Thermal and electrical management of photovoltaic panels using phase change materials – A review

Renewable and Sustainable Energy Reviews 92 (2018) 254–271

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

Thermal and electrical management of photovoltaic panels using phase change materials – A review ⁎

T

⁎

Adeel Waqasa,b, , Jie Jia, , Lijie Xua, Majid Alib, Zeashanc, Jahanzeb Alvia a

Department of Thermal Science and Energy Engineering, University of Science and Technology of China, 96 Jinzhai Road, Hefei City 230026, China Center for Advanced Studies in Energy (CASEN); National University of Science and Technology (NUST) H-12 campus, Islamabad 44000, Pakistan c Institute of Environmental Science and Engineering (IESE), National University of Science and Technology (NUST) H-12 campus, Islamabad 44000, Pakistan b

A R T I C LE I N FO

A B S T R A C T

Keywords: PV PCM, Solar energy BIPV

Thermal and electrical management of the PV systems integrated with Phase Change Materials has been discussed in this article. The main aim of this review article is to provide the current status of PV-PCM technology along with the research gaps and challenges being faced by this technology. A comprehensive literature review elaborating different aspects of this technology, such as system development, performance evaluation, PCM selection, heat transfer enhancement, simulation, and application in practice is presented. Cost incurred due to inclusion of PCM into PV system is also discussed in detail in this article. Major findings of the current review are that PV panel peak temperature can be reduced up to 20 °C with an increase in electrical conversion efficiency up to 5% by using PCM. At an average, ~ 2.6 kg of PCM is needed per meter square of the PV panel area to reduce the one-degree temperature of the PV panel during peak hours. The current review concludes that PV-PCM cooling system is not yet commercialized because of technological challenges, high system cost, and availability of appropriate PCMs. However, PV–PCM systems are still in the research phase, with great scope for practical applications. Suggestions for the future work along with current challenges are also presented.

1. Introduction The electrical conversion efficiency of PV cell is significantly affected due to the surface temperature of the PV panel [1]. A 1.0 °C increase in a typical PV cell surface temperature normally reduces the conversion efficiency by 0.08–0.1%, reducing power output by 0.45% over the nominal cell operating temperature of 25 °C [2]. Therefore the cooling of the PV panels becomes vital to remove the excessive heat generated by solar cells. The main aim of using thermal regulation techniques in PV panels is to bring down the temperature of the solar cells at a value which is as low as possible and close to Standard Test Conditions (STC) to increases the efficiency [3]. Cooling techniques are also observed helpful to increase the life of the solar cells as the thermal stresses are reduced due to cooling [4]. 1.1. Passive and active PV cooling techniques Various PV cooling techniques have been investigated and explored in past, including passive techniques and active techniques as summarized in Fig. 1 [2]. These include air based, liquid-based and PCM based PV cooling systems. All these cooling techniques can be used

⁎

actively and passively. In active techniques, excess heat generated by PV panel is removed by circulating air or water through PV panel. Circulating air or water acts as heat transfer fluid carrying heat away from the PV panel. Active heat removal systems commonly use pumps or blowers to maintain a flow of heat transfer fluid on the front or back of the PV panel for cooling purpose [1,5]. Active systems are observed efficient in removing excess heat from PV panel leading to higher PV efficiency compared to the passive systems. However, extra or parasitic power consumption and system maintenance costs are the key issues with active techniques [6]. Passive cooling systems are based on the techniques that don’t use pumps or blower for the circulation of the heat transfer fluid through the PV panel rather depend on natural convection, conduction and radiation heat transfer mechanisms. The main advantage of using passive cooling techniques is that they don’t require any parasitic energy for their operation and the maintenance cost is either zero or very low. A variety of passive cooling techniques has been reported for the cooling of PV panel. Most commonly used are air cooling and water cooling [7] while the simplest one is conductive passive cooling [7,8]. Researchers around the globe are working on innovative passive cooling techniques for PV panels that don’t need any extra energy and the maintenance cost is very low.

Corresponding authors. E-mail addresses: [email protected] (A. Waqas), [email protected] (J. Ji).

https://doi.org/10.1016/j.rser.2018.04.091 Received 18 September 2017; Received in revised form 11 April 2018; Accepted 15 April 2018 1364-0321/ © 2018 Published by Elsevier Ltd.

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PV Cooling

Air Based

Active Forced air circulation

Passive Transpired Cooling

PCM based

Liquid Based

Active

Passive

Active

Liquid Immersion heat pipe Hydronic cooling

Passive

Regenerated by forced heat removal

Regenerated by ambient cold

Water impingement

Fig. 1. Commonly used PV cooling techniques. [2].

explained in Section 2 while the scholarly articles published on PV-PCM systems are analyzed and discussed in Section 3. Theoretical and experimental studies are also summarized in Section 3. The effectiveness of PCM for thermal and electrical regulation of PV panels is discussed and analyzed in detail in the Section 4. PCM encapsulation techniques are elaborated in Section 5. Types of PCM along with the criteria to choose PCM for PV applications are discussed in Section 6. Heat transfer enhancement practices and their effect on PV-PCM system output have been discussed in Section 7. Indicators to access the performance of the PV-PCM system is discussed in Section 8 while the economic viability of the PV-PCM systems is elaborated in Section 9. Key issues and challenges with the PV-PCM system are discussed in Section 10 while conclusions are presented in Section 11.

1.2. PCM based PV cooling system Phase Change Material (PCM) based thermal regulation of PV panel falls under the category of the passive cooling technique [5]. Research in PCM based cooling systems are gaining interest as the excess heat generated by PV is absorbed by PCM without using any heat transfer fluid or moving part. In PCM based cooling systems excess heat generated by the PV panel is absorbed by PCM passively during sunshine hours when PCM is in the sold state. The absorbed heat is rejected to the ambient during non-sunshine hours when the ambient conditions are much cooler [9] turning PCM from the liquid state back to the solid state. PCMs are organic or inorganic in nature that undergoes a reversible phase change. PCM undergo melting phase if the absorbing surface temperature is more than the melting point of the PCM and vice versa and keep the absorbing surface temperature close to the melting point [10]. PCM based cooling of PV panels have gained interest since the year 2010 and most of the quality literature was published during the 2015–2017 while the first study was published in the year 2004 [11] by Dublin Institute of Technology. The published literature about PV-PCM has shown that PCM can effectively reduce the temperature of the PV panels up to 10 °C to 20 °C improving the electrical conversion efficiency up to 3–5% [8,12] but its economic viability is still under research phase. Availability of the PCM with required thermal and chemical properties with suitable price is also an issue as PCM is not commercially available in most of the countries. PV–PCM systems with numerous benefits, issues, and challenges offer further research opportunities for the researchers around the globe as PCMs not only reduce the temperature of PV panel but also provide thermal energy storage option for further utilization. Therefore the main aim of this article is to give the readers and researchers answers to the following questions that can arise related to the PV-PCM technology:

2. PV-PCM working principle PV panels integrated with the PCM storage is a hybrid technology in which PV panel and PCM are integrated into a single component to achieve the higher electrical conversion efficiency and lower PV surface temperature. A typical PV-PCM system is shown in Fig. 2. It consists of a PV panel and PCM capsulated in a suitable capsulation material and attached at the back side of the PV panel. In PV-PCM systems, PCM storage acts as a heat sink for PV panel, especially during sunshine hours, when the operating temperature of the PV panel is higher than the melting point of the PCM. PCM starts absorbing excess heat from PV panel keeping the temperature of the PV panel close to its melting temperature without any external energy input. Excess heat absorbed by PCM changes its phase from a solid state to liquid state at a certain temperature known as the melting point of the PCM. PCM keeps on absorbing the PV heat until the PCM is converted to the liquid phase. The liquefied PCM gets solidified again as the surrounding temperature and the PV surface temperature falls below the melting point of the PCM. Excess heat absorbed by the PCM from PV panel is released to the surrounding converting PCM back into the solid state. PCM solidification process normally occurs during non-sun shine hours when PV panel is not receiving any solar radiation. In this way, PV panel is cooled passively and electricity conversion efficiency is enhanced without using any parasitic energy from PV panel.

