and 𝛾𝑐 are PV cell temperature, back surface temperature of absorber, fluid temperature over cell, width of absorber, incident solar radiation, electrical efficiency of solar cell and packing factor of solar cell respectively. By solving the Eq (28), the equation for the mean solar cell temperature obtained as:

𝑇𝑐 =

𝜏𝑔[𝛼𝑐𝛾𝑐 + (1 ― 𝛾𝑐)𝛼𝑡𝑒𝑑 ― 𝜂𝑐,𝑒𝑙𝛾𝑐]𝐼𝑟 + 𝑈𝑐,𝑓𝑙𝑇𝑓𝑙 + 𝑈𝑐𝑡𝑇𝑏𝑓 𝑈𝑐,𝑓𝑙 + 𝑈𝑐𝑡

(29)

The average absorber temperature 𝑇𝑏𝑓 is dependent upon average fluid temperature 𝑇𝑤 within the inner tube and the radiation falling on this as given below [37] 𝑇𝑏𝑎 =

𝑁(𝛼𝜏)𝑒𝑓𝑓𝐼𝑟 + 𝑈𝑔,𝑡𝑒𝑑𝑇𝑓𝑙 + 𝑈𝑎𝑤𝑇𝑤 𝑈𝑔,𝑡𝑒𝑑 + 𝑈𝑎𝑤

(30)

Here, where N is the penalty factor due to presence of cell material, glass and ethylene vinyl acetate film. The overall heat transfer coefficient from glass to tedlar is calculated as [38] 𝑈𝑔𝑡 =

𝑈𝑐,𝑓𝑙 × 𝑈𝑐𝑡 𝑈𝑐,𝑓𝑙 + 𝑈𝑐𝑡

(31)

Page 16 of 41

After obtaining 𝑇𝑏𝑎 from Eq (30) the expression for an average solar cell temperature 𝑇𝑐 can be evaluated from Eq. (29). The maximum power output from a PV cell is evaluated which is given as 𝑃𝑚𝑎𝑥 = 𝑉𝑚𝑎𝑥.𝐼𝑚𝑎𝑥

(32)

Where Vmax and Imax are voltage and current at maximum power point respectively. The efficiency (𝜂𝑐) of the PV cell, which is dependent on STC conditions can be calculated using expressions from Evans [39] and Schott [40] 𝜂𝑐 = 𝜂𝑜[1 ― 𝛽(𝑇𝑐 ― 𝑇𝑆𝑇𝐶)]

(33)

While the PV panel efficiency depends upon the reflectivity, transmittance of the glass surface and cell efficiency. It is given as [41] 𝜂𝑚𝑜𝑑 = 𝜌𝑐𝜏𝑔𝜂𝑐

(34)

The instantaneous electrical efficiency of HCPVT system is calculated as [42] 𝜂𝑒𝑙 =

𝐹𝐹 𝐼𝑠𝑐𝑉𝑜𝑐 𝐴𝑐𝐼𝑟

(35)

The instantaneous thermal efficiency of the HCPVT system is [41] 𝜂𝑡ℎ =

𝑄𝑡ℎ

(36)

𝐴𝑎.𝐼𝑟

Where 𝑄𝑡ℎ = 𝑚𝑤𝑐𝑝𝑤 (𝑇𝑤,𝑜 ― 𝑇𝑤,𝑖)

(37)

The overall efficiency of the system is summation of the thermal and electrical efficiency and given as (38) 𝜂𝑡𝑜𝑡 = 𝜂𝑡ℎ + 𝜂𝑒𝑙

The Eq. (38) is based on first law efficiency and thus, the overall efficiency is obtained by summing up the two different efficiencies. However, some authors in the literature stated that kWel can not be directly compared to kWth, as the two are different grades of energy. Thus, it is more accurate to estimate the system efficiency in terms of primary energy saving efficiency (PES) 𝜂𝑃𝐸𝑆. The PES efficiency is obtained using following equation [43] 𝜂𝑃𝐸𝑆 =

𝜂𝑒𝑙 𝜂𝑝𝑜𝑤𝑒𝑟

+ 𝜂𝑡ℎ

(39)

Page 17 of 41

Where 𝜂𝑒𝑙 and 𝜂𝑡ℎ are electrical and thermal efficiency of the system. 𝜂𝑝𝑜𝑤𝑒𝑟 is defined as average efficiency of producing electrical power [44]. As per literature it varies from 0.4 to 0.6 depending upon climatic conditions of the country. For the present study it is taken as 0.5 for Indian Scenario. Accordingly, along with the overall efficiency, the average primary energy saving efficiency is also calculated for the data obtained and is discussed in Section 6. The design parameters which are taken for the analytical model are given in Table 3 Table 2 Design parameters of HCPVT system

Parameters Width of PV cell Thermal conductivity of the insulating material Thermal conductivity of the absorber tube (copper) Thermal conductivity of glass Thermal conductivity of the solar cell Absorptivity of solar cell (αc) Transmissivity of solar cell (τc) Transmissivity of glass (τg) Absorptivity of tedlar (αted) Emissivity of glass (εg) Emissivity of copper absorber tube (εt) Reflectivity of mirror (ρmi) Reflectivity of cell (ρc)

