Alternative designs of parabolic trough solar collectors

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Progress in Energy and Combustion Science 71 (2019) 81–117

Contents lists available at ScienceDirect

Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs

Alternative designs of parabolic trough solar collectors Evangelos Bellos∗, Christos Tzivanidis Thermal Department, School of Mechanical Engineering, National Technical University of Athens, Zografou, Heroon Polytechniou 9, 15780 Athens, Greece

a r t i c l e

i n f o

Article history: Received 4 June 2018 Accepted 24 November 2018

Keywords: Parabolic trough collector Solar energy Alternative design Secondary reﬂector Thermal enhancement Nanoﬂuids

a b s t r a c t Parabolic trough collector (PTC) is the most established solar concentrating technology worldwide. The conventional parabolic trough collectors are used in various applications of medium and hightemperature levels. However, there are numerous studies which investigate alternative designs. The reasons for examining different PTC conﬁgurations regard the thermal eﬃciency increase, the reduction of the manufacturing cost and the development of more compact designs. The objective of this review paper is to summarize the existing alternative designs of PTC and to suggest the future trends in this area. Optical and thermal modiﬁcations are examined, as well as the use of concentrating thermal photovoltaic collectors. The optical modiﬁcations include designs with secondary concentrators, stationary concentrators and strategies for achieving uniform heat ﬂux. The thermal modiﬁcations regard the use of nanoﬂuids, turbulators and the use of thermally modiﬁed receivers with insulation, double-coating and radiation shields. The concentrating thermal photovoltaics are systems with ﬂat or triangular receivers which can operate in low or in medium temperature levels with the proper alternative designs. It has been found that there are many promising choices for designing PTC with higher thermal performance and lower cost. The conclusions of this work can be used as guidelines for future trends in linear parabolic concentrating technologies. © 2018 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

∗

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The conventional PTC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. General description . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Basic mathematical modeling of the conventional 2.2.1. Geometry modeling . . . . . . . . . . . . . . . . . . 2.2.2. Thermal modeling . . . . . . . . . . . . . . . . . . . 2.2.3. Optical modeling . . . . . . . . . . . . . . . . . . . . 2.2.4. Heat transfer modeling of the ﬂow. . . . . . 2.2.5. Hydraulic modeling . . . . . . . . . . . . . . . . . . 2.2.6. Performance evaluation criteria. . . . . . . . . 2.3. The present work. . . . . . . . . . . . . . . . . . . . . . . . . . . Optical modiﬁcations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Modiﬁcations on the primary reﬂector . . . . . . . . . . 3.2. The use of secondary reﬂectors . . . . . . . . . . . . . . . 3.3. Summary of the optical modiﬁcation methods . . . Thermal modiﬁcations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. General design ideas . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Thermal loss coeﬃcient reduction . . . . . . . 4.1.2. Alternative designs of absorbers . . . . . . . . 4.1.3. Other ideas about PTC . . . . . . . . . . . . . . . .

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

https://doi.org/10.1016/j.pecs.2018.11.001 0360-1285/© 2018 Elsevier Ltd. All rights reserved.

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4.1.4. Summary of the general design ideas . . . . . . . . . . . . . . . . . . . . . . . Flow modiﬁcations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Flow inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Modiﬁed inner absorber geometry . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Nanoﬂuid-based solar collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Conventional PTC with nanoﬂuids . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Direct absorption PTC with nanoﬂuids . . . . . . . . . . . . . . . . . . . . . . 4.3.3. Applications with nanoﬂuid-based PTC . . . . . . . . . . . . . . . . . . . . . 4.4. Comparative studies about the thermal enhancement techniques in PTC . 5. The use of concentrating thermal PV with parabolic concentrators . . . . . . . . . . . 5.1. General approach for the concentrating thermal PV . . . . . . . . . . . . . . . . . . 5.2. Basic mathematical formulation for concentrating thermal PV . . . . . . . . . 5.3. Studies with parabolic thermal PV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Performance discussion of the examined studies . . . . . . . . . . . . . . . . . . . . 6.1.1. Discussion of optically modiﬁed designs . . . . . . . . . . . . . . . . . . . . . 6.1.2. Discussion of thermally modiﬁed designs . . . . . . . . . . . . . . . . . . . . 6.1.3. Discussion of concentrating thermal PV designs. . . . . . . . . . . . . . . 6.2. Financial discussion of the examined ideas . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Challenges and environmental beneﬁts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Basic modeling of nanoﬂuids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.

1. Introduction Solar energy utilization is a promising choice for facing numerous problems in the energy domain such as the fossil fuel depletion [1], the global warming [2], the increasing electricity demand [3] and generally the nature pollution. Solar energy is practically an abundant energy source with a huge potential [4]. The sun releases extremely high amounts of solar irradiation [1] and approximately the incident solar irradiation on the earth surface is about 1.7 × 1014 kW [4]. This energy rate is tremendous because if it was possible to utilize this amount only for 84 min then the yearly worldwide energy demand would be fully addressed [4]. Moreover, solar energy is a ﬂexible energy source because it can be directly converted either to useful heat or to electricity [5]. Sun is a radiation reservoir and this fact makes the solar energy a highpotential energy source [6]. As a result, the solar irradiation can be used in applications of a great temperature range and it is able to compete with the combustion technologies in the cases of concentrating solar systems [7]. The previous facts indicate that solar energy presents numerous advantages and thus it is an ideal energy source for covering the worldwide energy needs in a renewable and sustainable way. Solar thermal collectors are devices which capture the solar energy and convert it into useful heat energy at different temperature levels. On the other hand, photovoltaic cells are devices which capture the incident solar irradiation and produces directly electricity. The solar thermal collectors are separated into ﬂat and concentrating technologies. The ﬂat technologies include the ﬂat plate collector and the evacuated tube collectors. The ﬂat technologies are used in low-temperature applications like space heating, domestic hot water production and in solar cooling applications with singlestage machines (absorption or adsorption). The maximum obtained temperature levels are usually up to 90 °C with ﬂat plate collectors and up to 150 °C with evacuated tube collectors [8]. The concentrating technologies are able to operate at higher temperature levels and they are usually used in temperature levels between 150–500 °C [9]. The parabolic trough collector (PTC) is the most mature solar concentrating technology at this moment [10]. Moreover, the linear Fresnel reﬂector (LFR) is an an-

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tagonist of the PTC because both these collectors are linear concentrating technologies and they are usually used in the temperature range of 20 0–40 0 °C. The PTC presents higher thermal eﬃciency but the linear Fresnel collector tend to have lower speciﬁc installation cost. More speciﬁcally, the PTC has a speciﬁc installation cost of 275 €/m2 , while for the LFR it is estimated at 200 €/m2 [11]. The optical eﬃciency of a PTC can reach up to 75%, while the usual maximum limits of the LFR is about 65%. Other concentrating technologies are solar towers and solar dishes which are practically used only for high temperatures applications such as electricity production [12]. The PTC can be used in a great range of applications for solar cooling, refrigeration, industrial heat, desalination, chemical processes and electricity production [13,14]. The PTC usually operate with thermal oils as Syltherm 800, Therminol VP-1, Therminol 55, Dowtherm A, etc. for operation up to 400 °C [15]. Moreover, PTC can operate with water/steam for electricity production in Rankine cycles. Furthermore, there are applications with molten salts as work ﬂuids and especially nitrate salts which can operate up to 550∼600 °C [15]. A critical issue about the use of PTC and generally of solar thermal systems is the storage issue. The solar irradiation is not a continuous energy source and this fact creates problems in the solardriven applications. Thus, the solar irradiation is usually hybridized with other energy souses such as natural gas and biomass [16]. Comparing the PTC with PV panels, it can be said that the PV panels can produce electricity with a lower levelized cost of electricity (LCOE). However, the produced electricity by the PV panels is usually stored in batteries which face many limitations about their high cost, their negative environmental impact and the restricted lifetime (about 6–7 years) [17]. The alternative storage strategy is the pumped hydroelectric storage but it is not always possible to be done due to the restricted availability. On the other hand, the use of solar thermal systems (e.g., PTC) can store energy in storage tanks with thermal oils, molten salts and concrete which can be cost-effective choices compared to the batteries. Another competitive technology to the PTCs are the solar towers which operate with high concentrations ratios and so they can be coupled to high-eﬃcient power engines. Solar towers are high-

E. Bellos and C. Tzivanidis / Progress in Energy and Combustion Science 71 (2019) 81–117

Nomenclature A Be C cp D E f fr Gb h hout K k L m Nu Pel PEC PF Pr Q Re T u UL Vwind W Wp x y

area, m2 Bejan number, − geometrical concentration ratio, − speciﬁc heat capacity under constant pressure, J/kg K diameter, m exergy ﬂow, W collector focal distance, m friction factor, − solar direct beam irradiation, W/m2 heat transfer coeﬃcient, W/m2 K convection coeﬃcient between cover and ambient, W/m2 K incident angle modiﬁer, − thermal conductivity, W/mK tube length, m mass ﬂow rate, kg/s Nusselt number, − electricity production, W performance evaluation criterion, − packing factor, − Prandtl number, − heat rate, W Reynolds number, − temperature, K ﬂuid velocity, m/s collector thermal loss coeﬃcient, W/m2 K ambient air velocity, m/s collector width, m pumping work demand, W coordinate in x-axis, m coordinate in y-axis, m

Greek symbols α absorber absorbance, − β ratio of the nano-layer thickness to the original particle radius, γ intercept factor, − P pressure drop, kPa S entropy generation, J/K S T entropy generation due to temperature increase, J/K S P entropy generation due to pressure drop, J/K δ solar declination angle, o ε emittance, − ηex exergy eﬃciency, − ηel electrical eﬃciency, − ηopt optical eﬃciency, − ηovr overall eﬃciency, − ηPV photovoltaic cell electrical eﬃciency, − ηref reference photovoltaic cell electrical eﬃciency, − ηtot total eﬃciency, − ηth optical eﬃciency, − θ incident solar angle, o θz zenith solar angle, o ρ density, kg/m3 ρ tot total reﬂectance, − σ Stefan-Boltzmann constant, W/m2 K4 τ cover transmittance, − ϕ nanoparticle volumetric concentration, % ϕr rim angle, o ω hour angle, o

83

Subscripts and superscripts a aperture abs absorbed am ambient bf base ﬂuid c cover ci inner cover co outer cover en energy ex exergy fm mean ﬂuid in inlet heat heat production loss thermal loss max maximum nf nanoﬂuid np nanoparticle out outlet PV photovoltaic cell r receiver ref reference conditions ri inner receiver ro outer receiver s solar sky equivalent sky sun sun u useful 0 reference case Abbreviations CFD Computational ﬂuid dynamics CNT Carbon nanotube CPVT Concentrating thermal photovoltaic DAPTC Direct absorption parabolic trough collector LCOE Levelized cost of electricity LFR Linear Fresnel reﬂector MWCNT Multi-wall carbon nanotube ORC Organic Rankine cycle PTC Parabolic trough collector PV Photovoltaic cell SWCNT Single-wall carbon nanotube TIM Transparent insulation material eﬃcient solar systems with relatively low investment cost and this is the reason for their commercial expansion [18,19]. The solar tower power plants produce high power amounts and this fact leads to huge installations. So, the solar towers can be installed in locations with high area availability. On the other hand, the PTCs can be separated into small modules and they can be adjusted to any possible solar ﬁeld, from a building roof up to a huge solar farm. The thermal storage in the PTC solar ﬁelds is not so diﬃcult because it can be done in temperature levels of 30 0--40 0 °C for example and not in extremely high temperatures that the solar towers need (e.g., 700 °C or more). However, it is useful to state that the PTC systems for steam generation face problems with the energy storage. Furthermore, the PTCs are used in a great variety of applications such as solar cooling, industrial heating, desalination and polygeneration systems, while the PV panels and the solar towers are used only for power production. In any case, the existing values of the levelized cost of electricity with the PTC are about 0.20 €/kWh which is a not so attractive value [20]. Thus, there is a need for a reduction in the investment cost by increasing the thermal eﬃciency or reducing the manufacturing cost. It is expected that the levelized cost of the electricity with the PTC will be reduced to 0.13 €/kWh up to 2020 [21].

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tube (the cover). Usually, there are vacuum conditions between the absorber and the cover in order to eliminate the convection thermal losses of the absorber. The vacuum is practically an extremely low pressure of 0.013 Pa [23]. In order to keep the low-pressure, getters are used to absorb the gas molecules. Fig. 1b illustrates the cross-section of an evacuated tube. Usually, the outer surface of the absorber has a selective coating in order to reduce the radiation thermal losses to the environment. The Cermet coating is one usual choice for the PTC. The emittance of the Luz Cermet coating is 0.061 at 25 °C, 0.146 at 400 °C and 0.179 at 500 °C, while the absorbance is about 0.938 [24]. 2.2. Basic mathematical modeling of the conventional PTC 2.2.1. Geometry modeling The parabolic shape geometry is described by the following formula (see Fig. 2a):

y= Fig. 1. Alternative parabolic trough collector categories and goals.

The use of alternative and more ﬂexible designs is an answer to the problems of cost reduction, of designing light-weight systems and of improving the environmental impact of the solar collector. Moreover, alternative techniques for the performance enhancement (both optical and thermal) are also the key-points for achieving the reduced investment and operating cost. Furthermore, the operating of the PTC at higher temperature levels is an important goal in order to couple the PTC with advanced and high eﬃcient power cycles. In this direction, this review paper summarizes the existing alternative designs of the PTC in the literature. Optical and thermal modiﬁcations are examined, as well as the combination of PTC with photovoltaic cells (PV) is investigated. Fig. 1 brieﬂy summarizes the categories of the alternative PTC, as well the main goals that can be achieved with the alternative designs. Practically, the present study investigates three main classiﬁcations of the alternative PTC designs which are the optically modiﬁed systems, the thermal modiﬁed systems and the concentrating thermal PV with the parabolic trough. The results of this work can be used as guidelines for the future designs of PTC. 2. The conventional PTC 2.1. General description The conventional PTC consists of a linear parabolic shape concentrator, a linear tubular receiver and a metallic support structure. The PTC tracks the sun usually with a single axis mechanism in order for the solar rays to be delivered properly in the collector aperture. Usually, the PTC is placed with their axis in the North-South direction and they track the sun in the East–West direction [22]. This strategy leads to optimum exploitation of the incident solar irradiation during the summer period, when the solar potential is maximized. Otherwise, there is the option for placing the line PTC axis in the East–West direction and tracking the sun in the North– South direction, a strategy which leads to the better solar collection during the winter period. Usually, the PTC is separated into modules and many modules are placed linearly in series. Many series are connected in a parallel conﬁguration and they consist of the total solar ﬁeld. The module has usually a length close to 10 m and totally every series has about 10–15 modules in power plants. Fig. 1a depicts a typical PTC module. The receiver of the conventional PTC is an evacuated tube receiver with a metallic inner tube (the absorber) and an outer glass

x2 4· f

(1)

The rim angle (ϕ r ) is calculated using the aperture (W) and the focal distance (f) as below:

ϕr = arctan

f W f 2 W

8·

16 ·

(2) −1

The total collector aperture (Aa ) is the product of the width (W) and the length (L):

Aa = W · L

(3)

The absorber area (Aro ) is the outer area of the tube:

Aro = π · Dro · L

(4)

The geometrical concentration ratio (C) is deﬁned as the collector aperture (Aa ) to the absorber area (Aro ):

C=

Aa Aro

(5)

2.2.2. Thermal modeling The useful heat production of a PTC (Qu ) can be calculated using the energy balance on the ﬂuid volume:

Qu = m · c p · (Tout − Tin )

(6)

The available solar irradiation on the collector aperture (Qs ) is the product of the aperture area (Aa ) and of the direct beam solar irradiation (Gb ):

Qs = Aa · Gb

(7)

The thermal eﬃciency of the solar collector (ηth ) is the ratio of the useful heat (Qu ) to the available solar direct beam irradiation (Qs ):

ηth =

Qu Qs

(8)

The thermal losses of the solar collector (Qloss ) can be expressed as below, using the thermal loss coeﬃcient (UL ), the mean absorber temperature (Tr ) and the absorber outer surface (Aro ):

Qloss = Aro · UL · (Tr − Tam )

(9)

The thermal losses of the absorber to the cover (Qloss ) are practically the radiation thermal losses of the absorber tube to the cover [25]:

Qloss = Aro · σ ·

Tr4 − Tc4 1

εr +

1 −ε c

εc

·

Ari Aco

(10)

E. Bellos and C. Tzivanidis / Progress in Energy and Combustion Science 71 (2019) 81–117

85

Fig. 2. Conventional PTC (a) A typical PTC module (b) Cross-section of the evacuated tube receiver.