• What is a PV-PCM technology? How much temperature of the PV • •

panel can be lowered using PCM? How much mass of the PCM is needed to lower the PV temperature of PV panel? What should be the melting point of the PCM for PV cooling application? PCM should be organic or inorganic? Heat collected by the PCM from PV panel can be further utilized or not? What issues may arise if PV is integrated with PCM? What is the procedure to integrate PCM with PV panel? And finally, whether PV-PCM is economically feasible technology or not?

3. PV-PCM published scholarly literature In this section, experimental and theoretical literature related to the PV-PCM is analyzed and discussed. It is analyzed when and where the PV-PCM topic got the attention of the scientists. Universities, groups, and individuals working on the PV-PCM topic are also discussed. Later in the same section theoretical and experimental studies published on PV-PCM topic are analyzed and discussed in brief along with their

Therefore, sections in the current article are formulated in a way to answer the above-mentioned questions and provide a deep understanding of PV-PCM systems for academic researchers and implementation resource persons. PV-PCM system working principle is 255

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Fig. 2. Typical PV-PCM system [6].

the very old topic as the first study was published in the year, 2004 shown in Fig. 3. According to Fig. 3, about 50–60% of the research studies related to PV-PCM systems are published during the years 2015–2017. The first study was published during the year 2014 by Huang et al. [11] from Dublin Institute of Technology. The data collected shows that 27% of the published studies are purely experimental based while 36% are purely simulated studies validated from the already published data and 36% of the published studies are those which are mainly theoretical but a small indoor or outdoor experiment was conducted to validate the simulation results.

major findings. PCM integrated with PV panels for thermal regulation has attracted many researchers around the globe. The trend of the published literature on non-concentrated PV systems integrated with PCM is shown in Fig. 3. Research articles mentioned in Fig. 3 are based on the literature published in well reputed international journals have an impact factor. For this purpose, scholarly literature was searched using Scopus search engine and Thomson Reuters web of science. Articles published in conferences are not considered in formulating the Fig. 3 to avoid any duplication. According to the published literature PV-PCM system is not

Fig. 3. Published literature about PV-PCM. (a) Publication trend. (b) Nature of the published study. 256

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Fig. 4. (a) PV-PCM articles published by different publishers. (b) PV-PCM articles published by Elsevier journals.

significant. More uniform temperature distribution was observed within the PV-PCM system due to fins. It was observed that by using the internal metal fins in PCM container the temperature rise of the PV-PCM system can be reduced by more than 30 °C compared to the system without PCM [13]. A 2-D model [11,13] of the PV-PCM system was successfully upgraded to 3D model [14,15] by the same author to study the temperature, velocity fields, and vortex formation within the PCM with fins integrated into the PCM container. The model was used to investigate the temperature distribution and heat transfer in PCM due to high thermal conductive pin fins [14,15]. A PV-PCM system with aluminum fins studied theoretically [14,15] was investigated experimentally by Huang et al. [16] shown in Fig. 6. Fins helped in the uniform distribution of the temperature at the front surface aluminum plate. Stratification within the PCM was also controlled. Although metallic fins used by Huang et al. [16] (Fig. 6) improved thermal response and the heat transfer from PV to PCM but at the same the thermal regulation time period was shortened as the volume of the PCM was substituted by the metal mass of the fins. Increased weight of the system due to metal fins was observed also as a potential problem. Huang [17] worked out an innovative PV-PCM configuration to attain a rapid thermal dissipation response along with longer thermal regulation time period and reduced weight. The modified PV-PCM system was studied theoretically that utilized the triangular shaped cells in the PCM container as shown in Fig. 7 instead of metallic fins. Triangular shaped cells were integrated into the aluminum container with the PCMs having a different melting point as shown in Fig. 7. Combination PCM was expected to enhance the thermal regulation effect and improve the thermal response duration compared to the previously discussed metallic fins configurations [11,13,16]. Three consecutive days of June were selected to examine the thermal response of the PV-PCM system (Fig. 7) for the climatic conditions of UK. RT27–RT21 PCM combination efficiently kept the temperature of the PV surface under 22 °C for all three days as shown in Fig. 7. It was noticed that PCM with the lower melting point dominated the whole

According to the published literature (Fig. 3) about 81% of the articles have been published in the international journals available from Elsevier, 9% of the studies have been published in the international journals available from MDPI while 9% include other publishers like Springer, Wiley etc. shown in Fig. 4-a. A maximum number of articles about PV-PCM, in Elsevier, has been published by Solar energy journal, about 54%, followed by the journal of solar energy materials & solar cells and Renewable energy journal as shown in Fig. 4-b. Among all the published studies ~45% of the studies related to the PV-PCM system has been conducted by the University of Ulster, Dublin Institute of Technology and United Arab Emirates University. List of universities and groups involved in PV-PCM research along with the researcher details are provided in Table 1. Only those universities/ centers are tabulated in Table 1 who has published two or more than two articles. About 60–70% of the published researched have been conducted from these five universities listed in Table 1. 3.1. Theoretical and experimental studies on PV-PCM systems There are several experimental and computational studies that investigated the utilization of PCM to regulate the temperature of PV panels. A Very first study published in a well-known journal was conducted by Huang et al. [11]. Huang et al. [11] proposed a 2D numerical model to study the thermal behavior of the PV panel integrated with PCM. The numerical results were validated by the series of small-scale experiments. Three systems were experimentally and numerically investigated. System I was an aluminum plate to simulate a PV cell, System II was an aluminum plate attached to a container filled with PCM to simulate PV-PCM system, System III was an aluminum plate attached to a container filled with PCM and internal fins (shown in Fig. 5) to study the effect of fins on heat transfer in PCM. The temperature of the aluminum plate (PV panel) was reduced by 10 °C with PCM having a melting point of 32 °C and thickness 20 mm and up to 20 °C with a PCM thickness of 30 mm. The enhancement in the thermal performance due to metal fins in the PCM container was observed Table 1 Universities/ Centers involved in PV-PCM research. No

University/Country

Department/Center

Main researcher/s

Email

1 2 3 4

University of Ulster/ North Ireland UK Dublin Institute of Technology/ Ireland United Arab Emirates University / A.dhabi National Cheng Kung University, Taiwan

Center for Sustainable Technologies Dublin Energy Lab College of Engineering Research Center for Energy Technology and Strategy

[email protected] [email protected] [email protected] [email protected]

5

University of Palermo/ Italy

Department of Energy

M.J. Huang Brian Norton Ahmad Hasan A.O. Tanuwijava C.J. Ho Valerio Lo Brano Giuseppina Ciulla

257

[email protected]

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Fig. 5. PV-PCM system analyzed by Huang et al. [11,13].