Value 0.072 mm 0.041 W/mK 401 W/mK 1.04 0.036 W/mK 0.90 0.90 0.96 0.75 0.82 0.4 0.935 0.83

4. Statistical analysis The root mean square percent deviation (e) between the results obtained from theoretical simulations (Xi) and the experimental results (Yi) are evaluated using following equations: Root mean square percent deviation, 𝑒 =

Where 𝑒𝑖 =

[

∑(𝑒𝑖)2 𝑁

(40)

] × 100

𝑋𝑖 ― 𝑌𝑖 𝑋𝑖

5. Uncertainty analysis By using energy balance, an uncertainty analysis was carried out on both electrical and thermal efficiencies from the energy balance view-points. The experimental uncertainties were calculated with the help of analysis of errors in the experimental through various instruments employed. During the measurement, the observed values on the instrument indicated the level of uncertainty in a measurement. Since different measuring instruments were used for the experimental Page 18 of 41

measurement, the maximum error during the measurement can be calculated as the ratio of least count of the measuring instrument and minimum recorded value of the parameter. For the uncertainties calculations, the following equation is used [45]: 1

[∑( n

𝜔𝑅 =

i=1

2

)

∂𝑅 𝜔 ∂𝑥𝑖 𝑖

]

2

(41)

Where R is a function of ‘n’ independent linear parameters as R = R(x1,x2,x3…xn). Thermal efficiency of the HCPVT depends up on mass flow rate and inlet and outlet temperature of the fluid from HCPVT. So the Eq. (41) is solved for the all dependent variables in case of thermal efficiency and found resultant equation as: 𝜔𝑅 𝑄

=

[

𝑎2 𝑞2

+

𝑏2

(𝑇𝑖 ― 𝑇𝑜)2

―

𝑒2

]

1/2

(42)

(𝑇𝑖 ― 𝑇𝑜)2

Where 𝑞 = 𝑚𝑐 ∆𝑇, while 𝑇𝑖 and 𝑇𝑜 are the inlet and outlet temperature of the HCPVT respectively. And a, b, e are the percentage errors in the measuring instruments. Electrical efficiency of the PV panel depends up on current, voltage and solar radiation. Thus, the equation used for the all dependent variables in case of electrical efficiency is: 𝜔𝑅 𝜂

=

[

𝑎21 𝐼2

+

𝑏21 𝑉2

―

𝑒21

]

1/2

(43)

𝐺2

Where I, V and G are the current, voltage and solar radiation respectively. And a1, b1, e1 are the percentage errors in the measuring instruments. Based on Eq. (42) and Eq. (43), a code was made in MATLAB to evaluate the uncertainties related with the measuring instruments, whose values are presented in Table 4. Table 4 Uncertainty in the measured quantities and calculated parameters during the experiment. Parameters

Units

Maximum uncertainty (in experiment)

HCPVT inlet and outlet temperature

°C

±0.6

Ambient Temperature

°C

±0.6

PV panel temperature

°C

±1.6

Mass flow rate

Kg/s

±0.0008

Solar Radiation intensity

W/m2

±6

Open circuit Voltage

V

±0.005

Short circuit current

A

±0.015

Thermal Efficiency

%

±2.11

Electrical Efficiency

%

±0.25

Page 19 of 41

During the validation, this quantum of error may occur due to the assumptions which had taken during the thermodynamic modeling of the system, improper insulation of pipes, variation in coefficient of friction of materials used in simulation, and irregularities such as fitting and joints in experimental setup. The parameters taken during the simulation are shown in Table 5 as simulation matrix.

Table 5 Simulation matrix indicating the parameters varies during the theoretical modelling Parameters varied

Range

Solar Radiation

1050.74 – 525.73 W/m2

Ambient temperature

36.20 – 43.71 °C

Mass flow rate (inner tube)

0.008 – 0.050 kg/s

Mass flow rate (annulus)

0.075 – 0.120 kg/s

Length of PV Panel

1.5 – 6 m

Length of absorber tube

1.5 – 6 m

6. Results and Discussion 6.1 Variation of Annular flow rate by keeping inner tube flow rate as constant As discussed earlier, using MATLAB (2015a), an algorithm was developed to obtain solutions from the one dimensional mathematical model. The results were then validated with the data generated from the experimental setup. The setup was designed and fabricated in house at BITS Pilani and outdoor field testing was carried out for daily 6-hour during three consecutive days (26-28 May, 09-11 June and 14-16 June) in the month of May and June 2017. By keeping the parallel flow arrangement, the mass flow rate was varied from 0.083 kg/s to 0.117 kg/s through CPVT receiver. During 26-28 May, this was achieved by keeping mass flow rate through the inner tube as 0.075 kg/s and varying the annulus flow rate as 0.008 kg/s, 0.017 kg/s and 0.025 kg/s to get combined flow rate ranging from 0.083 kg/s 0.10 kg/s. During 09-11 June, the mass flow rate through the inner tube was kept at as 0.083 kg/s and during 14-16 June, the same was maintained at 0.092 kg/s, with varying annulus flow rate of 0.008 kg/s, 0.017 kg/s and 0.025 kg/s. The experiment was conducted by keeping parallel flow arrangement with in the receiver tube. Fig. 8 shows the obtained variation of solar radiation during the test days, along with the ambient temperature when the the inner tube flow rate was maintained at 0.091 kg/s and mass flow rate through HCPVT receiver varied from 0.008 kg/s, 0.017 kg/s and 0.025 kg/s.

