A Review of Heat Batteries Based PV Module Cooling—Case Studies on Performance Enhancement of Large-Scale Solar PV System

Several studies have concentrated on cooling the PV module temperature (TPV) to enhance the system’s electrical output power and efficiency in recent years. In this review study, PCMbased cooling techniques are reviewed majorly classified into three techniques: (i) incorporating raw/pure PCM behind the PV module is one of the most straightforward techniques; (ii) thermal additives such as inter-fin, nano-compound, expanded graphite (EG), and others are infused in PCM to enhance the heat transfer rate between PV module and PCM; and (iii) thermal collectors that are placed behind the PV module or inside the PCM container to minimize the PCM usage. Advantageously, these techniques favor reusing the waste heat from the PV module. Further, in this study, PCM thermophysical properties are straightforwardly discussed. It is found that the PCM melting temperature (Tmelt) and thermal conductivity (KPCM) become the major concerns in cooling the PV module. Based on the literature review, experimentally proven PV-PCM temperatures are analyzed over a year for UAE and Islamabad locations using typical meteorological year (TMY) data from the National Renewable Energy Laboratory (NREL) data source in 1 h frequency.


Introduction
Renewable energy sources are actively adopted as non-conventional energy sources to reduce fossil fuel consumption and global warming [1,2]. Among the alternative and renewable energies, solar energy being pioneer, photovoltaic, and thermal technologies thereby reducing the complex installation and economics of the system. Whenever the T PV rises, wetted cotton wick extracts the thermal energy from the PV module and reduces T PV by 30% [35]. However, most water-cooling technologies fail due to power consumption in operating the pumps for fluid circulation and the requirement of groundwater in case the fluid chosen is water.
Air, yet an abundant and well-explored coolant for PV modules, is one of the oldest and accustomed, due to which there are no restrictions in the usage of air to cool the PV module. Although natural airflow over and under the PV modules removes heat, particularly for cold regions, it is not significantly high to cool down the module to standard operating temperature. Hence, forced air circulation is widely adopted for cold climatic conditions [36,37]. For effective utilization of the resource, ambient air is pumped over to the back surface of the PV module by creating a cavity [37]. Again, this technique also requires additional power to blow the air. The technique is further improvised by sandwiching the PV module between the layers of glass to pass the air from both front and back surfaces of the PV module, and this technique achieved a greater reduction in T PV [38,39]. Sophisticatedly, a fin-aligned thermal collector is installed on the back surface of the PV module to create resistance in the air, which could increase the heat extraction from the PV module [36]. The air's density and specific heat capacity are less than water [40], thus achieving the effective T PV reduction, and the airflow duct should be in optimal range unless it is difficult to create the resistance in the air [41].
Besides water and air, heat sink-based T PV reductions are optional for real-time operations at gusty locations [42]. Other techniques include immersing the PV module into the water or other liquids, thermoelectric generator incorporation to reduce the T PV, but these techniques are still under investigation and have not been demonstrated for practical applications [43,44]. Currently, phase change material (PCM)-based T PV reduction techniques have gained considerable attention among research groups and have delivered promising results in real-time applications [45][46][47] as shown in Table 1. PCMs are often called heat batteries as it stores and discharges heat energy. In recent years, researchers are using the term heat batteries to attract readers and it directly promotes the usage of PCMs. Although PCM is an effective heat storage material, in real-time applications it is not as well known as electrochemical batteries. Initially, in the solid state, PCM stores thermal energy in the form of solid-specific heat capacity (J/g.K); following that, PCM reaches its melting temperature (T melt ) and then the material turns into a mushy state. Advantageously, mushy state stores thermal energy in the form of latent heat of fusion (J/g). This mushy state stores higher thermal energy without increasing its internal temperature, unlike specific heat capacity. Finally, in the liquid phase, the PCM utilizes its latent heat of fusion (L or Hm) completely and retains energy in a liquid-specific heat capacity J/g.K). Equation (1) is expressed below as three different forms of thermal energy storage in PCMs [48]. Generally, any specific heat material such as water stores thermal energy by raising its internal temperature (4 J/g.K). PCM also increases the internal temperature for a solid and liquid phase; moreover, its specific heat capacity is less than water (1.8-3 J/g.K) but H m is high (130-300 J/g) which does not exist in sensible heat storage materials (water), although H m makes PCM a unique heat storage material [49]. (1) Table 2 shows various review studies conducted on the PV module cooling. From this literature study, it is clear that several researchers have focused on finding the novelty mostly in categorizing the cooling techniques, namely active cooling and passive cooling techniques. It is noted that, so far, no review study has discussed the PCM thermophysical property on cooling the PV module. Figure 1 shows the formation of this review study according to the proposed objective.

