Design and development of compound parabolic concentrating for photovoltaic solar collector: Review

Renewable and Sustainable Energy Reviews 76 (2017) 1108–1121

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Design and development of compound parabolic concentrating for photovoltaic solar collector: Review ⁎

MARK

⁎

Ahed Hameed Jaaz , Husam Abdulrasool Hasan , Kamaruzzaman Sopian, Mohd Hafidz Bin Haji Ruslan, Saleem Hussain Zaidi Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

A R T I C L E I N F O

A BS T RAC T

Keywords: Compound parabolic concentrator Photovoltaic thermal Solar energy Solar concentrating photovoltaic

Despite about five decades of development, commercial solar energy has not yet unable to penetrate the electric and gas options. For this particular reason, designing compound parabolic concentrators-photovoltaic thermal solar collectors (CPC-PVT) continues until achieving similar or greater performance with a comparative cost. This paper outlines the various types of CPC-PV systems concerning design advantages and limitations. The article includes comparisons on used materials, optical tolerance and efficiency, and the range of the acceptance angle. The review focuses on the historical developments regarding the use of Fresnel lens for optimizing captured sunlight, 2- and 3-D CPC, parabolic trough, and materials used for coating. It is hoped that this review helps researchers to highlight the successful trends of designing CPC by sorting out the many layers and factors that are decisive in designing CPCs. It can be seen clearly the vast opportunities for developing better designs and utilizing the qualities of the material used for reflectance and absorbance. Flat plate collectors have shown an increasing drawback to deal with temperatures of more than 100 °C . The fixed orientation of CPC has a ydisadvantage due to the limitations of capturing sunlight; however, tracking mechanism could be employed to enhance the amount of the captured sunlight. The non-imaging system is also highlighted to show its efficiency over the imaging systems concerning larger accept angles, higher concentration ratios with less volume and shorter focal length, higher optical efficiency. However, for applications such as solar-to-electric conversion, imaging, and non-imaging Fresnel lens have shown almost same conversion factor. Throughout CPC designing, particular issues have to be considered such as the ratio of reflector-to-aperture size, the formation of hot spots, and the minimising of losing of multiple reflections concentrating photovoltaic (CPV) systems are still developing where new methods, designs, and materials are still being created in order to reach a low levelled cost of energy comparable to standard silicon-based PV systems. It is very important to note that non-imaging Fresnel lenses could bring a breakthrough in commercial solar energy concentration application technology very soon.

1. Introduction The conventional energy has caused severe damage to the environment in many aspects such greenhouse and global warming. The renewable energy has become the best alternative. However, the shift from the conventional energy to the green energy faces challenges characterized by availability, sustainability as well as the economy factor. Currently, the focus on solar and wind energy is the prime effort toward green energy [1]. Regarding the solar energy, the availability of the sun-light during half a day in most countries and the easy process of gathering the solar energy result in advancing this field scientifically and technologically to reach a very advanced level of reliability and acceptability during the last two decades. ⁎

In 1947, Winston [2] invented the first type of the compound parabolic concentrator (CPC) which is shown in Fig. 1 with most possible dimensions. The CPC can be stationary or with a compound rotation or translation [3]. The solar technology depends on gathering and reflecting the solar rays available in the sun spectrum. The efficiency of gathering and reflecting the sunrays depend on designing CPC. Regarding the use of the solar energy, it can be used directly for heating or store in photovoltaic cells (PVs) [4–8]. The direct conversion of solar radiation into electric power is traditionally more convenient than conversion into heat due to several reasons such a maintenance, safety, and long operation [9]. The solar power generation was reviewed by numerous articles [11–14] along with the analysis of the power generation [10,14–19].

Corresponding authors. E-mail addresses: [email protected] (A.H. Jaaz), [email protected] (H.A. Hasan).

http://dx.doi.org/10.1016/j.rser.2017.03.127 Received 2 December 2016; Received in revised form 5 February 2017; Accepted 27 March 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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MPPT P I V Pmax PVT CPV CCPC TEG

Nomenclature CPC PVs DC Ɵmax PMMA FPSC PVC

compound parabolic concentrators photovoltaic cells direct current angle of acceptance polymethylmethacrylate flat plastic static concentrator photovoltaic concentrating

maximum power point tracking power current voltage maximum current photovoltaic thermal solar collectors concentrating photovoltaic crossed compound parabolic concentrator thermoelectric generator

mounting. It is also possible for these systems to use standard PV cells that were made for non-concentrating applications. The drawback of these systems is to cool down the PV cells which requires difficult maintenance. The middle level of research, where the concentrating ratios between 10 and 100, requires only one-axis tracking and the concentrators are symmetric. This system is based on parabolic reflectors [33]. The design of the PV cells has to consider high intensities (high current) where the cells need cooling during the collecting light. The system of a medium level is not suitable for installation for houses and small buildings. The third level, the highest, where the concentration ratios are between 100 and 1000. For this system, the sunrays are to be tracked using 2-axis system. The purpose of the high-level system is to generate high temperature suitable for a steam turbine to generate electricity. For this high-level system, parabolic dish or spherical lenses are used to concentrate the light onto PV cells. In this paper, a brief review of the most important designs of solar energy, which highlights the principles of the technical developments and usages.

The surface modifications and design were discussed in [20–22]. The imaging and non-imaging systems are outlined in [23–27]. The PV cells are integrated into the solar system [7,28–31]. The non-imaging CPC was designed first in the mid-1960s and then developed by Winston [2]. The design of CPC consists of two parabolic reflectors at the two ends of the absorber. The 2-D CPC can receive radiation through a very big angular spread and can be still focused onto a linear receiver. The main purpose of the design was to collect as much as possible rays and then direct them by reflection either to a heat exchanger for heating or directly to PV cells collectors for electricity production. CPC is a non-imaging concentrator, which does not require the rays be parallel, or aligned with the axis of the concentrator. CPC consists of receiver, cover, and reflector. The receiver of silicon polymer (emissivity 0.4 and absorbance 0.9) is characterized by the highest possible absorbance of the sunrays and should be fabricated using metals of very high conductivity. The cover is made of transparent insulating materials to allow perfect passage of solar radiation such as a glass of 4 mm thickness. The third part of CPC is the reflector, which should have the ability focus the reflected beam onto a receiver. In designing a CPC, all these factors have to be considered. The performance of CPC with accurate design and suitable materials could reach the highest level with concentration ratio between 3 and 10. Recently there have been many attempts to improve the overall efficiency of the solar cell by utilizing the recent development in plastic technology, nanocomposites, molding techniques, and computer-aided technology. There are three levels of research in solar energy focusing on low, medium, and high concentration systems [32]. The first category is for concentration ratios between 1 and 10, where integrating the system into homes and small buildings becomes possible without complex