how low and high thermal conductive PCMs can affect PV temperature when capsulated in containers having high and low thermal conductivity. Thermal performance of the PCM with low thermal conductivity was improved in the aluminum container. Inorganic PCM achieved the best result in terms of temperature reduction at all radiation levels. CaCl2·6H2O having a melting point of 29 °C kept PV temperature 10 °C lower for 5 h at 1000 W/m2 while PCM with a melting point of 20 °C was able to provide a temperature difference of 10 °C for 1.8 h at 1000 W/m2. It was concluded that PCM with low thermal conductivity and low melting point provide better results for the PV operating at a lower temperature. PCM with higher melting point and higher conductivity work better for the PV operating at high temperature [18]. The temperature profiles for two PCMs encapsulated in two different containers are shown in Fig. 8-b also. Effect of the PCM container material can be observed clearly in Fig. 8-b. The temperature difference is more dominant for the PCM with low thermal conductivity. Micro-Encapsulated PCM (MEPCM) was first time used for PV cooling by Ho et al. [19,20] and Tanuwijava et al. [21]. The concept was studied numerically for winter and summer conditions. PCMs having a melting point of 26 °C and 34 °C were used for the simulations. The simulated system along with summer season results is described in

system performance. It was concluded that the current system with triangular metal cells performed better and extended the thermal regulation period compared to the system with fins discussed before in. Fig. 6 [16]. Instead of using internal fins as shown in Fig. 6, a system in which metallic fins were attached to the PCM container externally was studied by Atkin & Farid [5]. Graphite was mixed with PCM to increase the overall thermal conductivity of the organic PCM. The overall efficiency of the PV panel increased by 12.8% with graphite mixed PCM capsulated in a container with external fins. PV-PCM with external fins reduced the peak temperature of the PV panel from 77.5 °C to 61 °C. Effect of PCM thermal properties like melting point and thermal conductivity, on PV surface temperature under different solar radiation, was studied experimentally by Hasan et al. [18]. Indoor experiments were conducted with five different PCMs under three insolation levels (500 W/m2, 750 W/m2, and 1000 W/m2). PCMs used were of organic (RT20) inorganic (CaCl2·6H2O, Sp22) and Eutectic (capric–palmitic acid, capric–lauric acid) nature. The melting point and thermal conductivity of these PCMs were in the range of 25 °C ± 4 °C and 0.6 W/ m-K ± 0.4 W/m-K respectively. PCMs were capsulated in the containers made of Aluminum and Perspex having a thermal conductivity of 287 W/m-K and 0.189 W/m-K respectively as shown in Fig. 8-a. The main purpose of using these four types of containers was to observe

Fig. 6. PV-PCM system with internal aluminum fins [16]. 258

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PCM 2 PV

PCM 1 Aluminium rear plate Solar insolation

PCM 2

PCM 1 Aluminium front plate

Averaged temperature at the front surface of the PV/PCM system with the two different PCM combinations from 21st June till 22nd June (SE England) [17] PCM 2

Fig. 7. PV/PCM system with triangular metal cells for different PCM [17].

plastic bags was attached to the back side of the PV with the help of wire mesh as shown in Fig. 10 [22,23]. The experiments were conducted during the summer season in Palermo-Italy. It was observed that the peak temperature of the PV panel reduced by 3 °C to 4 °C due to PCM between 26 and 27 June 2010. It was observed that in the PV-PCM system main challenge is to make a perfect thermal contact between PV back surface and PCM for the efficient heat transfer otherwise PV-PCM system cannot achieve the expected results. Also if PCM cannot reject its accumulated heat to ambient desired results cannot be achieved for

Fig. 9. It was found that the MEPCM with melting point 26 °C kept PV panel at a lower temperature but when PCM with Tm of 34 °C started melting it showed lower PV temperature than PCM with Tm = 26 °C. Using MEPCM with an aspect ratio of 0.277 (W/H of PCM capsulation) and Tm = 26 °C minimum, PV efficiency improved by 0.13% and 0.42% during summer and winter season respectively. However, it was not mentioned how microencapsulated PCM can work better than macrocapsulated PCM. Commercially available PCM RT27 (Tm = 27 °C) encapsulated in

Fig. 8. PV-PCM system with different PCM capsulation material (a) and its effect on PV surface temperature (b). [18]. 259

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Fig. 9. PV integrated with MEPCM along with aspect ratio and weather conditions used for simulation [19,21].

Encapsulated PCM attached with PV

PCM encapsulated in Plastic bags

Outdoor test unit

Fig. 10. PCM encapsulated in plastic bags attached to the back side of PV panel [22,23].

observed more satisfactory in hot climate compared to cooler climate [24]. The PV-PCM system shown in Fig. 11 was integrated with building also to see how it can affect the indoor temperature of the space in the desert climatic conditions of UAE by Hasan et al. [12]. It was found that the PV power enhanced by 5.5% on daily average basis due to the addition of PCM at the back of the PV panel and indoor heat transmission was reduced by 7% that dropped the indoor air temperature by 3.5 °C during the daytime. Indoor cooling demand was also decreased as the PV-PCM system delayed the rise of peak temperature by 2.0 h. PCM RT42 having a melting point of 42 °C was used for this experimental study that was conducted in UAE. The PV-PCM system was investigated first time for the tropical climates like Malaysia by Mahamudul et al. [26]. The study reported a 10 °C decrease in the operating temperature of the PV panel for consecutive 4–6 h during a typical summer day that significantly affected the conversion efficiency of the PV panel as shown in Fig. 13-a. it can be observed in Fig. 13-a that with PCM, PV operating temperature can drop close to the ambient temperature. Most of the studies discussed before were related to inclined PV panels. Effect of PCM on the vertical PV system was firstly studied experimentally and numerically by Park et al. [27] for the climatic conditions of South Korea. It was observed that the peak temperature of the PV system decreased by 3 °C to 4 °C (Fig. 13-b.) and energy efficiency increased by 3% due to the addition of the PCM. The experiments were

the coming daytime. Finite difference model was developed for PV-PCM with PCM capsulated in plastic pouches [23] and results were validated by the experimental results [22]. Outdoor experiments on the PV-PCM system were conducted by first time for the hot and dry climatic conditions (Vehari-Pakistan) by Hasan et al. [24,25]. Results of the PV-PCM system for hot and dry climatic condition were compared when the same system was installed in the cooler European climate (Dublin, Ireland). Two PCMs, Eutectic (Tm = 22.5 °C) and Salt hydrate (Tm = 29 °C), were selected for the study. PCM was encapsulated in rectangular containers having internal dimensions of 600 mm * 700 mm* 40 mm. To enhance the heat transfer straight vertical aluminum fins were fabricated in the PCM container as shown in Fig. 11. Effect of the two PCMs (Tm = 22.5 °C & Tm = 29 °C) on temperature and power of the PV panel are shown in Fig. 12. It was concluded that PCM effect on PV temperature was more profound and dominant in hot climate compared to the cooler climate. The peak temperature of the PV in a hot climate was reduced from 63 °C to 46 °C and 42 °C by PCM1 (Tm = 22.5 °C) and PCM2 (Tm = 29 °C), respectively while in a cooler climate it was reduced from 49 °C to 43 °C by PCM1 and 39 °C by PCM2.as shown in Fig. 12. Similarly, PV power enhanced by 11% and 13% in hot climate while in cooler climate power increased by 4% and 5% as shown in Fig. 12. Comparing the two PCMs, salt hydrate PCM achieved a higher temperature drop and power savings compared to Eutectic mixture at both sites [25]. Financial benefits were 260

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Fig. 11. PV-PCM setup along with PCM container [24,25].