Author
Year Findings Elavarasan et al. [50] 2020 OM29 organic PCM poured directly under the PV module without using any intermediate layer.
A maximum 1.2 • C PV module temperature reduction is noticed.
Velmurugan et al. [49] 2020 Developed composite PCM integrated behind the PV module without using a physical contact to avoid the potential induced degradation (PID) loss. Different thickness of the PCM matrix is analyzed and validated experimentally. An increase in thickness of PCM enhanced the cooling but linearity was achieved at 2.5 cm thickness.
Poongavanam et al. [51] 2020 A novel eutectic PCM is developed for Chidambaram climatic conditions. Operational variation of PCM is analyzed for summer and winter. During high melting temperature of PCM enables higher cooling than a low melting temperature of PCM.
Velmurugan et al. [16] 2020 A novel cylindrical tube-based PCM matrix is developed for the hot climatic condition of Thailand. Developed cylindrical tube PCM container placed behind the PV module without physical contact to avoid the mechanical stress and PID loss. Cylindrical tube PCM container consumes less amount of PCM as compared to other techniques.
Savvakis et al. [52] 2020 RT27 and RT31 commercial PCM is examined for Chaina, Crete location. It is noted that RT31 reduced 7.5 • C which is 1.1 • C higher than RT27.
Velmurugan et al. [8] 2021 Eutectic cold PCM is developed for PV module cooling. A 3 cm and 5 cm thickness of cold PCM is examined resulting higher cooling effect achieved for 5 cm thickness.
He et al. [53] 2021 Stearic acid and Lauric acid are mixed to prepare the eutectic combination to cool the PV module. Developed eutectic PCM reduced the PV module temperature maximum of 20 • C.
Velmurugan et al. [54] 2021 Cascaded PCM structure is developed for tropical and subtropical climatic conditions. The PCM selection algorithm is developed based on the existing experimental results. India and France require to be cooled only in summer and Thailand require cascaded structure for both summer and winter.
Divyateja et al. [55] 2021 Simulation is conducted with RT25HC commercial PCM to cool the PV module, in addition, CuO is composited to enhance the thermal property of the PCM. Nano-enhanced PCM reduced maximum of 2.02 • C.
The main objective of this study is given below:  Typical meteorological year (TMY) data from National renewable energy laboratory (NREL) data source in 1 h frequency enables studying the sites' weather condition over a year. This would benefit in determining the PCM for varying environmental conditions. • With the reference values associated with experimentally proven PV PCM temperature from "Hasan et al. [56] and Waqas et al. [57]" to assess the performance of a typical MW scale solar PV system.

Author Objective Key Findings
Senthil et al. [58] Different types of solar thermal heat pipes are reviewed.
Velmurugan et al. [54] • Types of PCMs used for PV module cooling are reviewed. • A simplified PCM selection procedure is proposed.
• Following paraffin wax, commercial Rubitherm PCMs gained second place in cooling the PV module. • Organic eutectic PCMs are achieving proper endothermic and exothermic peaks though not involved majorly.

•
The study finds that Thailand requires a cascade structure as compared to France.
Anand et al. [59] • A comprehensive review is conducted for PV module cooling using water, air, thermoelectric generator, and PCM.
• The active cooling system enhances electrical output power generation, but it consumes more external sources to pump/circulate the fluids. • Thermoelectric generator found to be an efficient coolant medium with enhanced efficiency of 1-18%. • Using PCM and nanofluid enhanced the electrical efficiency by 8-10%.
Kumar et al. [60] PCM and nano-PCM based cooling techniques are reviewed.
• PVT system is mainly developed by aluminum and copper.