2. Concentration ratio As shown in Fig. 2, rays incident at the extreme angle of acceptance (θmax ) at the edge of the aperture are transported to the rim of the exit aperture [26]. If all rays are transported, the ideal case scenario is accomplished suggesting an ideal concentrator. This means that all rays can be gathered bounded by the phase space volume from a to a′ and by directional space ± θmax , which represents the limitations of the aperture. Fig. 2 shows the basic principles of the majority of designing architecture for the flat absorber. In Fig. 2(b), the rod AC is placed at the aperture tilted by θmax with the horizontal through which the parabolic absorber ac accepts all possible rays. This is the principle of the parabolic concentrator with its focal point at point d and the optical axis is cC . By applying Fermat's principle of an equal optical path, the edge ray yield (Eq. (1)):

Cc+ cd = ad + easinθmax

(1)

For symmetric shape of Fig. 2(b), Cc = ad , then, cd = ea sin θmax . The concentration ratio is defined as the entry aperture divided by the exit aperture (Eq. (2)),

C=

ea ea 1 = = cd easinθmax sinθmax

(2)

The geometry of 2D-CPC is shown in Fig. 3 with the basic construction of CPC of aperture (GF=W ), absorber (AB =b ), and the collectors of two parabolas, (AG) and (BF). Fig. 3(b) shows more details regarding the dimensions, the angles, and the axes of CPC. The distance between the reflector (AB) and the aperture (GF) represents the height of the CPC. The aperture area, Aaperture , is the product of W and the length (L ) of CPC (not shown in the figure) (Eq. (3)) while the absorber area, Aabsorber , is the product of b and the length (L ) of CPC as depicted in Eq. (4).

Aaperture = W *L

Fig. 1. Typical CPC with main dimensions [10].

1109

(3)

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Fig. 2. (a) CPC with flat absorber shows how all rays can be gathered within the concept of θmax and (b) applying string method for an ideal concentrator where the rod moves freely from A to C [32].

(4)

Aabsorber = b*L

The concentration ratio (C ) is defined as the ratio of the width (W ) of CPC and the length of the aperture (b ), and it describes the capability of collection systems to concentrate solar energy as in Eq. (5) [21]. The concentration ratio is expressed by a unit called sun where the area of the reflector and the aperture are the same value, and 2 sun when the area of the reflector is as twice as the area of the aperture.

C=

W b

(sun ),

b = Wsinθc

(5)

Fig. 3. The basic geometrical setting of 2D-CPC (a) the basics and (b) the geometrical details [32].

3. Basics and comparative CPC designs 3.1. Fresnel lens Since the 1950s when Poly(methyl methacrylate) PMMA became available, Fresnel lens, shown in Fig. 4, has been increasingly adopted in solar energy applications for its thermal stability up to 80 °C , the refractive index of 1.49, and excellent transmissivity [34]. With the aid of new molding of modern plastics supported by computer-controlled diamond turning machines, the materials composing Fresnel lens are manufactured with very high quality. As early as the 1980s, new developments were imposed on the traditional CPCs such as cooling techniques and geometrical shape with high efficiency. For the first time, a 300 W polar axis Fresnel lenses (40 cm by 40 cm) was developed with optical efficiency of 83% and 50% output power increase [34,35]. Non-imaging optics is applied in Fresnel lenses where many type of research is taking place [36].

Fig. 4. Conventional lens and Fresnel lens [34].

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characterized by the concentration ratio C = 1/sin θmax . It seems that all light rays within the acceptance angle can be collected as they hit the absorber. In the case of one important development, if the trough is filled with a dielectric material of index n , the concentration ratio C becomes C (n ) = n /sin θmax , which n times the previous one. The wedge CPC is another 2-D type of concentrator as schematically shown in Fig. 7 where the parameters of the parabolic mirrors are identically sharing same focal point. The vertex of the absorber, when the light hits, is located at the focal points of the two mirrors along the optical axis of the parabola. The light incident at θmax shown in Fig. 7 is concentrated to the focal point. At any other angle less than θmax , the light is focused at the absorber below the focal point. The concentration ratio C follows the same mathematical approach of C = 1/sin θmax . A recent study [44] explored the difference between CPC with a restricted 65° and 90° (known as CPC-65° and CPC-90°, respectively) and without restricted exit angle. CPC-65° and CPC-90° are PV-based systems and both having 20 ° half-acceptance angle and a geometrical concentration factor of 2x. The calculation showed that an increase of 3–5% and 8–10% annual performance for CPC-65° and CPC-90°, respectively.

3.2. Design of CPC For designing an CPC, the following parameters have to be taken into account: the type of CPC geometry, the initial conditions, the latitude or longitude, the thermal service needed, the size of the collector, the aperture, the acceptance angle, and the performance [14]. There are some differences in the designing CPC aimed at achieving a certain purpose such as CPC for refrigeration [37]; photocatalytic hydrogen production [38]; and for electrical production [39]. The design of a flat plastic static concentrator (FPSC) was carried out with an efficiency of 87.6% and 85.6% where mono-facial and bifacial PVC with a concentration ratio of 2 and 1.5, respectively [40]. The design relies on using an asymmetric concentrator with a wide range of acceptance angle. The dielectric materials used in the design has very low extinction coefficient to maintain a minimum loss [41] and to enhance the optical performance of CPCs [42]. CPCs are designed to concentrate solar radiation at the highest possible level on the aperture. In low-concentration systems, the solar rays were taken directly using a system known as a static solar connector. For tabular receivers, ideal for fluid uses, the highly reflective surface is used for parabolic section [21]. The CPC receiver represents a reactor through which the solar radiation is absorbed by photocatalyst before getting used or stored. The 2-D CPC is symmetric around one axis, which makes the illumination, directed to a line. The concentrating light is restrained theoretically by the second law of thermodynamics up to a theoretical limit and cannot extend to infinity. However, the maximum concentration ratio is derivable according to a strategy known as the concept of étendue [26]. The étendue of an optical system is a measure of the power transmitted along the beam. Étendue (ψ ) is defined as in Eq. (6):

