A comprehensive review of Uniform Solar Illumination at Low Concentration Photovoltaic (LCPV) Systems

A comprehensive review of Uniform Solar Illumination at Low Concentration Photovoltaic (LCPV) Systems

Renewable and Sustainable Energy Reviews 60 (2016) 1430–1441 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews jour...

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Renewable and Sustainable Energy Reviews 60 (2016) 1430–1441

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

A comprehensive review of Uniform Solar Illumination at Low Concentration Photovoltaic (LCPV) Systems Yasaman Amanlou a, Teymour Tavakoli Hashjin a,n, Barat Ghobadian a, G. Najafi a,nn, R. Mamat b a b

Biosystems Engineering Department, Tarbiat Modares University, Tehran, Iran Faculty of Mechanical Engineering/Automotive Engineering Centre, Universiti Malaysia Pahang, Pekan, Pahang, Malaysia

art ic l e i nf o

a b s t r a c t

Article history: Received 18 January 2015 Received in revised form 10 January 2016 Accepted 8 March 2016

Conventional high performance silicon solar cells have a potential to generate more electricity by using low concentrating reflectors. Static solar concentrators reduce the cost of photovoltaic systems for given electrical power demand. However, non-uniform illumination on the conventional rectangular photovoltaic panel causes ohmic drops, mainly due to the cell that operates locally at higher irradiance. In this research study a comprehensive review has been carried out regarding Uniform Solar Illumination at Low Concentration Photovoltaic (LCPV) Systems. Another objective of the present study is therefore, calculating the pattern of sun incident at low concentration ratios for reflective troughs (V-type, cylindrical and compound parabolic concentrators) and linear Fresnel reflectors. The geometrical parameters of these concentrators were studied to obtain uniform illumination on the common rectangular photovoltaic panels. The designed concentrator with most uniform flux distribution, high concentration ratio and low requirement of mirror was fabricated and tested at ambient conditions. The optical simulation output of different concentrators illustrated the linear Fresnel reflector had uniform irradiance on the photovoltaic panel with standard deviation less than 30% of total income radiation. The experimental results showed that the linear Fresnel reflector has the potential to harvest more energy when using standard silicon solar cells in a basic concentration configuration. Finally thermal, electrical and total performances of a photovoltaic/thermal flat collector were measured with and without concentrator. Using the concentrator improved thermal and overall efficiency by 16% and 17.5% respectively. The maximum overall efficiency for PVT collector with concentrator and without concentrator was 91% and 78% respectively. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Reflective concentrator Optical simulation Electrical performance Conventional solar cell Low Concentration Photovoltaic (LCPV) System

Contents 1. 2.

3.

4.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Concentration ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Uniformity definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Type of optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Flat concentrators or V-trough . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Parabolic concentrator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Cylindrical troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Linear Fresnel reflectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis different types of concentrators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Flat concentrators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Parabolic concentrator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. Tel.: þ 0098 21 48292318; fax: þ 98 21 48292200. Corresponding author. Tel.: þ 98 21 48292322; fax: þ98 21 48292200. E-mail addresses: [email protected] (T.T. Hashjin), g.najafi@modares.ac.ir (G. Najafi).