•
Modified heat transfer channels such as thin absorber sheet, porous air flow duct/heat pipe, fin-based heat pipe enhanced the heat transfer rate. • Nano-PCM with nanofluid enhanced the cooling effect.
Browne et al. [61] A comprehensive review was conducted on BIPV and concentrated PV cooling methods using PCM.
• This study found that the PCM technique enhanced the cooling effect than other sensible heat storage materials.

•
Water and air are efficient heat removing materials though it does not regulate the temperature of the PV module longer period.
• Nano fluid-based PVT system efficiency reached up to 61.23%.

•
The air cooling technique improved the performance maximum of 26%.

•
The radiative cooling technique is one of the promising techniques, yet the emissive factor must be developed to achieve greater impact.
Ghadikolaei et al. [63] A detailed review of water, jet impregnation, and PCM based cooling.

•
Heat pipes with internal and external fin enhanced the power output. • Selecting inappropriate PCM melting temperature deteriorates the efficiency of the PV system. • Nano fluid with thermal collector maintained the T PV under control for a longer duration.
Maleki et al. [64] Active and passive cooling methods are reviewed.
• Passive cooling methods tend to be the simplest and most efficient technique in turns of non-additional power consumption. • Active cooling methods mainly rely on fluid channel constructions.

Author Objective Key Findings
Mahian et al. [65] Building-integrated PCM-based PV module cooling techniques are reviewed.

•
Few research was found in the field of economic aspects and optimization. • A cooling system that is incorporated with HVAC, low operating temperature application seems to be good in economic aspect. • It is found that concentrated PV systems are not majorly incorporated with PCM-based cooling systems.

Defects in Water-Based Cooling
Although water is a practical material in removing heat commercially and technically, it is complicated to cool the PV module at a large scale due to resource and maintenance difficulties. Resource management becomes crucial due to the degrading statistics of the groundwater level and absence of water in arid and semi-arid regions, which are favorable sites for the PV plants, owing to their alternative advantages.

Water Spraying Technique
Water spraying is suggested to minimize the water consumption, effective utilization is subjected to the spacing between the PV modules as they act as sites for leakage. Despite recovering three-quarters of sprayed water, filtering fine dust and dirt particles adds to difficulties in maintenance and resource cost of the PV plant [28].

Syringe-Type Water Spraying Technique
Although a syringe-type of water spraying technique is installed to optimize the water utilization, an external pump is essential to maintain the constant pressure and flow in the water channel for uniform water spreading, which adds complexity to the existing system. However, this system is great for areas where the water resource is enormous, as shown in Figure 2a,b.

Cotton Wick Cooling Technology
Besides high-end technology in cooling the PV module, cotton wick with bottled water reservoir also reduces the temperature of the PV module. Perhaps the investment cost of the cotton wick and bottled water is not as expensive as water spraying and PVT techniques as shown in Figure 2e. However, the heat transfer rate as well as the reduction in the rate of TPV is so low that it was correspondingly found to be worth the investment for small and mid-sized power plants.

Immersing Cooling Technique
Other than any of the above-mentioned techniques, immersing the PV module inside the water bodies could restrain the PV module's temperature in higher order especially, by immersing the PV module in clean rivers rather than a lake or polluted river as shown in Figure 2c. The polluted river or lake could contaminate the glass surface of the module by producing unwanted layer formation that possibly reduces the solar radiation . PV module cooling using (a) stored rainwater spraying technique [28]; (b) nozzle type water spraying [66]; (c) immersing at different depth of water [32]; (d) heat pipe [67]; (e) wetted cotton wick [35].

Cotton Wick Cooling Technology
Besides high-end technology in cooling the PV module, cotton wick with bottled water reservoir also reduces the temperature of the PV module. Perhaps the investment cost of the cotton wick and bottled water is not as expensive as water spraying and PVT techniques as shown in Figure 2e. However, the heat transfer rate as well as the reduction in the rate of T PV is so low that it was correspondingly found to be worth the investment for small and mid-sized power plants.