ψ = n2a2 (θmax )2

3.4. Lens-walled CPC The design with lens-walled CPC by Guiqiang et al. [45] is shown in Fig. 8. The lens-walled CPC with air gap [46] was designed specifically for homes. The air gap between the lens and the reflector minimizes the total internal reflection and improves optical efficiency by more than 10% compared to the original design [47]. The mirror CPC was compared with another version of lens-walled CPC by [45] where a large acceptance angle was maintained. The lens-walled CPC has proven that it has more uniform flux distribution than the old version of mirror CPC with same concentration ratio. The flux distribution was investigated by fabricating a lens-walled compound parabolic concentrator photovoltaic (CCPC PV). The comparison shows that the fill factor (FF-factor) of the mirror CPC-PV dropped more sharply than that of the lens-walled CCPC-PV. This result indicates clearly that the lens-walled CCPC has better flux uniformity than the mirror CPC-PV. A new design of CPC is called air-gap-lens-walled CPC (ALCPC) shown in Fig. 8 was performed by Su et al. [48]. In this design, the area between the original and the modified curves is filled with a dielectric material [31].

(6)

where n is the refractive index, a is the aperture area and θmax the maximum extent of is still strike the exit aperture. The flat-plate concentrator (FPC) was modified with a static FPSC where the maximum power output increased by 2% [43]. The description of the CPC in r , θ coordinates, can be achieved by a series of equations (Eq. (7)) that are related to parameters in Fig. 5:

2fl 2f sin (φ − θmax ) r = Rsin (φ −θmax )−a′ = l z = Rcos (φ −θmax ) 1−cosφ 1−cosφ 2f cos (φ − θmax ) fl = a′[1 − cos (90 + θmax )] = a′(1 + sinθmax )2a′ = l 1−cosφ 2fl = 1−cos (90+θmax ) (7)

R=

3.5. V-trough concentrator The V-trough, shown in Fig. 9 is invented in 1952 [49]. The design is composed of two mirrors mounted at an angle α and were able to rotate around the axis of symmetry. The design requires that ray 1 incident at θmax as the ray strikes the edge of the exit of the aperture. It can be noted that any ray, say ray 2, strikes the mirror at the same angle, θmax , it will reflect out of the system. In this case, the design is not perfect because of the rays that reflected out of the system. The efficiency of V-trough is not promising; however, the design led to the construction of parabolic mirror design. The V-trough concentrator was experimented for higher maximum power and gain up to 1.5 the maximum power obtained from the traditional one [50].

3.3. Two-dimension CPC The 2-D design, shown in Fig. 3, is to be discussed first because it is considered as the basic principles of other designing architecture. The 2-D construction aims at achieving maximum concentrating ratio by utilizing a parabolic-mirror design enabling to avoid all those rays reflected out of the system as shown in Fig. 6(a). The 2-D CPC is

3.6. Multiple compound parabolic concentrator (MCPC) Another CPC, shown in Fig. 10, was designed to be used for photodegradation of carbaryl by using different geometric concentrator ratios and named as multiple compound parabolic concentrators (MCPC) [21]. In this design, a glass whose energy gap is 4.19 eV (equivalent to 296 nm), which acts as a UV filter before the threshold of 296 nm, was used in the MCPC. TiO2, as a catalyst, was immobilized over 1.40-m-long glass tube. At higher wavelength, the glass becomes transparent to radiation. By this spectrum, TiO2 can be excited since

Fig. 5. 2-D CPC the lower mirror is cut off by the dashed line to serve the parabolas [32].

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Fig. 6. (a) The basic principle of the parabolic optical mirror whose optical axis is the dashed line and its focal point is shown at f and (b) The original parabolas are rotated 20° from 1 to 1′ and from 2 to 2′ [32].

Fig. 9. A cross-sectional V-cone light consenser [32].

the maximum absorption of TiO2 is 330 nm which may decrease to 295 nm [51]. The results for MCPC degradation capability increased to levels of 54% and 92% depending on the MCPC solar concentrator [21]. 3.7. Three-dimension CPC The basic constructing principle of the 3-D CPC is performed by allowing the 2-D CPC shown in Fig. 5 to rotate around the z -axis. In this case, the light is composed of all rays in the meridian plane rather than only one conic section as in the 2-D CPC. The light is collected at the exit aperture similarly to the 2-D CPC cases. The most important difference here is that the light, which skews outside the acceptance angle, θmax is reflected back out of the concentrator instead of being collected at the aperture. The other difference is about the mathematical modelling of the 3-D CPC. Using polar coordinates (r , θ ), the following mathematical expressions could be achieved as shown in Eq. (8):

Fig. 7. The wedge CPC type where mirrors share same parameters including the focal points [32].

2fl sin (ψ ) sin (φ − θmax ) −a′sin (ψ ) 1−cosφ 2f cos (ψ ) sin (φ − θmax ) −a′cos (ψ ) y= l 1−cosφ x=

(8)

where ψ is the azimuth angle as illustrated in Fig. 11 which is introduced to accommodate the rotation of the 2-D CPC shown in Fig. 6. The design of the 3D CPC focuses on designing a two-stage concentration system rather than the normal design procedures

Fig. 8. Schematic air gap lese-walled CPC at normal direction [22].

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Fig. 10. MCPC (a) geometrical important parameters and (b) truncation process [21].

3.8. Asymmetric CPC The topography of the earth and the weather change make sun radiation arriving earth not symmetric. For this reason, an asymmetric CPC which allows light to enter between 10° and 60° from the horizontal becomes an option for some regions on earth. Fig. 12 represents an asymmetric CPC. The concentration ratio of asymmetric CPC is not as other CPC because the interval of acceptance is asymmetrical around the normal to the aperture. The derivation of C can be found elsewhere [32] where C is found to be 2.89. The asymmetric dielectric CPC with a wide range of acceptance angles is capable of collecting 40% solar radiation to be utilized in buildings in higher altitudes [56]. The maximum power ratio obtained from ACPC is up to 3.33× compared with the regular asymmetric CPC [16]. The asymmetric compound parabolic concentrator (ACPC) was modified by adding dielectric material (silicon elastomer Sylgard-184® of 90% transmittance value) to reach the total internal reflection and the construction can be rotated (RACPC) [57]. An asymmetric CPC with inverted absorber was developed by Farouk et al. [58] by introducing ray tracing to analyze the optical performance by adjusting the optical and investigate the possibility of using various receivers shapes such as tubes, wedges, and fins.