nn

http://dx.doi.org/10.1016/j.rser.2016.03.032 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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4.3. Cylindrical trough . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Linear Fresnel reflectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Experimental device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction With the growing problems surrounding global warming, solar photovoltaic (PV) technology is getting more attraction for electricity generation. PV cells are semiconductor devices that have the ability to convert the energy available in both dispersed and concentrated solar radiation into direct current (DC) electricity [1]. The development of the photovoltaic technology in the last years has been fuelled by the implementation of various supporting strategies [2–18]. Solar energy is probably the strongest-growing electricity generation technology, demonstrating recent annual growth rates of around 37% [19]. However, the use of photovoltaic (PV) system is limited due to its prohibitively high cost. In a PV system, the PV cell material contributes about 50% of the total cost. Thus, one of effective measures to reduce the cost of electricity generated by a PV system is to reduce the use of solar cells for given power demand, and this can be achieved by solar concentrators [20–30]. Various types of concentrators can be classified according to their concentration ratio namely low, medium and high concentration systems [31,32]. Low concentration photovoltaic (LCPV) systems have the potential to reduce the cost per kWh of electricity compared to conventional flat-plate photovoltaic (PV) [33– 36]. For medium and high concentration ratio, concentrating PV system seems to be difficult to implement. High concentrating PV system needs specially designed solar cells, complicated suntracking device and cooling techniques to control the high temperature of cell [37]. In case of medium and high concentration, common Si wafer based solar cells are not suitable because of negative temperature coefficient whereas about 80% to 90% of PV cells manufactured worldwide are Si wafer based solar cells [38–40]. Low concentration photovoltaic (LCPV) systems can make use of conventional high performance silicon solar cells (made for 1 sun application) [41]. In this technology, the commercial Si solar cell is used under the concentration of 2 suns to 10 suns. Therefore, low concentrator optics in static or quasi-static mode, wherein the continuous suntracking system is eliminated, was widely interested. Nevertheless, the success of the LCPV technology depends on two conditions: the first is that the optical surface collecting the light and redirecting it to the cells had to be cheaper than the cell area in replace; and the second is that the efficiency of the cells under concentrated sunlight should not decrease substantially. Uniform illumination is affirmed by the second condition in case of flat plate modules. Different optical designs for solar concentrators have been recommended by some researchers to concentrate light into the focal point or line, but there is not sufficient information about uniform concentration on the rectangular flat plate. Despite extensive research has been carried out on linear and twodimensional concentrators, but rare studies have focused on the design of concentrators which illuminate a flat plate rectangular PV panels uniformly [42–45]. In this study a comprehensive review has been carried out regarding uniform solar illumination at low concentration

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photovoltaic (LCPV) Systems. Then pattern of sun incident has been calculated at low concentration ratios for different conventional concentrators and adapt concentrators geometry to obtain uniform illumination on the common rectangular PV panels. Finally the best type of concentrator has been fabricated and applied to photovoltaic/thermal (PV/T) flat plate collector and performance of the collector has been reported with and without used concentrator.

2. Optical characteristic 2.1. Concentration ratio There are several definitions of concentration ratio in use. The most common is “geometric concentration ratio”. This is defined as the area of the primary lens or mirror divided by the active cell area [46–53]. The active cell area is the region of the cell that is designed to be illuminated. The non-illuminated edge of the cell is often provided with bus bars for electrical connection, and this need not result in an efficiency loss as would be the case in a flatplate module. In the other word, definition of concentration ratio (C) is the addimentional ratio of the area of aperture (Aa ) to the area of the receiver (Ar ). C¼

Aa Ar

ð1Þ

Another measure of the concentration is intensity concentration, or “Suns”. Since standard peak solar irradiance is often set at 0.1 (W/cm2), the “suns” concentration is defined as the focused light on the cell active area divided by 0.1 (W/cm2) [54,55]. 2.2. Uniformity definition The purpose of the optical system is to concentrate sunlight and direct it to the solar cell uniformly, but this might be uneasiness by the optical element and solar tracking system [56]. The non-uniformity in illumination profile causes several problems in the functioning of the concentrated photovoltaic (CPV) system. The non-uniform illumination produces ohmic drops higher than expected, mainly due to the cell operates locally at higher irradiance [57–61]. In a uniformly illuminated solar cell it is found that an internal current flows even in open-circuit conditions, which is directly proportional to the irradiance and the degree of non-uniformity [62]. The cell irradiance non-uniformity decreases the open-circuit voltage (or, equivalently, increases the effective series resistance) of the cell, thereby reducing its photovoltaic conversion efficiency [63–66]. Concentrating sunlight onto the small solar cell causes localized thermal heating. Non-uniform temperature distribution on the cell affected the heat transfer from cell and negative temperature coefficient reduces the thermal performance.

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Irradiance uniformity is also important because not only the cell efficiency depends on it, but also to assure the long term cell and concentrator reliability [67]. Point focus or linear focus concentrators generate hot spot on the solar cell. Controlling hot spots on the solar cell is a great challenge for researchers. Zou and Yang [68] designed a combine concentrator to achieve high irradiance uniformity on the solar cell in a high-concentration photovoltaic module. They reported that the enhanced cell irradiance uniformity enables a PV module to gain an increased photovoltaic conversion efficiency of 28%. Zhuang and Yu [69] improved uniformity irradiance distribution on the solar cell without using secondary optical element (SOE) in the concentrator photovoltaic (CPV) system to increase the solar cell conversion efficiency. To measure the intensity of non-uniformity some statistical term such as Standard Deviation, Mean and difference between maximum and minimum incoming flux are employed [70,71]. In many cases, uniformity of irradiance just compare between flux distribution pattern. Inaccurate designed concentrators can reflect part of the sunlight out of the target absorber (PV cells). Therefore the criterion is needed to explain the efficiency of a given concentrator. The optical efficiency is defined as the fraction of the solar radiation reflecting by concentrator that is absorbed by the PV collector and is given as:

η¼

Q ab I b Aa

ð2Þ

Useful length of mirror (L) is dependent on reflector angle (α) and width of PV panel. The L would be calculate by trigonometric relationships for different amount of α. Following Eqs. (3, 4) define the relationships between parameters: π  π ð3Þ β ¼ α   α ¼ 2α  2 2 L¼W

 sin 2α  π2  sin π2  α

ð4Þ

Kostic et al. [75] surveyed the influence of reflectance from flat aluminum concentrators on energy efficiency of PV/Thermal collector (Fig. 2). The flat aluminum has a simple structure and resulted in appropriate enhancement of thermal and electrical efficiency. The flat concentrators are interesting for their uniform concentration, simple structure and low cost. However, low concentration ratio (usually lower than 3) limited their application [75–78]. Sangani and Solanki [79] designed and fabricated a V-trough (flat-mirror) photovoltaic concentrator system to assess PV electricity cost ($/W) reduction. The PV concentrator system had 2 suns concentration intensity. They developed their PV concentrator system for different types of tracking modes such as seasonal, one axis north-south and two axis tracing. This concentrator increases the output power by 44% as compared to PV flat-plate system for passive cooled modules.

Where, Q ab refers to the energy absorbed by receiver, I b is the energy incident on the aperture per unit area.

3. Type of optics Concentrator is an optical devise it able to concentrate a large area of sunlight onto a small area. Various types of solar concentrators have been developed in recent years and have been tested for their suitability for various processes. Concentrator optics can classified to refractive lenses and reflective dishes and troughs. Also, concentrator optics have categorized to point-focus which have circular symmetry about their axis and linear focus, in which the lens has a constant cross section along a transverse axis (focus the light into the line) [72]. Linear concentrators include troughs (V-type, cylindrical and compound parabolic concentrators) and linear Fresnel concentrators. Linear Concentrators concentrate sun light on a linear focus. Therefore, these concentrators can be used to illuminate a linear array of photovoltaic cells [73,74]. Point-focus concentrators concentrate parallel sun rays at one focal point. Point-focus lens usually have high concentration ratio and they can’t be used commercially to illuminate a flat plate rectangular PV panel. The architectural designs and optical principles for various solar reflective concentrators for the LCPV systems are presented in the following section.

Fig. 1. The scheme of flat reflector.

3.1. Flat concentrators or V-trough Using a flat-plate mirror to reflect more sunlight on a flat plate rectangular PV is the simplest method for uniform sunlight concentration [75–77]. The simple scheme of flat reflector is shown in Fig. 1. Angle of mirror with horizon (α) specifies the useful length of mirror (L). The useful length of mirror (L) equals the width of mirror, which enables to reflect whole received sunlight to the photovoltaic plate. β is the incidence angle of reflective beam to the panel.

Fig. 2. Schematic diagram of the PV/T collector with solar radiation concentrators [75].

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Mosalam-Shaltout et al. [80] studied the performance of a LCPV for a year and compared the results with the performance of simple PV system. This research showed a good improvement in PV performance which was equipped with a flat plate concentrator (V-trough). The concentration ratio at this research was about 1.6 and trough was full tracing.

concentration ability of the system with flat receiver and cylindrical receiver is quantitatively analyzed and compared. Taneja et al. [91] studied the optical performance of a seasonally adjusted circular cylindrical solar concentrator in detail, using

3.2. Parabolic concentrator One of the best investigated solar concentrators is the 2-D CPC trough (Fig. 3); which is the most efficient solar concentrator because of its characteristics of collecting and concentrating all the rays within a specified acceptance angle [81,82]. Sellami and Mallick [82] determined the theoretical optical efficiency and the optical flux distribution at the photovoltaic cell of a 3-D Crossed Compound Parabolic Concentrator (CCPC) for different incidence angles of light rays. It was found that the CCPC with a concentration ratio of 3.6 represents an improved geometry compared to a 3-D Compound Parabolic Concentrator (CPC) for its use as a static solar concentrator. With accurate calculation of the concentration ratio and specific geometry, it can obtain an appropriate uniformity of input flux on the linear array of cells [83–87]. In Fig. 4, F and V are the focus and vertex of parabola curve respectively. Focal length and parabola depth are specified with the letters f and h. Parabolic curve is obtained from Eq. (4): y2 ¼ 4fx

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ð4Þ

Fig. 4. The curve of parabola.