Immersing Cooling Technique
Other than any of the above-mentioned techniques, immersing the PV module inside the water bodies could restrain the PV module's temperature in higher order especially, by immersing the PV module in clean rivers rather than a lake or polluted river as shown in Figure 2c. The polluted river or lake could contaminate the glass surface of the module by producing unwanted layer formation that possibly reduces the solar radiation penetration for energy conversion. On the other hand, cleaning the PV module is difficult and useless, as contamination can coat the surface in a short period. Based on the continuous drop in electrical efficiency and increase in thermal load, waste temperature from the PV module could be captured and reused for low and midtemperature applications. Thermal heat-removing sheets and tubes are placed on the Tedlar surface, whereas the PV module acts as a semiconductor device and flat plate thermal collector. Based on the electrical and thermal load demand, water flow arrangements are made in the flat plate thermal collector, as shown in Figure 2d. The major drawback in accompanying thermal collectors with PV modules is that it greatly increases the weight of the system. Therefore, safety and security become a questionable argument due to the constant pressure in the thermal collector and water storage tank which could cause an explosion. On the other hand, a photovoltaic thermal hybrid system creates noise and vibration during the water flow in the channels or tubes.
Non-uniform cooling occurs in PVT systems though it is widely adopted in cold regions under hybrid electrical and thermal energy generator models. This PVT system is not suitable for large-scale systems as the temperature from the PV module is not sufficiently high to run a turbine to generate electricity. It would increase the total investment with a long payback period. Overall, it is considered that water-based PV module cooling techniques are feasible under certain criteria, and it is permissible to deploy them in the common platform as a solution.

Defects in Air-Based Cooling
Likely, the air is an effective medium to cool the PV module than water because air is abundant in nature and can be applied as a coolant for any environmental conditions. However, air-based cooling techniques are not reported for large-scale systems, as it requires modifying the front or back surface of every PV module to facilitate heat extraction, which is practically impossible. Once the front or back surface of the PV module is modified, an external pump is necessary to assist in any climatic conditions as natural thermal dissipation is restricted in air-based techniques, as shown in Figure 3a. For commercialgrade implementation at a larger scale, air-assisted cooling methods are not appropriate. Especially in hot and humid climatic conditions, water can be sprayed over the PV module during the peak daytime hours to reduce thermal stress. The rest of the period can be set for natural dissipation without any effort. However, this condition is not applicable for air-based cooling techniques, since the front or back surface of the PV module becomes entirely suppressed by interacting with the ambient environment or surroundings. If the external air pump fails, T PV could increase exponentially than the unmodified PV module. These constraints depict that air is technically not recommended for large-scale systems; however, it can be widely used for a small and mid-sized PV system to warm the building or room temperature in cold climatic conditions. Reportedly, a PV module without an air pump can be adopted for hybrid application, although it is a demerit for PV module cooling because natural convection is capable to remove the heat from the PV module, but not for the entire duration of daytime hours.

Forced Air Flow Technique
In recent years, several researchers have recommended the forced convection method to enhance the electrical and thermal efficiency of the PV system where a fin-aligned thermal collector with forced convection doubles the benefit of cooling the PV module by creating the resistance in air flow which increases the heat absorption capability of the air as shown in Figure 3c. Notably, pressurized air in the thermal collector increases air leakage, which requires additional care in insulating the thermal collector. Glass wool or other high fire point insulation material is required to minimize the fire accident as short circuits in electrical connections are more vulnerable. Glass wool or other high fire point insulation material is required to minimize the fire accident as short circuits in electrical connections are more vulnerable.

Double Side Air Passing Technique
As mentioned earlier, the double side air passing method cools the PV module effectively and acts as a double cooling agent on both the front and back surface of the PV module as shown in Figure 3b. This modified front surface of the PV module lacks an antireflection coating that affects the absorption of solar radiation. Overall, the air-based technique is advised for small and mid-sized power plants this technique is subjected to several operational issues for scaled-up applications.