Page 20 of 41

1200

42.0

800

40.0

600

38.0

Solar Radiation (W/m2)

1000

Ambient Temperature (°C)

44.0

36.0

400

34.0

200

32.0

0

30.0 10

11

12

13

14 Time (Hr)

Solar Radiation (14 June) Solar Radiation (16 June) Ambient temperature (15 June)

15

16

17

Solar Radiation (15 June) Ambient temperature (14 June) Ambient temperature (16 June)

Fig. 8 Variation of solar radiation and ambient temperature during test days (14-16 June, 2017)

80

80

70

70 Temperature (°C)

Temperatrue (°C)

The observations obtained throughout the day are represented on hourly basis. It is observed that during the time of experiments, the solar radiation ranged between 558.2 W/m2 to 1050.8 W/m2 while the ambient temperature varied from 36.2 °C to 41.8 °C. The dataset for solar radiation and ambient temperature was also observed within the close range with maximum deviation of ± 3.31% and ± 1.37% respectively.

60 50

14 June, 0.008 kg/s, e=6.93%

60 50

15 June, 0.017 kg/s, e = 6.54%

40

40

30

30 10

11

12 13 Time (Hr)

14

HCPVT Inlet temp HCPVT outlet (Exp) HCPVT outlet (Simulated)

15

16

10

11

12 13 Time(Hr)

14

15

16

HCPVT Inlet temp HCPVT outlet (Exp) HCPVT outlet (Simulated)

Page 21 of 41

80

Temperature (°C)

70 60 50

16 June, 0.017 kg/s, e = 7.59%

40 30 10

11

12

13 Time (Hr)

HCPVT Inlet temp HCPVT outlet (Simulated)

14

15

16

HCPVT outlet (Exp)

Fig. 9. Simulated and experimental values of HCPVT inlet and outlet temperature during the test days (14-16 June) at various annulus flow rates

The variation observed during the inlet and outlet temperature of HCPVT during the experimentation is graphically represented in Fig. 9, along with the error bars at 5%. The experimental results obtained during the testing are validated with the theoretical results obtained using analytical approach. From the analysis, it is found out that the temperatures obtained during the experiment are slightly on the lower level, as compared to the theoretical results due to experimental errors. From Fig. 9 it is observed that HCPVT inlet temperature ranged between 35.40 °C to 37.55 °C for all annular flow rates. When the inner tube flow rate was kept constant at 0.091 kg/s, the maximum experimental outlet temperatures obtained was 66.7 °C, 64.1 °C and 60.1 °C for annulus mass flow rate of 0.008 kg/s, 0.017 kg/s and 0.025 kg/s respectively. This implies that at an annulus flow rate of 0.008 kg/s, the maximum temperature difference observed was 29.47 °C. Other external factors like incident solar radiation and CR of the collector, also account for the variation in HCPVT outlet temperature. From the statistical analysis, it was observed that the root mean square percent deviation (e) varied from 6.54% to 7.59%, which indicates that the experimental and theoretical results are in a good agreement. Fig. 10 illustrate the PV panel temperature obtained on an hourly basis during cooling and without cooling condition on test days. The test results in without cooling conditions depicts that under high concentration, the PV output increase along with the panel temperature. In long term, the high temperature of PV panel reduces its cell efficiency. From the experiments, it was observed that in no cooling conditions the PV panel temperature ranged from 80.5 °C to 87.8 °C, 81.0 °C to 88.4 °C and 81.7 °C to 89.1 °C during 14, 15 and 16th June 2017 respectively. On the other hand, PV panel temperature dropped with water cooling from 59.6 °C to 65.9 °C, 59.6 °C to 65.6 °C and 59.3 °C to 65.2 °C at constant inner flow rate of 0.091 kg/s with 0.008 kg/s, 0.017 kg/s and 0.025 kg/s and annulus flow rate respectively. It is visible from the graph that with an increase in solar radiation, the panel temperature increases which in turns increases the water temperature. The experimentally observed module temperature was validated with the simulation

Page 22 of 41

90

90

80

80 Temperature (°C)

Temperature (°C)

results and found to be in good agreement with root mean square percent deviation (e) ranging from 6.82% to 8.13%.