Guiqiang [88] has used compound parabolic concentrators (CPCs), as non-tracking concentrators, when concentration ratio is below 3. They also compared the performance of the common flat and solar concentrating PV/T systems. They observed a gradual reduction at the quantity of photovoltaic cells efficiently, but also provided a higher temperature heat for space heating or water heating (Fig. 5). 3.3. Cylindrical troughs Cylindrical trough is the part of a cylinder that is cut longitudinally so that its cross-sectional area is the sector of circle (Fig. 6). Factors involved in the design of cylindrical concentrators are: radius (R), depth (d) and angle towards the horizon (α). Nicolas and Duran [89] studied cylindrical concentrators for any incidence angle of the solar rays. Hongfei et al. [90] investigated the flat receiver and cylindrical receiver respectively as well as determining the relationship between the receiving beam and the incident ray. By light tracking simulation, the distribution, width, eccentric magnitude of the image, and efficiency of the concentrated light varying with the incident angle can be calculated and visualized. Furthermore, the

Fig. 3. Comparison between CCPC and 2-D CPC [82].

Fig. 5. PV/T-CPC module [88].

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three different absorber configurations. The effect of angle of incidence of direct solar radiation, mirror rim angle, absorber size, mirror shading and absorber defocusing/lateral shift on the concentration characteristics was evaluated for the three absorber configurations. Cylindrical trough is not popular to concentrate light on a line since its geometrical properties. If their design is not accurate, two blurred focal line will occurr. This property limited the usage of cylindrical troughs to concentrate light on linear absorber in one hand. At the other hand, it inspires an idea making a concentrator for a flat plate PV.

inclination of each mirror element with respect to the base of the concentrator and the width of the mirror element are determined so that incident solar radiation, after reflection, is intercepted within the absorber and, hence, illuminates the absorber from one end to the other. The approach allows a variation in the width of the constituent mirror elements. In the second approach, the concentrator is generated from a prespecified equal width of the mirror elements and an appropriate size of the absorber which will intercept all the solar energy reflected from the constituent mirror elements and which is determined for the desired aperture of the concentrator.

3.4. Linear Fresnel reflectors A Fresnel typical solar concentrator is a linear Fresnel reflector solar concentrator (LFRSC). This type of concentrators is made of a series of long plane mirror elements positioned parallel on a framework. The angle of inclination of each mirror element with respect to the base of the concentrator is variable. An appropriate space is introduced between two consecutive mirror elements in order to avoid blocking of solar radiation reflected from the subsequent mirror elements. From the fabrication point of view, the concentrator is very simple and inexpensive [92–94]. However, the disadvantage of this concentrator is that because of the space introduced between the mirror elements, only a fraction of the base area of the concentrator covered by reflecting surfaces contributes to the concentration on the surface of the absorber. Nevertheless, when mounted on a simple framework, the existence of the gaps between the mirror elements offers an advantage, when considering that wind loading can be considerably reduced. Two different approaches have been proposed for designing LFRSC [95]. In one approach, the concentrator is generated from the prespecified size of the absorber. In this approach, the angle of

4. Analysis different types of concentrators In this study, the Monte Carlo ray tracing method (MCRTM) was employed to evaluate the optical performance and flux distribution on the receiver (PV panel). The MCRTM can perform well with a good agreement in dealing with special complex geometry problems [96,97] and it is a technique that computes the outcome of random processes [98]. Optical analysis was carried out based on the following assumption: Reflecting surfaces ideal and free from fabrication errors; the transmittance, reflectance and absorbance of the receiver (PV panel) are constant. The MCRTM reduces the volume of huge data and simulation time [99]. The optical performance was predicted by aggregating the result of several beams being traced. 1000 radiation sources were randomly distributed over the width of the aperture. Although in all calculations width (W) and height (H) of receiver are equal to 80 and 100 cm (common width of commercial fortyfour arrays PV panels) respectively. Thus at each simulation a pair of known concentrator have taken place at lateral sides of PV panel. Incoming light is normal to the PV panel and trough axis. 4.1. Flat concentrators

Fig. 6. Cylindrical trough cross section.