Fig. 11. The azimuth angle [32].

concerning the geometrical shape and the flux distribution [52]. 3D CPC has attracted researchers for several decades due to its advantage of the high quantity of radiation within the acceptance angle cone. In establishing a high power generator, high-temperature receivers are equipped with the 3D CPC in order to minimize the infrared and convection losses by using a small aperture size. The design of 3D CPC has progressed since 1989 when Welford and Winston [53] studied the 3D CPC transmissivity performance as a function of incident light. The major development was conducted by Lipinski and Steinfeld in 2006 [54] for re-designing the 3D CPC to capture any spilled solar radiation [54]. The efficiency of 3D CPC was finally improved by 30% [55]. The 3-D design is imperfect and, hence, it needs work to be improved to achieve an idealistic 3-D CPC construction. One of these improvements could be introduced by filling the space inside the CPC with a dielectric material of n > 1. Filling the space with specific dielectric material could be very expensive which reflects higher cost of CPC pushing it away from the commercial use. The trend of having big CPC size may be altered to manufacture small sizes filled with air and the only part that be filled with the dielectric material is the aperture area. In this case, the concentration ratio will increase by n as noted earlier for 2-D CPC or increase the acceptance angle. Briefly, keeping the size of the aperture intact and reducing the total size of CPC result in better-commercialized 3-D CPC. Although reaching the ideal theoretical limit of C = n2 /sin2 (θmax ) is not achievable at this time, the modification, which includes small size and filling the aperture area by dielectric material, may be considered a very important step towards that goal.

Fig. 12. Asymmetric CPC which allows light to enter between 10° and 60° with horizontal [32].

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Fig. 13. (a) Asymmetric truncated wedge CPC and (b) a special form of asymmetric truncated where the parabola is tilted by 25 ° [32].

CPC is considerably more than the stationary CPC because of better performance. In addition to using dielectric materials, lens-walled [72] and air-gap-lens-walled [73], are used in some cases. Improving reflectivity of CPC was discussed thoroughly in [7,30,65,67,74,75]. The third column of Table 1 shows the input parameters, which includes the half-acceptance angle, concentration gain, aperture size, concentration ratio, and the orientation. The most important parameter is the half-acceptance angle which varies from very low of 3° [76], to a medium level of 30° [11], and to the maximum level of 75 ° [77]. The concentration gain was found to vary from 1.53 [62] to the highest possible gain of 6.1 [66]. The third parameter is the aperture size which was found to be as small as 1 cm2 [70] to 32 m2 [78]. The concentration ratio is the fourth parameter presented in Table 1. The lowest concentration ratio of 0.92 for mini-CPC [79] while the highest ratio of 2.5 for lens-walled CPC [72]. It is also noted in Table 1 that the concentration ratio might be fixed or variables [29,75,80,81]. The orientation of CPC was introduced to deal with the latitudes which make this factor less important than the other previous parameters and it was only considered in [11,13] for E-W orientation. The findings are shown in the last column of Table 1. The findings cover several issues; however, the overall performance is the main issue. The annual performance has increased by about 80% [72] which represents the highest increase due to the use of lens-walled CPC while a moderate performance of 50% [23] due to the increase of the surface area. The optical efficiency has shown an increase by 95% [29] by using a variable concentrating technique, which allows more rays collection. A 60% improvement of average optical efficiency was reported by [82] due to using a tracking technique. The power generated was also determined by [1,63,83] and found that the highest increase was from 25.86 to 44.80 mW for 2-D extrusion symmetrical dielectric CPC.

The asymmetric truncated CPC shown in Fig. 13(a) was designed to be placed on a horizontal surface. The two mirrors are symmetrical; however, it is symmetrically truncated to collect as much radiation as possible. This type of CPC contains only one mirror which is able to collect radiation on both sides due to a circular section found between the end points of the parabolas. The asymmetric truncated CPC is characterized by designing mirrors with same focal length whose acceptance angle ranges between 20° and 65°. The concentration ratio C can be calculated from the formula C = 1/sin θmax , (20°<θmax <6°) which corresponds to C between 2.92 and 1.10. Fig. 13(b) shows that the front reflector is removed and the absorber is turned slightly. The parabola is rotated 25° to enable the mirror accepting radiation at a solar altitude above 25°. The efficiency of CPC is related to the truncation which is used in order to minimize the size and the cost of the CPC. These limitations were studied and analyzed at annual average performance for a variety of absorber surfaces. The heat flux and temperature distribution on the absorber were evaluated. It was found that the most effective truncation occurs at incident angle of 20 ? [23]. 3.9. Challenges and concerns CPC is a very important device in solar energy collection [37]. Throughout this review, some challenges and concerns in designing CPCs can be identified. Firstly, the incident rays that hit the reflector near the acceptance angle may cause hot spots on the absorber [59]. The hot spots and irregular temperature distribution on the receiver may cause undesirable effects [26] which widen the concerns of scientists regarding the annual radiation collection [13]. Another concern is about the east-west or north-south orientation of the CPC cells, which adds difficulty in determining the acceptance half-angle. For example, the east-west is favorable for higher altitudes while the optimal acceptance half-angle was found at 25.97° [13]. The other concern is the limitation of services needed for utilizing CPC cells based on the temperature level in residential or industrial establishment [60]. One of the drawbacks of CPC is the drop of FF factor as the temperature gradually increases. To overcome this drawback, the passive cooling mechanism such as using high-grade silicon or semiconductor reflectors can reliably reduce the effect of hot spots [61]. Table 1 contains the most important historical events of designing CPC with different geometrical types and dimensions. The main purpose of this table is to show the developments that have accompanied the construction of CPC. Table 1 includes the type of CPC, the input parameters used, and the findings of each type along with the reference. A comparison can be seen between utilizing 2-D [1,62–64], and 3-D [30,65–67], CPC regarding the symmetry [11,19,62,66,68], whether CPC is stationary [11] or rotating [65,66], or the use of the dielectric material [63,66,69–71]. It seems that use of the rotational