70

70

60

60

14 June, 0.008 kg/s, e=6.82% 50 10

11

12 13 Time (Hr) Without cooling Theoretical

14

15 June, 0.017 kg/s, e= 8.13%

50 15

16

10

Experimental

11

12

13 14 Time (Hr)

15

16

Without cooling Theoretical

Experimental

14

16

Temperature (°C)

90 80 70 60

16 June, 0.025 kg/s, e=7.68% 50 10

11

12

Without cooling

13 Time (Hr) Experimental

15 Theoretical

Fig. 10 Simulated and experimental values of PV panel temperature in the HCPVT system during the test days at various annulus flow rates.

Fig 11 shows the variation of electrical efficiency of HCPVT without cooling and with cooling, along with PV panel temperature variation for different mass flow rates. In cooling mechanism, when inner tube flow rate was kept constant at 0.091 kg/s and annulus flow rates varied from 0.008 kg/s, 0.017 kg/s and 0.025 kg/s, the maximum experimental electrical efficiency obtained ranged between 11.98% to 12.39%, 12.00% to 12.39% and 12.02% to 12.41% respectively. In the case of no flow condition, the electrical efficiency varied from 10.45% to 11.01%. This variation indicates that with an increase in PV panel temperature and without any cooling mechanism, the efficiency decreases. It was also observed that during peak afternoon, the Page 23 of 41

efficiency was on the lower side as compared to the values obtained during the morning and evening period. 13.0

13.0

14 June, m = 0.008 kg/s

12.5 Electrical Efficiency (%)

Electrical Efficiency (%)

12.5 12.0

12.0

11.5

11.5

11.0

11.0

10.5

10.5

10.0 10

11

12

13 14 Time (hr)

η (no-cooling) 13.0 Electrical Efficiency (%)

15 June, m = 0.017 kg/s

15

16

10.0

η (cooling)

10

11

12

13 14 Time (hr)

η (no-cooling)

15

16

η (cooling)

16 June, m = 0.025 kg/s

12.5 12.0 11.5 11.0 10.5 10.0 10

11

12

13 Time (hr) η (no-cooling)

14

15

16

η (cooling)

Fig. 11 Electrical efficiency of HCPVT system with and without cooling during the test days for various flow rates

The hourly variation of thermal efficiency of HCPVT system during the test days (14-16 June) at different mass flow rates is graphically represented in Fig 12. It was observed that at inner tube flow rate of 0.091 kg/s and experimental thermal efficiency of the system varied from 48.53% to 63.46%, 47.25% to 60.52% and 44.83% to 56.60% for annulus flow rates of 0.008 kg/s, 0.017 kg/s and 0.025 kg/s respectively. It was obserevd that during peak afternoon, the thermal efficiency was on the minimum side as compared to values observed during morning and evening. This may be because due to peak sunshine, the ambient temperature increases which also increases the thermal losses due to convection. When the data obtained from experiment Page 24 of 41

was compared with the simulation results, it was found out to be in good agreement with root mean square percent deviation varying from 9.61% to 10.57%. 80 75

15 June, m = 0.017 kg/s, e = 10.57%

75 70

70 Thermal Efficiency (%)

Thermal Efficiency (%)

80

14 June, m = 0.008 kg/s, e = 9.61%

65 60 55 50 45

65 60 55 50 45

40

40

35

35

30

30 10

11

12

13 14 Time (hr)

Experimental

80

10

16

Simulation

11

12

13 14 Time (hr)

Experimental

15

16

Simulation

16 June, m = 0.025 kg/s, e = 9.66%

75 Thermal Efficiency (%)

15

70 65 60 55 50 45 40 35 30 10

11

12

Time13(hr)

Experimental

14

15

16

Simulation

Fig. 12 Thermal efficiency of HCPVT system during the test days at various flow rates

Using Eq. (38), the overall efficiency of HCPVT system was calculated and is shown in Fig. 13. It was found out that during test days, the mean overall efficiency of HCPVT comes out to be 66.36%, 64.37% and 61.71% for annulus mass flow rate of 0.008 kg/s, 0.017 kg/s and 0.025 kg/s respectively. It was observed that the overall efficiency decreases with an increase in annulus flow rate at a constant inner tube flow rate.

Page 25 of 41

Overall efficiency (%)

90 85 80 75 70 65 60 55 50 45 40 10

11

14 June, m = 0.008 kg/s

12

13 Time (hr) 15 June, m = 0.017 kg/s

14

15

16

16 June, m = 0.025 kg/s

Fig. 13. Overall efficiency of HCPVT system during the test days at various flow rates

As discussed in Eq. (39), the average primary energy saving efficiency was calculated for annulus mass flow rate of 0.008 kg/s, 0.017 kg/s and 0.025 kg/s and it was found to be 78.49%, 76.51% and 73.88% respectively. Thus, it is observed that though the overall efficiency for mass flow rate of 0.025 kg/s was 61.71%, the primary energy saving efficiency was on higher side with 73.88%. This means that the system can save primary energy, with the use of both thermal and electrical, up to 73.88% at this flow rate. 6.2 Variation of Inner tube flow rate by keeping annulus flow rate as constant As discussed earlier, the experiments were carried out in sets of three consecutive days (26-28 May, 09-11 June and 14-16 June). In the previous section, the analysis was carried out when inner flow rate was kept constant at 0.091 kg/s and the annulus flow rate varied from 0.008 kg/s, 0.017 kg/s and 0.025 kg/s during test days of 14 June, 15 June and 16 June. During 26th May, the inner flow rate was maintained at 0.075 kg/s, on 9th June, it was maintained at 0.083 kg/s and on 14th June, the same was maintained at 0.091 kg/s. In the current section, the variation of the inner tube rate is discussed when the annulus flow rates was maintained at 0.008 kg/s. The variation of solar radiation along with the ambient temperature during the test days, i.e. 26 May, 9 June and 14 June, are shown in Fig. 14.

