Thermal management systems for Photovoltaics (PV) installations: A critical review

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ScienceDirect Solar Energy 97 (2013) 238–254 www.elsevier.com/locate/solener

Thermal management systems for Photovoltaics (PV) installations: A critical review Dengfeng Du ⇑, Jo Darkwa, Georgios Kokogiannakis Centre for Sustainable Energy Technologies, University of Nottingham, Ningbo, China Received 12 March 2013; received in revised form 31 July 2013; accepted 15 August 2013 Available online 14 September 2013 Communicated by: Associate Editor Brian Norton

Abstract Strong solar radiation and high ambient temperature can induce an elevated Photovoltaic (PV) cell operating temperature, which is normally negative for its life span and power output. Different temperature dependences for PV performance have been reported and it has been found that the efficiency of crystalline silicon cells drops at a rate of around 0.45%/°C. Various cooling methods have been proposed to achieve lower PV cell temperature in favour of higher cell efficiencies. Passive cooling by heat spreader or heat sink can provide enough cooling to get a relatively low cell temperature even for Concentrator PV (CPV), but the heat sink surface area can be extremely large. Natural ventilated systems can achieve PV temperature in a range of 50–70 °C and forced ventilated systems are found to achieve a lower temperature range of 20–30 °C at the price of parasite electric consumption. Forced de-ionized liquid immersion cooling, jet impingements techniques and heat pipe cooing mainly applicable to CPV systems and can achieve a temperature range of 30– 96 °C. Phase change material (PCM) system due to a choice of melting temperature, amount of material to be used, and different system designs is a promising thermal management of flat plate PV and can maintain PV temperature below its melting temperature e.g. 27 °C for a relatively long time. A facility to re-utilize of the heat energy stored in PCM is encouraged. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Photovoltaic cell; Thermal management; Phase change material

1. Introduction In recent years there has been a significant resurgence of interest in the renewable energy systems due to the concerns of global warming and skyrocketing prices of fossil fuels. Photovoltaics (PV) cells absorbing photons to generate free electrons, can convert light to electricity, which has been largely employed in the past years (Tsakalakos, 2010). The effect of temperature draws a great deal of research attention since relative high cell temperature during cell operation contributes to short-term efficiency loss as well as long-term irreversible cell degradation (Royne et al., ⇑ Corresponding author. Address: 199 East Taikang Road, Ningbo China. Tel.: +86 574 88180319; fax: +86 574 88180313. E-mail address: [email protected] (D. Du).

0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.08.018

2005). Many studies have been focused on the temperature response of PV performance since operating temperature is one of the biggest factors that affect the conversion efficiency. Skoplaki and Palyvos (2009) summarized both implicit and explicit correlations for deriving the PV operating temperature based on factors such as wind speed, type of cell and system configuration. PV conversion efficiency usually decreases with increasing cell temperature since a big drop in open circuit voltage Voc overwhelms a slight increase in short circuit current Isc (Lorenzo, 1994). The temperature coefficient b which measures efficiency percentage change against unit temperature increase was summarized to have an average value of 0.45%/°C at Tref = 25 °C from a number of references for mono-Si, aSi, Poly-Si cells and also a number of Photovoltaic and Thermal (PV/T) applications (Skoplaki and Palyvos,

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Nomenclature PV/T photovoltaic/thermal PCM phase change material gc thermal electric conversion efficiency bref temperature coefficient BIPV/T building integrated photovoltaic/thermal MEPCM microencapsulated phase change material Voc open circuit voltage

2009). The temperature responses for different types of PV cells have been greatly examined. Radziemska and Klugmann (2002) through their experimental investigation, revealed that the output power declines at 0.65%/°C for crystalline PV cells, which is actually much higher than the theoretical value of 0.4%/°C. Nevertheless, Taguchi et al. (2005) reported that the temperature coefficient of HIT thin-film cell, a silicon based heterojunction structure cell has been reduced from 0.33% to 0.25%/°C by a new process of manufacturing, which shows a much smaller temperature effect on the reduction of efficiency compared to conventional silicon cells. They attributed the lower temperature coefficient to a higher Voc in excess of 710 mV and even 743 mV that HIT cell can achieve (Mishima et al., 2011). Nishioka et al. (2006) reported the temperature coefficient of conversion efficiency for InGaP/InGaAs/Ge triple-junction solar cells to be 0.248%/°C and also investigated the relationship between temperature coefficient and concentrator ratio. Results demonstrated that for this triple-junction cell, high concentration ratio is beneficial since there is a big reduction of temperature coefficient with an increasing concentration ratio. In addition to those negative temperature coefficient, Riedel et al. (2004) reported a positive temperature coefficient, showing a positive effect of temperature on the power output of polymer–fullerene bulk-heterojunction solar cells due to thermally excited intrinsic charge carriers. It is evident that the effect of temperature is quite different on PV cell performance of different types, and there is a trend of decrease with the negative effect on conversion efficiency for newly developed cells. However crystal silicon cells are still dominant in market and their performance are heavily affected by high cell temperature. Therefore, thermal management systems for PV system are still of significance. A number of investigations have been carried out to seek for thermal management systems for different silicon PV applications. This paper thus reviews various thermal management systems for PV installations based on crystalline silicon.