Ljubljana showed that the peak temperature of the PV panel was 75.5 °C that was reduced to 44 °C due to the addition of PCM shown in Fig. 14. 32 °C reduction in the PV surface temperature was not observed in the previous studies. The average difference between the temperature of PV panel and the PV-PCM system was observed ~ 15 °C. The modified PV panel used by the Stropnik & Stritih [8] is shown in Fig. 14. PV panels integrated with PCM can attain better economic yield compared to the standard PV was evaluated by Japs et al. [31]. Three PV panels were used for the study. One was the standard PV other two were integrated with PCM having the same melting point but different thermal conductivity and specific heat. Graphite was mixed with the conventional PCM to increase the thermal conductivity of the PCM. Daily energy yield and economic outputs of the PV-PCM system were almost negative despite promising results during the first half of the day. The PV-PCM temperature was observed higher than standard PV during the 2nd half of the day because of the thermal insulation effect of the PCM pouches attached at the back of the PV panels. However, the PCM with higher thermal conductivity was observed more suitable for PV cooling than conventional PCM. The main conclusion of the study was that PCM pouches suppress the convection currents from PV back resulting higher PV temperature during the afternoon. Effect of the thermal conductivity of PCM on PV output was also

conducted during the month of May and June for the climatic conditions of South Korea. PV with more PCM thickness absorbed more heat and regulated the PV temperature for the larger duration. At the same time, overall weight and the cost of the system was also increased According to the simulation results optimal melting point of the PCM was 25 °C that increased the power output by 1.0–1.5% compared to the conventional PV on yearly basis. Vertical PV-PCM system facing south was observed more beneficial during summer season compared to the winter season [27]. Numerical model developed to study the PV-PCM thermal and electrical behavior successfully reported a 5% increase in the electrical efficiency of the PV panel when integrated with the PCM container in the hot climatic condition like India and Saudi Arabia [28,29]. It was reported that higher wind velocity and PV inclination angle leads to a lower operating temperature of PV module.[29]. When the tilt angle of the PV-PCM system was increased from 0° to 90° PVPCM temperature decreased from 43 °C to 34 °C. Maximum gain in the efficiency was reported for the vertical PV-PCM system compared to the horizontal PV-PCM system [30]. Commercially available TRNSYS software was used to model the PV-PCM system to study the thermal and electrical behavior. Modeling results were validated with the outdoor experimental results [8]. Experiments that were conducted during the month of October in

Fig. 12. Effect of PCM on PV temperature and power in Pakistan and Ireland [24,25]. 261

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Fig. 13. Effect of PCM on PV surface temperature (a) Mahamudul et al. [26] (b) Park et al. [27].

Fig. 14. PV-PCM panel configuration [8].

was increased by an average of 5.8%. The effectiveness of the PCM on the water temperature, when combined with conventional PV/T, was studied by Browne et al.[10]. Novel PV/T/PCM system was aimed to generate electricity, heat and pre-heat the water under the ambient conditions of Ireland. PCM used was of Eutectic nature having a melting point of ~ 22 °C. Seven to eight times more thermal energy was removed by PCM from PV panel compared to the system without PCM. Temperature attained by water due

studied by Hachem et al. [32]. Conventional PCM having a melting point of 45 °C was mixed with 20% copper and 10% graphite to enhance the thermal conductivity. The resulted thermal conductivity of the new PCM was 92.1 W/m-K compared to the convention one that was only 0.18 W/m-K. Conventional PCM reduced the temperature of PV up to 6.5 °C with an average of 2.7 °C. Electrical output was enhanced by an average of 3.0%. Altered PCM reduced the peak temperature of PV up to 6.3 °C with an average of 5.6 °C. Electrical output

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Table 2 Thermal and eectrical regulation of PV panel using PCM. Parameters

Study Type

Ref

Exp Theo

Tm (°C) Tpeak _PV (°C) TPV-PCM (°C) *ΔTPV_peak (°C) mspec (kg/m2 of PV area) m_eff_Peak kg/(m2-°C) Increase in electrical energy yield (%) Increase in ηelect (Max & Avg) Study condition Climatic area

[8]

[33]

✓

✓

28 75.5 44 32 31.1 0.97 ** 7.4

42 53 43 10.0 24 2.4 ** 5.9

+

N.A Outdoor UAE

2.8 & b1.2 Outdoor Slovenia

[27]

[35]

[32]

[5]

[18]

[12]

[25]

✓

✓

✓

✓

✓

✓

✓

✓

✓

42 72 60 12.0 24 2.0

25 36 32 5.0 23 5.8 ** 1.5

27.17 61 47 14.0 23 1.6 a 2.2

27 62.2 56.2 6.0 15 2.5 a,b 7.4

40 79 67 ~ 12 22 1.8 a 9.6

29 57 45 12 38 3.16 N.A

42 65 58.4 6.6 24 3.6 ++ 5.5

29.8 63 42 21 48.5 2.3 a,b 7.7

29.8 49 39 10 48.5 4.9 a,b 1.8

b

N.A Indoor

a,b

+ a

N.A Indoor

N.A Outdoor UAE

N.A Outdoor Pakistan

N.A

2.0 Outdoor S. Korea

2–5 Outdoor N.A

2.5 Indoor

Ireland

[28]

[29]

[17]

[30,34]

✓ 31 91 78 13 26 2.0 N.A

✓ 35 61 54 7 16 2.3 a,+ 4.0

✓ 27–31 52 32 20 30 1.5 a,++ 20

✓ 26.6 54 35 19 16 1.2 N.A

N.A sim S.Arabia

N.A sim India

N.A sim UK

N.A sim N.A

N.A Not Available. sim = simulation study. *Temperature difference between PV-PCM and PV at peak PV time. ** Yearly basis. + Maximum gain in efficiency (Peak). ++ Daily average basis. a Observed during experimental/simulation time period. b Average increase.

temperature. Similarly, authors [8] have reported 2.8% increase in the electrical energy conversion efficiency during the experimental time period. The annual increase in the energy yield due to PCM was reported 7.4%. Hasan et al. [25] conducted an experimental study for hot climate and cool climate using the PCM having melting point of 29 °C. The specific mass of the PCM for both climates was kept same i.e. 48.5 kg/ m2. The results showed that the effective mass coefficient (m_eff) for cold climate was 4.9 kg/(m2-°C) while for the hot climate it was 2.3 kg/(m2°C) showing that PCM was more effective in reducing PV temperature in hot climatic conditions compared to the cold climate due to higher insolation in hot climates. Therefore in hot climates, less mass of the PCM can be more effective. Similar PCM was also more effective in increasing the electrical yield in hot climate compared to the cold climate as tabulated in Table 2 under reference [25]. Most of studies have reported the peak PV temperature reduction in the range of 10 °C to 20 °C [5,17,18,25,28,30,33,34] with very few studies reporting above 20 °C [8,25] Peak electrical efficiency has been reported to increase up to 2.0–3.0% due to PCM. It is observed that at an average 2.6 kg of PCM per meter square of PV area has been used to reduce 1 °C peak PV temperature showing that to reduce the peak temperature of the PV panel up to 10 °C 26 kg of PCM may be needed. On the other hand PV panel may be very heavy in weight due to PCM addition. One meter square of PV panel without PCM has the weight of ~ 18 kg and with the PCM cooling option, it can as high as 44 kg with a 10 °C reduction in PV peak temperature. The overall increased weight of the PV-PCM system may be an important factor from PV panel mounting structure point of view. More costly and heavy mounting structure may be needed due to the increased weight of the PV-PCM panels. Overweight PV panel issue has not been addressed by the authors.

to the PCM was 5.5 °C higher compared to the PV/T system without PCM. The study showed that PCM can also improve the performance of the whole system when it is integrated with convention PV/T system. The above-mentioned studies have clearly shown that PCM can be used effectively to reduce the PV surface temperature and improve the electrical conversion efficiency of the PV panel. In the coming sections, it will be analyzed and summarized how much mass of the PCM can be effective in regulating the temperature and efficiency of the PV panel. Also, it is discussed how much mass of PCM has been used by different authors in reducing the one-degree temperature of the PV panel. 4. Effectiveness of PCM for thermal and electrical regulation of PV panel The main purpose to use PCM with PV panel is to reduce its working temperature and regulate the electrical efficiency which is related to the operating temperature of the PV panel. Therefore in the current section effectiveness of PCM to reduce the PV panel surface temperature along with its effect on the electrical yield is discussed. Results and findings of different studies have been summarized and tabulated in Table 2. Following terms has been used to analyze the results tabulated in Table 2:

•T •T

peak _PV PV-PCM

= Peak temperature of the PV panel without PCM = Temperature of a PV-PCM panel at peak time.