Page 26 of 41

50

1,000

45

800

40

600

35

400

30

200

25

0

Temperature (°C)

Solar Radiation (W/m2)

1,200

20 10

11

12

13

14

Time (hr) Solar Radiation (26 May) Solar Radiation (14 June) Ambient temperature (9 June)

15

16

17

Solar Radiation (9 June) Ambient temperature (26 May) Ambient temperature (14 June)

Fig. 14 Variation of solar radiation and ambient temperature during test days (26 May, 9 June and 14 June, 2017)

80

80

70

70 Temperature (°C)

Temperature (°C)

During the time of experiments (26 May, 9 June and 14 June), the solar radiation varied from 543.2 W/m2 to 1025.4 W/m2, which implied that maximum deviation observed during these days was ±5.76%. Similarly, the ambient temperature varied from 37.1 °C to 43.7 °C, which gave the maximum deviation of ±5.73%. This indicated that the dataset of the test days was within the permissible range.

60 50

26 May, m = 0.075 kg/s, e = 6.01%

60 50 9 June, m = 0.083 kg/s, e = 6.41%

40

40

30

30 10

11

12

13 14 15 Time (Hr) HCPVT Inlet temp HCPVT outlet (Exp) HCPVT outlet (Simulated)

16

10

11

12

13 Time (Hr)

14

15

16

HCPVT Inlet temp HCPVT outlet (Exp) HCPVT outlet (Simulated)

Page 27 of 41

Temperature (°C)

80 70 60 50

14 June, m = 0.091 kg/s, e = 6.93%

40 30 10

11

12

13 Time (Hr)

HCPVT Inlet temp HCPVT outlet (Simulated)

14

15

16

HCPVT outlet (Exp)

Fig. 15. Simulated and experimental values of HCPVT inlet and outlet temperature during the test days (26 May, 9 June and 14 June) at various inner tube flow rates

Similar to the analysis of inlet and outlet temperature, cell temperature, electrical and thermal efficiencies of HCPVT for varying annulus flow rate, the hourly representation of these parameters with varying inner tube flow rate are shown in Fig. 15 - Fig 19. The hourly variation of inlet and outlet temperature of HCPVT, as observed in Fig. 15, shows that inlet temperature during the test days varies from 35.4 °C to 37.6 °C for all inner tube flow rates when annulus flow rate was kept constant at 0.008 kg/s. During this period, the outlet temperature observed during the experimental testing varies from 58.4 °C to 65.3 °C, 60.1 °C to 66.5 °C and 61.2 °C to 66.7 °C for inner tube flow rate of 0.075 kg/s, 0.083 kg/s and 0.091 kg/s respectively. This implies that at inner flow rate of 0.091 kg/s, the maximum temperature difference between inlet and outlet observed was 29.47 °C. The simulation results show the root mean square percent deviation (e) ranging from 6.01% to 6.93% indicating the validation of theoretical results with the experimental data.

Page 28 of 41

100

90

Temperature (°C)

Temperature (°C)

90 80 70 60

80

70

60

26 May, m = 0.075 kg/s, e = 8.40%

9 June, m = 0.083 kg/s, e = 7.13%

50

50 10

11

12

13 Time (Hr)

Without cooling Theoretical

14

15

16

10

11

12

13 14 Time (Hr)

Without cooling Theoretical

Experimental

15

16

Experimental

Temperature (°C)

90

80

70

60 14 June, m = 0.091 kg/s, e = 6.82%

50 10

11

12

Without cooling

13 Time (Hr) Experimental

14

15

16

Theoretical

Fig. 16. Simulated and experimental values of PV panel temperature in the HCPVT system during the test days at various inner tube flow rates.

The effect of varying inner tube flow rate by keeping the annulus flow rate constant was apparent on PV panel temperature, as represented graphically in Fig. 16. In case of no flow condition (without cooling), the PV panel temperature reached the maximum of 88.1 °C, 88.8 °C and 87.8 °C during 26 May, 9 June and 14 June respectively. It is evident from the fact that under high concentration, the PV panel temperature increases and reaches a maximum during peak afternoon. When cooling was provided by varying inner tube flow rate from 0.075 kg/s, 0.083 kg/s and 0.091 kg/s, the PV panel temperature reduced and varied from 62.8 °C to 69.0 °C, 61.4 °C to 67.8 °C and 59.6°C to 65.9 °C respectively at constant annulus flow rate of 0.008 kg/s. The Page 29 of 41

13.0

13.0

12.5

12.5 Electrical Efficiency (%)

Electrcal Efficiency (%)

panel temperature observed from the theoretical analysis is also shown in the Fig. 16 and found out that they were in good agreement with experimental data having root mean square percent deviation (e) ranged from 6.82% to 8.40%.