CPV DT gTref Dg Isc

concentrator photovoltaic temperature difference gc at reference cell temperature conversion efficiency difference short circuit current

of photons of energy over the bandgap. The energy surplus after experiencing photovoltaic effect is converted to heat. On the other hand, if the photon energy is below the band gap energy of the cell, it is not utilized by the solar cell and goes directly to heat the solar cell. Therefore, there will be a high cell temperature given a strong solar radiation and ineffective cooling/heat dissipation for PV cells, which highlights the necessity of a good thermal management strategy for PV. In order to curb the temperature rise in PV cells, a number of conventional cooling techniques would be reviewed. 2.1. Natural or forced air ventilation systems 2.1.1. Passive cooling by heat spreader/heat sink Concentrator PV (CPV) provides the possibility of a high efficiency, but the thermal condition of the cells tend to deteriorate due to dense heat influx. Araki et al. (2002) proposed a simple module structure for concentrator PV made by printed heat conductive epoxy and copper sheet on aluminum plate. Schematically shown in Fig. 1, the cell surface was cooled by natural convection and its back surface is glued by the epoxy paint. The heat spreader that spreads heat in the cell by a metal plate and the 3 mm thick normal grade aluminum was found to have sufficient heat spreading coefficient and suppress the temperature rise. The outdoor experiment confirmed only a 21° temperature rise under 400 non-imaging square Fresnel lens. Min et al. (2009) also investigated metal plates in a similar configuration to Fig. 1 as heat sinks for cooling a concentrating PV system of a 400 concentration. The focus of the study was on the temperature dependence of the cell area of the heat sink for different concentration ratios. The area

2. Thermal management techniques for PV applications Only a small part of the solar incident radiation is converted to electricity with the remaining being converted to heat. Photovoltaic effect takes place only for the absorption

Fig. 1. Heat spreading concept for concentrator module (Araki et al., 2002).

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Fig. 2. Temperature profile of the outdoor test for 400X concentrator PV (Min et al., 2009).

of the heat sink was 700 times larger than the solar cell. The results showed that for a fixed concentration ratio, the cell temperature did reduce as the heat sink area increased. The heat sink area needs to increase linearly as a function of the concentration ratio if cell temperature is to be kept constant. The monitored cell temperature profile as shown in Fig. 2 demonstrates a stabilized mean temperature profile of 37 °C for the effective cooling performance of the heat sink. Luque et al. (1997) demonstrated a trough-type photovoltaic concentrator technology, the EUCLIDES, in which, thermal energy is passively transferred to the ambient through a lightweight aluminium-finned heat sink. The optimized fin dimensions for the 32 parabolic geometrical concentration PV is 1 mm thick, 140 mm long and spaced about 10 mm apart. Measured operating cell temperature was around 58 °C. Natarajan et al. (2011) developed a numerical study of solar cell temperature for 10 concentrating PV. Based on a thermal model, the result showed that four numbers of uniform fins of 5 mm height and 1 mm thickness, 4 mm spacing can be effectively used to reduce the solar cell temperature to 49.6 °C. Solanki et al. (2008) indicated that light concentration can be achieved using mirrors or reflectors arranged in V-shape. The V-trough walls can be used for light concentration to achieve 2 Suns concentration as well as heat dissipation from the cells. The configuration is shown in Fig. 3. Heat dissipation area was found to be 4 times higher than the case without V-trough walls. The cell temperature remains same as that of a cell temperature in a flat plate PV module, around 60 °C (as against to 80 °C without heat sink effect) under 750 W/m2 despite light concentration, showing a relatively good cooling effect. 2.1.2. Natural ventilated PV/T or PV facßade systems A photovoltaic/thermal hybrid solar system (or PV/T system for simplicity) is a combination of photovoltaic and solar thermal components/systems which produce both electricity and heat from one integrated component or system and a review on PV/T hybrid solar technology was done by Chow (2010). Tonui and Tripanagnostopoulos (2008) presented air cooling of a commercial PV

module configured as a ventilated PV/T design by natural flow. The air flow channels were configured in three ways to improve convection heat transfer from the duct walls to air stream, shown below. (see Fig. 4.) The heated PV rear surface radiates heat to the back wall and transfers heat to the circulating air by convection. The back wall surface temperature rises by absorbing radiation and then heats the air circulating over it. PV temperature against channel depth was studied for both glazed and unglazed PV of the three configurations as presented in Fig. 5. The plot of the PV temperature showed that the module temperature initially decreased sharply until a minimum value was obtained at the optimum channel depth and thereafter increased as the channel depth continued to increase. Beyond the optimum depth, the TMS system presents lower PV temperature than the reference system for both glazed and unglazed configurations while the FIN system presents lower temperature beyond the optimum depth for the unglazed configuration and for all channel depths for the glazed configurations. Results showed that at a channel depth of 15 cm, the unglazed TMS and FIN systems have about 3 °C lower temperatures than the REF system, which corresponds to improvements of about 1–2% in electrical efficiency. Mei et al. (2003) developed a natural ventilation strategy for cooling PV facßade which simultaneously utilizes surplus heat available from the ventilated PV (see Fig. 6). However the focus of the study was concerned with room temperature and building cooling/heating load rather than the PV temperature drop. Although detailed cooling effect for PV was not given, the PV facßade outlet air temperature was demonstrated in Fig. 7 for both simulation and practical measurements. The outlet air temperature fluctuated and the peak values were approaching 50 °C. Since the heat is transferred from PV cell, it can be

Fig. 3. (a) Cross-sectional view of concentrator V-trough PV module with trough made of a continuous metal sheet and (b) complete view of prototype V-trough PV module (2008).