ΔTPV _peak = Tpeak _PV − TPV − PCM

(1)

• Specific Mass of PCM = m •

2 spec = mPCM / PV area (kg/m ) Specific mass of PCM (mspec) is used to analyze how much mass of the PCM per meter square of the PV area has been used by different authors. Effective mass coefficient = m_eff = mspec / ΔTPV_peak kg/(m2-°C).

5. PCM capsulation for PV-PCM systems

The effective mass coefficient is used to analyze how much specific mass of the PCM has been used to decrease one-degree peak PV temperature. Maximum reduction of 32 °C in peak PV temperature was reported by Stropnik& Stritih [8] using PCM having a melting point of 28 °C. Specific mass of the PCM (mspec) used during the experiments was 31 kg/m2. The effective mass coefficient (m_eff) was 0.97 showing that the author/s have used 0.97 kg/m2 of PCM to reduce 1 °C of the peak PV

PCM has to be encapsulated to hold the material as it changes phase from liquid to solid and solid to liquid during its operation otherwise material may flow away. Two major capsulation techniques have been studied and reported widely [36] that are: i. Macro Capsulated PCMs 263

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Thermal grease

PV panel 5mm

6.1. The choice between organic and inorganic PCMs

PCM container 30 mm

Organic PCMs are preferred over inorganic PCMs as they do not react and rust with the PCM capsulated material. Therefore, leakage issues with organic based PCMs are very rare. Organic PCMs also exhibit a very low degree of subcooling compared to inorganic PCMs which is another advantage. Lower thermal conductivity (0.1–0.2 W/mK) is a major issue of using organic PCMs. Due to lower thermal conductivity, some heat transfer enhancement techniques have to be used otherwise expected results may not be achieved [42]. Although inorganic PCMs have high thermal conductivity they have failed to attract many researchers in PV-PCM field. Corrosive nature and sub-cooling are the two major reasons that caused the limited usage of inorganic PCM for PV cooling purpose. Due to corrosive nature PCM may react with its capsulation causing leakage from its containers. Leaked PCM can also react with the PV panel damaging the solar cells. Commercially available organic PCMs have been frequently used for the cooling of PV panels. About 77% of the published studies have preferred paraffin based PCMs for PV applications. Only 9% have used salt hydrates while 14% preferred Eutectic PCM as shown in Fig. 17-b. List of the PCMs that have been frequently used for the cooling of PV panel is provided in Table 3. One of the studies [18] compared the performance of the organic PCM and inorganic PCM for cooling down the PV panel. It was observed that the organic PCMs having lower thermal conductivity may be beneficial for low solar radiation locations and for PVs with low operating temperature. While inorganic PCMs having higher thermal conductivity like slat hydrates can provide good results for PVs working under high solar radiation. Also, it is reported that for organic PCMs which have low thermal conductivity may require high thermal conductivity capsulation material that can be costly from an economic point of view. Another study reported that salt hydrate (CaCl2·6H2O) kept PV temperature 3–4 degree lower compared to the Eutectic mixtures having lower thermal conductivity [25]. Still, it is not reported how the sub-cooling phenomena can be addressed when inorganic PCMs are used for PV cooling specially CaCl2·6H2O which shows a significant degree of subcooling.

Aluminium Frame 1 mm

Fig. 15. Rectangular PCM container attached to the back of PV Panel [35].

ii. Micro Capsulated PCMs PCM is said to be macro-capsulated when capsulation size exceeds 1 mm. In macro capsulation, PCM is capsulated in tubes, pouches, rectangular plates or spherical balls. PCM is said to be microencapsulated when the capsulation size is in between 1 µm and 1000 µm [37]. From the PV-PCM point of view, most of the published literature has reported macro-capsulated PCM [8,10,29–33,35,38–41] only a few have worked out microencapsulated PCMs [20,21]. Macro capsulation of PCM is cheaper compared to micro capsulation. Also, Macro capsulation can accommodate a large quantity of PCM as desired and easy to repair if damaged. In most of the studies published on the PV-PCM topic, PCM is capsulated in a rectangular container made of aluminum that can be easily fitted at the back side of the PV panel without any alteration to PV panel. PCM capsulated in plastic pouches was also used by [22,23] and attached at the back of the PV panel with wire mesh as shown in Fig. 16-a. PCM capsulations randomly used for PV cooling purposes are shown in Fig. 16. Rectangular PCM containers which are generally used with PV-PCM systems are made of aluminum and bonded to the back side of the PV panel with suitable thermal grease and epoxy as shown in Fig. 15. In very few cases it is observed that PCM filled aluminum containers are bolted at the back side of the PV panel and thermal grease of higher thermal conductivity is applied on the surface of PV panel and container to make perfect thermal contact [32]. PCM capsulation also acts as a heat exchanger between PV and PCM so the material of the capsulation should be selected in such a way that it can enhance the heat transfer from PV to PCM. Therefore in most of the cases, aluminum having a thermal conductivity of 287 W/m-K has been used as capsulation material. PCM capsulated in Polymethyl methacrylate has also been tested but the reported results are not as satisfactory as for aluminum capsulation. Depending on the quantity of PCM to be used different depths of the PCM filled containers have been used that include 35 mm [8], ~ 40 mm [12,18,24,25,33], 20 mm [32], 30 mm [35], and 50 mm [27]. PCM encapsulated in aluminum pouches and aluminum panels having enhanced heat transfer characteristics and frequently used for buildings may be used for PV cooling as shown in Fig. 16-e. Aluminum plates shown in Fig. 16-e can easily accommodate salt hydrates without any sign of leakage or rust.

6.2. Selection of the melting point of PCM for PV application There is no well-defined rule till now for choosing the melting point of the PCM especially of PV cooling, however, Hasan et al. [33] discussed the criteria to choose the melting point of PCM. This criterion is based on the average night temperature during the summer season and average PV temperature during winter days. The night temperature can be obtained from the metrological department of the specific location and the average PV temperature during winter days was calculated using the following relation:

0.32 ⎞G Tpv = Ta ⎛ ⎝ 8.91 + 2Vw ⎠ ⎜

6. Type of PCMs used for PV cooling

⎟

(2)

In Eq. (2) Ta is the ambient temperature, Vw is the wind speed and G is the average solar radiation. Using this relation Hasan et al. [33] found that average night time temperature during June in Al Ain (UAE) was 34 °C and average PV temperature during daytime in December was 46 °C. So PCM with a melting point of 42 °C was selected which was higher than 34 °C and lower than 46 °C. PCM melting points that have been used by different researchers [5,8,10–14,16–19,21–32,35,38,40,45–48] to cool down the PV panel are shown in Fig. 17-a. According to the Fig. 17-a, 65% of the published studies have used PCMs with a melting point in the range of 25 °C to 35 °C. In 19% published studies PCMs with a melting point above 35 °C have been used while 15% of the studies have used PCM with a melting point below 25 °C. PCMs with a melting point above 40 °C are mostly used for the hot and dry climatic conditions like UAE and Saudi Arabia. PCMs have been widely used for free cooling of buildings [49]. On

PCMs are generally divided into three main groups: organic compounds, inorganic compounds and eutectic mixtures [43]. These PCM compounds have a different melting point, latent heat and thermal conductivity that determine their usability for a specific application. An ideal PCM should have large latent heat, large thermal conductivity, chemically stable, non-toxic, inexpensive and non-corrosive [44]. Details of the thermo-physical properties of an ideal PCM can be found in the study by Islam et al. [1]. The phase change temperature of the selected PCM should be within the desired temperature range. In this section, PCMs that have been frequently used for the PV cooling applications are discussed and analyzed. Criteria to select the melting point of the PCM for a specific location are also discussed.