The parameter (σ ) is the Stefan-Boltzmann constant which is equal to 5.67•10−8 W/m2 K4 . Moreover, the absorber emittance (ε r ) and the cover emittance (ε c ) are used in the previous equation. The thermal losses of the cover to the ambient (Qloss ) are assumed to be the same as the thermal losses of the absorber to the cover because the system is evaluated in steady-state conditions. There are both radiation and convection thermal losses from the cover to the ambient [25]:

4 Qloss + Aco · σ · εc · Tc4 − Tsky + Aco · hout · (Tc − Tam )

radiation thermal losses

(11)

convection thermal losses

In the previous calculation, the mean cover temperature (Tc ), the ambient temperature (Tam ) and the sky temperature (Tsky ) are involved. The sky temperature (Tsky ) can be estimated by the following equation [26]: 1. 5 Tsky = 0.0552 · Tam

(12)

The heat transfer coeﬃcient between cover and ambient (hout ) can be calculated using the following formula [27]: 0.58 .42 hout = 4 · Vwind · D−0 co

(13)

The previous equation can be applied for cross-ﬂow on cylindrical objects and it uses the wind speed (Vwind ) and the cover outer diameter (Dco ). A typical value of the heat transfer coeﬃcient is around 10 W/m2 K, for wind speeds close to 1 m/s [25]. 2.2.3. Optical modeling The energy balance on the absorber indicates that the absorbed solar irradiation (Qabs ) is separated into useful heat (Qu ) and thermal losses (Qloss ):

Qabs = Qu + Qloss

(14)

The previous formula takes into consideration the cosine losses and the end losses of the PTC and it gives the optical eﬃciency reduction of the collector with high accuracy. For a tracking strategy with the PTC axis in North-South direction, the cosine of the incident angle is calculated as below [22]:

cos (θ ) =

cos2 (θz ) + cos2 (δ ) · sin (ω ) 2

The zenith solar angle (θ z ), the solar declination angle (δ ) and the solar hour angle (ω) are used in the previous calculation. The maximum optical eﬃciency (ηopt, max ) is a product of various parameters. Every parameter regards a different optical loss:

ηopt,max = ρtot · γ · τ · α

2.2.4. Heat transfer modeling of the ﬂow The heat transfer from the absorber to the working ﬂuid is modeled using the heat transfer coeﬃcient (h). So, the useful heat production (Qu ) can be written as:

Qu = Aro · h · Tr − T f m

Qabs = ηopt · Qs

Tf m =

The optical eﬃciency changes with the solar irradiation incident angle and it can be modeled using the incident angle modiﬁer (K) and the maximum optical eﬃciency (ηopt, max ) which is observed for zero incident angle:

ηopt (θ ) = K(θ ) · ηopt,max

(16)

The incident angle modiﬁer is a function of the geometric characteristics of the PTC, as well as of the incident angle (θ ). An analytical mathematical expression for this parameter is given below and it has been suggested by Gaul and Rabl [28]:

K (θ ) = cos (θ ) −

f · L

1+

2

W 48 · f 2

· sin(θ )

(17)

(19)

The absorber absorbance (α ), the cover transmittance (τ ), the intercept factor (γ ) and the total reﬂectance (ρ tot ) are the used parameters in the maximum optical eﬃciency calculation. The intercept factor is usually close to 1 for optimized commercial designs. The absorbance and the transmittance take values close to 90–95% and they are variable from system to system. The total reﬂectance includes various factors [29] as the concentrator reﬂectance (ρ tot ), the tracking errors, the shading factors, the clearness factors and any other possible reason for the optical loss. The concentrator reﬂectance takes values usually close to 90–93%, while the other factors can be about 98–99%. The total reﬂectance takes values close to 80–85%. Finally, the maximum optical eﬃciency of a typical PTC is around 75% [30].

The absorbed solar energy (Qabs ) is calculated using the optical eﬃciency of the solar collector (ηopt ) and the available solar irradiation (Qs ):

(15)

(18)

(20)

The mean ﬂuid temperature (Tfm ) can be estimated as:

Tin + Tout 2

(21)

The heat transfer coeﬃcient (h) can be calculated using the Nusselt number:

h=

Nu · k Dri

(22)

Usually, the ﬂow inside the absorber of a PTC is turbulent and in this case, the Nusselt number (Nu) can be calculated using the Reynolds number (Re) and the Prandtl number (Pr), according to the Dittus–Boelter model [31]:

Nu = 0.023 · Re0.8 · P r 0.4

(23)

The previous equation is usually applied for Reynolds number over 10,0 0 0.

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2.2.5. Hydraulic modeling The hydraulic modeling of the PTC is important in order to calculate the pressure drop (P) and the pumping work demand (Wp ). The pressure drop (P) is calculated as:

P = f r ·

L · Dri

1 2

· ρ · u2

(24)

The mean ﬂuid velocity (u) is calculated as:

u=

m

ρ·

π 4

2

(25)

· Dri

The friction factor (fr ) can be estimated using the Moody equation for turbulent ﬂow [32]:

fr = 0.184 · Re−0.2

(26)

The pumping work demand (Wp ) is calculated as:

Wp =

m · P

(27)

ρ

2.2.6. Performance evaluation criteria The overall performance of the PTC has to take into consideration both the useful heat production and the pumping work demand for the ﬂuid movement. So, various criteria can be applied in order to make a suitable evaluation. In the conventional PTC, the thermal and the exergy eﬃciencies are the most usual evaluation indexes. However, there are extra criteria which are useful when alternative designs are evaluated and compared with the reference design. Exergy eﬃciency: The exergy eﬃciency (ηex ) evaluates the useful heat production as the maximum equivalent work that a Carnot thermal engine is able to produce. Moreover, it takes into consideration the pressure drop along the absorber tube and the pumping work demand for the ﬂow movement. The exergy eﬃciency is deﬁned as the ratio of the useful exergy production (Eu ) to the exergy ﬂow if the solar irradiation (Es ):

ηex =

Eu Es

(28)

The useful exergy production (Eu ) is calculated as [33]:

Eu = Qu − m · c p · To · ln

T out

Tin

−

m · T o · P ρ · Tf m

(29)

used for converting the pumping work demand to the equivalent primary energy. Performance evaluation criterion: The performance evaluation criterion (PEC) is a ﬂow criterion which evaluates the heat transfer coeﬃcient enhancement of an alternative design compared to the reference case. This parameter is usually calculated for the evaluation of the heat augmentation techniques. This index evaluates the increase of the heat transfer coeﬃcient under the equivalent conditions of “same pumping work demand” [36]:

Nu

P EC =

N u0 fr

(32)

13

fr,0

The (Nu0 ) and the (f0 ) are the Nusselt number and the friction factor of the reference case respectively. The reference case is usually the conventional PTC case. If the (PEC) is over 1, then there is an enhancement in the ﬂow, otherwise, the ﬂow is not improved and the examined method is not effective. Practically, the values of (PEC) greater than 1 indicate an enhancement in the heat transfer coeﬃcient. The enhancement of the heat transfer coeﬃcient leads to higher heat transfer rates in the ﬂuid and more useful heat is produced. Moreover, higher heat transfer coeﬃcients lead to lower receiver temperature, the fact that reduces the thermal losses and increases the thermal eﬃciency. Entropy generation ratio: The generation of the entropy (S) is something expected but it is not favorable. A criterion for checking if the ﬂow has been enhanced is by checking the entropy generation. Thus, the ratio of the entropy generation between the examined design and the reference one (Ns ), is an important criterion. If this criterion takes values lower than 1, then there is an enhancement in the ﬂow.

Ns =

S S 0

(33)

Usually, the Bejan number (Be) is also calculated in order to check the impact of the pressure drop in the total entropy generation. This number is the ratio of the entropy generation due to the temperature increase (ST ) to the total entropy generation [37]. The total entropy generation is a result of entropy generation due to the temperature increase (ST ) and of the pressure drop (SP ).

Be =

S T S T + S P

(34)

The exergy ﬂow of the solar irradiation (Es ) can be calculated using the Petela model [34]. This formula takes into consideration that the sun is a radiation reservoir and not a thermal reservoir. This equation is ideal for the undiluted solar irradiation [34]. The PTC exploits only the beam irradiation and not the diffuse, so this model seems to be the proper one for the PTC.

In PTC, the Bejan number takes values close to 1 because the pressure drop is generally low. However, it is able to indicate in any case the amount of the irreversibility due to the pressure drop. The exception is the cases with gas working ﬂuids where the entropy generation due to the pressure drop can be more intense.

4 To 1 Es = Qs · 1 − · + · 3 Tsun 3

2.3. The present work

T 4 o Tsun

(30)

The temperature levels in the previous equations have to be in Kelvin units, the reference temperature (To ) can be selected at 298.15 K and the sun temperature (Tsun ) at 5770 K. Overall eﬃciency: The overall eﬃciency of the PTC (ηovr ) indicates the net primary heat production of the solar system. Practically, the equivalent primary energy consumption for the ﬂuid pumping along the collector is reduced by the useful heat production and so the overall eﬃciency can be written as below [35]: W

ηovr =

Qu − η p el Qs

(31)

The electrical eﬃciency (ηel ) is the average electrical eﬃciency of the grid and it takes values close to 33% [35]. This parameter is

The mathematical modeling of the conventional PTC proves that numerous parameters are used in order ﬁnally the thermal performance of the collector to be calculated. By combing the Eqs. (5), (7)–(9) and (14)–(16), it can be written:

ηth = ηopt,max · K (θ ) − UL ·

Tr − Tam C · Gb

(35)

Eq. (35) that some possible ways for enhancing the thermal performance are the following: (a) The increase of the maximum optical eﬃciency (ηopt, max ) by improving the material properties or by adding a booster reﬂector. (b) The increase of the incident angle modiﬁer (K) by improving the tracking strategy or reducing the end losses.

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87

Fig. 3. The suggested design with the non-continuous reﬂector (a) The design principle of the collector (b) The movable receiver for end losses elimination [38] (License Number: 4402620040390).

(c) The decrease in the thermal loss coeﬃcient (UL ). It can be achieved by reducing the radiation losses or by adding, for example, extra insulation to the system. (d) The reduction of the receiver temperature by increasing the heat transfer rates on the system. This fact can be achieved using any thermal enhancement method such as the use of ﬂow inserts or internal ﬁns in the absorber. (e) The increase of the concentration ratio which can be achieved with the reduction of the absorber area and simultaneously by keeping the optical performance at high levels. It is obvious that the previous ideas are reasonable and they can be easily supported by Eq. (35). In the literature, many researchers have examined alternative designs of PTC in order to increase the collector performance. Many of them examined ideas as the previously stated in points (a)–(e). Moreover, many alternative designs aim to reduce the cost of the PTC which is another important issue. This work summarizes and discusses the examined alternative designs of PTC in the existing literature. Different ideas which try to enhance the optical and thermal eﬃciency are presented. Moreover, ideas that try to create a system with lower cost and more ﬂexible design are also included. To our knowledge, there is no other study which summarizes all these ideas together and so this work has to add something new and important to the existing literature. The following sections include the presentation of the alternative PTC designs. 3. Optical modiﬁcations The optical modiﬁcations on the PTC aim to increase the optical eﬃciency and to enhance the heat ﬂux uniformity over the absorber. They are associated with changes in the primary reﬂector, as well as with the use of a secondary reﬂector. The alternative designs aim to suggest conﬁgurations with lower cost and relatively acceptable optical eﬃciency. 3.1. Modiﬁcations on the primary reﬂector Various ideas have been suggested for the proper modiﬁcation of the primary receiver. These ideas try to create a more compact design and to increase the optical eﬃciency when it is possible.

Zhu et al. [38] designed an alternative PTC with non-continuous primary absorber which is similar to an LFR design idea, as it is depicted in Fig. 3a. They also used a secondary reﬂector for enhancing the optical eﬃciency. Furthermore, they used a movable receiver in order to eliminate the end losses (see Fig. 3b). According to their results, this system is able to reach up to 66% thermal performance with transcal oil, while the thermal loss coeﬃcient is about 1.32 W/m2 K. They stated that this system presents higher performance than the conventional linear Fresnel reﬂector (LFR) and it has a comparative eﬃciency (a bit lower) than the conventional PTC. They stated that the examined idea has a potential for reduced investment cost compared to the conventional PTC. Tsai [39] suggested a methodology for achieving uniform heat ﬂux over the PTC absorber. He used a free-form geometry parameterization for designing the primary concentrator and the author ﬁnally managed this goal. According to the results, the uniformity is increased close to 97%, while the uniform heat ﬂux distribution enhances the thermal performance of the collector compared to the conventional design with water as the working ﬂuid. The suggested conﬁguration is found to have a high thermal eﬃciency which is about 0.5% lower than the ideal case of an ideally uniform heat ﬂux distribution over the absorber periphery. However, these results are found for an extremely low concentration ratio (close to 2) and so it is debatable if the suggested method can be applied in real systems with concentration ratios around 30. The use of a variable concentration ratio design along the PTC length has been suggested by Wang et al. [40] for enhancing the methanol reforming process. The new suggested conﬁguration is depicted in Fig. 4. This idea aims to achieve the optimum heat ﬂux distribution along the receiver and consequently to create the ideal temperature levels in the different parts of the receiver. Finally, it is found that this idea is able to increase the process eﬃciency up to 16% compared to the conventional PTC case. This idea is a promising choice for applications of methanol reforming but is not clear if it is beneﬁcial for conventional solar thermal systems. Maatallah et al. [41] proposed the design of a PTC with the stationary primary receiver. They selected to use a cylindrical shape primary reﬂector and the receiver tracks the sun. This design is a low-cost design because it does not include complex tracking mechanisms and it is an important advantage of this idea. They optimized the collector and they found that the optimum concentration ratio is

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Fig. 4. (a) Typical PTC (b) Variable concentration ratio PTC [40] (License Number: 4402651446305).