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Fig. 4. Sketches showing the three configurations studied, along the flow direction. (a) Reference(REF), (b) thin metal sheet (TMS) and (c) fin (Tonui and Tripanagnostopoulos, 2008).

Fig. 5. Variation of PV temperature with channel depth for the three systems (glazed and unglazed) (Tonui and Tripanagnostopoulos, 2008).

Fig. 6. PV facade structure (Mei et al., 2003).

predicted that PV cell temperatures should fluctuate in a similar way and that the peak PV temperature can be projected to be above 50 °C in summer. This reveals that the fluctuation of the PV temperature and a relatively high peak temperature are potential drawbacks of the ventilated facßade system. Yun et al. (2007) investigated a ventilated PV facßade system as a natural ventilation system in summer (see Fig. 8). In summer, due to the stack effect, airflow from the room will be naturally induced to enter the vent in the bottom and driven upwards by buoyancy effect. As shown in Fig. 9, the air convection helped to reduce monthly PV module temperatures from 1.0 (December) to 5.0 °C (April). This leads to an approximate 15% improvement (about 1.3% of the absolute PV efficiency improvement)

Fig. 7. PV facade outlet temperature in summer (Mei et al., 2003).

Fig. 8. Ventilated PV facade operation strategy in summer (Yun et al., 2007).

of the PV module efficiency. Fig. 10 represents the efficiency and module temperature profiles for the hottest recorded day for the PV module. It shows a relatively phenomenal cooling effect of the airflow and great benefits in energy conversion efficiency. 2.1.3. Forced ventilated PV/T or PV facßade systems Forced circulation is more efficient than natural circulation owing to better convective and conductive heat transfer coefficients, but the required fan power reduces the net electricity gain. Hegazy (2000) looked at four different models of forced air type PV/T, as shown in Fig. 11. By setting up the air flow rate ranging from 0.005 to

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Fig. 9. Monthly PV module temperatures with and without the provision of ventilation behind PV modules (Yun et al., 2007).

Fig. 11. Schematics of the various PV/T models by forced convection (Hegazy, 2000).

Fig. 10. PV efficiency and module temperature during the day when the hottest PV module temperature occurs (Yun et al., 2007).

0.040 kg/sm2, parameters such as fan power consumption, maximum outlet inlet air temperature difference, net electricity production and net overall efficiency were obtained. Comparing the four models, Model (a) PV/T collector was reported to have the lowest overall electricity production, while the other models exhibited comparable data up to a specific mass rate of 0.02 kg/sm2. For higher mass flow values, Model (c) has the highest overall electricity production followed by the Model (d). Generally, at a same mass flow rate, Model (c) was reported to have the highest temperature difference of outlet and inlet air flow, i.e. the biggest heating effect for the air. This may indicate Model (c), which places PV between single pass channels, has the best cooling performance for a PV module. Forced circulation was also identified for open-loop building-integrated photovoltaic/thermal (BIPV/T) systems. Candanedo et al. (2010) has developed a general model useful for design or control purposes which allows for steady-state or transient analysis. Pantic et al., 2010 investigated the energy performance of three different open loop air BIPV/T systems (shown in Figs. 12–14) that utilize recovered heat for home heating. For configuration 1, results showed when wind speed increased from 1 to 7 m/ s over 4.8 m long roof, the PV panel temperature and outlet air temperature dropped by about 25 °C and 15 °C. For the same wind speed range, electrical efficiency increases from 12% to 13% and thermal decreases from 14% to 8%. Fig. 15 shows that for a typical summer day the PV temperature in configuration 1 and 2 were both 66.5 °C however

in configuration 3, PV temperature reached 102 °C due to the addition of a glazing cover. Regarding optimization of the BIPV/T system, an experimental study for the development of convective heat transfer correlations for an open loop air-based BIPV/T system was carried out by Candanedo et al. (2011) in which correlations for the average Nusselt number for the top and bottom surfaces in the channel as a function of Reynolds number were developed. Bambrook and Sproul (2012) investigated quantified additional electricity generated by PV/T air system at different air flow rates accounting for fan power requirement. Additional electrical PV output in excess of the fan energy requirement was reported for air mass flow rates in the range of 0.03–0.05 kg/sm2. This was made possible through energy efficient hydraulic design using large ducts to minimize the pressure loss. The electrical and thermal energy generated by the system are compared with the fan energy requirement for a range of air mass flow rates and the output values are shown in Fig. 16 after adjusting to have a same insolation for each test day. 2.2. Hydraulic cooling 2.2.1. Hydraulic PV/T system He et al. (2006) proposed a hybrid PV/T collector technology using water as the coolant aiming at improving the energy performance (Fig. 17). The system outputs, including the voltage, current, temperature and wind conditions, were collected daily through a data logger over a 3 month period spanning from early August to early November. Results showed the hybrid PV/T collector technology as an effective solution for improving the energy performance. Through good thermal-contact between the thermal