Defects in TEG and Others
TEG is a well-known energy converter and is mainly used in high-temperature applications such as thermal plants, industrial furnaces, heat engines, and others to convert waste heat into useful electrical energy. TEG requires a higher temperature difference Overview of PV module cooling using (a) single passing air [37]; (b) double passing air [38]; (c) aligned internal fins [36]; (d) variable duct depth air [41].

Double Side Air Passing Technique
As mentioned earlier, the double side air passing method cools the PV module effectively and acts as a double cooling agent on both the front and back surface of the PV module as shown in Figure 3b. This modified front surface of the PV module lacks an antireflection coating that affects the absorption of solar radiation. Overall, the air-based technique is advised for small and mid-sized power plants this technique is subjected to several operational issues for scaled-up applications.

Defects in TEG and Others
TEG is a well-known energy converter and is mainly used in high-temperature applications such as thermal plants, industrial furnaces, heat engines, and others to convert waste heat into useful electrical energy. TEG requires a higher temperature difference between the hot and cold sides, whereas the charge carriers are excited on the hot side and move towards the cold side, generating voltage. In this case, T PV is the TEG source applied on the hot side while the cold side faces the ground in an open environment. Converting T PV into useful electrical energy boosts the system's electrical output technically, but economically, TEG failed to meet the return on investment. TEG is economically favorable when the temperature difference between the hot and cold sides is high. Mostly, T PV lies around 50-70 • C during peak daytime hours and approximately 30 • C of the temperature difference between T PV and ambient can be noticed. TEG will not make the effective electrical conversion that makes TEG not economically favorable with this temperature difference. Other than TEG, placing the heatsink behind the PV module could enhance the heat transfer. It requires a proper mounting structure unless the Tedlar surface of the PV module can break easily because of its brittleness and fragile nature conditions.

Drawbacks of Non-PCM Technique
The major drawback in non-PCM-based cooling technology is non-uniformity in PV module surface cooling. The higher temperature difference between the sensible heat material and PV module enables a higher heat transfer rate; water and air absorb a high amount of heat energy when it enters into the header or footer section of the PV module. During the motion of the fluid from header to footer or footer to the header, the fluid gains a higher temperature and fails to absorb the same heat energy resulting in non-uniformity in cooling to occur. The same situation occurs for other sensible heat storage materials. That is why PCM has gained significant attention in cooling the PV module because PCM is a stationary unit and removes the heat uniformly during the entire period of operation.