4. Solar cells 4.1. Basic principles and advantages of PV cells The development of using CPC based PV has shown that by 2011, PV was introduced in more than 80 countries and was considered as the fastest power generation technology with about 79% increase compared to the year of 2010 [44]. There are two types of PV cells depending on the nature of their operation: maximum power point tracking (MPPT) and direct-coupling system. MPPT works with DC-DC convertor [84] while the coupling system is connected directly to electrolyze [85]. The power (P ) is defined by the product of the current (I ) and the voltage (V ) or, simply, as P = IV . For the photocell, the maximum power generated occurs at maximum voltage and maximum current as Pmax = Imax Vmax . Based on these principles, the fill factor FF is defined as in Eq. (9): 1114

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Table 1 Summary of various CPC designs and their applications. Reference

Type of CPC

Input parameters

Findings

[62]

Concentration gain 1.53, exit aperture width 14.4 cm, total height 12 cm and half-acceptance angle 35°

Increased the annual energy output by 2.6% compared with the non-concentrating system. CAP is 0.88

Concentration gain 2.41, exit aperture width 1 cm, total height of 2.7 cm and half-acceptance angle 36.8°

Increased the electrical power from 25.86 mW to 44.80 mW when compared with non-concentrating PV. CAP is 1.44

Half-acceptance angle is 30°

[7]

2D extrusionsymmetrical reflective CPC 2D extrusionsymmetrical dielectric DCPC Reflective 3D rotationally symmetric CPC Dielectric 3D rotationally symmetric CPC Reflective ACPC

Concentration gain 6.1, refraction index 1.5 and half-acceptance angle 10° Concentration gain 2

[69]

Dielectric ACPC

Concentration gain 2.8

[67]

Reflective 3D CCPC

[70]

Dielectric 3D CCPC

[64] [83]

Two reflective 2D CPC N/A

[80] [30]

CPC-CPV/T Reflector 3D CPC

[76]

External CPC

Concentration gain 3.61, half-acceptance angle 30°, total height of 1.616 cm and aperture 1 cm by 1 cm Concentration gain 3.61, total height 1.616 cm, half-acceptance angle 35° and aperture 1 cm by 1 cm half-acceptance angle 30° Concentration ratio 4, half acceptance angle as low as 12.5°, concentration ratio C=2.5 Concentration ratio 4× Optical polar-axis aligned CPCs according to extra-terrestrial radiation Concentration ratios 3.06 (3×) and 6.03 (6×) and half-acceptance angles of 10° and 3°, respectively.

The transmission-angle curve of the reflective design results in almost ideal step-like behavior CPC/V produces a 5.7 more short circuit current when compared with a bare solar cell. CAP is 0.43 62% more maximum electrical output power compared with a non-concentrating system, maximum optical efficiency is 85.85% 80.5% more maximum optical efficiency, increased electrical power ratio to 2.27 when compared with non-concentrating system Generated a maximum power concentration of 3 when compared non-concentrating module. CAP is 0.95 Maximum optical efficiency 73.4%, maximum power ratio 2.67 when compared with non-concentrating design. CAP is 1.09 N/A Optical 0.71 0.5 kWp, yearly output 250 kWh of electricity per square metre, 800 kWh of heat at low temperatures Eliminating multiple reflections EW-CPCs annual collection about 65–74%) (1.26–1.45)

[74]

Reflectors CPC

Glazed photocell, trade-off between electricity and heat

[23]

Reflector CPCs

Improved acceptance angle, surface area increased by 5%

[77]

Double tube configuration, acceptance angles of 30°, 40° and 60°

[79]

Elliptical single receiver CPC Mini-CPC

[75]

Reflective CPC

Variable concentration ratios from 3.6× to 4×

[29] [19] [73] [44] [81]

3D CPC Symmetrical CPC Air-gap-lens-walled CPC CPC variable acceptance angle A linear asymmetric CPC

[68]

Satisfactory CPC

Concentration ratio 3.6×, square entry and exit aperture Concentration ratio 2.3×. Half acceptance angle 35° Both CPC-65 and CPC-90 are identical in the acceptance halfangle (20°) and geometrical concentration factor (2x). Incident angles of 0° and 55, geometrical concentration ratio 2.8×. PCM tank and CPC was integrated into one piece for simple structure for PCM storage in same unit

[71]

Dielectric-filled CPC

[13]

CPC-Oriented based

[82]

Tracking CPC

[1]

Fresnel 2-D CPC

[11]

Stationary low concentrated CPC

[78]

Simplified CPC

A new simulation model at latitude 29°52’N, longitude 31°21’E and elevation 141 m Acceptance angle 30°, concentration 1.2, maximum concentration (C) between 1 and 2, occur for (C=2), East–West orientation, concentration of the truncated cavity between (1.0–1.2) Total aperture area of 32 m2

[72]

Lens-walled CPC

Concentration ratio 2.5, nominal half acceptance angle 23.5°

[63]

[65] [66]

FF =

Imax Vmax Isc Voc

Concentrating ratio 0.92

Integrated optics on the rear side of a planar bifacial silicon solar cell together with a 25% Er3+ doped hexagonal sodium yttrium fluoride (β-NaYF4:Er) UC phosphor East–west aligned symmetric CPCs, acceptance half-angle between 25.3° and 26°, yearly optimal tilt-angle with a deviation less than 1° Concentration ratio of 2.3

FF0 =

(9)

where Voc is the open-circuit voltage and Isc is the short circuit current. In another approach, FF of an ideal characteristics (FF0 ) was determined analytically [86] by Eq. (10):