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Fig. 12. Configuration 1–unglazed BIPV/T system (Pantic et al., 2010).

absorber and the PV module, waste heat was absorbed by thermal absorber from PV, thus both the electrical efficiency and the thermal efficiency were raised. In addition, they spotted that fin performance of the heat exchanger is crucial towards achieving a high overall energy yield. Zakharchenko (2004) presented a PV/T design, placing PV cell on the surface of solar collector in which cold water flows. One experimental finding is that the conventional PV is not appropriate for use in the hybrid PV/T systems due to a poor heat transfer from PV cell to solar thermal collectors. Thus, aluminum (Al) substrate was chosen with an adhesive coating to provide best possible thermal contact between the PV and solar collectors. The PV panel was cooled by approximately 10 K, and its conversion efficiency increased by 10%. Zakharchenko et al. also observed that flat plate collector partially covered with PV module would give better thermal and average cell efficiency and PV cell

should be put at the portion of the collector where the coolant enters. This suggestion for system design was agreed by Dubey and Tiwari (2008) through their thermal modeling of a combined system of photovoltaic thermal (PV/T) solar water heater. Fraisse et al. (2007) addressed two points that attention should be paid for a hydraulic PV/T system. If with a glazing cover for the PV/T system, the annual photovoltaic cell efficiency decreased 28% to be 6.8% in comparison with a conventional non-integrated PV module. This is due to a temperature increase given by the cover. Another point is demonstrated through the PV cell temperature evolution during a summer day (June). Shown in Fig. 18, it can be seen the PV temperature exceeded 100 °C at peak values. This is due to the fact that the heating needs are null during summer so that water inside the collector are with high temperature.

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Fig. 13. Configuration 2–unglazed BIPV/T system with solar air collector (Pantic et al., 2010).

2.2.2. Liquid immersion cooling Russell (1981) configured an elongated tube filled with the non-conductive liquids as optical concentrator as well as a cooling system for PV arrays. Shown in Fig. 19, PV cells are mounted inside the tube and immersed in a clear non-conducting liquid and the elongated tube with the liquid has a refractive index suitable for concentrating the sunlight onto the cells. Similar devices employing dielectric liquid immersing PV cells have been reported by Ugumori and Ikeya (1981), Abrahamyan et al. (2002), Tanaka (2007) and Han et al. (2013). A common finding is an improved energy conversion efficiency of the PV cells operating in liquid, for example, Abrahamyan et al. reported that a dielectric liquid thin-film can increase the efficiency of common silicon solar cells by 40–60% and Han et al. (2013) reported an efficiency increase of 8.5–15.2% for 1.5 mm thickness liquid layer over the cell surface. Unfortunately, the cooling effect by the liquid dielectric was rarely investigated in these literatures and the enhanced conversion efficiency was resulted from two independent physical phenomena, an increase in output current due to a reduced surface carrier recombination, and enhanced collection of light through refraction and inner reflection of light in the liquid.

Wang et al. (2009) investigated the performance of silicon solar cells operated in liquids. It started to acknowledge that the cooled solar cell temperature that contributed to an enhanced efficiency for the CPV was a good result of dielectric liquid immersion, however no detailed data regarding cooling effect have been revealed. Later, Liu et al. (2011) developed a small-scale experimental system to study the heat dissipation performance of solar cells by dielectric liquid immersions under different medium concentration ratios. Results showed a uniform temperature distribution achieved for the module along the flow direction in turbulent flow and the maximum temperature difference is less than 3 °C. Also it was found that when the thermal load is 47.3 kW/m2, the module average temperature can be cooled down to about 30 °C and the corresponding heat transfer coefficient is around 1000 W/ (m2 °C). Zhu et al. (2011) adopted de-ionized liquid immersion cooling to eliminate the contact thermal resistance of back cooling and to improve cell performance for 250 concentrator PV. The results showed that the module can be cooled down to 45 °C at 940 W/m2 direct normal irradiance and 17 °C ambient temperature with 30 °C inlet water temperature. The temperature distribution on the PV

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Fig. 14. Configuration 3–glazed BIPV/T system (Pantic et al., 2010).

surface was relatively uniform (maximum temperature difference was less than 4 °C) and the calculated overall convective heat transfer coefficient of approximately 6000 W/m2 K demonstrated that the cooling capacity of immersion cooling was prominent. Xiang et al. (2012) did a three dimensional numerical simulation for a cylindrical liquid-immersed solar receiver used in a dish concentrating PV system subject to conditions of 250 concentration ratio, 1000 W/m2, 25 °C inlet temperature and 15% sunlight-to-electricity conversion efficiency. It demonstrated inlet velocity and fin number are determinants of cell temperature. Shown in Fig. 20, at an inlet velocity of 0.04 m/s, the maximum cell temperature is observed as 84.7 °C. When the inlet velocity increases to 0.06 m/s, 0.08 m/s, 0.16 m/s, 0.32 m/s, 0.48 m/s and even 0.80 m/s, the peak value decrease to 77.6 °C, 72.8 °C, 59.4 °C, 49.9 °C, 44.9 °C and 40.0 °C, respectively. Meanwhile, a higher velocity at inlet causes a stronger turbulence

inside the channel and at the inter-fin gaps, which gives rise for lower cell temperature gradient along the flow

Fig. 15. Typical summer BIPV/T energy production for the three configurations (Pantic et al., 2010).