12.0 11.5 11.0 10.5

12.0 11.5 11.0 10.5

10.0

10.0 10

11

12

13 14 Time (hr)

η (no-cooling)

15

16

10

η (cooling)

11

12

13 14 Time (hr)

η (no-cooling)

15

16

η (cooling)

13.0

Electrical Efficiency (%)

12.5 12.0 11.5 11.0 10.5 10.0 10

11

12

13 Time (hr)

η (no-cooling)

14

15

16

η (cooling)

Fig. 17. Electrical efficiency of HCPVT system with and without cooling during the test days for various flow rates

With varying mass flow rate the PV panel temperature decreased as compared to no flow condition. This leads to an increase in electrical efficiency of the PV panel. The variation observed in the electrical efficiency with and without flow condition is shown in Fig. 17. When no cooling was provided, the electrical efficiency varied from 10.46% to 11.08%. This changed with cooling of PV panel as electrical efficiency varied from 11.77% to 12.18%, 11.85% to 12.28% and 11.98% to 12.39% for inner tube flow rate of 0.075 kg/s, 0.083 kg/s and 0.091 kg/s respectively. This implies that with cooling the panel efficiency increased by 1.19% to 1.47% during the test days. The efficiency thus observed, was on the higher side during morning and Page 30 of 41

evening as compared to the values obtained during afternoon period. It is because during the morning and evening period, the solar radiation was less which results in less panel temperature and higher PV efficiency. 80

80

26 May, m = 0.075 kg/s, e = 10.28%

75

70 Thermal Efficiency (%)

Thermal Efficiency (%)

70 65 60 55 50 45

65 60 55 50 45

40

40

35

35

30

30 10

11

12

13 14 Time (hr)

Experimental 80 Thermal Efficiency (%)

14 June, m = 0.083 kg/s, e = 7.14%

75

15

16

Simulation

10

11

12

13 14 Time (hr)

Experimental

15

16

Simulation

14 June, m = 0.091 kg/s, e = 9.98%

70 60 50 40 30 10

11

12

13 Time (hr)

Experimental

14

15

16

Simulation

Fig. 18. Thermal efficiency of HCPVT system during the test days by varying inner tube flow rates

Fig. 18 shows the hourly variation of thermal efficiency of HCPVT system obtained at different inner tube flow rates while keeping annulus flow rate constant at 0.008 kg/s. It was found out that the thermal efficiency varied from 44.35% to 60.73%, 47.5% to 60.92% and 48.53% to 63.46% for inner tube flow rate of 0.075 kg/s, 0.083 kg/s and 0.091 kg/s respectively. As per the trends of electrical efficiency, the thermal efficiency was also on the higher side during the morning and evening period due to less thermal losses to the ambient. The root mean square percent deviation observed by validating simulation results with experimental data ranged Page 31 of 41

between 7.14% to 10.28%. The variation between the results may arise due to various factors including losses from pipe junctions, convection losses and insulation losses during the experiment.

Overall Efficiency (%)

The overall efficiency of HCPVT system is shown in Fig. 19. It was observed that during test days, the mean overall efficiency of HCPVT came out to be 61.42%, 64.61% and 66.36% for inner tube mass flow rate of 0.075 kg/s, 0.083 kg/s and 0.091 kg/s respectively. It was seen that the overall efficiency increased with an increase in inner tube flow rate when the annulus flow rate is kept constant. 90 85 80 75 70 65 60 55 50 45 40 10

11 26 May, m = 0.075 kg/s

12

13 Time (hr) 9 June, m = 0.083 kg/s

14

15

16

14 June, m = 0.091kg/s

Fig. 19. Overall efficiency of HCPVT system during the test days at various flow rates

Similar to estimation of the average primary energy saving efficiency as per Eq. (39), the mean primary energy saving efficiency was found to be 73.36%, 76.62% and 78.49% for inner mass flow rate of 0.075 kg/s, 0.083 kg/s and 0.091 kg/s respectively. The primary energy saving efficiency was also observed to be on higher side than the overall efficiency. 6.3 Discussion It is evident from the analysis carried out that throughout the day, the overall efficiency of HCPVT system first decreases then increases, with respect to change in solar radiation. By varying inner tube and annulus mass flow rate, the efficiencies vary throughout the test days. In order to find out the optimized flow rate to achieve maximum efficiency of the designed novel HCPVT system, the mean overall efficiency observed during all the test days are compared at different flow rate. The graphical representation of the variation observed, is shown in Fig. 20. From the figure, it is observed that with increase in inner tube flow rate, the overall efficiency increases. However, with an increase in annulus mass flow rate, the overall efficiency decreases. This implies that maximum overall efficiency could be achieved with a high inner tube flow rate and low annulus flow rate. The simulation results show that the optimum efficiency could be achieved at an inner tube flow rate of 0.1 kg/s with annulus flow rate of 0.008 kg/s. Thus at the combined flow rate of 0.108 kg/s, the mean overall efficiency of 69.19% could be achieved. This means that at this flow rate, Page 32 of 41