Intermediate temperature within 80 °C and 250 °C, daily thermal efficiencies of 3× and 6× CPC collectors can reach 40% and 46% at the collecting temperature of 200 °C, respectively. Reducing PV cell material by a factor 2–3, increase heat, less electricity Annual performance is 50% better than truncated CPC, reducing hot spots Heat transferred to the cooling water by 21%, 19.8% and 18.3% improve the electrical efficiency by 3.3% and 48.6% when solar radiation and water temperature are 800 Wm-2 and 20 °C Maximum value reached 321.5 K at the front wall under 50° incidence Maximum optical efficiency 95%. Power output is higher than that of the flat PV by 39–23% optical efficiency of 83.0% CPV-90 annually concentrated about 3–5% more radiation on solar cells as compared to CPC-65 based An increase of 16% in the average power output and extra 6% by using a reflective film. Thermal efficiency varies from 40–50% for clear day and around 40% for semi-cloudy day. The overall efficiency of the PVT collector between 55–63% for clear-day and around 46–55% for semi-cloudy day An efficiency increase of 32% under sub-band-gap, external quantum efficiency (EQE) from 1.33% for the non-concentrating to 1.80% for a solar cell with integrated optics Maximum annual average optical concentration ratios ranged from 1.45 to 1.74 Average optical efficiency is over 60%, TCPC collectors during the test period increased by 1.9–2.3 times higher than that at the FM Average laser output power of 1.27 W in Winter, 2 W in Spring, 5 W in Summer and 4.68 W in Autumn Cost-benefit relationship enable to conclude that a good choice for a well-designed collector Produce steam exceeding 200 °C with pressure ranging from 0.10 to 0.55 MPa, efficiency over 0.30 Achieve about 80% of the solid CPC's performance and 20–30% larger than the mirror CPC.

Voc − ln (Voc+0. 72) Voc+1

(10)

The advantages of PV cells can be summarized as follows: PV cells replace the expensive silicon or GaAs with low-cost of material used, higher efficiency, electric power producing, and fewer materials used in 1115

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successful attempts to achieve better performance. One of these attempts was performed by generating electricity using a V-trough CPC with an average concentration ratio of 1.9 reaching the 1.9 A [90]. In another attempt, an analysis has shown that the maximum power ratio of 3.7 was reached, however with the mechanical loss, the power ratio was declined to 3.4 [91]. A third attempt was performed by proposing a low concentration ratio with a sun tracker built on one-axis three positions angles to track the sunlight in the morning, noon, and afternoon. The experimental results show that the PV power generation can increase by about 23% while by using one-axis three positions, the total increase may hit 56% [92]. There are two designs worth to be explained for their specific importance. The first one aimed at replacing the expensive crystalline silicon by a thin film PV module based on Cu(In,Ga)Se2 [93]. The other design includes a concentrating photovoltaic (CPV) module and concentrating dielectric compound parabolic concentrator for outdoor applications having acceptance half angles up to 55 ° and concentration ratio of 2.8. The CPC module of area 300 mm × 300 mm with 2 strings of 14 solar cells in series. The design was operated during a cloudy and rainy day with sunny interval environment and the maximum power was increased for the same environment by 2.22 times the flat-plate module while in rainy days, the maximum power was reported at 2.17 time the corresponding power with the flat-plate module [94].

PV cells. On another hand, the disadvantages can be seen in two issues: the operating temperatures increases as solar radiation increases which negatively affects the performance of PV cells and the second disadvantage is that PV cells use direct sunlight which requires a tracking system [9]. Experimental implementation of CPC based PV started when a comparison was made between symmetric and an asymmetric CPC and the results of an asymmetric CPC-PV of concentration factor of 2.01 has shown that the performance increased by 62% while the temperature of the solar cell increased by only 12 ? [7]. In an attempt to increase FF, Hatwaambo used semi-diffuse reflective materials as CPC reflectors and the results showed that the performance has slightly increased [61]. However, there is an adverse result of using semidiffuse reflectors by creating non-uniformity of the reflected rays [44]. Regarding the uniformity of the distributing solar intensity, it is found that CPC-V-trough is better than the traditional CPCs despite the fact that CPC-V-trough is not an ideal concentrator [87,88]. 4.2. Integrating PV As of 2013, there is a huge missing technical information about the Photovoltaic Thermal Solar Collectors (PVT) collectors which, unfortunately, led to imperfect the design [89]. However, there are very

Fig. 14. CPC cross-sectional of (a) U-tube receiver with no fins and (b) a hybrid bifacially flat receiver [98].

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more compact CPC operating at low temperatures. In addition to Li et al. [76] work, a hybrid solar hot water, and Bi2Te3-based thermoelectric generator (TEG) unit with a mini-compound parabolic concentrator (m -CPC) was proposed [79]. The investigation shows that m -CPC can significantly improve the electrical efficiency. The temperature is improved from 100 ? to 200 ? by using geometrical concentrator ratio at 0.92. Another advanced design was performed for n -sided inlet and outlet polygonal CPCs as shown in Fig. 15. The formation of polygonal CPS is explained graphically in the figure by circumscribing regular polygons about the circular apertures of an underlying revolved CPC with minimum gap loss for applications, which require multiple absorbers. Using Monte-Carlo ray tracing, the variation of the optical efficiency with reflectivity, geometry, and surface errors were studied. The calculations showed that the square CPC (n = 4) has some favorable anomalous behavior over the designs of n = 5 and 6 sides for small acceptance angles [65]. The one-axis-three positions sun-tracking polar axis (3P-CPCs) was proposed and theoretically studied with PV-applications. The design contains a trough-like oriented in the polar axis while the aperture can be daily adjusted in three directions in the morning and mid-day was investigated analytically to determine the performance. The results showed that the half-acceptance angle is the primary factor in determining the performance [30]. Finally, one of the very important developments that aimed at working towards more efficient solar photovoltaic modules due to the lower efficiency of the traditional PVs. Recently, two technologies have emerged under the name of solar photovoltaic thermal hybrid technology, which is able to produce both electric power and heat. However, one drawback of this technology comes from the low solar energy absorption and high thermal resistance at the interface between PV and the cooling medium. A possible solution was proposed by [99] to laminate a copper oxide– water nanofluid sheet attached to the silicon cell in order to reduce the thermal resistance. Experimentally, it was proven that the nanofluid made a significant improvement in the thermal performance compared to water. At the end of this section, it is important to point out the basic and most recent papers with the parameters they impose on the PV design, the efficiency, and the nature of work. A comparison among selected