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Fig. 16. Daily PVT system energy output and fan power versus air mass flow rate (adjusted for 5.75 kW h/m2/day) (Bambrook and Sproul, 2012).

variation with fin number are shown in Fig. 21 at the inlet velocity of 0.32 m/s. It can be seen that the cell module with 11 fins shows the best thermal performance with the maximum temperature of 47.2 °C and the temperature difference of 14–15 °C. The predicted results indicate that liquid immersion cooling together with a good fin design has relatively good cooling effect for CPV.

Fig. 17. Experimental set-up of the hydraulic hybrid PV/T collector (He et al., 2006).

direction. However, increasing liquid inlet velocity will cause the pressure drop and thus require more pump power consumption but no quantified value of power consumption was given. The simulated results of the temperature

2.2.3. Water impingement cooling Impinging jets are capable of extracting a large amount of heat because of the very thin thermal boundary layer that is formed in the stagnation zone directly under the impingement, and it is used to cool bodies with high heat flux. Royne and Dey (2007) also proposed a jet impingement device for cooling packed photovoltaic cells (Fig. 22). The device consists of an array of jets where the cooling fluid is around the sides in the direction normal to the surface of densely packed PV cells. A model was developed to predict the pumping power for different device configurations and showed that a higher number of nozzles per unit area will perform better than a lower nozzle number. Typical plots of the total and net

Fig. 18. The PV cell temperature and efficiency evolutions (Fraisse et al., 2007).

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Fig. 21. Variation of solar cell array temperature over the length of the cell array center-line with different fin numbers (Xiang et al., 2012).

Fig. 19. A perspective view of the optical concentrator and cooling system for PV cells invented by (Russell (1981)).

Fig. 22. Schematic drawing of jet configuration with drainage direction normal to the impingement surface (side drainage) (Royne and Dey, 2007).

Fig. 20. Variation of solar cell array temperature over the length of the cell array center-line as a function of inlet velocity (Xiang et al., 2012).

(pumping power subtracted) PV output for different conditions were shown in Fig. 23. A higher average heat transfer coefficient results in a lower cell temperature and a subsequent higher cell output. For 200 concentrations in both models, PV temperature drops from its origin of 60 °C to a value slightly higher than 30 °C at the maximum power output point. For 500 concentrations in both models, PV temperature drops from its origin of 110 °C to a value slightly higher than 40 °C at the maximum power output point, therefore good cooling effects were achieved for the jet impingement system. 2.3. Heat pipe cooling A heat pipe is a sealed chamber whose inner surface is lined with a capillary wick material and the inner space is filled with working fluid, which is vaporized by an external heat source at the evaporator. The use of heat pipes for

thermal management has been widely investigated (Garimella and Sobhan, 2001). The miniaturization of electronic devices results in an increase in power densities and poses a great challenge to cooling strategy. However, heat pipes due to their ability to provide effective heat transfer with minimal losses have much potential for managing heat in electronic equipment. Flat heat pipes of various types have been investigated (Xuan et al., 2004) while loop heat pipes are also claimed to be highly efficient heat-transfer devices. Flat miniature heat pipes with micro capillary grooves can spread heat flux across a heat sink. Models of the structure have been developed and shown that their dissipated power can reach about 110 W/cm2 without heat transfer limitations (Gillot et al., 2003, Xiao and Faghri, 2008). Russell (1982) developed a heat pipe cooling system which uses linear Fresnel lenses, with each focusing the light onto a string of cells mounted along the length of a heat pipe of circular cross-section (see Fig. 24). Several pipes are mounted next to each other to form a panel. The heat pipe has an internal wick that pulls the liquid up to the heated surface. Thermal energy is extracted from the heat pipe by an internal coolant circuit, where inlet and outlet is on the same pipe end, ensuring a uniform temperature along the pipe. The coolant water is fed and extracted by common distribution pipes.

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Fig. 23. Cell and total power output and cell temperature plotted for cooling system average heat transfer coefficient for the two models and concentrations: (a) 200 suns, Martin model; (b) 200 suns, Huber model; (c) 500 suns, Martin model and (d) 500 suns, Huber model (Royne and Dey, 2007).

Fig. 24. Uniform temperature heat pipe for PV (Russell, 1982).

Heat pipes as a cooling solution for concentrating photovoltaic (CPV) cells reported by Akbarzadeh and Wadowski (1996) and Farahat (2004) have proven to be effective on cooling performance and increasing PV efficiency (Fig. 25). The CPV at high temperature transfers heat to the evaporator section at high heat flux in the heat pipe, and the heat is transferred internally to its condensation section. Then heat is rejected to fins, which is cooled by natural convection, at a much lower heat flux. Experimentally, Akbarzadeh and Wadowski (1996) exposed the heat pipe cooling system to a solar radiation of 20 concentration for a period of 4 h to evaluate the cooling performance for this system and the result showed that temperature at the surface of solar cell was kept within 46 °C. In comparison, cell surface temperature rose to 84 °C when there is no fluid in the heat pipe (heat pipe does not function). The output power dropped to 10.6 W from 20.6 W for the case when the system cooling was in operation. This shows that the proposed cooling system has substantially improved the performance of the solar cells.