Ma et al. [42] designed a compact PTC with lenses as primary reﬂectors which is illustrated in Fig. 5. This design is similar to a great cylinder with lenses in the upper part and the receiver tube in the center. They found that this design reaches up to 84% optical eﬃciency and it is able to operate at 250 °C with thermal oil (YD320). Moreover, they stated that the receiver has to be located 150 mm over the focal point in order to improve the optical performance in a greater range of incident angles. Another interesting idea is the rotation of the solar ﬁeld in order to follow the sun azimuth angle. Qu et al. [43] studied a PTC solar ﬁeld in a rotating platform, as it is depicted in Fig. 6. The examined conﬁguration was a porotype with a nominal power of 300 kW operating with Dowtherm A. Their results showed that the daily performance of this system can be increased by up to 5% compared to the conventional system with single-axis tracking. Moreover, they found that the daily performance with this system is about 63% in the summer and 40% in the winter, while the mean annual eﬃciency is about 50%. Finally, the authors stated that the rotating platform is a promising idea for reducing the collecting area and creating more cost-effective designs with higher thermal eﬃciency. However, it is important to test this idea with a detailed ﬁnancial analysis for commercial systems of greater nominal power.

3.2. The use of secondary reﬂectors

Fig. 5. The examined compact PTC with lenses. There is a cylindrical absorber and the lenses in the top part rotate in order to track the sun [42] (License Number: 4402660012328).

about 20 with a rim angle of 80°. Moreover, they stated that the optimum orientation of the collector is in the East-West direction. They concluded that their system is able to reach optical performances up to 70–75% in the solar noon, while the optical performance during the day has acceptable values. The suggested design has lower performance than the conventional PTC but it has lower cost due to the elimination of tracking demand. So, this is a promising choice in cases where there is no restriction for high eﬃciency but there is a need for low investment cost.

The use of a secondary reﬂector has been examined by various researchers in the literature and different types of secondary reﬂectors have been studied. The general idea is to capture the solar rays which do not reach the absorber and so to increase the intercept factor and consequently the optical eﬃciency. Furthermore, the utilization of a secondary reﬂector can lead to a more uniform heat ﬂux distribution over the absorber periphery because the solar irradiation is not only concentrated in the low part of the absorber, as in the conventional PTC, but in the entire periphery of the absorber. Moreover, the use of a secondary reﬂector reduces the sensitivity of the system in the tracking errors because the secondary receiver is able to manipulate properly the solar rays which do not reach directly to the absorber after the primary reﬂections. Abdelhamid et al. [44] investigated a two-stage PTC with a secondary compound parabolic shape concentrator in order to increase the total concentration ratio. Fig. 7 depicts the examined conﬁguration which presents a concentration ratio around 60 and it can operate at 365 °C outlet temperature with thermal oil (Duratherm 600), while the thermal eﬃciency was about 37%. It is important to state that the use of two-stage concentration with high concentration ratio increases the need for a precise design, something that increases the installation cost. Wang et al. [45] suggested the use of a secondary parabolic trough concentrator and

Fig. 6. PTC ﬁeld with a rotating system. The PTC modules are mounted on a rotating system in order to track the sun in two directions [43] (License Number: 4402620618811).

E. Bellos and C. Tzivanidis / Progress in Energy and Combustion Science 71 (2019) 81–117

Fig. 7. The use of a compound parabolic concentrator for increasing the total concentration ratio [44] (License Number: 4402640667035).

Fig. 8. PTC with a secondary ﬂat plate reﬂector [46] (License Number: 4402640898504).

they moved the absorber close to the primary concentrator. This technique aids to enhance the uniformity of the heat ﬂux distribution over the absorber and so to reduce the thermal stresses. The ﬁnally found that the maximum temperature of the absorber is 6 K lower than the conventional case with Syltherm 800 as the working ﬂuid but there is a penalty of 4% in the optical eﬃciency. They stated that this technique leads to lower performance but to more reliable systems with lower failure possibilities.

89

Fig. 10. Various secondary reﬂectors inside the evacuated tube a) reﬂective glass surface b) reﬂecting annulus insulation c) aplanatic secondary reﬂector d) tailored seagull secondary reﬂector [48] (License Number: 4402641367833).

Rodriguez-Sanchez and Rosengarten [46] suggested the use of a ﬂat secondary concentrator over the absorber in order to increase the concentration ratio (see Fig. 8). Their idea is to reduce the absorber diameter and to concentrate the solar energy in a smaller absorber area. They managed to increase the concentration ratio up to 80% for collectors with great focal distance and rim angles close to 80°. They stated that this idea is a promising choice for cases with space restrictions like the building rooftops. Furthermore, the use of a trapezoidal cavity receiver has been examined optically by Liang et al. [47]. They found the optical eﬃciency of the total conﬁguration to be around 85% while the optical eﬃciency of the absorber tube to be about 45%. Moreover, they stated that this system has satisfying performance in the cases with increases tracking errors of the primary reﬂector (Fig. 9). Various secondary reﬂectors inside the evacuated tube have been examined by Wirz et al. [48] in order for the optical and the thermal eﬃciencies to be enhanced. Fig. 10 illustrates the examined ideas which are a reﬂective glass, a reﬂecting insulation layer, aplanatic mirrors and tailored seagull secondary reﬂector. They found that the initial design has 65% optical eﬃciency and only the aplanatic mirrors are able to increase the optical eﬃciency at 65.1%. The thermal analysis was performed with synthetic oil as working ﬂuids and for temperature level at 390 °C. In the thermal performance, they found that all the conﬁgurations enhance the

Fig. 9. A PTC with a trapezoidal cavity receiver (a) Total system (b) The examined receiver [47] (License Number: 4402641022466).

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Fig. 11. The use of a PTC for lighting purposes (a) The total conﬁguration (b) The PTC design [49] (License Number: 4402641209765).

collector performance. The benchmark design has 58.8% thermal eﬃciency for a reference case, while the case with the glass reﬂector 59.8%, the case with reﬂective insulation 59.7%, the aplanatic mirrors 60.4% and the tailored seagull 59.6%. It is essential to state that the heat ﬂux distribution over the absorber is the key factor for the thermal enhancement of the examined cases. The use of PTC for lighting purposes is a very interesting idea which has been examined in the literature. Ullah and Shin [49] studied the use of a multi-stage PTC for lighting purposes as Fig. 11 exhibits for a location of 37.5° latitude. They used optical ﬁbers for transferring the reﬂected solar rays inside a building. They found that this technique can lead to adequate indoor lighting. More speciﬁcally, they found that this technique creates higher interior illuminance uniformity than the traditional lighting systems. Lastly, they stated that the examined idea presents a high cost and there is a need for the proper optimization for reducing the investment cost. Li and Yuan [50] examined a similar system with a PTC of 8 m2 for providing lighting and heating for a building of 500 m2 ﬂoor area. The total eﬃciency was found 39.4% which is separated to 16.3% lighting eﬃciency and 23.1% thermal eﬃciency. Moreover, they found that the electricity savings can reach up to 90% and the payback period of this investment is up to 10 years for the location of Los Angeles. However, in other locations, the payback period can be higher with the Seattle to be the less suitable location with a payback period of 25 years for commercial applications.

3.3. Summary of the optical modiﬁcation methods The analysis of Sections 3.1 and 3.2 make obvious the high interest in the optical modiﬁcations of PTC. Various ideas about the primary concentrator and the receiver design have been tested experimentally and numerically. Table 1 includes the studies of this work which are associated with optical modiﬁcation ideas. Generally, these studies try to achieve the following goals: • Increase the concentration ratio in order to achieve operation at higher temperatures [45]. • Create a more uniform heat ﬂux over the absorber in order to reduce the thermal losses [39]. • Reduce the complexity and the cost of the system [47]. • Improve the eﬃciency of a process, like the methanol reforming [40]. • Create systems which can operate without tracking system or not be so sensitive in the tracking errors.

• Use the solar irradiation for lighting purposes in an indoor space [49,50]. The previous goals are reasonable and they have been achieved by the presented studies. This fact indicates that the research on the optical improvements/modiﬁcations of the PTC is important in order to design sustainable and highly eﬃcient systems. It can be said that the suggested designs lead to high optical eﬃciencies which directly lead to increases in useful heat production yield. For instance, the use of cylindrical design with lenses [42] leads to 84% optical eﬃciency and the rotating platform leads to daily thermal performance increase close to 5%. The use of a non-continuous primary reﬂector is able to reduce the mechanical diﬃculties of the system and to achieve an acceptable eﬃciency by up to 66% [38]. Moreover, it is important to state that the use of secondary reﬂectors inside the evacuated tube is able to increase the thermal eﬃciency up to 2.7% due to the increased optical eﬃciency and the more uniform heat ﬂux distribution over the absorber periphery [48]. An alternative idea is the use of PTC for lighting purposes which seems to be an interesting idea for future applications. However, this technology faces limitations due to high attenuation losses in the optical ﬁbers and maybe it is more sustainable for solar dish collectors. It has to be stated that in any case, the increased cost and the possible manufacturing diﬃculties play an important role in the establishment of the optical enhancement ideas. The cost of the system is increased in the cases where there is a need for an extra mechanism or for a precise design. For instance, in the case of a rotating platform [43] the investment cost is higher and also a more intelligent control system is needed. On the other hand, the investment cost is not high in the case that there is a stationary system [41] or in the cases without evacuated tube [47]. However, in the cases with lower cost, there is a penalty in the eﬃciency and so there is a need for deeper ﬁnancial analysis for these systems. 4. Thermal modiﬁcations The thermal enhancement methods aim to increase the thermal performance of the solar collector by reducing the thermal losses or by increasing the heat transfer rates between the absorber and working ﬂuid. More speciﬁcally, the thermal enhancement can be achieved by reducing the thermal loss coeﬃcient (UL ) and by increasing the heat convection coeﬃcient inside the ﬂow (h). Moreover, the use of new working ﬂuids, the nanoﬂuids, is usual in the literature because these ﬂuids present superior optical and thermal properties.

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91

Table 1 Summary of the studies with optical modiﬁcations on the PTC. Study

Description

Working ﬂuid

Main ﬁndings

Zhu et al. [38]

Non-continuous primary reﬂector and a movable receiver (C = 13) Free-form design of the primary reﬂector (C ≈ 2)

Trancal oil

Variable concentration ratio along PTC length (Initial concentration ratio C ≈ 37) Stationary primary receiver (Variable concentration ratio, optimum value C = 20) Cylindrical compact design with lenses (C = 10) Rotating solar ﬁeld platform (C ≈ 27) Use of a compound parabolic secondary concentrator for increasing the concentration (C = 60) Use of a compound parabolic secondary concentrator and move down the absorber (C ≈ 35) Flat secondary reﬂector in order to increase the concentration ratio (C = 20) Trapezoidal cavity design (C = 20) Secondary reﬂectors inside the evacuated tube (C ≈ 27)

Water/methanol

High wind resistance, 66% maximum thermal eﬃciency, higher eﬃciency than LFR, similar eﬃciency with conventional PTC 97% increase in the heat ﬂux uniformity, 0.5% lower thermal eﬃciency compared to the ideal uniform heat ﬂux distribution case 16% eﬃciency increase in the methanol reforming process compared to the case with conventional design

Tsai [39]

Wang et al. [40]

Maatallah et al. [41]

Ma et al. [42] Qu et al. [43] Abdelhamid et al. [44]

Wang et al. [45]

Rodriguez-Sanchez and Rosengarten [46]

Liang et al. [47] Wirz et al. [48]

Ullah and Shin [49] Li and Yuan [50]

Lighting application with PTC (C ≈ 67) Lighting application with PTC (C = 21)

Fig. 12. Insulation material inside the absorber tube [51] (License Number: 4402650505732).

4.1. General design ideas 4.1.1. Thermal loss coeﬃcient reduction The ﬁrst part of the studies aims at the reduction of the thermal loss coeﬃcient. These thermal losses are mainly radiation thermal losses of the absorber to the cover. One interesting idea in the literature is the use of insulation material in the top part of the evacuated tube collector. This insulation material is located between the absorber and the cover, as Fig. 12 shows. The insulation is located in the upper part because this part does not absorb important amounts of solar irradiation. So, the penalty in the optical

Water

–

Lower cost and lower performance than the conventional PTC, maximum optical eﬃciency up to 75%

YD3O (thermal oil)

84% optical eﬃciency, operation at medium temperature levels (∼250 °C) 5% daily thermal eﬃciency increase compared to the single axis system, 50% mean yearly eﬃciency Thermal eﬃciency of 37% for operation at 365 °C

Dowtherm A Duratherm 600

Syltherm 800

6 K lower peripheral temperature deviation in the absorber, increase in the uniformity of the heat ﬂux distribution and lower failure possibility

–

Increase in the concentration ratio up to 80%, ideal for application with space restrictions (e.g. rooftops)

–

Satisfying performance with tracking errors

Synthetic oil

Thermal eﬃciency enhancement up to 2.7% compared to the initial design with 65% optical eﬃciency and 58.8% thermal eﬃciency (at 390 °C) Higher interior illuminance uniformity than the traditional systems 90% electricity savings, 10 years payback period for Los Angeles and 25 years for Seattle

– –

eﬃciency is small, while there is a high potential for reduction in the thermal losses. Al-Ansary and Zeitoun [51] examined this idea in a PTC with air in the tube cavity (non-evacuated tube). They found that the use of insulation in a PTC with non-vacuum is able to enhance the performance and to lead to more cost-effective systems, ideal for low and medium temperatures. Moreover, Chandra et al. [52] found that the use of insulation is able to decrease the thermal loss coeﬃcient of a non-evacuated tube PTC up to 20% for operation with Therminol VP-1. Moreover, the use of insulation has been highlighted as an effective way of reducing the cost of the receivers up to 20% and so it seems to be a cost-effective technique for application in PTC. Osorio et al. [53] studied a PTC with a transparent insulation material between the absorber and the cover, as Fig. 13 exhibits. They found that this conﬁguration leads to higher thermal eﬃciency for temperature levels over 300 °C. After this temperature level, the radiation thermal losses of the conventional PTC are higher than the insulation conduction thermal losses in the alternative design and thus this collector is recommended for operation at high temperatures. To conclude about the use of insulation material, it can be said that this idea can compete with the non-evacuated tubes and it is a choice for designing low-cost receivers with a relatively acceptable eﬃciency. The transparent insulation is a promising choice but more research is needed for this technology. The next part of the literature studies is devoted to modiﬁcation about the reduction of the radiation thermal losses. Wang et al. [54] examined the idea of using a radiation shield in the upper part of the absorber tube. Fig. 14 shows this conﬁguration which has a metallic part between the absorber and the cover. This part aims to reduce the thermal losses of the PTC. According to the results, it is found that this idea is able to reduce the thermal losses

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Fig. 13. The PTC with the transparent insulation material (TIM) [53] (License Number: 4402650639224).