5. Recent developments In this section, the most important recent developments in the field of CPC and CPV are added. The goal of this section is to show how the performance and/or the technology have been evolving in the recent years. In the first two recent developments, the overall efficiency is discussed. The first one was about filling the CPC with air and tested for performance in outdoor conditions. The simulation results revealed that the average optical efficiency for a half acceptance angle of 35° was about 60% and the overall efficiency of ALCPC has reached 47.3% and the final temperature obtained was 70 ? [73]. In another design known as the crossed compound parabolic concentrator (CCPC) which was designed and experimented generating 3 times the more maximum power than non-crossed one [29,67]. Another a new technique was developed to reduce the heat associated with CPC by introducing fins at the flat absorber. In [95], several parameters such as the length, packing factor, and flow rate are considered to determine the performance and compare it with un-finned CPC-PVT. It was found that the thermal gain of 1% has been achieved, while the annular electric gain was 3% higher than the un-finned CPC-PVT. The thermal performance efficiency of replacing the traditional reflector of CPC by truncated the size of the reflectors has resulted in improving the thermal performance efficiency improved up to 71% [96]. A new CPC design which was started by Jadhav et al. [97] and continued by Kuo et al. [14] showed that the vertical position of the receiver was adjusted by setting the height of the receiver to 0.46 times the aperture width. By this modification, it is found that greater collection range of incident rays was achieved without affecting the concentration ratio. Another design to reduce the losses caused by truncation was performed analytically by assuming the area of the aperture as 2.1 m2, acceptance angle at 30°, and keeping the concentration ratio at 1.8 [20]. Another design, shown in Fig. 14, combines the hybrid U-tabular shaped receiver and a bifacially flat receiver CPC which results in achieving medium temperature range between 150 °C and 300 °C [98]. The peculiarity of this design suggests that CPC favors parabolic trough collectors because CPC does not need tracking and the ability to collect some diffuse radiation. In 2013, Li et al. [76] developed and investigated a cheaper and

Fig. 15. Flux distribution at the outlet at circular and various polygonal CPCs with half acceptance angle (a) 5 °, (b) 30 °, (c) 45 ° [65].

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traditional use of silicon while the efficiency increased to higher values due to using other depositing layers of InGaP/InGaAs/Ga. The nature of the study is reflected in the last column of Table 2 which is either thermal, electrical, or both using simulation [108,118,128], modelling [124,129–132], or both [121,128,133–135].

published papers regarding the tracking technique, PV type, efficiency, and the nature of the study are shown in Table 2. The tracking mechanism of CPVT can be one- or two-axes. Table 2 shows both tracking mechanisms; however, the majority related to 2axis mechanism. The second column is about the type of PV cell. Table 2 shows five PV types which still reflect the traditional type of using silicon [100–107] or mono silicone [33,83,100,108–117]. Other PV types are InGaP/InGaAS/Ge [108,118,119], GaAs [120], GaAs/Ge [121], ad GalnP/GaAs/Ge [122,123]. The efficiency (electrical/thermal) is tabulated in the fourth column. The highest efficiency –electric wise- ranges from the lowest value of 2.5% [124–126] to the highest reported value of 41.5% [127]. In addition, the efficiency –thermal wise- started at 16% [103] and was increased to 69% [123]. The efficiency variation, electrical or thermal, could be attributed to the

6. The cost versus design The ever lasted increasing demands for electricity and with the increasing security demand for safe electric power supplies, the need for other sources of electricity has become a must [9]. The stationary non-evacuated CPC solar collectors are of great interest compared to the parabolic trough and flat plate collectors. The first CPC collector for absorbing and trapping maximum solar radiation was in 1999 [159]. In

Table 2 Comparison of tracking, PV type and efficiency of CPVT. Reference

Tracking

[109,110,136] [33,100,137] [100,101] [108,118]

One-axis One-axis One-axis One-axis

[119]

Two-axis

[113] [108] [138] [129] [139] [140] [141] [142] [130] [101] [143] [120] [121] [133] [144] [131] [145] [146] [147] [148,149] [122] [127] [132] [102] [150] [151] [134] [114] [135] [152] [104] [153] [123] [154] [124–126] [105] [155] [156] [115] [116] [106,157] [158] [83] [117] [107]

PV type

Efficiency (%)

Nature of study

Electrical

Thermal

8 11 8 20–25

50 58 45–70 50–60

Thermal/electrical Thermal/electrical Thermal/electrical Thermal/electrical

19–25

50–60

Thermal/electrical modelling, simulation, and parametric studies

None None Two-axis None One-axis Two-axis None None None Two-axis Two-axis Two-axis NA Two-axis Two-axis NA

Mono-Si Mono-Si Si InGaP/InGaAs/ Ge InGaP/InGaAs/ Ge Mono-Si Mono-Si Si Mono-Si Mono-Si Si N/A N/A Si Si NA GaAs GaAs/Ge Si Mono-Si Si

5–10 10.4 N/A 9.83 11.99 18 9 17.5 16 N/A 25 19.9 15.6 N/A 15.2 7.3

50–60 62.2 N/A 55.9 N/A N/A 66 N/A 71 N/A 53 N/A 72.2 65.8 49.9 40

NA NA NA One-axis Two-axis None NA Two-axis NA MonoOne-axis None None None Two-axis One-axis Two-axis Two-axis None None One-axis None None Two-axis None One-axis One-axis None None None

Si Si N/A N/A GaInP/GaAs/ Ge Si NA Si Si NA NA Mono-Si NA MJPV Si MJPV GaInP/GaAs/ Ge a-Si NA Si Mono-Si NA Mono-Si Mono-Si Si Mono-Si Mono-Si Mono-Si Si

N/A 20–25 N/A 8.5 34.75 41.5 24 15.8 10.02 N/A 8.5 3.35 N/A 20.5 N/A 18 20 N/A 2.5 11 16.4 30 5.1 3.69 N/A 6.4 N/A N/A 13.9