Fig. 25. Schematic of heat pipe based cooling system for CPV (Akbarzadeh and Wadowski, 1996).

Anderson et al. (2008) examined a copper heat pipe with water as the working fluid. A series of CFD analyses were run to determine the optimum fin size and spacing for rejecting heat by natural convection. A prototype heat pipe was then designed, fabricated, and tested. The heat flux from PV cell was 40 W/cm2, and the heat pipe rejected this heat to the environment by natural convection. The confirmed cell-to-ambient temperature difference was 40 °C, showing a relative good cooling performance. Huang et al. (2012) fabricated a novel hybrid-structure flat plate heat pipe (NHSP heat pipe) for a concentrator

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photovoltaic. The feature of the design was a sinteredmetal structure and a supporting structure, which gives rise to a much lowered thermal resistance of NHSP heat pipe compared to conventional heat pipes. The NHSP heat pipe contributed to a better performance for the concentrator photovoltaic, which can increase PV conversion efficiency by approximately 3.1%, compared to an aluminum substrate. 2.4. Phase change material systems Latent heat storage due to its large heat storage capacity and the ability to keep a relative stable temperature during the heat absorbing process, has been massively investigated. Phase change material (PCM) mainly utilizing latent heat storage has witnessed a lot of application in thermal management systems. There is a bulk of literature of PCM applications identified for electronics, especially for intermittent power sources (Chidambaram et al., 2011 and Kandasamy et al., 2007; Tan and Tso, 2004). Due to relative low thermal conductivity of most PCMs, fins play an important role to enhance heat transfer in PCM systems. Fin incorporated heat sinks filled with PCM also have been studied separately as independent cooling devices that can be applied on different heat sources. The number of fins filled with n-eicosane were studied (Setoh et al., 2010) while different configurations of fins e.g. plate fin and pin fin heat sinks incorporating a PCM have been evaluated (Yoo and Joshi, 2004). Results showed the heat sink configurations are crucial for the effectiveness of PCM systems. Huang et al. (2004) were the first to develop a numerical model for a system using PCM to moderate the temperature rise of PV. The model has been validated successfully by comparison with experiments. As shown in Fig. 26, a PCM layer was attached to the PV cell from its rear surface, thus allowing convection to take place on the surface of the PV and plenum. In the experiment, the PV/PCM system was subjected to different ambient temperatures and the surface temperature of PV was plotted in Fig. 27. It shows PCM systems during the phase change period can maintain relatively long time of stable temperature thus preventing the rise of PV cell temperature. Huang et al. (2006) also investigated fins in PV/PCM system. Fins enhance heat transfer from PV and PCM and are helpful to retain PV temperature near PCM phase change temperature. One of the fin incorporated PV/PCM system designs was demonstrated in Fig. 28 with aluminum fins directly inserted into PCM layer. The presence of fins and the fin width were studied and temperature curves in Fig. 29 showed that although the PV/PCM test system without fins reduced the temperature rise of the PV, the presence of the fins reduced the temperature rise more significantly as compared to systems without fins. Performance of PCMs with different melting temperature was also analyzed by Huang et al. (2006). Two differ-

Fig. 26. Schematic drawing of a PV/PCM system (Huang et al., 2004).

Fig. 27. Average temperature evolution at the front surface of PV/PCM system with ambient temperatures at 10, 15 and 20 °C with an insolation of 1000 W/m2, and an initial system temperature of 20 °C (Huang et al., 2004).

Fig. 28. Experimental setup of PV/PCM system (Huang et al., 2006).

ent PCMs were compared. One PCM is RT 25 with a melting temperature at 25 °C and the other GR40 is a granular PCM with a melting temperature at 40 °C. Those two PCMs were filled in the hollow of the aluminum structure with thirty-one aluminum fins of 27 mm wide subject to 750 W/m2 insolation. Results in Fig. 30 revealed that

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Fig. 29. Comparison of front surface temperatures measured for four systems exposed to insolation of 750 W/m 2 and an ambient temperature of 23 ± 1 °C (Huang et al., 2006).

a metal wall which is bent into a triangular shape to divide and hold those two PCMs (Fig. 31) can alleviate the volume expansion problem since the wall can be a pressure release pathway for the melted PCM. The metal wall of the cell provides a good thermal transfer to the two PCMs and is helpful for a longer temperature control. Shown in Fig. 32, PV front surface temperature evolutions for different combinations of two PCMs were simulated and recorded. It can be seen within the first hour that the temperature on the front surface of the PV/PCM system has the lowest temperature rise for the system with the combination of RT27-RT21 due to its lower melting temperature, but at a later stage RT27-RT27 were proved more effective on curbing the temperature rise of PV cell. Also, targeting PCM for BIPV system, Hasan et al. (2010) conducted experiments to evaluate the performance of five different PCMs in four different PV/PCM systems. Results found that a maximum temperature reduction of 18 °C was achieved for 30 min while 10 °C temperature reduction was maintained for 5 h at 1000 W/m2 insolation. Selecting PCM with high melting temperature, Maiti et al. (2011) configured a V-trough structure to enhance solar insolation and incorporated paraffin wax with a melting range of 56–58 °C to the rear of the PV module to limit its temperature rise. The problem of low thermal conductivity of the wax was solved with the help of packed metal turnings wherein the wax resided. Employing a 0.06 m thick bed of the PCM matrix, the module temperature in the indoor experiment could be maintained at 65–68 °C for 3 h whereas in its absence the temperature rose beyond 90 °C within 15 min. In outdoor studies, the module temperature in V-trough could be reduced from 78 to 62 °C