Fig. 15. Double coating PTC [55] (License Number: 4402651053523).

Fig. 14. The PTC with a radiation shield [54] (License Number: 4402650961387).

by about 24% for both selecting and non-selective absorber coatings. Yang et al. [55] proposed a double coating PTC which uses different coatings in the upper and in the down part of the absorber. They found that this idea is able to reduce the thermal losses close to 31%. More speciﬁcally, they stated that for operation at 500 °C with molten salt, the conventional PTC has 64.7% thermal eﬃciency, while the double coating has 68.1%. Furthermore, they calculated that the levelized cost of electricity with the suggested idea can be lower than the conventional PTC with a reduction of 5%. This is an important result which proves that the trade-offs between the higher energy production and the higher investments costs are positive and lead to a viable conﬁguration (Fig. 15). 4.1.2. Alternative designs of absorbers A lot of research has been focused on alternative designs of PTC absorbers because the conventional evacuated tubes have high cost and also they present durability problems in high-temperature applications. The use of a ﬂat absorber with an asymmetrical reﬂector for steam generation was examined by Bortolato et al. [56]. Fig. 16 illustrates this conﬁguration which presents 82% optical efﬁciency and 64% thermal eﬃciency for temperature levels close to 100–120 °C. The authors stated that the examined design is a ﬂexible conﬁguration which gives the possibilities for an overall costeffective design. Halimi et al. [57] studied a PTC with a U-tube absorber, as it is given in Fig. 17. They examined different locations of the internal U-tubes and they found the parallel conﬁguration of

Fig. 16. Flat receiver with an asymmetrical reﬂector [56] (License Number: 4402651125736).

Fig. 17. PTC with a U-tube absorber [57] (License Number: 4402651193617).

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Fig. 19. PTC with a V-cavity receiver [59] (License Number: 4402660122330). Fig. 18. Cavity receiver with insulation material and glass cover [58] (License Number: 4402651305056).

Fig. 17 to be the best design with a thermal eﬃciency of 40%. Liang et al. [58] examined a cavity receiver with ﬂat cover glass and insulation, as it is depicted in Fig. 18. The authors claimed that this design is better the solar ﬁelds with many modules in series. They found that the examined prototype has 48% thermal eﬃciency at 80 °C and 34% thermal eﬃciency at 160 °C with synthetic oil. Chen et al. [59] studied a PTC with a V-cavity receiver as it is depicted in Fig. 19. They studied the conﬁguration with and without additional ﬁns inside the ﬂow. They concluded that this collector is a low-cost design which is able to operate over 300 °C with thermal oil. In low-temperature levels, this system has about 55% thermal eﬃciency operating with water, while at higher temperature the eﬃciency signiﬁcantly reduces. Moreover, they found that the use of internal ﬁns leads to higher heat transfer rates.

The previous ideas seem to be interesting but they need more investigation in order to compete with the conventional PTC. The most eﬃcient choices are the use of a ﬂat absorber with an asymmetrical reﬂector [56] which leads to 64% thermal eﬃciency at 120 °C, as well as the V-cavity receiver [59] which operates at 300 °C with 55% thermal eﬃciency. Other researchers have suggested signiﬁcantly novel designs which are not close to the conventional PTC image. Wang et al. [60] studied a PTC with a thermosiphon receiver, as it is given in Fig. 20. This design has a horizontal evaporator, a condenser, a riser, and a downcomer with a U-turn. They suggested that this system is a good choice for the temperature ranges of 20 0–40 0 °C operating with Dowtherm A and it has not the conventional diﬃculties of other natural phase circulation systems, due to its unique design. Good et al. [61] examined a PTC with air as working ﬂuid

Fig. 20. A PTC with a thermosiphon system [60] (License Number: 4402660769701).

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Fig. 21. Alternative PTC design operating with air as working ﬂuid [61] (License Number: 4402660844497).

Fig. 22. Light-weight PTC structure (a) back side of the PTC (b) front side of the PTC [63] (License Number: 4402660932423).

with an alternative design. This PTC has a dome and inside it, there is the receiver, as Fig. 21 depicts. They stated that this system is able to operate up to 500 °C. Moreover, in another work about this collector by Bader et al. [62], it is found that this collector has thermal eﬃciency up to 65% for temperature levels at 125 °C and up to 42% for temperature levels at 500 °C with air as working ﬂuid. 4.1.3. Other ideas about PTC It is essential to include some other ideas which are associated with the PTC. Forman et al. [63] designed a novel PTC with an optimized structure. This PTC has independent parts merged to light-weight and solid system, as it is given in Fig. 22. This system can be a low-cost and compact design for the future PTCs. Anwar et al. [64] examined a receiver of a PTC with phase change materials (LiNO3 ) in order to store thermal energy. This conﬁguration is depicted in Fig. 23 and it is like a thermal battery. The receiver rotates in order to have uniform heat ﬂux in its periphery. Moreover, Nation et al. [65] studied a thermal battery with NaS inside the absorber tube. In this collector, the heat transfer ﬂuid (thermal oil) ﬂows in the annulus between the storage material and the absorber tube, as Fig. 24 shows. They found that this collector has a thermal eﬃciency between 61% and 75%.

4.1.4. Summary of the general design ideas It is found that there are numerous ideas with thermal modiﬁcations in the receiver. These modiﬁcations try to reduce the thermal losses and also to create ﬂexible and cost-effective designs. Table 2 includes these studies with a brief description and their main ﬁndings. Generally, it has been found that the thermal modiﬁcations reduce the thermal losses close to 20–30% [54,55]. The use of a double coating [55], as well as the use of a radiation shield [54], seems to be the most effective ways for enhancing the performance. About the utilization of insulation material between the cover and the absorber, it has been found that it is a promising choice which can compete with the non-evacuated tubes in terms of eﬃciency [51–53]. The use of cavity receiver is able to give adequate useful heat production at medium temperatures [56,58,59], while the use of alternative designs with air can lead to eﬃcient operation at 500 °C [61,62]. The use of a ﬂat absorber with an asymmetrical reﬂector [56] and the use of a V-cavity receiver [59] are the most eﬃcient choices for medium and high-temperature levels respectively. Moreover, it has been found that the use of thermal storage inside the PTC is a promising future choice [64,65]. Lastly, the use of alternative designs is able to manufacture systems with lower

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Fig. 23. Thermal battery – storage with phase change materials (a) the PTC (b) the rotating receiver [64] (License Number: 4402661047869).

Fig. 24. Thermal battery design inside a conventional PTC [65] (License Number: 4402661139182).

weight and lower cost because of the less material utilization [63]. However, it has to be stated that the commercialization of the alternative ideas needs many steps in order to achieve low manufacturing costs and to have the proper reliability levels.

4.2. Flow modiﬁcations The use of ﬂow modiﬁcations aims to increase the heat transfer rates inside the ﬂow. The higher heat rates (or heat transfer

coeﬃcients) lead to lower receiver temperature and consequently to lower thermal losses. So, the thermal eﬃciency is enhanced. The use of modiﬁed absorbers creates passive vortexes inside the ﬂow and so there is higher effective thermal conductivity. Moreover, there is better mixing in the ﬂow and the heat input of the solar energy is delivered in the entire ﬂuid domain with a better way. The previous mechanism describes the reasons for the heat transfer coeﬃcients enhancement. However, the thermal enhancement techniques usually are associated with problems such pressure drop increase, higher investment cost and need for creating

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E. Bellos and C. Tzivanidis / Progress in Energy and Combustion Science 71 (2019) 81–117 Table 2 Summary of the studies with thermal modiﬁcations on the PTC. Study

Description

Working ﬂuid

Main ﬁndings

Al-Ansary and Zeitoun [51]

Air

Cost effective systems, ideal for low and medium temperatures 20% thermal losses reduction compared to the non-evacuated absorber Higher thermal eﬃciency for temperature levels over 300 °C 24% thermal losses reduction

Chen et al. [59]

Insulation material between absorber and cover in the upper part Insulation material between absorber and cover in the upper part Transparent insulation material between absorber and cover Radiation shield in the upper part of the absorber tube An absorber with double coating Flat absorber with an asymmetrical reﬂector U-tube absorber Cavity receiver with ﬂat cover glass and insulation V-cavity receiver

Wang et al. [60] Good et al. [61] Bader et al. [62]

Thermosiphon receiver Alternative PTC with air working ﬂuid Alternative PTC with air working ﬂuid

Dowtherm A Air Air

Forman et al. [63]

PTC with independent parts merged to light-weight and solid system Rotating thermal battery Thermal battery inside the PTC with NaS

–

Chandra et al. [52] Osorio et al. [53] Wang et al. [54] Yang et al. [55] Bortolato et al. [56] Halimi et al. [57] Liang et al. [58]

Anwar et al. [64] Nation et al. [65]

Fig. 25. Twisted tape inserts design inside the absorber tube (License Number: 4403060850043).

reliable designs. So, the proper criteria have to be applied for evaluating the overall heat transfer enhancement and the overall performance improvement in any case. 4.2.1. Flow inserts The ﬁrst category of studies regards the use of ﬂow inserts inside the ﬂow. There are different ideas about the ﬂow inserts. The most usual is the twisted tape insert which is located centrally in the absorber tube and aims to mix the ﬂow along the tube. Mwesigye et al. [66] studied the use of a wall-detached twisted tape insert in a PTC operating with the thermal oil Syltherm 800, as it is depicted in Fig. 25. They found a 58% decrease in the entropy generation, 168% higher heat transfer coeﬃcient and 68% lower circumferential temperature difference in the absorber tube. Moreover, they stated that the thermal eﬃciency of the PTC can increase up to 10%, while there is an important increase in the friction factor up to 14 times. Zhu et al. [67] examined a wavy-tape insert in a PTC with a sinusoid shape, as it is given in Fig. 26, for operation with Syltherm 800. They found that this idea reduces the thermal losses by 33%, while there is an increase in the Nusselt number 300% and on the friction factor 400%. Sahin et al. [68] examined

Therminol VP-1 – –

31% thermal losses reduction 82% optical eﬃciency and 64% thermal eﬃciency at 120 °C 40% thermal eﬃciency 48% thermal eﬃciency at 80 °C and 34% thermal eﬃciency at 160 °C Maximum thermal eﬃciency ∼55%, operation over 300 °C Operation in the range 20 0–40 0 °C Operation at 500 °C Thermal eﬃciency up to 65% at 125 °C and up to 42% at 500 °C Light-weight and low-cost design

Molten salt Water/steam – Therminol 55 Sklan-460 (mineral oil)

Phase change material (LiNO3 ) Syltherm 800

Fig. 26. Wavy insert 4402670110413).

Important energy storage Thermal eﬃciency between 61% and 75%

inside

the

absorber

tube

[67]

(License

Number:

the use of a wire coil insert (see Fig. 27) inside and absorber tube and they found 200% increase in the Nusselt number and many times higher pressure drop compared to the empty tube for operation with water. Song et al. [69] studied a screw tape insert inside the absorber tube which is depicted in Fig. 28. They used Syltherm 800 and they found that the thermal losses can signiﬁcantly be reduced by up to 67%, while the pressure drop increases about 5 times compared to the smooth case. Chang et al. [70] examined the use of an eccentric rod insert in the absorber operating with molten salt, as it is depicted in Fig. 29. They found the optimum location to be in the upper absorber part with a small eccentricity. The Nusselt number can be increased up to 7 times, while the friction factor can be increased up to 14 times with this technique. Bellos and Tzivanidis [71] suggested the use of a star ﬂow insert in a PTC, as it is depicted in Fig. 30. They found that the thermal eﬃciency for operation with Syltherm 800 can be enhanced by 1%, while the thermal losses are decreased by up to 15%. The heat transfer enhancement is found to be about 50–70%, while there are important increases in the pressure drop. However, the pumping work demand was extremely low in all the cases. Also, they stated that the examined idea reduces the thermal stresses in the absorber and this fact is able to reduce the failure rate in the ab-

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Fig. 27. A wire-coil insert for the PTC absorber [68] (License Number: 4402670191939).

Fig. 28. A screw-tape insert for the absorber tube [69] (License Number: 4402670274638).

Fig. 29. Different locations of an eccentric rod insert inside the absorber tube [70] (License Number: 4402670396913).

sorbers. So, the overall ﬁnancial performance of the system can be enhanced because of the lower replacement rate of the absorbers. Jamal-Abad et al. [72] examined the use of metal foam inside the absorber tube operating with water, as it is given in Fig. 31. They found thermal eﬃciency enhancement up to 3%, 8 times greater Nusselt number and extremely high-pressure drop increase (over

100 times). Mwesigye et al. [73] studied the perforated plate inserts of Fig. 32 and they found an overall eﬃciency enhancement between 1.2% and 8% for operation with Syltherm 800. Ghasemi and Ranjbar [74] examined the use of porous rings (see Fig. 33) inside the absorber which operates with Syltherm 800 and they found 50% Nusselt number increase and also they stated there is

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Fig. 33. Porous 4402670992740).

discs

inside

the

absorber

tube

[74]

(License

Number:

Fig. 30. A star ﬂow insert for the absorber tube [71] (License Number: 4402670539771).

Fig. 34. Internal multi-ﬁn array for parabolic solar air heater [75] (License Number: 4402671081740).

Fig. 31. Metal foam insert for the PTC absorber [72] (License Number: 4402670615611).

pressure drop increase with this method. Furthermore, Nems and Kasperski [75] examined the use of a solar air-heater with an internal multiple-ﬁn array, as it is given in Fig. 34. They found that this collector has a maximum eﬃciency of about 40% and thermal loss coeﬃcient close to 1.6 W/m2 K. The previous studies, with a brief description and their main ﬁndings, are summarized in Table 3. It is obvious that the use of ﬂow inserts enhances the Nusselt number but the thermal eﬃ-

Fig. 32. Perforated plate inserts inside the absorber tube [73] (License Number: 4402670723222).