N/A N/A N/A N/A N/A 0.0378 48 65 16 N/A 60 33.5 52 70 N/A 56 69 N/A 69 55 50 35 26 36.98 N/A 45 N/A N/A N/A

General proposed design Thermal/electrical performance (various concentration ratios) Optical modelling, simulation, parametric studies, and optimization Thermal/electrical modelling, investigate weather conditions effect Optical modelling, simulation, and parametric studies Optical modelling and simulation and electrical performance assessment Optical ray tracing simulation and thermal and electrical modelling and numerical simulation Thermal/electrical testing and performance assessment Thermal/electrical modelling and performance assessment Description and preliminary analysis of a built prototype Optical, thermal, and electrical performance assessment Validation of a developed model using a test prototype Thermal, electrical, and finite element fatigue modelling, simulation, and parametric studies Thermal/electrical dynamic modelling, simulation, and validation Optical, thermal, and electrical modelling and performance assessment Genetic algorithm optimization of HTF and optical, thermal, and electrical modelling, performance assessment, and parametric studies Electrical modelling and validation of CPV silicon cells Optical/electrical performance assessment Thermal/electrical modelling and parametric studies Thermal/electrical modelling, performance assessment, and parametric studies Thermal/electrical modelling and simulation Thermal/electrical performance assessment Thermal/electrical modelling and parametric studies Thermal/electrical performance assessment Thermal/electrical modelling, simulation, and performance assessment Modifications description Thermal/electrical modelling, simulation, and performance assessment Thermal/electrical modelling, simulation, and performance assessment Thermal/electrical modelling, performance assessment, and parametric studies Thermal/electrical performance assessment Lifecycle assessment Thermal/electrical, and economic performance assessment Thermal/electrical, and economic modelling, performance assessment, and parametric studies Thermal/electrical performance assessment Thermal/electrical modelling, performance assessment, and parametric studies Optical, thermal, and electrical simulation and performance assessment Thermal/electrical performance assessment Thermal/electrical performance assessment Thermal/electrical performance assessment Thermal/electrical performance assessment and optimum orientation optimization Ray tracing and optical, thermal, and electrical modelling, simulation, and parametric studies Standardized testing procedure and thermal and electrical performance assessment Optical, thermal, and electrical performance assessment Optical, thermal, and electrical performance assessment Optical and electrical performance assessment

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[10] Kim Y, Han G, Seo T. An evaluation on thermal performance of CPC solar collector. Int Commun Heat Mass Transf 2008;35:446–57. [11] Fraidenraich N, Tiba C, Brandão BB, Vilela OC. Analytic solutions for the geometric and optical properties of stationary compound parabolic concentrators with fully illuminated inverted V receiver. Sol Energy 2008;82:132–43. [12] Tripanagnostopoulos Y, Yianoulis P, Papaefthimiou S, Zafeiratos S. CPC solar collectors with flat bifacial absorbers. Sol Energy 2000;69:191–203. [13] Tang R, Wu M, Yu Y, Li M. Optical performance of fixed east–west aligned CPCs used in China. Renew Energy 2010;35:1837–41. [14] Kuo C-W, Yen P-S, Chang W-C, Chang K-C. The design and optical analysis of compound parabolic collector. Procedia Eng 2014;79:258–62. [15] Antonelli M, Baccioli A, Francesconi M, Lensi R, Martorano L. Analysis of a low concentration solar plant with compound parabolic collectors and a rotary expander for electricity generation. 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2010, another group designed a CPC collector which provides a vacuum enclosure for the collector and absorber by which the performance has increased but with increasing cost [160]. A hybrid PV/Thermal collector which was improved to deal with temperatures as high as 200 °C. The reflector on CPC, the most expensive part, was subjected to a numerous number of improvements. For this reason, a modified CPC reflector curves were designed and tested to overcome the multiple reflections and big size. The modified CPC was tested for lowtemperature range steam generation [96]. The cost associated with the solar energy due to photovoltaic cell still hinders the economic use of this source of energy. In order to minimize the gap, concentrating solar system represents the most important step towards the goal of affordable solar energy. The number of the PV modules or thermal receivers can be adjusted through increasing the outputs the using optical materials to increase concentrating radiation on a small area such as the use of compound CPC or truncated CPC [77]. 7. Conclusions The journey through the continual developments on CPC, CPC-PV, CPC/V, and CPC/T has asserted the strong willingness of researchers to develop better designs with a lesser cost to avoid conventional sources of energy. The design of CPC and PV is the cornerstone of these developments. It was found through this review that the geometrical shape, the size and type of the reflectors, and the photocell materials are the essential parts of the design. The 3-D CPC has shown better performance than the 2-CPC. It was also noted that truncation plays an important role in lowering the cost. Tracking mechanism has also shown a positive effect on performance, yet it might feasible to design multi CPCs that enables capturing sunrays without the need for tracking system due to technical difficulties. CPC with large acceptance angle may not require implementing tracking mechanism; however, for short periods, this type of applications requires higher concentration ratio. The non-imaging Fresnel lens is suitable for large collection of sunlight; however Fresnel convex-lens and dome-shaped have been recently introduced and proven better solar rays collection. Nonimaging Fresnel lens systems are very competitive solar collectors because of their high optical efficiency, light-weight and cost effectiveness. Fresnel lens solar concentration systems are expected to be used extensively in the field of commercial solar power generation. Adversely to non-imaging systems, imaging systems are very sensitive to tracking and may create undefined focal areas which leads, in particular, to hot spots problem on oversized receivers. To minimize the cost due to employing tracking mechanism, the requirements of this mechanism should be kept at minimum. References [1] Abdel-Hadi YA, Ghitas A, Abulwfa A, Sabry M. Simulation model of a new solar laser system of Fresnel lens according to real observed solar radiation data in Helwan of Egypt. NRIAG J Astron Geophys 2015;4:249–55. [2] Winston R. Principles of solar concentrators of a novel design. Sol Energy 1974;16:89–95. [3] Welford W, Winston R. High collection nonimaging optics academic. San Diego, Calif, vol. 19892; 1989.p. 55. [4] Adsten M, Helgesson A, Karlsson B. Evaluation of CPC-collector designs for standalone, roof-or wall installation. Sol Energy 2005;79:638–47. [5] Gang P, Guiqiang L, Xi Z, Jie J, Yuehong S. Experimental study and exergetic analysis of a CPC-type solar water heater system using higher-temperature circulation in winter. Sol Energy 2012;86:1280–6. [6] Hatwaambo S, Hakansson H, Nilsson J, Karlsson B. Angular characterization of low concentrating PV–CPC using low-cost reflectors. Sol Energy Mater Sol Cells 2008;92:1347–51. [7] Mallick TK, Eames PC, Norton B. Non-concentrating and asymmetric compound parabolic concentrating building façade integrated photovoltaics: an experimental comparison. Sol Energy 2006;80:834–49. [8] Harmim A, Merzouk M, Boukar M, Amar M. Performance study of a box-type solar cooker employing an asymmetric compound parabolic concentrator. Energy 2012;47:471–80. [9] Zahedi A. Review of modelling details in relation to low-concentration solar concentrating photovoltaic. Renew Sustain Energy Rev 2011;15:1609–14.

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