Fig. 30. Temperature evolution on the test system front surface when employing solid–liquid PCM RT25 and solid–solid (granular) PCM RT40 compared with the aluminum base plate datum condition when exposed to 750 W/m2 and an ambient temperature of 23 ± 1 °C (Huang et al., 2006).

GR40 was less effective compared to RT25 at maintaining a steady temperature due to its smaller latent heat of fusion and reduced bulk thermal conductivity by the air between the solid granules. However compared with the temperature rise for the base case condition (without PCM), GR40 still delayed the temperature rise and kept the PV front surface temperature under 51 °C (Huang et al., 2006). In order to achieve a quick thermal dissipation response with longer thermal regulation, a modified PV/PCM system integrated with two PCMs with different phase transient temperatures for improving the heat regulation was investigated for BIPV (Huang, 2011). The system having

Fig. 31. Schematic diagram of PV/PCM system with metal cells for different PCMs (Huang, 2011).

D. Du et al. / Solar Energy 97 (2013) 238–254

Fig. 32. Predicted front surface temperatures evolution for different PCMs (Huang, 2011).

with the PCM assembly and operation could be sustained throughout the day. The molten wax formed during operation re-solidified during the evening and night and could be re-used. A few recent simulation work carry on, Ho et al. (2012) did parametric simulations of the thermal and electrical performance of a BIPV integrated with a microencapsulated phase change material (MEPCM) layer. The MEPCM layer was attached to the PV back side to absorb the heat transferred from PV as a passive way of cooling. Different aspect ratio and MEPCM with different melting temperature was investigated in winter, summer and normal climate conditions. The simulation results showed that for an aspect ratio of 0.277, the MEPCM with a melting temperature of 26 °C can reduce peak PV temperature from 49 to 47 °C, increasing conversion efficiency from 17.86% to 17.99%. However, the same system in winner case, was able to reduce peak PV temperature from 35 to 30.5 °C, increasing conversion efficiency from 19.07% to 19.49%. Biwole et al. (2013) proposed a detailed mathematical and numerical modeling of heat and mass transfers in coupled PV/ PCM system. Results showed that adding a PCM on the back of a solar panel can maintain the panel’s operating temperature under 40 °C for 80 min under a constant solar radiation of 1000 W/m2 while the same temperature was reached by the panel after only 5 min without the PCM. Recent research was on the optimizing designs of PV/ PCM finned system. Effects of convection in the PCM on heat transfer and the problem of PCM crystalline segregation PV/PCM systems have been evaluated by Huang et al. (2011), but it is noted that the addition of internal fins improved the temperature control of the PV in a PV/ PCM system. 3. Discussion and conclusion Cooling by forced/natural ventilation, hydraulic cooling, heat pipe cooling and phase change material system are most common thermal management systems employed on PV installations and each of them has its own applicability but performance varies in different systems. A

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summary is provided in Table 1 for the various systems that have been reviewed. Passive cooling by heat spreader or heat sink can provide enough cooling to get a relatively low cell temperature even for CPV, but the heat sink surface area can be extremely large if increasing with growing solar concentration ratio. Literature on natural/forced ventilated PV/T or PV facßade systems are found to focus more on the performance of simultaneous utilization of solar thermal and electrical energy rather than on the investigation of cooling effect for PV, which implies more PV temperature related parameters need to be collected to investigate the system cooling performance for PV. Generally, natural ventilated systems can achieve PV temperature in a range of 50–70 °C, in comparison forced ventilated systems are found to achieve a lower temperature range of 20– 30 °C, however the parasitic power consumption by fan/ pump should be accounted. Many literatures did not quantify the parasitic power consumption but there exist a few demonstrating net power gain can be achieved after deducting the parasitic power consumption. Hydraulic PV/T system was similar to ventilated PV/T system which lacks a detailed investigation of PV temperature. It revealed that an glazing cover for PV/T system will induce higher PV temperature and in summer the circulating water itself is at high temperature which could not effectively cool PV cell. De-ionized liquid immersion cooling was reported to cool CPV down to a temperature range of 30–45 °C and jet impingements have achieved 60 °C for 200 CPV, both without mentioning parasitic power consumption. Heat pipe systems were also found to be mainly useful for concentrator PV due to the presence of a much dense heat flux. De-ionized liquid immersion cooling, jet impingement and heat pipe systems generally require more complicated system design and are most suitable for densely packed CPV. In contrast, this review has identified so far that very rare PCM thermal management systems are applied on concentrating PV systems. Although systems examined may adopt a single kind PCM or a combination of several PCMs with different melting temperature, both proved themselves relatively effective for regulating PV temperature. Another advantage is its versatility in the choice of melting temperature. Also, the thickness of the PCM layer can determine the period of time that a relatively stable PV temperature can be maintained. It is believed that PCM thermal management system can be more intelligent and the heat stored has a potential to be re-utilized. There are limitations in heat storage and heat transfer capabilities of the PCMs. These limitations may necessitate an additional facility to utilize or at least dissipate the heat stored in the PCM. Although using multiple PCM components and optimizing fin designs can partially alleviate the problems of low thermal response and crystalline segregation, the waste heat stored in the PCM is rarely utilized in comparison with PV/T system. An integrated PCM system with waste heat utilization facility capable of simultaneously cooling down PV cell and converting the waste