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Table 3 Summary of the studies with ﬂow inserts. Study

Description

Working ﬂuid

Main ﬁndings

Mwesigye et al. [66] Zhu et al. [67] Sahin et al. [68] Song et al. [69] Chang et al. [70] Bellos and Tzivanidis [71] Jamal-Abad et al. [72] Mwesigye et al. [73] Ghasemi and Ranjbar [74] Nems´ and Kasperski [75]

Twisted tape insert Wavy-tape insert Wire coil insert Screw-tape insert Eccentric rod insert Star ﬂow insert Metal foam Perforated plate inserts Porous rings internal multiple-ﬁn array

Syltherm 800 Syltherm 800 Water Syltherm 800 Molten salt Syltherm 800 Water Syltherm 800 Syltherm 800 Air

Up to 10% thermal eﬃciency enhancement 300% Nusselt number increase 200% Nusselt number increase 67% thermal loss reduction 600% Nusselt number increase 1% thermal eﬃciency enhancement 3% thermal eﬃciency enhancement Up to 8% thermal eﬃciency enhancement 50% Nusselt number increase Maximum eﬃciency of 40% and thermal loss coeﬃcient close to 1.6 W/m2 K

Fig. 35. Longitudinal vortex generators in the down part of the absorber tube [76] (License Number: 4402671239305).

ciency is not so enhanced. This fact is explained by the relatively low thermal losses of the PTC due to the evacuated tube collector. Moreover, the use of ﬂow inserts leads to a great increase in the pressure drop, a fact that has to be taken into consideration in the overall evaluation. The pressure drop can be many times increased, compared to the reference case, but it has been found that the overall performance is enhanced [73] and the pumping work demand value is generally low [71]. Among the previously examined cases, the use of twisted tape inserts [66] and of perforated plate inserts [73] are the cases which lead to the highest thermal eﬃciency enhancements. 4.2.2. Modiﬁed inner absorber geometry Another important part of the literature is devoted to modifying the inner absorber geometry in order to create more turbulent ﬂow conditions. These ideas are applied to the entire absorber surface or in the down part because, in this area, there is high solar heat ﬂux concentration. Cheng et al. [76] studied the use of longitudinal vortex generators in the down part of the absorber, as Fig. 35 shows. They found 13% thermal loss decrease and a 100% increase in the pressure drop for operation with Syltherm 800. Xiangtao et al. [77] examined the use of ﬁn pin arrays in the down part of the absorber operating with D12 thermal oil, as Fig. 36 illustrates. According to their results, there is a small Nusselt number and friction factor increase. Bellos et al. [78] studied the use of internal longitudinal ﬁns in all the periphery of the absorber for operation with Syltherm 800. Fig. 37 exhibits the eight exam-

Fig. 36. Absorber tube with pin ﬁn arrays (a) Longitudinal side (b) Cross-section [77] (License Number: 4402671358754).

ined internal ﬁns which are symmetrically located in the absorber tube. They ﬁnally found that this technique is able to enhance the thermal eﬃciency by up to 1.5%, while there is a penalty in the pressure drop. Moreover, Bellos et al. [79] optimized the number and the location of the internal longitudinal ﬁns inside the absorber tube. This work has performed for a constant ﬂow rate of 150 L/min with Syltherm 800 as the working ﬂuid. They found that the optimum design has three ﬁns in the down part and in this case, there is 0.51% thermal eﬃciency enhancement. The use of helical ﬁns inside the absorber (see Fig. 38) has been examined by Munoz and Abanades [80] and it is found to enhance the thermal eﬃciency of about 3% with Syltherm 800 as the working ﬂuid. They also stated that the examined idea has positive ﬁnancial potential. More speciﬁcally, they stated that the use of internal ﬁns is estimated to increase to collector initial cost at 5% and this increase will lead to an overall power plant cost increase of 0.5%. The power plant power production increase will be about 2% and so the overall sense is that the use of the helical internal ﬁns leads to a viable investment. Fuqiang et al. [81] examined the use of an asymmetric outward convex corrugated tube which is given in Fig. 39. They used the

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Fig. 37. Internal longitudinal ﬁns along the absorber tube [78] (License Number: 4402671497882).

in the Nusselt number and in the friction factor for operation with Therminol VP1. Bellos et al. [83] studied a converging-diverging absorber tube (see Fig. 41) which increases the thermal eﬃciency of about 4.5% with thermal oil as the working ﬂuid. In another recent work, Bitam et al. [84] investigated a sinusoidal absorber tube of Fig. 42 which presents 3% thermal eﬃciency enhancement and a 50% increase in pressure drop for operation with Syltherm 800. The previous studies show that the use of internal modiﬁcations leads to performance enhancement with a relatively low penalty in the pressure drop. Thus, it can be said that the limitation of the pressure drop increase is not as intense as in the studies about ﬂow inserts. Table 4 summarizes the previous studies with their main ﬁndings. It remarkable to state that the highest thermal eﬃciency enhancements are found in the cases of helical ﬁns [80] and converging-diverging absorber geometry [83]. Fig. 38. Design with helical internal ﬁns inside the absorber tube [80] (License Number: 4402680117785).

thermal oil D12 and it was found an increase of 150% in the Nusselt number. Huang et al. [82] studied a dimpled absorber tube which is depicted in Fig. 40 and they found up to 20% increase

4.3. Nanoﬂuid-based solar collectors The use of nanoﬂuids in PTC has been examined by many researchers. Nanoﬂuids are special working ﬂuids with improved thermal and optical properties. The term nanoﬂuid has been deﬁned by Choi and Eastman in 1995 [85]. The nanoﬂuids are created by dispersing some nanoparticles in a base ﬂuid. The most usual

Fig. 39. Asymmetric outward convex corrugated absorber tube [81] (License Number: 4402680275826).

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Table 4 Summary of the studies with modiﬁed inner absorber geometry. Study

Description

Working ﬂuid

Main ﬁndings

Cheng et al. [76] Xiangtao et al. [77] Bellos et al. [78] Bellos et al. [79] Munoz and Abanades [80] Fuqiang et al. [81] Huang et al. [82] Bellos et al. [83] Bitam et al. [84]

Longitudinal vortex generators Pin ﬁn arrays Eight longitudinal ﬁns Various longitudinal ﬁns Helical ﬁns Asymmetric outward convex corrugated absorber Dimpled tube Converging-diverging absorber Sinusoidal absorber tube

Syltherm 800 Thermal oil D12 Syltherm 800 Syltherm 800 Syltherm 800 Thermal oil D12 Therminol VP-1 Thermal oil Syltherm 800

13% thermal loss decrease Small enhancements in the Nusselt number Up to 1.5% thermal eﬃciency Optimum design with three ﬁns in the down part 3% Thermal eﬃciency enhancement 150% increase in the Nusselt number 30% increase in the Nusselt number 4.5% thermal eﬃciency enhancement 3% thermal eﬃciency enhancement

Fig. 40. Dimpled absorber tube [82] (License Number: 440268040 0 071).

nanoparticles are the following [86,87]: Cu, CuO, Al, Al2 O3 , TiO2 , SiO2 , ZnO, Au, SiC, CeO2 , MWCNT, SWCNT, CNT, etc. The base ﬂuids are usually thermal oils or water, while the use of molten salts is more restricted. Fig. 43 shows the nanoﬂuid samples (oil/MWCNT) for the solar concentrating application. The nanoﬂuids present increased thermal conductivity which leads to greater heat transfer rates inside the ﬂow. Moreover, the viscosity is increased; the fact that leads to higher pressure losses and this is a limitation of nanoﬂuids. The density of nanoﬂuid is higher than in the base ﬂuid, while the speciﬁc heat capacity is lower in nanoﬂuids. But, the product of the density-speciﬁc heat capacity (or the volumetric speciﬁc heat capacity) of the nanoﬂuid is close to the respective of the base ﬂuid, so this pa-

Fig. 41. (a) Smooth absorber tube (b) Converging-diverging absorber tube [83] (License Number: 4402680519005).

rameter does not play a so signiﬁcant role on the collector performance, except some seldom cases. The basic mathematical modeling for the calculation of the nanoﬂuid thermal properties is given in Appendix A. Furthermore, the nanoﬂuids are ideal working ﬂuids in transparent collectors because they absorb high amounts of solar irradiation. The nanoﬂuids can be used for optical ﬁltering of the sunlight and so they can be applied in novel designs. However, the use of nanoﬂuids is conjugated with many limitations about their utilization. First of all, the nanoﬂuids have high cost and they increase the initial investment cost of the system. Moreover, the operation cost is increased due to the higher pump-

Fig. 42. Sinusoidal absorber tube [84] (License Number: 4402680638025).

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Fig. 43. Nanoﬂuid samples (oil/MWCNT) [88] (License Number: 4402680735977).

ing work demand, a result of the increased viscosity. Furthermore, there are stability problems with nanoﬂuids because of the agglomeration. These stability problems are usually faced with the use of proper surfactants. The mechanical and the chemical erosion, as well as the toxicity of nanoﬂuids, are other important issues. All the previous factors make the use of nanoﬂuid to be more diﬃcult at this time and they indicate the need for further studies and research in this area. In any case, the potential for the higher energy harvesting with nanoﬂuids is clear in the literature and thus they are a promising idea.

4.3.1. Conventional PTC with nanoﬂuids The use of Al2 O3 nanoparticle is the most usual choice in nanoﬂuid-based PTC. Subramani et al. [89] found experimentally that the use of water/Al2 O3 leads to 8.5% thermal eﬃciency enhancement. Mwesigye et al. [90] found that the Syltherm 800/Al2 O3 nanoﬂuid leads to 7.6% thermal eﬃciency enhancement with a 40% penalty in pressure drop. Moreover, Bellos et al. [83] found that the use of oil/Al2 O3 nanoﬂuid is able to enhance the PTC thermal performance close to 4.3%. In another study, Wang et al. [91] proved 1.2% thermal eﬃciency enhancement with Syltherm 800/Al2 O3 nanoﬂuid. Moreover, there are studies which compare the Al2 O3 nanoﬂuid with other nanoﬂuids. Bellos et al. [87] found that the Syltherm 800/CuO nanoﬂuids lead to 1.26% thermal eﬃciency enhancement, while the Syltherm 800/Al2 O3 to 1.13%. Ghasemi and Ranjbar [92] found that the water/CuO nanoﬂuid has a bit higher heat transfer coeﬃcient enhancement (∼35%) compared to the water/Al2 O3 nanoﬂuid (∼28%). They stated that the pressure drop increase with both nanoﬂuids is about 40%. On the other hand, Rehan et al. [93] found that the water/Al2 O3 leads to higher thermal eﬃciency enhancement (13%) compared to the water/Fe2 O3 (11%). In another comparative study, Allouchi et al. [94] found that the use of Synthetic oil-based nanoﬂuids leads to performance enhancements 1.06%, 1.14%, and 1.17% with CuO, TiO2 and Al2 O3 , respectively.

In a recent work, the SWCNT dispersed in Therminol VP-1 has been examined by Mwesigye and Meyer [95]. They used a CFD model and they found 4.4% thermal eﬃciency enhancement and also the nanoﬂuid utilization leads to 70% lower entropy generation. Furthermore, it is essential to state that Kasaeian et al. [96] found that the oil/MWCNT leads to 0.5% thermal eﬃciency enhancement using a numerical model. Moreover, they conducted a ﬁnancial analysis and they determined the payback period of the nanoﬂuid utilization to be 3.8 years for inlet temperature at 100 °C and 4.7 years for inlet temperature at 30 °C. In another work, Kasaeian et al. [97] found in an experimental study about 5% thermal eﬃciency enhancement with the use of oil/MWCNT. In another interesting study, Kolb et al. [18] proved that the nanoﬂuidbased thermal oils lead to higher thermal eﬃciency enhancement than the molten salt-based nanoﬂuids, compared to the respective pure base ﬂuids in every case. An alternative idea is the use of hybrid nanoﬂuids in a PTC. Bellos and Tzivanidis [98] found that the use of Syltherm 800/(Al2 O3 - TiO2 ) hybrid nanoﬂuid leads to 1.8% thermal eﬃciency enhancement while the respective mono nanoﬂuids lead to only ∼0.7% thermal eﬃciency enhancement. Additionally, Minea and ElMaghlany [99] studied the water/(Cu - MgO) hybrid nanoﬂuid and they found 6% thermal eﬃciency enhancement, 20% increase in the heat transfer coeﬃcient and approximately 20% increase in the pressure drop. The previous studies show that the nanoﬂuids are able to enhance the thermal eﬃciency of the solar collector. The penalty in the pressure drop is generally reasonable and this fact proves the minor importance of this parameter on the overall system performance. Table 5 summarizes the previous studies with the proper details. It is useful to state that the highest thermal eﬃciency enhancements have been found with water-based nanoﬂuids and especially water/Al2 O3 and water/Fe2 O3 [89,93]. In some comparative studies, the use of CuO [87,92] is the best case. Moreover, the use of oil/MWCNT [97] seems to be another promising choice. 4.3.2. Direct absorption PTC with nanoﬂuids The last years, there is an increasing number of literature studies about the use of direct absorption parabolic trough collectors (DAPTC). This collector is similar to a conventional PTC but it has a double glass receiver. Inside the receiver, there is a base ﬂuid (for instance water) or usually a nanoﬂuid which directly captures the incident solar irradiation. It has been found that the nanoﬂuids are ideal working ﬂuids for absorbing the solar irradiation [100]. A typical image of the DAPTC is given in Fig. 44. It is obvious that the solar irradiation reaches in the working ﬂuid inside the double transparent tube. Kasaeian et al. [102] examined a direct absorption PTC with ethylene glycol/MWCNT nanoﬂuid for volumetric concentrations up to 0.3%. They stated that the optical eﬃciency of the solar collector reaches up to 71.4% for the case with 0.3% MWNCNT. In this case, the solar collector has a 17% higher thermal performance compared to the pure base ﬂuid case. Menbari et al. [103] studied the use of water/CuO nanoﬂuid for volumetric concentrations of 0.002% up to 0.008% and they found thermal eﬃciency enhancements from 18% to 52% respectively. Furthermore, O’Keeffe et al. [101] proved that the concentration of about 2% is adequate for maximizing the DAPTC thermal performance. Fan et al. [104] suggested a double pass direct absorption PTC with Syltherm 800/Al nanoﬂuid which is depicted in Fig. 45. This conﬁguration has the absorption ﬂuid in the region between the cover and the absorber tube, while there is also heat transfer ﬂuid inside the absorber. They stated that their system has relatively high eﬃciency and it can substitute the selective PTC with a sacriﬁce in the eﬃciency up to 4%. Dugaria et al. [105] investigated a ﬂat volumetric receiver for direct absorption in a PTC, as it is depicted in Fig. 46. They found that this design has a

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Table 5 Summary of the studies with conventional nanoﬂuid-based PTC. Study

Nanoﬂuid

Main ﬁndings

Subramani et al. [89] Mwesigye et al. [90] Bellos et al. [83] Wang et al. [91] Bellos et al. [87]

Mwesigye and Meyer [95] Kasaeian et al. [96] Kasaeian et al. [97] Bellos et al. [10] Bellos and Tzivanidis [98]

Water/Al2 O3 Syltherm 800/Al2 O3 Oil/Al2 O3 Syltherm 800/Al2 O3 Syltherm 800/CuO Syltherm 800/Al2 O3 Water/CuO Water/Al2 O3 Water/Al2 O3 Water/Fe2 O3 Oil/CuO Oil/TiO2 Oil/Al2 O3 Therminol VP-1/SWCNT Oil/MWCNT Oil/MWCNT Oil/CuO, Molten salt/CuO Syltherm 800/(Al2 O3 - TiO2 )

Minea and El-Maghlany [99]

Water/(Cu - MgO)

8.5% thermal eﬃciency enhancement 7.6% thermal eﬃciency enhancement 4.3% thermal eﬃciency enhancement 1.2% thermal eﬃciency enhancement 1.26% thermal eﬃciency enhancement with CuO 1.13% thermal eﬃciency enhancement with Al2 O3 35% heat transfer enhancement with CuO 28% heat transfer enhancement with Al2 O3 13% thermal eﬃciency enhancement with Al2 O3 11% thermal eﬃciency enhancement with Fe2 O3 1.06% thermal eﬃciency enhancement with CuO 1.14% thermal eﬃciency enhancement with TiO2 1.17% thermal eﬃciency enhancement with Al2 O3 4.4% thermal eﬃciency enhancement 0.5% thermal eﬃciency enhancement About 5% thermal eﬃciency enhancement Higher enhancements in the thermal oil 1.5% thermal eﬃciency enhancement, higher enhancements with the hybrid nanoﬂuid 20% increase in the heat transfer coeﬃcient

Ghasemi and Ranjbar [92] Rehan et al. [93] Allouchi et al. [94]

[107] found that the use of gas phase nanoﬂuid with air and CuO is able to perform 65% thermally for temperature levels of 150 °C. They stated that this design suffers from huge nano-powder deposition within the receiver pipe, due to humidity, as it is depicted in Fig. 47.