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(a)

(b)

PCM filled

Fin width 50 mm

Fin space 75 mm

Attached at PV back

plastic pouches

Fins Height 700mm

PCM encapsulated in Plastic bags and attached at the back of PV [22,23] Width 600mm

Depth 50 mm

PCM filled aluminium container [12,18,24,25,33] (d) (c)

PCM aluminium container with metal fin [16] [11]

(e)

PCM encapsulated in Aluminium container and attached at the back of PV [27] (f)

PCM filled aluminum pouches and panels [42] PV/PCM system with and without internal fins [43] Fig. 16. Macro Capsulated PCMs used for the PV cooling applications.

when used [51]. In some studies, during peak hours PV back temperature is observed as high as 91 °C especially in desert conditions [28]. In such conditions, it is vital to make sure that organic PCM is safe to use up to the peak temperature limit. Flammability issues can be avoided by using Inorganic PCMs.

average PCM melting point used for the PV cooling applications is 29.9 oC shown in Fig. 17-a. In free cooling PCM gets cold from ambient for solidification purpose. It is reported that for free cooling application PCM melting temperature should be at least 5 °C to 6 °C lower than the minimum ambient temperature otherwise PCM may not be solidified during short summer nights [42,50]. Such criteria can be followed here also as in PV-PCM systems PCM need cold from ambient for solidification purpose. A complete list of PCMs that have been used by authors for PVPCM systems along with its latent heat is provided in Table 3 that can be helpful for any researcher working in the PV-PCM field.

7. Heat transfer enhancement techniques for PV-PCM system The most common limitation and issue with PCM are its low thermal conductivity. Paraffin that has been widely used (Fig. 17-b) for thermal regulation of PV panels has very low thermal conductivity (0.1–0.2 W/ m-K). Therefore certain techniques have to be used to enhance the heat transfer to make sure that PCM efficiently absorb the excess heat from PV and reject the absorbed heat efficiently. Therefore in this section techniques used to enhance the heat transfer in the PV-PCM system has been discussed and summarized.

6.3. Flammability of the PCM As discussed in above sub-sections most of the studies have used organic PCMs for thermal regulation of PV panels. Paraffin and fatty acid based PCMs are highly flammable and need careful precautions 265

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Fig. 17. (a) PCM melting points used by different researchers. (b) Type of PCMs used for PV cooling.

potential problem [16]. A modified PV-PCM system integrated with two different PCMs with different melting points to improve the heat regulation was investigated by [17] shown in Fig. 18-a. PCM capsulation was equipped with triangular shaped metal cells to accommodate different PCMs. The triangular shaped metal cells also worked as fins and provided a high rate of heat transfer from PV back to PCM. The average temperature of the PV front surface observed was much lower in the current system as compared to the vertical fins system studied by [11,13]. It was also observed that the thermal regulation time period increased up to 20% compared to vertical fin system. The average temperature on the front surface of the PV panel with and without triangular shaped metal cells is shown in Fig. 18-b. Temperature difference and increase in a thermal regulation time period can be observed clearly.

Table 3 PCMs used for cooling of Photovoltaic. PCM Name

PCM Type/ Category

Melting point (°C)

Latent heat (kJ/ kg)

Reference

RT20 RT21 Capric–palmitic acid (C–P) RT22 SP-22 Capric–lauric acid (C–L) RT25 Not available Not available RT27 RT28 ZDJN-28 Not available CaCl2·6H2O RT31 Not available Sasolwax RT35 RT40 RT42 Parafina Merck GR40

Paraffin Paraffin Eutectic mixtures Paraffin Blend Eutectic mixtures Paraffin Paraffin Paraffin Paraffin Paraffin Paraffin Paraffin Salt Paraffin Paraffin Paraffin Paraffin Paraffin Paraffin Paraffin Eutectic mixtures Paraffin Paraffin Paraffin

20 21 21.2–22.6

140.0 134 190

[18] [17] [10,18,24,25]

22 24 24.66

190 182 172

[38] [18] [18]

25 25 26 & 34 27 28 28 28 29.66 31 32 32–36 35 40 42 43 43

170 184 172 184 245 204.5 210 213.2 165 251 162 240 180 145 N.A 82

[11,13,14,30] [27] [19,21] [16,17,22,23,47] [8,31] [35] [46] [18,24,25] [28,48] [11] [16] [16,26,29] [5] [12,40] [38] [13]

48 55 57

N.A 170 255

[32] [40] [45]

N.A RT55 Not available

7.2. PCM capsulation integrated with external metal fins A PV-PCM system with external fins was studied by Atkin & Farid [5]. The modified PV-PCM system is shown in Fig. 19-a. PCM was mixed with graphite to increase the overall thermal conductivity while external metal fins were attached to PCM container to enhance the heat transfer from PCM to ambient. The modified system decreased the PV front surface temperature from 75 °C to 55 °C resulting 12% increase in the electrical conversion efficiency of PV panel. The temperature profile of the PV panel without PCM, with PCM and PV-PCM with integrated external fins, is shown in Fig. 19-b. the difference in the thermal regulation can be clearly observed.

7.1. PCM capsulation integrated with metal fins

7.3. PCM capsulation manufactured with high thermal conductivity material

PCM container integrated with internal fins to enhance the heat transfer between PCM and PV was discussed by Huang et al. [11,13]. The results showed that fins can be beneficial for uniform temperature distribution within the PV-PCM system. [13]. Although the metal fins inserted in the PCM container [11,13] improved the heat transfer but at the same time, thermal regulation time period declined as the volume of the PCM was substituted by the mass of the metal fins. The increased weight of the PV-PCM system due to the metal fins was also observed a

Hasan et al. [18] concluded experimentally that PCMs with low thermal conductivity should be capsulated in capsulations having high thermal conductivity in this way their performance can be enhanced. This effect of capsulation having high and low thermal conductivity on PV surface temperature is shown in Fig. 8-b. It can be seen and observed that PCM in aluminum capsulation maintained PV front surface at a lower temperature than PCM capsulated in Perspex capsulation for both Eutectic and CaCl2·6H2O PCMs. 266

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Fig. 18. PV/PCM system with metal cells for different PCMs.

7.4. PCM mixed with high thermal conductive metals Petroleum jelly PCM having a thermal conductivity of 0.18 W/m-K was mixed with 20% copper and 10% graphite [32]. By mixing graphite and copper overall thermal conductivity changed from 0.18 to 92 W/mK although PCM mass was reduced by 30% due to additive metals. It can be seen in Fig. 20 that by the addition of graphite and copper in PCM, thermal regulation duration increased without much fluctuation in the surface temperature compared to the pure PCM. Overall results were observed very promising and it was found that the PCM mixed with graphite and copper can increase the thermal regulation time period as well as further reduce the PV surface temperature by 20 °C compared to the pure PCM. Therefore in most of the studies either graphite is mixed with PCM to increase the overall thermal conductivity of the PCM or PCM capsulation integrated with fins is used for heat transfer enhancement. These two methods are seen promising in producing uniform temperature over PV surface and increasing thermal regulation time period. Metals fins can be an issue from PV weight point of view as the overall weight of the PV panel can be increased due to metal fins.