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Table 1 A summary of the systems reviewed. Work

Configuration

PV Type

PV cell temperature

Araki et al. (exp.)

Natural convection, Aluminum heat spreader

CPV (400)

Min et al. (exp.) Tonui and Tripanagnostopoulos (Theoretical.)

Natural convection, large heat sink Natural convection, refercence PV/T (glazed)

CPV(400) Flat plate PV facßade

21 °C temperature rise than ambient 37 °C 70 °C

Luque et al. (exp.) Natarajan et al. (theoretical) Mei et al. (exp.)

Natural Natural Natural Natural Natural Natural Natural Natural

Yun et al. (exp.)

Natural convection, PV facßade

Solanki et al. (exp.) Hegazy (theoretical) Candanedo et al. (theoretical) Pantic, Candanedo and Athienitis (theoretical) Candanedo et al. (exp.) Bambrook and Sproul (exp.) He et al. (exp.) Zakharchenko et al. (exp.) Fraisse et al. (theoretical) Russell (exp.)

Natural convection, V-trough heat sink Forced air convection, PV/T Forced air circulation, open-loop BIPV/T Forced air circulation, open-loop BIPV/T

Ugumori and Ikeya (exp.) Abrahamyan et al. (exp.) Tanaka (exp.) Han et al. (exp.) Wang et al. (exp.) Liu et al. (exp.) Zhu et al. (exp.) Xiang et al. (Theoretical) Royne and Christopher (theoretical) Russell (exp.) Akbarzadeh and Wadowski (exp.) Farahat (exp.) Anderson et al. (theoretical) Huang et al. (exp.) Huang et al. (theoretical) Huang et al. (exp.) Huang et al. (theoretical) Hasan et al. (exp.) Maiti et al. (exp.) Ho et al. (theoretical) Biwole et al. (theoretical)

convection, thin metal sheet (glazed) convection, rectangular fin (glazed) convection, refercence PV/T (unglazed) convection, thin metal sheet (unglazed) convection, rectangular fin (unglazed) air convection, aluminium-finned heat sink convection, finned heat sink convection, PV facßade

Forced air circulation, open-loop BIPV/T Forced air circulation, unglazed open loop PVT air system Forced water circulation, PV/T Water circulation, PV/T Water circulation, PV/T Water immersion, elongated tube with liquid as optical concentrator and cooling system Dielectric liquid immersing/optical cpv Dielectric liquid immersing/optical cpv Dielectric liquid immersing/optical cpv Dielectric liquid immersing/optical cpv Dielectric liquid immersing/optical cpv Dielectric liquid immersing/optical cpv Dielectric liquid immersing/optical cpv Dielectric liquid immersing/optical cpv with fins Hydraulic jet impingement Heat pipe cooling Heat pipe cooling Heat pipe cooling Heat pipe cooling with fins Flat plate heat pipe Phase change material system Phase change material system with fins Phase change material system, two materials separated in triangles Phase change material system Phase change material system, V-trough metla-wax matrix Phase change material system, BIPV Phase change material system with fins

heat into electrical energy could be the way forward towards future development. Experimental work is still important for determining the best method of cooling for a given application, but the comparisons in this review may provide a good background.

66 °C 63 °C 51 °C 49 °C 49 °C 58 °C 49.6 °C Peak value higher than 50 °C

CPV (32) CPV (10) Flat plate PV facßade Flat plate PV facßade CPV(2) Flat plat PV/T Flat plat BIPV/T Flat plat BIPV/T

– 66.5 °C

Flat plat BIPV/T Flat plat PV/T Flat plat PV/T Flat plat PV/T Flat plat PV/T CPV

– 22–28 °C – Cooled by 10 °C – –

CPV CPV CPV CPV CPV CPV CPV (250) CPV (250) CPV (200 and 500) CPV CPV (20) CPV CPV CPV Flat plate PV Flat plate PV Flat plate PV Flat Flat Flat Flat

plate plate plate plate

PV PV PV PV

5 °C reduction on Average module temperature 60 °C

– —— —— —— 30 °C 45 °C 40.0 °C 60 °C (200) 96 °C(500)

46 °C

40 °C higher than ambient 37.5 °C 27 °C 28 °C 18 °C reduction 62 °C (outdoor) 65 °C (indoor) 30.5 °C (winter) 47 °C (summer) 40 °C

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