Outline of PV Module Cooling Using PCM
In the recent decade, researchers have been using PCM as coolant material to enhance the electrical efficiency of the PV module [54,68]. This passive cooling technique does not require a flowing medium, or external energy needed to circulate the coolants, thus making PV-PCM systems/technology more economical with low maintenance costs compared to conventional PV-thermal systems. The compactness of the auxiliary design and reliability is yet another reason for the massive adaption of this cooling system. PCM has a high energy density and stores thermal energy with a negligible temperature rise. This makes the material and the technology robust and safe-fail; thus, even to be used in complex projects such as the Pioneer-Venus spacecraft in 1979 [69]. Later, in the late 1990s, Hausler et al. demonstrated the simplest PV-PCM system to boost the PV module performance by improving the thermal conductivity of the PCM [70]. One of many ways to improve the thermal conductivity is by simply fitting a metal container filled with PCM to the PV module Tedlar surface [71].
Effective recovery and utilization of thermal energy dissipating through a rise in temperature of the PV modules are of higher importance, particularly in renewable energy systems for the generation of power. PV-PCMs can be regarded as the best existing solution for designing future hybrid systems for better energy conservation and utilization [8,9]. Despite these facts that the technology is familiar and offers unmatchable benefits, low thermal conductivity, low heat storage density, leakage, and high material costs are the main areas of concern that need further investigation. Thus, on the whole, understanding the properties and types of PCM and identifying the appropriate PCM for the application plays a crucial role in developing a matured PV-PCM system. Classification of PCM cooling systems based on PCM type is shown in Figure 4 and their respective material properties are discussed broadly in the forthcoming sections.
any further processing and can be integrated behind the PV module as such. Secondly, considering the change in phase of the PCM it can be filled in the container to avoid leakage, which makes the system robust. Generally, organic PCMs are identified as non-corrosive and have ease of maintenance compared to their inorganic types of counterparts. showed that PCM integration is more effective for ground mounting than the rooftop PV system. The reason is that the PCM container's back surface does not get exposed to the ambient or surrounding, which results in thermal stress occurrence around the PCM container, leading to restriction in dissipating its heat. Therefore, it is necessary to consider the thermal dissipation factor of the PCM container as one of the key parameters to enhance the electrical efficiency of the PV module [72]. Notably, building integrated photovoltaic (BIPV) with PCM integration also failed to reduce the TPV due to resistance in thermal dissipation and a lack of wind interaction [73]. As mentioned earlier, PCMs are filled in a container to avoid leakages during the phase change and most of the time PCMs are packed in a metal container to maintain perfect physical contact with the PV module. Nikolaos Savvakis et al. developed a three-segment PCM container constructed using galvanized steel to increase the mechanical strength as well as physical contact closeness ratio with the PV module. In this study, RT27 commercial organic PCMs were filled in the segmented PCM container and integrated on the Tedlar surface. With the help of perfect physical contact, a segmented PCM container enabled higher heat transfer, resulting in the reduction in TPV to the maximum of 20 °C for 150 min compared to PV without PCM [74]. Several researchers used thermal conducting adhesive material between the PV module and PCM container to attain 100% physical contact since there is a different property for every adhesive material used, thus limiting the heat transfer. Considering this issue, liquid PCMs are directly filled on the PV module Tedlar surface and sealed on the backside of the PCM using a glass/metal sheet [75,76]

Overview of Pure PCM-Based PV Module Cooling
Primarily, there are several types of PCM available in the market and each one of them is unique in its way in terms of major benefits and storing and discharging heat with minor defects. Commercially available pure PCMs becomes an alternative organic PCM type that stand tops in terms of the simple material design and usage, likely most of them are paraffin wax, fatty acids, or oils. While most of the PCM integrated systems need to be tailored to the requirement, pure PCM, which means the raw PCM, does not require any further processing and can be integrated behind the PV module as such. Secondly, considering the change in phase of the PCM it can be filled in the container to avoid leakage, which makes the system robust. Generally, organic PCMs are identified as non-corrosive and have ease of maintenance compared to their inorganic types of counterparts. Figure 5 shows the overview of PV-PCM operational conditions using a different construction technique. Yuli et al. showed that PCM integration is more effective for ground mounting than the rooftop PV system. The reason is that the PCM container's back surface does not get exposed to the ambient or surrounding, which results in thermal stress occurrence around the PCM container, leading to restriction in dissipating its heat. Therefore, it is necessary to consider the thermal dissipation factor of the PCM container as one of the key parameters to enhance the electrical efficiency of the PV module [72]. Notably, building integrated photovoltaic (BIPV) with PCM integration also failed to reduce the T PV due to resistance in thermal dissipation and a lack of wind interaction [73]. As mentioned earlier, PCMs are filled in a container to avoid leakages during the phase change and most of the time PCMs are packed in a metal container to maintain perfect physical contact with the PV module. Nikolaos Savvakis et al. developed a three-segment PCM container constructed using galvanized steel to increase the mechanical strength as well as physical contact closeness ratio with the PV module. In this study, RT27 commercial organic PCMs were filled in the segmented PCM container and integrated on the Tedlar surface. With the help of perfect physical contact, a segmented PCM container enabled higher heat transfer, resulting in the reduction in T PV to the maximum of 20 • C for 150 min compared to PV without PCM [74]. Several researchers used thermal conducting adhesive material between the PV module and PCM container to attain 100% physical contact since there is a different property for every adhesive material used, thus limiting the heat transfer. Considering this issue, liquid PCMs are directly filled on the PV module Tedlar surface and sealed on the backside of the PCM using a glass/metal sheet [75,76]. The direct filling PCM technique transfers the PV module heat to PCM without any intermediate adhesive material resulting in the achievement of a higher heat transfer rate. This method attained a PV module temperature reduction of a maximum of 10-12 • C under Malaysian climatic conditions [77,78]. Even though this technique limits the thermal barriers, PCM leakage becomes the biggest issue and thus is not an affordable one for solar power plants.