Fig. 20. Effect PCM mixed with graphite and copper on PV surface temperature [32].

Fig. 19. PV-PCM system with external fins [5]. 267

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8. Indicators to access the performance of PV-PCM systems

thermal performance enhancement as an indicator to access the thermal performance of PV and PV-PCM system which is given in Eq. (9).

In this section indicators used to access the performance of the PVPCM systems has been summarized and discussed. Many authors have used the electrical efficiency as one of the major indicators to access how much efficiency can be improved by using the PCM. Most of the numerical and theoretical studies have used the formulation shown in the Eq. (3) to find out the efficiency of PV panel from the surface temperature of the PV panel [5,19–21,29,38,46].

η = ηref (1 − β (Tc − Tref ))

Enhancement (%) =

Pout IA

(3)

t=n

Γ=

It has been observed and discussed in previous sections that PCM can effectively reduce the peak temperature of the PV panel up to 10 °C to 20 °C, as a result, electrical conversion efficiency can be improved up to 5%. The economics of PV–PCM system is greatly dependent on the PV energy generation of a location; higher solar radiation, more energy generation means higher PV temperature and a need for thermal regulation [2]. In this section economic viability of the PV-PCM system is discussed. The cost involved in the manufacturing of PV-PCM system was analyzed and discussed by Hasan et al. [24]. Two systems of same capacity using the same PCM were manufactured and developed in Ireland and Pakistan. The cost involved in the development of these PVPCM systems is shown in Fig. 21. Although the PCM cost for both countries is same the PCM capsulation material cost is 33% lower in Pakistan compared to Ireland. Similarly, the cost involved in the manufacturing of the whole system was 99% lower in Pakistan compared to Ireland. So the overall cost incurred on the PV-PCM systems to regulate PV temperature was 40% lower in Pakistan compared to Ireland. PV-PCM system working in the climatic conditions of Pakistan was observed 3.1% more efficient compared to the climatic conditions of Ireland due to higher drop in PV panel surface temperature that was achieved in Pakistan. The economic benefits of the system were observed higher in Pakistan compared to Ireland due to the lesser cost incurred during production and higher improvement in the electrical efficiency in Pakistan. A very simple economic analysis of PV-PCM was conducted by Radziemska [38] for a 1 kWp PV panel. The analysis showed that the modified 1 kWp PV-PCM system (1 kWp capacity) can cost 8.5% higher than the cost of simple PV panel producing 7% more energy over a period of one year. Assuming 30 year lifetime of the PV panel, the

8.1. Power & energy enhancement percentage (PEP)& (EEP) Hachem et al. [32] introduced two indicators to access the effectiveness of PCM on PV performance. One was the power enhancement percentage (PEP) and second was efficiency enhancement percentage (EEP) which are defined below in Eqs. (5) and (6).

EEP = 100*

Pout _PCM − Pout _ref (5)

ηout _PCM − ηout _ref ηout _ref

(6)

Pout_PCM and Pout_ref are the power out from the PV panels with and without PCM and ηout_PCM and ηout_ref is the efficiency of the PV panel with and without PCM respectively. 8.2. Energy efficiency Hasan et al. [25,33] used the energy efficiency as an indicator to access the performance of PV panel with and without PCM. Energy efficiency was defined as:

η=

Ps Pe, ref

Ireland

Pakistan

500

(7)

450

In the equation, (7) η is the energy efficiency and Ps is the energy savings (Wh) due to the integration of PCM which is calculated as given in Eq. (8)

400

Cost in Euros

Ps = Pe _PV _PCM − Pe, ref V I Pe = oc sc FF

(10)

9. Economic viability of PV-PCM systems

(4)

Pout _ref

∑ (TPV ,t − TPV −PCM ,t ) t=0

In Eq. (4) Pout is the output power of PV panel, A is the area of the PV panel and I is the incident solar radiation. Where Pout = V*Ic (V is the voltage and Ic is the current). Some specific indicators that have been used in different studies to monitor the performance of the PV–PCM system are described below.

PEP = 100*

(9)

In Eq. (9) TPV and TPV-PCM are PV panel front surface temperature with and without PCM. The difference in the surface temperature of the PV panel with and without PCM was used as a parameter to measure the total thermal regulation enhancement ( Γ) by Hasan et al. [18].Γ was defined as given in Eq. (10)

In the Eq., (3) electrical efficiency is coupled with the operating temperature of the solar cell (Tc). ηref, β, and Tref are PV panel's electrical efficiency, temperature coefficient, and temperature at STC. The difference in the efficiency of PV with and without PCM is used as an indicator of improvement in the efficiency of the PV panel with PCM. The electrical efficiency of the PV panels with and without PCM for the experimental studies is also calculated using the following relation [24,29,32,33].

η=

(TPV − PCM − TPV ) *100 TPV − PCM

(8)

350 300 250 200

150

In the Eq., (8) Pe is the electric power of the PV panel measured from open circuit voltage (Voc) and the short-circuit current (Isc). FF is the fill factor taken between 0.72 and 0.75.

100 50 0 PCM

8.3. Enhancement in thermal performance

Capsulation Material

Manufacturing

Fig. 21. Total cost involved in manufacturing a PV-PCM system in developed and developing country [24].

Kibria et al. [28] conducted a simulation study and introduced 268

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PV-PCM system issues and challenges General System issues and challenges

Heat Transfer Issues and challenges

PCM heat transfer issues

Overall system heat transfer

Overall weight Cost effectiveness and of the system commercialization

Flammability of the PCM

Further utilization of heat absorbed by PCM Low thermal conductivity

Subcooling

Convection suppression from back of PV Panel

Availability and cost of the PCM

Thermal contact between PV and PCM

Fig. 22. Key issues and challenges related to PV-PCM system.

reduce the overall temperature of the PV module is briefly highlighted in this section. The efficiency of the conventional PV panels can be further increased by increasing the amount of solar radiation falling on the PV modules using a concentrator. Access solar radiation falling on PV module due to concentrator can increase the PV module temperature more than non-concentrated PV module [45]. To prevent this, PCM heat sink placed at the back side of the concentrated PV module can be effectively used to reduce the overall PV module temperature. Highgrade thermal energy absorbed by PCM from concentrated PV modules can be further used for different thermal energy applications. Ceylan et al. [52,53] conducted an experimental study in which electrical gains from the PV module was increased by using the concentrator while the access heat generated was stored in the paraffin PCM placed at the back side of the concentrated PV module. The heat stored in the PCM was used for the greenhouse for the drying of the product. The melting point of the PCM was 47 °C. Maiti et al. [45] observed that the module temperature could be kept below 65 °C by using PCM having a melting point of 56–58 °C melting. It was observed that in the absence of the PCM the temperature of the concentrated PV module can be as high as 95 °C. Concentrated PV modules integrated with the PCM heat sinks is still under investigation and attracting many researchers as high-grade thermal energy can be obtained by using concentration along with more electrical gains. Due to new research field, very few studies are available who have evaluated the thermal performance of the concentrated PV module.