Fig. 44. Typical idea of a direct absorption PTC [101] (License Number: 4402680885090).

maximum optical eﬃciency close to 90% with water/SWCNT, while the conventional pure water system gives optical eﬃciency close to 82%. This is a promising result which indicates important enhancement margin with the nanoﬂuid-based direct absorption systems. Furthermore, the use of gas phase nanoﬂuids in DAPTC has been also studied. De Risi et al. [106] studied a water/steam nanoﬂuid with a mixture of CuO and Ni. They found that the optimum design leads to 62.5% thermal eﬃciency. Potenza et al.

4.3.3. Applications with nanoﬂuid-based PTC The use of nanoﬂuid-based PTC in applications has been examined by various researchers in the last years. The literature studies are summarized in Table 6 with the basic details. The applications are generally for electricity and refrigeration/cooling production. The examined studies are theoretical works and there are no experimental works about nanoﬂuid-based solar systems coupled with applications. The general sense of the Table 6 data indicates an important enhancement in the system performance with the use of nanoﬂuid-based solar systems. Firstly, the studies about refrigeration and cooling are given. Abu-Hamdeh and Almitani [108] studied a regenerated liquid desiccant cooling system driven by nanoﬂuid-based PTC (see Fig. 48). The best nanoﬂuid was found to be the water/ZnO which leads to thermal eﬃciency enhancement up to 50%. Ratlamwala and Abid [109] studied a multi-effect absorption cooling system driven by nanoﬂuid-based PTC. According to their results, the water/Al2 O3 nanoﬂuid is able to enhance the heat transfer rates up to 56%. In electricity production applications, there is a greater variety of studies. Toghyani et al. [110] studied a Rankine cycle driven by

Fig. 45. (a) Conventional design (b) Direct absorption PTC with outer and inner heat transfer ﬂuids [104] (License Number: 4402681076984).

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Fig. 46. Flat volumetric receiver for direct absorption [105] (License Number: 4402681190521).

Fig. 49 illustrates the examined conﬁguration. According to their results, the use of Syltherm 800/Cu leads to maximum thermal efﬁciency enhancement which is close to 1.8%. The last part of the literature studies regards the multigeneration systems. Abid et al. [114] examined a multi-generation system with a Rankine cycle for electricity production and an electrolyzer for hydrogen production (see Fig. 50). They applied the water/Al2 O3 and water/Fe2 O3 nanoﬂuids in PTC. According to their results, the system is able to produce about 8.17 kW electricity and 0.02454 g/s hydrogen. Bellos and Tzivanidis [115] studied a solar driven trigeneration system with nanoﬂuid-based PTC. The examined conﬁguration includes an Organic Rankine Cycle (ORC) and an absorption heat pump operating with LiBr-H2 O. In this work, Syltherm 800/CuO and Syltherm 800/Al2 O3 nanoﬂuids are tested, while the authors studied different organic working ﬂuids in the thermodynamic cycle. The ﬁnal results proved that the best case is the one with toluene in the ORC and water/CuO nanoﬂuid in the solar ﬁeld. The optimum nanoparticle concentration was found to be 4.35% which leads to 1.8% overall system eﬃciency enhancement. Fig. 47. The problem of nano-powder deposition in the receiver for the gas-phase nanoﬂuid [107] (License Number: 4402681310076).

nanoﬂuid-based PTC. They investigated various oil-based nanoﬂuids. The maximum exergy eﬃciency enhancement of the system was about 11% with the oil/Al2 O3 nanoﬂuid. Alashkar and Gadalla [111] examined the use of nanoﬂuid-based PTC for feeding a Rankine power cycle. According to their results, the maximum thermal eﬃciency enhancement was up to 12%. Moreover, they stated that the use of nanoﬂuids is able to decrease the levelized cost of electricity from 0.0411 €/kWh with pure Therminol to 0.0407 €/kWh with Therminol/Ag nanoﬂuid (4% volumetric concentration). Abid et al. [112] also studied a Rankine power cycle with nanoﬂuidbased PTC. They found the exergy eﬃciency of the system to be around 23%. The use of nanoﬂuid-based PTC coupled to an Organic Rankine cycle has been studied by Bellos and Tzivanidis [113].

4.4. Comparative studies about the thermal enhancement techniques in PTC The comparative studies of the thermal enhancement methods compare different techniques under the same operating conditions and so the comparison can be conducted in a proper way. Firstly, it is essential to state a theoretical study about the thermal enhancement methods in PTC which has been conducted by Bellos et al. [116]. They found that the thermal enhancement of a commercial and high eﬃcient PTC can reach up to 2.0–2.5%. This small enhancement range is explained by the low thermal losses of the PTC which gives small enhancement margins in PTC with evacuated receivers. Too and Benito [117] compared the use of porous foam, twisted tape inserts, wire coils and dimpled absorber tube. According to their results, the use of dimpled absorber is the most promising

Table 6 Summary of the studies with nanoﬂuid-based PTC in applications. Description Study Abu-Hamdeh and Almitani [108] Ratlamwala and Abid [109] Toghyani, et al. [110] Alashkar and Gadalla, [111] Abid et al. [112] Bellos and Tzivanidis [113] Abid et al. [114] Bellos and Tzivanidis [115]

Nanoﬂuids

Application

Water with ZnO, Al2 O3 , Fe3 O4 Water/Al2 O3 Oil with CuO, TiO2 , Al2 O3 , SiO2 Syltherm 800, Therminol VP-1 with Ag, Cu Water with Al2 O3 , Fe2 O3 Syltherm with Al2 O3 , CuO, TiO2, Cu Water with Al2 O3 , Fe2 O3 Syltherm 800 with Al2 O3 , CuO

Liquid desiccant refrigeration Refrigeration Electricity Electricity Electricity Electricity (ORC) Multi-generation Trigeneration

Main ﬁndings Enhancement 24 % < ηen < 50% 56% heat transfer enhancement Enhancement ηex : 11% Enhancement ηen : 12% ηex : 23% Enhancement ηen : 1.8% ηex : 23.08% Enhancement ηen : 1.8%

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Fig. 48. Regenerated liquid desiccant cooling system driven by nanoﬂuid-based PTC [108] (License Number: 4402681403589).

Fig. 49. Organic Rankine Cycle driven by nanoﬂuid-based PTC [113] (License Number: 4402681496310).

choice which leads to 0.8% thermal eﬃciency enhancement for operation with air. This choice was determined using the performance evaluation criterion which takes into consideration both the thermal eﬃciency enhancement and the increase in the pressure drop. Moreover, Huang et al. [118] found that the use of dimples is better than the use of helical ﬁns and prostrations in the PTC absorber. They found the performance evaluation criterion of the dimples case to be between 1.23 and 1.37, for operation with Therminol VP-1. Bellos and Tzivanidis [25] examined the use of internal ﬁns, twisted tape inserts and perforated plate inserts in evac-

uated and non-evacuated tube collectors. They found that the use of ﬁns is the best technique according to the thermal, the exergy and the overall eﬃciency criteria. The use of twisted tape inserts is the second choice, while the use of perforated tape inserts is the less effective technique. The thermal eﬃciency enhancement with the evacuated tube collector was found up to 1.5%, while with the non-evacuated tube collector up to 2%. Practically, the higher thermal losses of the non-evacuated tube collector give greater margin for thermal enhancement.

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Fig. 50. Multi-generation system driven by nanoﬂuid-based PTC [114] (License Number: 4402690 0870 03).

The last part of the comparative studies compares the use of geometrical modiﬁcations and nanoﬂuids in PTC. Bellos et al. [10] compared the use of internal ﬁns and nanoﬂuids. They found that the use of Syltherm 800/CuO leads to 0.76% thermal eﬃciency enhancement, while the use of internal ﬁns to 1.10%. The combination of these techniques is the best choice with 1.54% thermal eﬃciency enhancement. About the pumping work increase, they found that the nanoﬂuids lead to 25% higher pumping work, the internal ﬁns to 100%, while the combination to 150%. However, the pumping work takes low values in all cases and so it is not an important limitation of the application of the thermal enhancement methods in PTC. Recently, Okonkwo et al. [119] compared the use of internal ﬁns, converging-diverging tube and twisted tape inserts with pure oil and oil/Al2 O3 . They found that the use of the converging-diverging tube is the best technique which enhances the exergy performance by 0.65% with pure oil and 0.73% with the nanoﬂuid. To sum up, the previous studies indicate that the performance enhancements of PTC are no so great and they are ranged from 0.5% to 2%. The use of dimples, internal ﬁns and convergingdiverging tube are the most effective techniques. The nanoﬂuids lead to lower enhancements compared to the geometric modiﬁcations. The combination of nanoﬂuids with geometrical modiﬁcations seems to be the best technique. About the limitations of the examined studies, it is important to state that the pressure drop increase is something that has to be taken into consideration. However, it has been found that the higher pumping work demand due to pressure drop increase is not able to eliminate the heat transfer enhancement and the overall collector performance is enhanced with the turbulators and the nanoﬂuids. It is remarkable that the ﬂow inserts are associated with the highest increases in the pressure drop. Another limitation of the examined ideas is the cost increase which is more intense in the nanoﬂuids. Moreover, the nanoﬂuids present extra disadvantages which are associated with the stability issues, the toxicity, the mechanical and the chemical erosion. So, the ﬁnal selection evaluation of the thermal enhancement techniques has to take into consideration both the thermal enhancements and the limitations.

5. The use of concentrating thermal PV with parabolic concentrators 5.1. General approach for the concentrating thermal PV The thermal PVs are hybrid solar collectors which can produce simultaneously useful heat and electricity. In these collectors, there is a heat transfer ﬂuid which removes heat from the PV cell and produces useful heat. Moreover, this heat transfer ﬂuid acts as a coolant and reduces the temperature of the PV cell and so increases the electrical eﬃciency of the cell. It is generally known that the electrical eﬃciency of the PV cell (ηPV ) is depended on the cell temperature with an equation as the below [120]. More speciﬁcally, the electrical eﬃciency of the PV reduces linearly with the cell temperature increase.

ηPV = ηre f · 1 − b · Tpv − Tre f

(36)

The reference temperature (ηref ) is depended on the PV cell type and it takes values close to 15–20%. The temperature dependence coeﬃcient takes small values with the value of 0.005 K−1 to be a typical one [120]. Moreover, it is useful to state that the reference temperature (Tref ) is usually selected at 25 °C and these results regard a solar irradiation of 10 0 0 W/m2 . The use of a concentrator is able to increase the incident solar energy on the PV cell and so higher amounts of electricity to be produced. However, in concentrating PVs, the use of a coolant is extremely important in order not to operate at extremely hightemperature levels. The use of a parabolic shape reﬂector gives the opportunity for operating in high concentration ratios and to produce high electricity amounts with a small PV cell area. This design is a cost-effective conﬁguration because the higher cost regards the PV in a system and not the reﬂector material. In the literature, there are some review papers which investigate concentrating thermal PV (CPVT) designs [121–124]. Zhang et al. [121] performed a review in 2012 for various CPVT designs and they suggested the use of a compound parabolic concentrator with a simple tracking system as the best choice. Sharaf and Orhann [122,123] conducted a double-study with many details about the design of the

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CPVT. Recently, Kasaeian et al. [124] performed a review about the parabolic shape concentrator and the linear Fresnel reﬂectors coupled to PVT. They stated that there is a need for examining experimentally the CPVT in large-scale systems. It is obvious that there is interest in the area of CPVT and many steps have to be done in order to commercialize this technology. 5.2. Basic mathematical formulation for concentrating thermal PV The basic mathematical formulation about the concentrating thermal PV mainly regards performance eﬃciency equations. First of all, the electricity production (Pel ) can be calculated as below:

Pel = APV · C · Gb · P F · ηPV

(37)

The (PF) is the packing factor and this parameter is depended on the design of the concentrating thermal PV. The concentration ratio (C) is depended on the reﬂector dimensions, as well as the absorber area. The useful heat production (Qheat ) can be calculated using the energy balance on the ﬂuid volume:

Qheat = m · c p · (Tout − Tin )

(38)

The available solar irradiation (Qs ) can be calculated as:

Qs = Aa · Gb

(39)

The electrical eﬃciency (ηel ) is calculated as:

ηel =

Pel Qs

(40)

The thermal eﬃciency (ηth ) is calculated as:

ηth =

Qu Qs

(41)

The system total (or energy) eﬃciency (ηtot ) is calculated using the useful production (Qheat ) and the electricity production (Pel ). This criterion evaluates the useful outputs in the same way and it does not take into account the different quality between them.

ηtot =

Qheat + Pel Qs

(42)

The exergy eﬃciency (ηex ) is calculated by converting the useful heat to the equivalent exergy (or work) quantity. This index takes into account the different quality between the useful outputs. The exergy of the solar irradiation is calculated using the Petela model [34].