the hybrid system would work to give peak efficiency. However, the current lab scale system being smaller in size provides the limited output. The output could be increased by increasing the receiver length. By doing do, the fluid temperature increases which is desirable for such hybrid systems. However, after certain length, the effect of cooling of PV panel would diminish as instead of cooling, the high temperature of fluid would increase the PV panel temperature. This would reduce the panel efficiency and may damage the cell integrity. Thus, there is a trade-off required in the length of absorber tube and length of PV panel. In order to estimate the effect of large length of receiver, the simulation were carried out to observe PV panel temperature and HCPVT outlet temperature for inner tube flow rate of 0.1 kg/s with annulus flow rate of 0.008 kg/s. It was observed that with increase in length of absorber, the outlet temperature of HCPVT system and PV panel temperature increases. In order to improve the system performance, the panel temperature needs to maintain less than 80 °C. Thus the length of PV panel may be kept below 2.5 m, while the absorber tube length can be increased as desired to achieve higher thermal energy. The output thus obtained is at threshold panel temperature with sufficient thermal energy which could be used for solar based distillation units for the continuous supply of fresh water.

Overall efficiency (%)

70 68 66 64 62 60 58 56 54 0.07

0.075

0.08 0.085 Inner tube flow rate (kg/s)

0.09

0.095

Annulus flow rate of 0.008 kg/s (Exp) Annulus flow rate of 0.008 kg/s (Sim) Annulus flow rate of 0.017 kg/s (Exp) Annulus flow rate of 0.017 kg/s (Sim) Annulus flow rate of 0.025 kg/s (Exp) Annulus flow rate of 0.025 kg/s (Sim) Fig. 20. Overall efficiency of HCPVT system during the test days at various flow rates

The proposed HCPVT system is a newly designed prototype for cooling of CPVT system. The literature, as discussed in beginning, has also investigated different designs of CPVT system. The current system is also compared with the different prototypes available in literature. It would be apt to compare the current design by taking existing CPVT system as baseline. Accordingly, a comparison is made and is represented in Fig. 21.

Page 33 of 41

70 58

Efficiency (%)

60

51.3

50

45

50

54.24

34.53

40 30

Electrical

20 10

6.4

11

10.2

10

12.12

Thermal

3.67

0 Bernardo et Smeltink al. [15] and Blakers [16]

Xu et al. [17]

Cappelletti Coventry et et al. [18,19] al. [10]

Present Work*

Fig. 21. Comparison of efficiency obtained from current system with existing systems in literature

It is evident from Fig. 21 that the current system exhibits maximum thermal efficiency of 54.24% and electrical efficiency of 12.12%, which is comparable with the thermal and electrical efficiencies obtained in CPVT systems available in literature. This indicates that the current system is at par in terms of thermal efficiency and provides higher electrical efficiency as compared to existing CPVT systems. 7. Environmental cost analysis Energy utilization for heat or power would impact environment, especially when resources are used, it leads to carbon emissions. Thus the cost associated with carbon emissions is prominent factor for environmental assessment. The proposed HCPVT system was also evaluated in terms of carbon credit earned on life time basis for the price of CO2 emitted annually. The enviroeconomic study would help to estimate yearly carbon mitigation along with its environmental cost. For the estimation, it is taken that coal in power plant would generate average of 980 g CO2/kWh of emissions for electricity production, which eventually utilized for various purposes including heating water. With transmission and distribution losses, taken from the literature, it comes out to be 2.08 kg CO2/kWh [46]. The methodology for the estimation of carbon credit was adopted from Agarwal and Tiwari [47] by estimating total thermal gain of the system over the year and calculating associate cost of energy. The total thermal gain was evaluated using the hourly rate of thermal energy (Qth) as evaluated in Eq. (37) and summing up to get daily, monthly and hence annual thermal gain. Thus, for total thermal gain the carbon mitigation per annum would be given as [46] 𝐶𝑀𝐶𝑂2 =

𝐸𝐶𝑂2 × Qth,total

(41) 103 Where 𝐶𝑀𝐶𝑂2is carbon mitigation (t CO2/annum), 𝐸𝐶𝑂2is average emission of CO2. It is taken as 2.08 kg CO2/kWh.

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In case, the reduction in carbon emission is traded at current rate of €22/tCO2, the cost of reduction by the novel HCPVT system per annum would become [48] (42)

𝐸𝐶 = 𝐶𝑃 × 𝐶𝑀𝐶𝑂2 Where CP is the carbon price and is taken as €22/tCO2 and EC is the environmental cost.