Different length of flat mirror and its angle effect on the concentration ratio was studied (Fig. 7). The calculated values of L and C for different amount of α are listed in Table (1). The results showed that by increasing the angle of the mirrors from horizon (α), the concentration ratio would be increased, though, it does not exceed 3. Actually, to achieve concentration ratio more than 2, the required length of the mirror increases rapidly. Therefore, manufacturing cost grows rapidly instead small C incensement.

Fig. 7. Pattern of irradiation and irradiation intensity on a 80  100 cm PV collector that illuminated by flat concentrator.

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Optical simulation indicates that flat concentrators make a uniform distribution of irradiation on the absorber. Fig. 7 illustrates optical simulation of the flat concentrator at 60 degree (C ¼2) when incoming flux is 1200 W/m2. Due to refractive characteristic of glass which covers the PV collector, a little bit of nonuniformity occurs at some angles of the mirror. To surmount very low concentration ratio of flat concentrators, Butler et al. [36] employed a four-facet flat reflector (Fig 8). Using this, technique they could achieve geometric concentration ratio of C ¼3 in theory and actual concentration ratio of C ¼2.68 when taking reflectivity into account. Their idea is very close to using Fresnel parallel reflective elements. 4.2. Parabolic concentrator The most effective factor that is involved in the design of the parabolic curve is the focal length (Fig. 9). By increasing the focal length, the aperture length increases. Since the concentration ratio is calculated using aperture area divided by the area of the receiver, therefore on increasing the focal length, concentration ratio is enhanced. By varying the depth and focal length, various patterns of reflection have been occurring. For instance, look at Fig. 10 and Table 2. A parabolic concentrator with a focal length of 40 cm, irradiance light on the PV plate is more uniform than when the focal length is 30 cm.Geometric concentration ratio (C) is 4 when focal length is 40 cm, also as Table 2 shows, the maximum and average irradiation are 2021 and 1294, respectively. These figures indicate that the concentrator cannot concentrate sun light as much as the theoretical concentration ratio. By decreasing focal length from 40 to 30 cm, the maximum irradiation reaches to 5261 (W/m2), but uniformity of irradiation on PV plate decreases rapidly. Overall, studying more states, it is found that, larger focal length increases the uniformity of irradiation and decreases the received sunlight. It also smaller focal length decreases the

theoretical concentration ratio and non-uniformity. This characteristic of parabolic concentrators makes them suitable to collecting sunlight on the linear absorber not plates. 4.3. Cylindrical trough The effect of radius (R), depth (d) and angle (α) has been studied on irradiation intensity and pattern in cylindrical trough (Fig 11). The results indicated that the angle has a great impact on the received maximum radiation and uniformity. For example, when radius and depth are 60 and 20 cm, respectively, by varying angle from 30 to 70 degree, average and maximum incident rays are changing dependently (Table 3). When angle towards the horizon equals 60 degree, both of maximum and average radiation obtains the maximum value. (Table 4) 4.4. Linear Fresnel reflectors The impacts of the width and the number of parallel mirrors (as Fresnel reflector Fig. 12) on the intensity of the radiation pattern have been analyzed. In Fig. 13, the simulation results can be seen in four various numbers of parallel mirrors.

Table 1 The calculated values of useful length (L) and concentration ratio (C) in flat concentrators (W ¼ 80 cm). α (degree)

L (cm)

C

50 60 70 80

22 80 180 433

1.35 2 2.53 2.89

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Fig. 9. Parabolic concentrator.

Fig. 8. Low concentration ratio flat concentrator with 4 flat reflectors [36].

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Fig. 10. Pattern of irradiation and irradiation intensity on a 80  100 cm PV collector that illuminated by CPC. (a) the focal length ¼ 30 cm and (b) the focal length¼ 40 cm. Table 2 Optical characteristics of parabolic concentrator.

Table 4 Optical characteristics of Fresnel concentrators.