ηex =

Qheat · 1 −

Qs · 1 −

4 3

·

To Tsun

To Tf m

+

1 3

+ Pel ·

To 4 Tsun

(43)

The temperature levels of the previous equations have to be in Kelvin units, the reference temperature (To ) is equal to 298.15 K and the sun temperature (Tsun ) is equal to 5770 K. 5.3. Studies with parabolic thermal PV There are several designs with thermal PV coupled to parabolic shape concentrators. The most usual design includes a ﬂat absorber with a PV cell, while there are cooling channels in the back side of the PV cell. The channel design can be tubular or rectangular, while there are also other designs with triangular absorbers. Li et al. [125] studied a parabolic shape concentrator with a ﬂat PV absorber which is given in Fig. 51. On the back side of the PV cell, there is a great tubular channel with internal ﬁns and water as the working ﬂuid. They found the electrical eﬃciency to be close to 10% and the thermal eﬃciency to be about 50% with GaAs cell array. Moreover, they found the electrical eﬃciency to be 7.5% and

Fig. 51. CPVT with a ﬂat absorber and tubular ﬂuid channel with ﬁns [125] (License Number: 4402690318316).

the thermal eﬃciency of 42.5% with a low-cost silicon PV array. In the previous results, the inlet ﬂuid temperature was 20 °C, while the outlet was 29.6 °C with the silicon cell and 33.9 °C with the GaAs cell. Finally, the authors concluded that the suggested technology leads to the competitive cost of the produced electricity compared to the conventional ﬂat PV. Karathanasis et al. [126,127] studied a CPVT with a parabolic concentrator and a ﬂat absorber (see Fig. 52). The heat transfer ﬂuid (water) was ﬂowing in rectangular channels in the back side of the PV cells. They found 50% total eﬃciency which is separated into 44% thermal and 6% electrical eﬃciency, for low-temperature levels (25–50 °C). They stated also that the cost of the system is estimated at 1.75 €/W which is a competitive value compared to other concentrating technologies. Moreover, the use of a ﬂat PV area with a cover glass has been studied by Yazdanifard et al. [128]. This design is depicted in Fig. 53 and it is similar to an inversed thermal PV with a parabolic concentrator. They found that the use of glass is beneﬁcial for the total and the thermal eﬃciency while it reduces the electrical eﬃciency. Generally, they found about 7% electrical eﬃciency, 55% thermal eﬃciency, 62% total eﬃciency and 9.6% exergy eﬃciency for operation at inlet temperature at 25 °C with water/TiO2 nanoﬂuid. They also studied the use of water/TiO2 nanoﬂuid and they found that it is able to enhance the electrical eﬃciency by up to 0.5% and the thermal eﬃciency up to 4% in laminar ﬂow regime. Finally, they stated that a preliminary analysis indicates the suggested system to be roughly cost-effective compared to the conventional ﬂat PV systems. The use of a triangular absorber has been studied by Calise et al. [129] and the examined conﬁguration is given in Fig. 54. They found the electrical, thermal and total eﬃciencies at 21%, 62%, and 83%, respectively for water inlet temperature equal to 70 °C. Srivastava and Reddy [130] examined various conﬁgurations with PV cells in the outer surface of the absorber tube, in a conventional PTC (see Fig. 55). They also studied the effect of the nanoﬂuid water/Al2 O3 on the results. They found that this collector presents 19% electrical eﬃciency and 70% thermal efﬁciency for inlet temperature equal to 27 °C, while they stated that the nanoﬂuid enhances the electrical eﬃciency by 0.6%. Moreover, there are extremely alternative designs in the literature about CPVT. Jiang et al. [131] studied a double stage system, as it is given in Fig. 56. They used a secondary parabolic shape concentrator which reﬂects partially the solar rays, using a proper optical ﬁltering. The PV cells are located in the down part of the main concentrator. They found that the useless heat for the PV cell can be reduced up to 20.7%, while totally there is a 10% increase in the utilized solar energy by the receiver. They also stated that the optical eﬃciency of the system was 76.4%. Stanley et al. [132] studied a double-pass absorber in a CPVT system. The absorber is de-

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Fig. 52. CPVT with a ﬂat absorber and ﬂow channels (a) The total system [126] (b) the cooling system design [127] (License Numbers: 4402690477951 & 4402690604005).

Fig. 53. CPVT with ﬂat absorber, tubular ﬂuid channel and cover glass [128] (License Number: 4402690710157).

Fig. 54. Triangular absorber with PV and tubular cooling channel [129] (License Number: 4402690811999).

picted in Fig. 57 and there are two tubes; the upper for the cold stream (water) and the down for the hot stream (propylene glycol). The advantage of this design is the possibility for operation at higher temperature levels compared to the other systems. This system presents a relatively low electrical eﬃciency up to 5%, while the thermal eﬃciency is close to 50% for temperature levels up to 120 °C. They also stated that the examined design presents a high cost and there is a need for conducting more work on reducing the production cost. Widyolar et al. [133] investigated a double stage

CPVT with a primary parabolic concentrator and a secondary compound parabolic concentrator. The greater part of the secondary reﬂector gas PV cells and the total system is given in Fig. 58. They found that this design is able to produce 8% electrical eﬃciency and 37% thermal eﬃciency at 365 °C with Therminol VP-1. The previous studies indicate that relatively high electrical and thermal eﬃciencies can be achieved with the CPVT. Table 7 summarizes brieﬂy all these studies. It is useful to state that the alternative designs of refs [129,130] give high electrical eﬃcien-

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Fig. 55. Tubular absorber with PV arrays in the outer down part [130] (License Number: 4402690932987).

Fig. 56. Double stage CPVT with two parabolic concentrators [131] (License Number: 4402691044525).

Fig. 58. Double stage CPVT with PC cells in the secondary compound parabolic concentrator [133] (License Number: 4402691271554).

Fig. 57. Double pass absorber of a CPVT [132] (License Number: 4402691163587).

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E. Bellos and C. Tzivanidis / Progress in Energy and Combustion Science 71 (2019) 81–117 Table 7 Summary of the studies with concentrating thermal PC. Study

Description

Working ﬂuid

Main ﬁndings

Li et al. [125] Karathanasis et al. [126]

Flat absorber, tubular ﬂuid channel Flat absorber, rectangular ﬂuid channels Flat absorber, tubular ﬂuid channel, glass cover Triangular design with tubular ﬂuid channel Conventional absorber with PV cells in the outer surface Double stage CPVT with two parabolic concentrators Double-pass absorber in a CPVT system

Water Water

ηel = 7.5%, ηth = 42.5% at 20 °C ηel = 6%, ηth = 44% at 25∼50 °C

Water/TiO2 (nanoﬂuid)

ηel = 7%, ηth = 55% at 25 °C

Water

ηel = 21%, ηth = 62% at 70 °C

Water/Al2 O3 (nanoﬂuid)

ηel = 19%, ηth = 27% at 27 °C

–

Double stage CPVT with a primary parabolic concentrator an a secondary compound parabolic concentrator

Therminol VP-1

20.7% lower heat duty on the PV cell, ηth = 10.5%, ηopt = 76.4% ηel up to 5%, while ηth close to 50% at 120 °C ηel = 8% and ηth = 37% at 365 °C

Yazdanifard et al. [128] Calise et al. [129] Srivastava and Reddy [130] Jiang et al. [131] Stanley et al. [132] Widyolar et al. [133]

cies close to 20% and they are promising choices. Moreover, the Refs [132,133] give high thermal eﬃciency at medium and hightemperature levels. So, it is obvious that the ﬁeld of CPVT with parabolic concentrators is open and there are numerous interesting ideas that can be improved and studied in the future. 6. Discussion The discussion section is devoted to presenting the advantages and the limitations of the examined designs and to discuss them in a more comparative way. Also, the most promising ideas are highlighted. Finally, the environmental effects of the alternative, as well as the future challenges which regard their utilization are given. 6.1. Performance discussion of the examined studies In this work, numerous alternative designs and ideas about the PTC are given. The main goals of the alternative collectors are listed below: • Increase of the thermal eﬃciency of the solar collector in order to achieve higher useful heat production. • Increase of the optical eﬃciency in order to increase the absorbed solar irradiation in the absorber. • Decrease in the manufacturing cost in order to create more viable systems. • Create uniform heat ﬂux distribution over the absorber in order to reduce the hot spots and to enhance the thermal performance. • Reduce the temperature deviation in the absorber periphery and so the thermal stresses are reduced. This fact eliminates the possibilities for failure of the absorber. • Create more compact systems which can be used in applications with space limitations. • Better performance with greater tracking errors in order to reduce the need for highly precise tracking systems with a high cost. • Operation at higher temperature levels in order to use the solar systems in applications with higher temperature needs. • Increase of the overall performance of the systems with thermal and electrical production (cogeneration systems). The previous goals can be achieved in different ways and the ﬁeld is open; for new and different ideas. In this work, the examined collectors are separated into three main categories: the optically new design, the thermally new designs and the alternative concentrating thermal PV.

Water and propylene glycol

6.1.1. Discussion of optically modiﬁed designs The optical modiﬁcation ideas aim to increase the optical performance of the proposed designs. Zhu et al. [38] suggested the movement of the receiver in order to reduce the end losses, while Qu et al. [43] suggested the rotation of the solar ﬁeld in order to eliminate the optical losses due to the sun position. The receiver movement with a secondary reﬂector and a non-continuous primary absorber (Ref. [38]) leads to an acceptable thermal eﬃciency at 66% while the total conﬁguration faces low mechanical diﬃculties due to the reduced wind loads. Moreover, the idea of Ref. [43] seems to be interesting with a 5% increase in the daily performance but it has to be tested in greater solar ﬁelds than the 300 kW. Another promising idea for reducing the land utilization and given a more compact system is the use of lenses. This idea has been examined by Ma et al. [42] and it is able to give an optical eﬃciency of 84%, higher than the conventional PTCs which have about 75%. Maatallah et al. [41] suggested a stationary concentrator with a moving receiver and they stated that this system is a low-cost design with acceptable eﬃciency, especially close to the solar noon. However, this design is not favorable in cases with high thermal eﬃciency needs. The use of secondary reﬂectors in PTC has been examined by many researchers. The goals of this technique are to capture the lost solar rays, to create a more uniform heat ﬂux over the absorber and to create a higher concentration ratio if it is needed. Widyolar et al. [44] used a compound parabolic secondary reﬂector which is able to operate at 375 °C with a thermal eﬃciency of 37%. Moreover, Wang et al. [45] used a similar idea for creating uniform heat ﬂux over the absorber periphery. In the same direction, Wirz et al. [48] examined different secondary designs and they found that the thermal eﬃciency can be enhanced by up to 2.7% with a secondary reﬂector. Another interesting and new idea is the use of PTC for lighting purposes in a building which can reduce the electricity consumption for lighting up to 90% [49,50]. The previous brief summary of the optical modiﬁcation studies indicates that the use of secondary reﬂectors seems to be a promising idea for operation in high temperatures and for increasing the thermal performance of the PTC. Furthermore, the use of lenses [42] is an interesting concept leading to compact designs. Also, the use of a rotating tracking system [43] increases the daily yield because the system follows the sun in a more suitable way. Important enhancements have been found in the optical eﬃciency and this fact is the most important indicator for the application of these technologies. However, it is important to take into consideration the higher investment cost and the manufacturing diﬃculties in the optically improved systems.

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6.1.2. Discussion of thermally modiﬁed designs The ﬁrst part of the studies about thermally modiﬁed absorbers regards ideas for reducing the thermal losses of the receiver. Chandra et al. [52] found the use of an insulation material in the evacuated tube is able to reduce the thermal losses about 20%, while Wang et al. [54] found that the use of a radiation shield reduces the thermal losses about 24%. Moreover, Yang et al. [55] stated that 31% thermal loss reduction can be achieved with the use of double coating in the absorber surface. At this point is essential to state that the high quality evacuated tubes have a high cost, so the use of an alternative receiver is a key factor in the development of lowcost systems. One interesting idea is the use of a V-shape receiver [59] with insulation in order not to use the high cost evacuated tubes. Furthermore, a low-cost design with a light-weight structure has been suggested by Forman et al. [63]. In this work, two independent parts are merged into a light-weight and solid system. Other interesting ideas are associated with a special design of a solar air collector which has been studied in Refs. [61,62] with details. This design is able to operate eﬃciently in temperatures over 500 °C. The use of thermal batteries [64,65] is an alternative idea which has to be investigated more in the future. About the ﬂow enhancements methods, the use of turbulators has been found to be more eﬃcient than the use of nanoﬂuids. The studies of Bellos et al. [10] and Okonkwo et al. [119] clearly state the above argument. Moreover, it is essential to state that the combination of both techniques (turbulators and nanoﬂuids) is the best way of maximizing the heat transfer rates from the absorber to the working ﬂuid [10]. However, the use of turbulators leads to higher pressure drop than the nanoﬂuids which is a point that has to be taken into consideration in any design process. Among the turbulators, the use of internal ﬁns [78], dimples in the absorber [82] and the converging-diverging inner absorber surface [83] seem to be the most effective choices. About the nanoﬂuids, the use of Cu/CuO nanoparticles [87,92], the use of Al2 O3 [89,93], as well as the use of MWCNT [97] seem to be the most eﬃcient choices. Finally, it can be said that all the thermal enhancement techniques can improve the performance of the PTC but they lead to higher investment cost. The uses of ﬂow modiﬁcations (inserts, ﬁns, dimpled absorber) are most eﬃcient techniques than the use of nanoﬂuids. The combination of both techniques is the superior choice but this fact has to be checked with real installations in great solar ﬁelds. The nanoﬂuids are also associated with many problems such as the high cost and the stability issues due to agglomeration, as well as erosion issues. On the other hand, the ﬂow modiﬁcations lead to higher pressure drop compared to pressure drop in nanoﬂuid cases. However, the evaluation criteria indicate that this pressure drop is not so important in energy terms. Lastly, it has to be said that the use of ﬂow inserts and the use of nanoﬂuids can be applied in the existing systems, while the use of modiﬁed absorber geometries (e.g., internally ﬁnned absorber) can be used only to new systems. So, the use of the ﬂow inserts or of the nanoﬂuids can be applied for retroﬁtting the existing systems. A comparison between the thermal and the optical enhancement methods indicates that the optical studies may have a higher potential because the PTC faces higher optical than thermal losses. A typical PTC presents a performance of 65% which means 25% optical losses and 10% thermal losses. The thermal enhancement margin is higher in the optical losses because they are generally more than the thermal ones. So, the optical enhancement gives a higher potential for improvement. However, the commercial systems use highly precise tracking systems and material with good optical properties (transmittance, absorbance and reﬂectance). These facts make the optical improvement to be a more diﬃcult concept than the thermal improvements.