surplus energy produced over the 30-year life will generate 10% profit. Hasan et al. [33] observed that 13.5 kWh/m2 extra energy can be produced by integrating PCM with conventional PV panel. Multiplying this by international electricity rate (@ 0.15 Euro/kWh) PV-PCM system may produce an economic benefit of 2.2 $/m2 of PV area. 27 kg of PCM was used for one m2 of PV area. The extra amount incurred due to the addition of the PCM (@ 1.0 $/kg for paraffin wax) can produce a payback within 10–12 years. Inorganic PCMs like slat hydrates which are much cheaper than paraffin wax (0.14–0.24 $/kg) can give a payback within 3–4 years. Atkin &Farid [5] observed that the cost of the PV-PCM system was 10% higher compared to the conventional PV-PCM system generating ~ 10% more energy compared to the conventional PV panel. The Economic viability of the PV-PCM system was studied by Hendricks et al. [48] using a simplified heat balance model. It was observed that system can be more economical if the PV surface temperature is kept close to the theoretical STC conditions otherwise the PV-PCM system may not be the economically viable solution. To make PV-PCM systems financially viable heat collected by PCM should be used for space heating or water heating purposes. The Payback period was calculated using the following relation [47]:

PBP (yr ) =

PPCM MPCM EPCM COE

(11)

In Eq. (11) PPCM as the price of the PCM (€/kg), MPCM is the weight (kg/m2), EPCM is the energy enhancement (kWh/ m2/year) with PCM, and COE is the price of the electricity (€/kWh). EPCM in the Eq. (11) is calculated as given in Eq. (12).

EPCM = Eref [1 + d (Tref − TPV − PCM )] Eref = AηαϕΔt

11. Key issues, challenges and further research areas for PV-PCM systems Although thermal regulation of the PV panels with PCM is an attractive option compared to other convention PV cooling techniques there is still much to be researched to improve the working and commercialization of these systems. Some of the key issues and challenges related to the PV-PCM systems are elaborated in Fig. 22 and discussed here. Issues and challenges related to the PV-PCM system can be divided into two main groups. One is the heat transfer issues due to PCM low thermal conductivity, subcooling etc. and PV-PCM contact surface issues while others are the issues and challenges related to the whole system like cost and payback of the system, overall weight of the whole system, further utilization of the heat absorbed by the PCM etc.

(12)

Eref is the energy output from PV without PCM and d is a percentage of output change per degree Kelvin. d was 65% for silicon PV panels which was experimentally determined. Overall the Payback period of the PV-PCM system depends on the mass of the PCM used, cost of the PCM and the manufacturing cost of the PV-PCM system. Manufacturing cost may be lower in some countries but PCM cost which is normally imported from the developed countries may be higher. Locally produced PCM may be beneficial from payback point of view. Overall still payback period of the PV-PCM system is 12–15 years which is very high. The payback period can be reduced by enhancing the benefits by collecting the absorbed heat from PCM and utilizing it for further applications.

✓ Most of the PCMs with high energy storage density have low thermal conductivity especially paraffin wax. In order to deal with the low thermal conductivity, some heat transfer enhancement techniques are needed. Some authors have used internal fins [11,13,24,25] while some have used external fins fixed with PCM container [5]. Although fins can increase the overall heat transfer from PV to PCM but at the same thermal regulation time period can be decreased as discussed before. The overall increased weight of PV-PCM system due to fins can also be an issue and a challenge during the

10. PCM coupled with concentrated PV modules The main aim of the current article is to investigate the effectiveness of the PCM heat sink coupled with the conventional flat PV modules. However, due to increasing interest in concentrated PV modules, the effectiveness of the PCM integrated with concentrated PV modules to 269

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implementation phase. ✓ Salt hydrates may have very good thermal conductivity but their subcooling can be a big challenge during PCM solidification time period if used for PV cooling. Although some author/s [18] have studied the behavior of salt hydrates the subcooling issues and its consequences on the PCM solidification have not been addressed. Salt hydrates with high thermal conductivity can be a good replacement for paraffin wax if the issues like subcooling and reaction with PCM capsulation can be resolved. ✓ Natural convection from the back of the PV panel is totally suppressed as the PCM container fully covers the back of the PV. In one of the studies [31], it is noticed that in the afternoon when the whole PCM is melted the PV panel with PCM may experience higher temperature compared to the normal PV panel due to the poor convention from the back of PV panel with PCM. Therefore it can be a further research area how to design PCM capsulation that can assist and increase the natural convection from the back of the PV panel rather suppressing it. ✓ Efficient removal of heat from PCM and recovering it for further utilization is also a challenge. The overall efficiency of the PV-PCM can further be enhanced if the recovered heat can be used. Therefore, suitable heat transfer system should be researched out to ensure effective utilization of the energy absorbed by the PCM. This will also improve the economic viability of PV-PCM systems. ✓ The PV-PCM system may be 40–50% heavy in weight compared to the conventional PV panel. Such heavy panels may cause extra cost during their installation time period due to heavy and more stable mounting structure. It has been observed that currently 20–30 kg of PCM per meter square of the PV panel is needed to reduce 10 °C to 20 °C PV temperature. Weight may be reduced by using PCMs that have high latent heat so that less mass of the PCM be used but such PCMs are rarely available and a major research topic. ✓ The payback period of the PV-PCM systems is seen very high compared to the other conventional passive cooling techniques [18,33]. This factor may be challenging for the acceptance of the PV-PCM systems for commercialization. ✓ Availability of the PCM having high latent heat, high thermal conductivity, and low subcooling is a challenge and need to be addressed. Cost-effectiveness of the PCM is also a big hurdle since only a few companies around the globe are dealing in good quality PCM.

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12. Conclusions Conventional PV system integrated with PCM has been critically reviewed. It has been observed that PCM can effectively reduce the operating temperature of the PV panel to certain degrees resulting better and improved electrical conversion efficiency of the PV panels. Key issues and challenges currently faced by the PV-PCM system along with possible research gaps have also been highlighted and discussed. Elaborated research gaps, issues and challenges in PV-PCM can be overcome by more extensive research and development work by academia sector in collaboration with the commercial sector. Based on the reviewed literature following are the key conclusions:

• PCM can effectively reduce the operating temperature of the PV •

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locations with high ambient temperature and high solar radiation compared to cooler locations with low solar radiation. Organic PCMs even with low thermal conductivity are seen more attractive for thermal and electrical regulation of PV panels. On the other hand, economic benefits in terms of payback period are observed more favorable with the inorganic PCMs due to their low cost and high thermal conductivity. At an average, ~ 2.6 kg of PCM per meter square of PV area may be needed to decrease one degree of peak PV temperature. Such high quantity of PCM can increase PV panel overall weight, up to 40%, making them heavier for the mounting and installation. PCM capsulation with internal and external fins is seen beneficial in enhancing the heat transfer between PV and PCM that can further lower the temperature of the PV panel due to rapid heat transfer. For rapid heat transfer PCM mixed with some metals like graphite is also seen useful as the overall thermal conductivity of the PCM is enhanced mixed with metals. Very limited studies are being conducted under real outdoor conditions. Experiments under real out conditions can be beneficial for further understanding of PV-PCM performance. Round year studies under real outdoor conditions will also be helpful in clearing the uncertainties about the payback time period of the PV-PCM system.

panel up to 20 °C enhancing the overall electrical conversation efficiency of PV panel up to 5%. PCM with a melting point in the range of 25 °C to 35 °C has been observed more suitable for the PV cooling application. PCM with a melting point greater than 35 °C are observed suitable for the hot climatic conditions. The upper limit of the PCM melting point is seen 42 °C which is used when PV temperature rises up to 90 °C. However, the selection of the PCM melting point completely depends on the geographical location and the climatic conditions of the study area. PCMs are observed suitable and economically beneficial for the

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