Focal length (cm)

Geometric concentration ratio (C)

Maximum flux (W/m2)

Average flux (W/ m2)

Standard Deviation

Optical efficiency (%)

Number of parallel mirrors

Width of Maximum each mir- flux (W/m2) ror (cm)

Average flux (W/ m2)

Standard Deviation

Optical efficiency (%)

30 40

4 4

5261 2021

2587.5 1294.54

1013.75 141.81

64.7 32.4

2 3 4 5

40 40 20 20

2007.5 2112.5 2140 2497.5

457.125 414.687 830.5 757.25

83.6 88 89.1 91.3

pair pair pair pair

2745 2882 3453 3782

Fig. 11. cylindrical trough. Table 3 Maximum and average incident rays on a 80  100 cm PV collector that illuminated by cylindrical trough at different angles. 2

Fig. 12. Linear Fresnel reflectors.

2

Radius (cm)

depth(cm)

α(degree)

Maximum(W/m )

Average(W/m )

160 160 160 160

20 20 20 20

30 45 60 70

1882 2492 4278 4024

1367 2021 3199 3087

number cause the pattern that looks like cylindrical reflectors. Despite this, uniformity of Fresnel reflectors is better than cylindrical troughs for the same concentration ratio.

5. Experimental device The results of optical simulation indicated that increasing the number of mirrors and decreasing their width, increases light intensity on the PV panel. Likewise, an increase in maximum irradiation and non-uniformity are the result of high degree of intensity. Generally, few number of wider mirrors cause the uniform pattern of irradiation looks like a flat concentrator. On the other hand, reducing the width of the mirror and increasing their

The simulation results of the different designs of concentrators were compared with each other and the best design with most uniform flux distribution, high concentration ratio and low requirement of mirror was chosen in this research work. In order to verify the optical simulation results, linear Fresnel reflectors with five pair of 20 cm width mirrors was fabricated and evaluated under ambient condition. Due to recovery of waste heat in the PV

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Fig. 13. Pattern of irradiation and irradiation intensity on a 80  100 cm PV collector that illuminated by Linear Fresnel concentrator,(a) two pair of 40 cm width mirror, (b) three pair of 40 cm width mirror, (c) five pair of 20 cm width mirror and (d) six pair of 20 cm width mirror.

panel, a hybrid photovoltaic/thermal (PVT) solar collector was used. Hybrid PVT system, which simultaneously converts solar radiation to thermal and electrical power, was increasingly considered. The results of various studies have shown that such hybrid systems are more efficient in comparison with both individual photovoltaic and thermal systems [100–104]. To examine the effect of using linear Fresnel concentrator, the PVT collector which has been successfully tested by Mortezapour et al. [105] was employed. This PVT collector with fabricated concentrator is shown in Fig. 14. The frame of the mirrors is adjustable to adapt the mirror angles for seasonal solar tracking. Tests were performed in Tehran, Iran. The PV solar collector was installed at a tilt angle of 43° equal to the latitude of the city. The solar collector has a wooden duct of dimensions 1.05, 0.805 and 0.1 m that is glazed by a glass layer and the PV module is positioned equidistant from the glazing glass and the bottom insulator of the duct. The bottom insulation consisted of wood and glass wool [106]. The mass flow rate of inlet air was constant for all tests and it was 0.02 kg/s.

The tests were carried out in the month of September from 9 am to 15 pm. The PV module was a single-crystalline silicon and its characteristics at standard testing condition were: open circuit voltage Voc ¼30 V, short-circuit current Isc ¼ 4 A, electrical efficiency 12%. To monitor the PV panel temperature as an effect of irradiation distribution, 9 temperature sensors were attached to the back surface of the PV panel. In order to measure the temperature of inlet and outlet flowing, two temperature sensors (SMT 160) were installed before and after the collector duct. Simultaneously, a solar power meter (TES 1333 R) measured the solar radiation intensity on the collector surface, during the test. A multimeter (DEC330FC) was used to measure the open circuit voltage (Voc) and short circuit current (Isc).

6. Experimental results The hourly variation of solar radiation intensity for a typical day of September 2014 under the condition of Tehran, from 9 a.m. to

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Fig. 14. Fabricated concentrator and PVT collector.

Fig. 15. Hourly variation of solar radiation intensity for a typical day September 2014 under the condition of Tehran.