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6.1.3. Discussion of concentrating thermal PV designs The use of concentrating thermal PV is an interesting idea for the production of high amounts of electricity and useful heat (cogeneration systems). The use of a heat transfer ﬂuid for the PV cell cooling is vital in concentrating systems because of the high amounts of the incident solar energy on the PV cells. It has been found that the use of ﬂat absorbers is able to give an electrical eﬃciency of 7–8% [125–128], while the use of a triangular about 20% [129,130]. It has been also proved that the use of nanoﬂuids is able to give a small electrical eﬃciency enhancement of 0.5% [128,130]. Moreover, the study of Stanley et al. [132] suggested an interesting double-pass absorber which can operate at 120 °C with 5% electrical eﬃciency and 50% thermal eﬃciency. Widyolar et al. [133] found that a double stage system is able to operate with 8% electrical eﬃciency and 37% thermal eﬃciency at 365 °C. These results prove that the operation at medium-high temperatures is achievable with the alternative designs. The previous results show that there is great potential for designing eﬃcient systems with the use of concentrating thermal PV and parabolic concentrators. Especially the triangular design gives extremely high potentials because of the increased electrical eﬃciencies. In the future, the study of large-scale concentrating thermal PV is a key factor for the establishment and the commercialization of these systems. Lastly, it is important to establish the proper legislation in order to face the concentrating thermal photovoltaic as cogeneration systems and to ensure the proper electricity price tariff. 6.2. Financial discussion of the examined ideas The ﬁnancial evaluation of the alternative PTC designs is an important issue in order to check the viability of the examined cases. Practically, the increase in the income due to the enhanced performance has to be evaluated together with the possible increase of the investment cost in order to check if the new designs are beneﬁcial in an overall evaluation. However, in the present studies, there are only a few aspects of the ﬁnancial viability of the examined designs. Below, a summary of the most important conclusions about the ﬁnancial point of view of some alternative designs are given. Maatallah et al. [41] stated that the use of a stationary concentrator is able to reduce a lot the total cost of the system and this is an encouraging result. Moreover, Chandra et al. [52] found that the use of insulation inside the absorber tube is able to create a receiver with 20% lower cost, compared to the conventional evacuated tube. The authors of the previous ideas stated that these designs are promising ideas for the development of low-cost PTC with a reasonable performance. Yang et al. [55] stated that the use of a double-coating absorber tube leads to a 5% decrease in the LCOE. The use of a nanoﬂuidbased PTC coupled with a Rankine cycle has been found to reduce about 1% of the LCOE, according to the study of Alashkar and Gadalla [111]. About the power plants, Munoz and Abanades [80] stated that the helical ﬁns in the inside absorber surface lead to 2% power plant electricity production, while the increase of the investment cost of the plant is estimated to 0.5%. So, they concluded that the overall ﬁnancial performance of the system is beneﬁcial with the internal ﬁns. Bellos and Tzivanidis [71] calculated lower thermal stresses with a star ﬂow insert in a PTC and they concluded that this fact is able to reduce the failure rate of the absorbers and so to reduce their replacement cost. About the use of nanoﬂuids, Kasaeian et al. [96] found that the use of oil/MWCNT leads to a payback period of about 4 years which a promising value. About the CPVT, Karathanasis et al. [126] found that the use of a parabolic concentrator with a ﬂat PVT receiver presents a system cost competitive with other solar concentrating antagonists.

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The previous summary indicates that the found ﬁnancial data in the literature are promising and they indicate the viability of the examined idea. Especially the use of helical ﬁns [80] and of the nanoﬂuid utilization [96,111] leads to the most promising results. Moreover, the use of a double-coating absorber leads to 5% lower LCOE, an interesting result about this design. In any case, extra advantages of alternative designs such as the lower thermal stresses in the absorber [71] can increase the overall impact of the thermal enhancement methods in PTC. 6.3. Challenges and environmental beneﬁts It has been proved that alternative designs present important eﬃciency advantages, as well as some of them, can reduce the cost of solar systems. However, it is essential to discuss the possible environmental beneﬁts of these alternative designs, as well as their challenges. First of all, it has to be stated that the increase in the thermal eﬃciency is able to produce higher amounts of useful heat production (or electricity with CPVT) and so higher amounts of fossil fuels can be substituted. This fact has a direct environmental effect with the reduction of the CO2 emissions. Moreover, the more eﬃcient systems are able to reduce the demanded collecting aperture, especially in the higher solar ﬁeld. This fact makes possible the reduction of land utilization, important issues especially for locations with land restrictions (for instance overpopulated cities). It is also useful to state that the cost reduction of the alternative PTC will lead to more investments in solar thermal systems and consequently to more green energy production. Another advantage of the alternative designs is associated with the lower material utilization which leads to lower cost and to lower need for a lower number of processes about the PTC construction. So, the total life cycle of the PTC can make them a more environmentally friendly technology. However, the alternative ideas are conjugated with possible problems which are associated with the complexity and the reliability of the new systems. A lot of work has to be performed before the alternative designs to be commercially ready and many experiments and durability tests are a need in this direction. Moreover, it is important to take into consideration the increase of investment cost through the use of the alternative technique and to evaluate it properly. The cost-effective choices have to be determined in order to be commercialized on a great scale. Moreover, about the challenges of the alternative designs, it can be said that all the ideas about the ﬂow enhancements (inserts, ﬂow turbulators, nanoﬂuids, etc.) lead to higher pressure drop and consequently to increased pumping work demand. The result is the increased operating cost because of the higher consumption in the pumping system, as well as an increase in the installation cost of the hydraulic circuit. However, it has been found that the increase of the pumping work is not so important and this fact makes this limitation not an obstacle in the utilization of the ﬂow enhancement methods. About the nanoﬂuid utilization, the limitation and the problems in these cases are more because of the high cost of these ﬂuids and the stability problems due to nanoparticle agglomerations [134]. Furthermore, there are problems with the power deposition in the gas-phase nanoﬂuids according to the Ref. [107]. 6.4. Future work There is a need for conducting future works about the establishment of the most eﬃcient alternative designs. The experimental studies are needed in order to determine the performance enhancement and to examine the durability of the alternative designs. Furthermore, there is a need for experimental work with large-scale systems in order to estimate the real enhancement in

the scale of real solar ﬁelds. This technique is able to make the alternative designs more attractive for investments. Especially for the use of turbulators in the PTC, the majority of the studies are CFD work or thermal models are used. So, it would be critical to measure experimentally the thermal enhancement with the use of ﬂow inserts, internal ﬁns and of the similar techniques. Additionally, there is a need for future comparative works which will examine different alternative designs under the same operating conditions and with a respective methodology. These studies are able to compare properly the different designs (or ideas) and to give reliable conclusions. In this direction, there are some studies in the literature, mainly about the ﬂow enhancement methods. However, there is a need for more comparative studies. An interesting idea on this issue would be the establishment of standard methods about the way of conducting comparative studies with simulations or experiments. Some reference cases could be agreed and in order for the found results to be properly compared. So, the results of different studies could be easily compared and combined. Another remark about the future work is associated with the combination of optically and thermally modiﬁcation methods. It is obvious that the combination of more enhancement techniques is able to create extremely high eﬃcient and compact systems. Especially in the cases with operation in high-temperature levels, the combinations of optical and thermal modiﬁcations can be really important in order to develop systems which can compete for the fuel and well-established technologies. The cost analysis of the various alternative designs is critical in order to evaluate the viability of these ideas. The cost analysis has to be conducted about the investment cost and the operation cost in order to assess the life economic cycle. Furthermore, there is a need for highlighting the possible manufacturing diﬃculties in intelligent designs because in many cases the eﬃciency increase is conjugated with various technological restrictions.

7. Conclusions The parabolic trough solar collector is one of the most mature solar concentrating technologies for operation in medium and high temperatures. The objective of the review article is to present and to discuss alternative parabolic trough designs which can lead to more sustainable conﬁgurations. The examined alternative designs of this work are separated into three categories; the optically modiﬁed systems, the thermally modiﬁed systems and the concentrating thermal photovoltaics. Various ideas have been studied in the literature and the main goals of these ideas to enhance the thermal performance, to operate at higher temperature levels and to design cost-effective systems. The most important conclusions of this work are summarized below: • The optical modiﬁcations of the PTC are able to increase the concentration ratio and to give the possibility for operation at higher operating temperatures. The use of a secondary reﬂector is a promising choice which can help to achieve these goals. Various secondary designs such as trapezoidal, ﬂat and with a parabolic shape have been tested. Moreover, the use of lenses leads to more compact designs. • The improvements in the collector tracking systems and the optical eﬃciency enhancements are able to increase the daily energy yield of the PTC solar ﬁeld about 5%. • The thermal modiﬁcations of the PTC are able to enhance the thermal performance and to increase the useful output. The thermal enhancements are usually low (up to 2%) because of the low thermal losses of the conventional PTC with an evacuated receiver which gives small thermal enhancement margin.

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• The use of alternative receivers with insulation, radiation shield or double coating are promising choices for designs low-cost systems with satisfying performance. Especially the use of insulation in the upper part of the absorber reduces the receiver cost about 20%. • The use of internally helical ﬁns in the absorber and the use of nanoﬂuids are found to be the most promising choices ﬁnancially. More speciﬁcally, the use of nanoﬂuids can lead to a payback period of about 4 years. Also, the use of the doublecoating is an idea which can reduce the LCOE by 5%. • The use of turbulators is a more eﬃcient technique than the use of nanoﬂuids for enhancing the thermal performance of PTC. Especially, the use of internal ﬁns, absorber with dimples or a converging-diverging absorber tube seems to be the most ideal choices. Moreover, the nanoﬂuids with Cu/CuO, Al2 O3 and MWCNT nanoparticles seem to be the best solutions. • There is a need for conducting research on the ways of restricting the limitations of the various technologies. Especially for the nanoﬂuids, it is important to reduce the investment cost and to create more stable working ﬂuids, especially at higher temperature levels. The problems of the toxicity and the erosion have also to be studied. • The optical modiﬁcation methods are generally more effective than the thermal modiﬁcation methods because the optical losses are generally greater than the thermal losses in PTC. So, there is a higher performance enhancement margin with the optical improvement than the thermal. However, the thermal enhancement methods can be more important in cases of operation at high-temperature levels. • The concentrating thermal PV are promising alternative choices for producing high electricity amounts and important amounts of useful heat. Alternative designs with triangular absorber, double stage concentrators or special double-pass absorbers give the opportunity to produce useful heat at medium temperature levels with simultaneous electricity production and to develop highly eﬃcient cogeneration systems. These systems are able to achieve high electrical eﬃciencies close to 20%. The conclusions of this work can be used as guidelines for future studies and projects on the domain of alternative PTC designs. There is a need for more experimental and comparative studies not only in laboratory scale but also in real solar ﬁelds. Furthermore, there is a need for cost analysis studies in order to ﬁnd in which cases the thermal eﬃciency increase is enough to cover the investment cost increase. Acknowledgments Dr. Evangelos Bellos would like to thank “Bodossaki Foundation” for its ﬁnancial support. Appendix A. Basic modeling of nanoﬂuids The thermal properties of the nanoﬂuid are calculated by modifying the respective thermal properties of the base ﬂuid. The most important thermal properties for the solar thermal system calculations are the thermal conductivity (k), the dynamic viscosity (μ), the density (ρ ) and the speciﬁc heat capacity (cp ). In the literature, there are numerous studies which present many models for the nanoﬂuid thermal properties, especially for the dynamic viscosity and the thermal conductivity [135–138]. The reason for this great variety is based on the dependency of the nanoﬂuid thermal properties by numerous parameters as the nanoparticle concentration, the nanoparticle diameter, the preparation temperature and the nanoparticle shape. The given equations are applicable both in water and oil-based nanoﬂuids and they are usually applied in the literature studies.

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Table A1 The thermal properties of the usual nanoparticles [9]. Nanoparticle

Density (ρ ) (kg/m )

Speciﬁc heat capacity (cp ) (J/kgK)

Thermal conductivity (k) (W/mK)

8933 60 0 0 5180 4230 3960 2200

397 551 670 692 773 765

393 33 6.9 8.4 40 1.4

3

Cu CuO Fe2 O3 TiO2 Al2 O3 SiO2

The nanoﬂuid is symbolized with (nf), the base ﬂuid with (bf) and the nanoparticle with (np). The volumetric concentration is (ϕ ) of the nanoparticle to the nanoﬂuid is an important parameter. It takes values up to 8%, but in the majority of studies, it has values up to 2%. The thermal properties of the most usual nanoparticles are given in Table A1 [9]. The density (ρ ) of the nanoﬂuid is calculated as [139]:

ρn f = ϕ · ρnp + (1 − ϕ ) · ρb f

(A1)

The speciﬁc heat capacity (cp ) is calculated according to the following equation [135]:

c p,n f =

ϕ · ρnp · c p,np + (1 − ϕ ) · ρb f · c pb f ρn f

(A2)

The thermal conductivity (k) of the nanoﬂuid can be estimated using the model of Yu and Choi [140].

kn f = kb f ·

knp + 2 · kb f + 2 knp − kb f · (1 + β ) · φ 3

knp + 2 · kb f − knp − kb f · (1 + β ) · φ 3

(A3)

In the previous equation, an important parameter is the ratio of the nano-layer thickness to the original particle radius (β ). This parameter can be taken equal to 0.1, according to the Ref. [141]. The nanoﬂuid dynamic viscosity (μ) is usually estimated using the Bachelor model [142]. It is important to state that this equation is a theoretical formula which underestimates the dynamic viscosity at higher concentrations and thus it usually applied for concentrations up to 4%.

μn f = μb f · 1 + 2 . 5 · φ + 6 . 2 · φ 2

(A4)

About the Nusselt number in the turbulent regime (Re > 2300), the model of Maiga et al. [143] is a usually used model:

Nu = 0.086 · Re0.55 · P r 0.5

(A5)

PTC usually operates with thermal oils and thus the emphasis is given in oil-based nanoﬂuids. Fig. A1 [9] illustrates the thermal properties of various oil-based nanoﬂuids with Syltherm 800 as a base ﬂuid and CuO, Al2 O3 , TiO2 and Cu nanoparticles. This ﬁgure shows the speciﬁc heat capacity, the density and the thermal conductivity of these nanoﬂuids for different temperature levels and volumetric concentrations. It is important to state that the density and the thermal conductivity of the nanoﬂuid are higher than the base ﬂuid. On the other hand, there is a reduction in the speciﬁc heat capacity for the nanoﬂuid. Moreover, Fig. A2 [10] shows that the dynamic viscosity of the nanoﬂuid is higher than the base ﬂuid for all the temperature levels. The main advantage of the nanoﬂuids is the increase in thermal conductivity which leads to higher heat transfer coeﬃcients between the absorber and the heat transfer ﬂuid. The increased viscosity leads to a higher pressure drop and it is a limitation that has to be taken into account in any case.

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Fig. A1. Thermal properties of various oil-based nanoﬂuids [113] (license number: 4403060140458).

Fig. A2. Dynamic viscosity of pure oil (Syltherm 800) and nanoﬂuid (Syltherm 800/CuO with 6% concentration) [10] (license number: 4403060375697).

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Alternative designs of parabolic trough solar collectors

Alternative designs of parabolic trough solar collectors

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