The estimated carbon emissions mitigated and associated cost is calculated for three different scenarios. In Case I, it is taken that the HCPVT would operate at fixed inner tube flow rate and annulus flow rate varies during its operations. For the conditions of Pilani, India, the carbon mitigation and its cost is evaluated for a year. Similarly in Case II, the annulus flow rate is kept constant and inner tube flow rate varied during its operations. Case III is related to optimum flow rate which was obtained previously. The results are then compared with a conventional PVT system taken from the Agarwal and Tiwari [47] as Case IV and are shown in Table 6 and Fig. 22. Table 6 Annual carbon mitigation and environmental cost associated per annum of HCPVT system for the conditions of Pilani, India

Scenarios

Carbon Mitigation per annum

Environmental cost

(tonnes CO2)

(€/annum)

Case I

32.878

723.31

Case II

35.837

788.41

Case III

40.230

885.06

Case IV

10.614

233.50*

*adjusted to current carbon trading rate

1200

40

1000

35

800

30 25

600

20

400

15 10

200

5 0

Environmental cost (€/annum)

Carbon Mitigation t CO2/annum

45

0 Case I

Case II

Case III

Case IV

Fig. 22 Carbon mitigation and environmental cost per annum for HCPVT system in different cases

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8. Conclusion In the current investigation, an experimental and theoretical analysis has been carried out to assess the performance of proposed HCPVT system in terms of thermal and electrical efficiency. The experimental testing was carried out during the month of May and June 2017 in Pilani, Rajasthan for different annulus and inner mass flow rates. The major conclusions derived from the analysis are: 

The theoretical values of HCPVT outlet temperature, PV panel temperature and thermal efficiency obtained from the simulation are in good agreement with the results obtained from the experimental study.



Simulation results showed that without cooling, i.e. no flow inside inner tube and annulus, PV panel temperature ranged from 82.5 °C to 88.1 °C, 81.3 °C to 88.9 °C and 80.8 °C to 88.4 °C during 26, 27 and 28 May respectively. With cooling, temperatures dropped in the range of 70.6 °C to 74.5 °C, 71.1 °C to 75.0 °C and 69.7 °C to 73.7 °C at fixed inner tube flow rate of 0.075 kg/s and annulus flow rate of 0.008 kg/s, 0.017 kg/s and 0.025 kg/s respectively.



Experimentally, with cooling, the PV panel temperature observed slightly lesser than the simulation results and ranged from 62.8 °C to 69.0 °C, 63.6 °C to 70.1 °C and 61.8 °C to 68.1 °C at respective flow rates. The mean square percent deviation between simulation and experimental PV panel temperature comes out to be in the range of 7.17% to 8.87%. This may be due to the one dimensional assumptions taken during the theoretical model.



It was observed that maximum difference of 28.7 °C was observed between inlet and outlet temperature of HCPVT for annulus mass flow rate of 0.008 kg/s. For higher flow rate of 0.017 kg/s and 0.025 kg/s, it reduced to 23.71 °C and 20.53 °C respectively. This may be due to the fact that at lower mass flow rate, the fluid inside the annulus gains more heat from the surroundings as more time is available for heat transfer.



During fixed inner flow rate conditions, the electrical efficiency, with cooling, was observed varying from 11.77% to 12.18%, 11.69% to 12.13% and 11.84% to 12.25% for annulus flow rates of 0.008 kg/s, 0.017 kg/s and 0.025 kg/s respectively. It varied from 11.77% to 12.18%, 11.86% to 12.28% and 11.98% to 12.39% for inner flow rates of 0.075 kg/s, 0.083 kg/s and 0.091 kg/s respectively, when annulus flow rate was fixed.



The electrical efficiency of PV system is on higher side during the morning and evening period as the solar radiation is low. However during peak sunshine hours, with increase in radiation, the ambient temperature also increases. This leads to increase in overall fluid and PV temperature, which reduces the cell efficiency.



The mean experimental thermal efficiency of the system was observed to be 49.48% for the lower annulus mass flow rate.

Page 36 of 41



The maximum mean overall efficiency of the system obtained was up to 61.42%, for the annulus mass flow rate of 0.008 kg/s. Slight variation was observed in thermal efficiency with increase in mass flow rate.



At optimum flow rate, such novel HCPVT system would helps to reduce carbon emission by 40.3 t CO2/yr with associated environmental cost of 885.06 €/annum.



With combined heat and power, the proposed system could be used for low grade thermal applications like distillation, heating etc.

Acknowledgments The authors would like to acknowledge the support from the ‘Center for Renewable Energy and Environmental Development’ and ‘Central Workshop Facility’ of Birla Institute of Technology and Science - Pilani Rajasthan, for this research. References [1]

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Highlights     

A hybrid concentrated photovoltaic thermal system was proposed and analyzed using mathematical model and experimental investigation The mass flow rate was varied from 0.083 kg/s to 0.117 kg/s using two different flow configurations. At optimum flow rate of 0.108 kg/s the mean overall efficiency of the system reaches up to 69.19%. Environmental cost analysis shows the carbon mitigation of 40.2 t CO2/annum at optimum conditions. The proposed HCPVT system could be used for low grade thermal applications.

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