3 p.m. is denoted in Fig. 15. The solar radiation intensity varied from 589 W/m2 at 9:00 to 1019 W/m2 at 12:30. In order to evaluate the performance of collector assisted by linear Fresnel concentrator, the PVT collector was studied under typical solar intensity and the results were compared with those from PVT collector stand-alone (without concentrator). The CPV/T system performance is combination of the two parts’ performance: Photovoltaic conversion and thermal utilization [107]. Power generation of the PV device under concentrated illumination condition can be expressed as PPV ¼ APV :FF:VOC :ISC

ð3Þ

where APV is the total area of the PV devices in the CPV/T system; FF, VOC and ISC are the fill factor, open circuit voltage and shortcircuit current of the solar cell under concentration condition, respectively. Thermal performance of a solar collector is described by an energy balance that indicates the distribution of incident solar energy into useful energy gain thermal losses and optical losses. In steady state the useful energy output of a flat plate collector of area APV is difference between the absorbed solar radiation and thermal loss: Q th ¼ APV ½S  UL ðTPV  Ta Þ

ð4Þ

Fig. 16. Electrical performance of PVT collector with and without concentrators.

where S is solar energy absorbed by collector per unit area of absorber; UL is coefficient of thermal energy lost from collector; TPV and Ta are the mean absorber plate temperature and ambient temperature, respectively [72]. Fig. 16 shows the performance of collector with and without concentrators. Almost in all points, electrical performance of collector which is assisted by concentrator is higher but around the noon the performance of collector without concentrator is little more than the performance of concentrator assisted collector. This problem occurs because of PV temperature profile. The cell temperature grows with an increase in concentration of light and being a semiconductor material, it has a negative temperature coefficient of open-circuit voltage. At the noon, the mean temperature of PV and standard division of temperature distribution are more than the other times. Nevertheless, using concentrator can improve the electrical performance by 4.22 percent. Thermal performance and overall performance are presented in Figs. 17 and 18. Using concentrator improves thermal and overall efficiency by 40.2 and 36 percent respectively. The maximum overall performance for PVT collector with concentrator and without concentrator is 2185 (W) and 1413 (W) respectively. Sangani and Solanki [79] reported that a concentrator with geometric concentration ratio of 2 and tracking system could improve the output power by 44%.

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3. Cylindrical concentrators with large radius and shallow depth have uniform pattern of reflecting light. Compromising between radius, depth and angle; uniform illumination is available. 4. Fresnel linear reflectors which have more number of parallel mirror by narrow width reflect the light same as cylindrical reflectors. Usually the low cost of manufacturing is the main criteria to select this type of concentrators. Also the pick of solar radiation intensity plays a main rule in the decrement of electrical efficiency. 5. The experimental findings show that LCPV has the potential to harvest more energy when using standard Si solar cells in a basic concentration configuration as used in this study. Using concentrator improves thermal and overall efficiency by 4.22%, 40.5% and 36% respectively.

Acknowledgment Fig. 17. Thermal performance of PVT collector with and without concentrators.

The authors acknowledge the financial support of Tarbiat Modares University (TMU) renewable laboratories authorize and Iranian fuel conservation company (IFCO) to carry out this investigation.

References

Fig. 18. Overall performance of PVT collector with and without concentrators.

In comparison with the results obtained from stand-alone collector, the concentrated PV panel has a higher mean surface temperature. The fabricated linear Fresnel reflector with four pair of 20 cm width mirror doesn’t change temperature distribution on the PV until solar radiation intensity is lower than 850 W/m2. When solar radiation intensity reaches to 1012 W/m2, the standard deviation of temperature distribution on the concentrated PV surface increases 30% as compared with non-concentrated collector. This result shows a good agreement with the simulated results.

7. Conclusions The conclusion drain from the results achieved from optical simulation of four type of low concentration reflectors namely flat, parabolic, cylindrical and Fresnel; forward sentences are as follow: 1. Flat concentrators make the most uniform irradiation pattern at the angle of 60 degree (C ¼2). Concentration ratio hardly increases more than 2, therefore, cannot accumulate very much sunlight on the PV panel. 2. Parabolic concentrators increase the maximum intensity, but they create a non-uniform pattern of light. This problem would be worsen if the focal length is small. This type of concentrator is inappropriate to concentrate the sunlight on the rectangular PV plate.

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