PV-PCM Operational Difficulties
Other than the construction and placement of the PCM container in the given system, several researchers focused on the thermal properties of the PCM such as T melt , H m, and congruent in the material. In most cases, organic PCMs are examined for cooling the PV module as they are congruent in material with high H m . Selecting PCM T melt is one of the major parameters in cooling the PV module as PCM is a temperature-dependent material and sensitive to climatic conditions [79]. To illustrate this, Indartono et al. [80] studied two different T melt PCM materials on Indonesian climatic conditions. The first one was made up of coconut oil as this sort of PCM failed to reduce the T PV effectively due to higher T amb, while on the other hand PCM made up of crude oil reduced the T PV to a better extent rather than that of coconut oil. It was noted that the PCM material with low T melt turns to liquid before the peak sunshine which results in restriction in the transfer of heat between the PV module and PCM. Following Indartono et al., the study by Eqwan M. R et al. [81] revealed that examining with inappropriate PCM adversely affects the heat removal from the PV module, even to the extent that under certain conditions, it rather increases the T PV than the conventional PV module. The main reason behind this negative impact is due to H m not being appropriately utilized [79,82]. Precisely, to optimize the PCM T melt , Waqas et al. [83] conducted a numerical simulation using a different melting range of PCM (30 • C, 35 • C, 40 • C, and 44 • C) for hot climatic conditions in Pakistan. It was noted that 44 • C of PCM T melt cools the PV module effectively with a higher T PV reduction of 28 • C. From this study, it was concluded that to select the appropriate PCM, summer T amb must be averaged and from that 10 • C (average T amb + 10 • C = PCM T melt ) must be added to optimize the PCM T melt . Following the PCM T melt selection, optimizing the PCM thickness for a balanced cooling effect is necessary. It is well known that an increase in thickness of PCM directly increases the PCM's total energy storage capacity (Table 3), resulting in H m , sustained for a longer period, and heat energy from the PV module is effectively transferred to the PCM [84]. Waqas et al. revealed that by increasing the PCM thickness beyond 2 cm, the linear cooling effect is disturbed, suggesting it is the optimum thickness for the reported climatic conditions [83]. Sourav Khanna et al. [85] examined the necessity of increasing the thickness of PCM under variable wind azimuth angles and wind speed for the existing experimental work of M. J. Huang [86] and Pascal Henry Biwole [87]. Moreover, Sourav Khanna showed the necessity of PV module inclination (0-90 • ) to find the effective heat transfer for an existing experimental work by Taieb Nehari [76]. The increase in inclination of PV-PCM reduced the T PV in higher order for up to 45 • and beyond that, a reduction in T PV was not as effective due to a restriction in the liquid PCM circulation inside the container. This restriction in liquid PCM circulation creates a convection barrier and is forced to conduct the heat by conduction mode [88].
It is well known that the thermal conductivity of PCM (K PCM ) lies from 0.2-2.0 W/m.K which is lower than most of the sensible heat storage material. Reportedly, several researchers found that a low K PCM may increase the thermal resistance that directly affects the cooling effect. As mentioned earlier, the direct filling method minimizes the heat transfer loss to a certain extent; however, it is not practically applicable. The only way to reduce the heat transfer loss during the peak daytime period is through increased PCM thermal conductivity. Through such measures, the contact loss with PV and PCM containers can be negotiable. It can be achieved by imparting a thermal distribution fin or modifying the material by blending with expandable graphite (EG) or metal scrap. However, copper powder and metal foams with PCM play a major role in reducing the overall thermal resistance and a detailed discussion is provided in the following subsection.   [86]. In the beginning, the thermal distribution fin was considered a thermal enhancer that was submerged/projected into the liquid PCM from the top surface to the bottom surface of the container. The physical dimension of the thermal distribution looks like a heat sink with a series of thin metal plates (equal thickness) arranged in parallel with equal spacing between them. M J Huang cross-examined different morphologies patterns of thermal fins to optimize the spacing between them. He found that a decrease in spacing between each fin (4 mm) adequately gained higher composition with PCM and enhances the heat transfer rate. Unfortunately, it eventually affects the cooling effect due to a decrease in the overall PCM quantity. Notably, 20 mm spacing of each fin gained a high amount of PCM, but the heat transfer rate was ineffective due to less volume occupancy of thermal fins (high thermal conduction stress). This study showed that 8 mm and 12 mm spacing thermal fins minimized the conduction barrier without deteriorating the PCM performance compared to 4 mm and 20 mm [86]. Further, Nehari et al. [94] performed a 2D numerical model for the experimental setup of M. J. Huang [86] with a modified inter-fin length (0-40 mm with an increment of 5 mm). This study revealed that PCM with no fin is ineffective while 5-20 mm inter-fin performed moderately. An increase in the length of the fin improved the heat transfer rate but under this condition, a 25-35 mm fin length reduced PCM thermal stress; however, 40 mm becomes ineffective. This is because a 40 mm fin length interconnects the PCM container chamber that restricts the PCM internal convection as shown in Figure 6. Later, the same author performed a simulation for different inclinations (0-90 • ) for finned PCM containers. For the inclination, which is less than 45 • , natural convection dominates inside the PCM and helps to melt the PCM layer by layer as shown in Figure 7. Beyond 45 • , pure conduction occurred during the entire simulation period and reduced the T PV reduction as compared to an inclination of less than 45 • [95]. It is noted that M. J. Huang's experimental designs have been widely re-examined for further studies to develop an effective cooling system. Pascal et al. solved a Navier stroke equation using a 2D finite element model for the same system geometry of M. J. Huang. Still, in this case, different thicknesses of PCM are filled in the plexiglass-based container rather than a metal container. As a result, the temperature of the front surface of the T PV was maintained below 50 • C for more than 89 min under the constant insolation of 1000 W/m 2 . A further increase in the thickness of the PCM container leads to a reduction in the T PV value to the maximum of 39 • C for one hour, and for PV without PCM in a couple of minutes, T PV was raised higher than PV with PCM [96]. Following other researchers, Pascal Henry Biwole et al. [87], Khanna et al. [97], and other researchers also reexamined the M. J. Huang system design to achieve effective heat transfer between PV and PCM to enhance the electrical efficiency of the PV module. as shown in Figure 7. Beyond 45°, pure conduction occurred during the entire simulation period and reduced the TPV reduction as compared to an inclination of less than 45° [95]. It is noted that M. J. Huang's experimental designs have been widely re-examined for further studies to develop an effective cooling system.

PV-PCM Metal Scrap
Other than fins (as for thermal conductivity enhancer), Subarna Maiti et al. [98] used a metal scrap as composting material to improve the heat transfer; advantageously, since metal scrap is a low-cost material, it is easy to incorporate with PCM and is as effective as thermal distribution through the inter-fin design as shown in Figure 8a. This study takes 5.5 kg of paraffin wax as PCM by solving the energy balance equation. The performed indoor experimental result showed that 23 • C of average T PV reduction is monitored for a maximum of 3 h. After a continuous 3 h of experimentation, PCM completely turns to liquid resulting in an increase in the value of T PV , 62 • C, which is lower than the value compared to the T PV value of PV without PCM. Pascal et al. solved a Navier stroke equation using a 2D finite element model for the same system geometry of M. J. Huang. Still, in this case, different thicknesses of PCM are filled in the plexiglass-based container rather than a metal container. As a result, the temperature of the front surface of the TPV was maintained below 50 °C for more than 89 min under the constant insolation of 1000 W/m 2 . A further increase in the thickness of the PCM container leads to a reduction in the TPV value to the maximum of 39 °C for one hour, and for PV without PCM in a couple of minutes, TPV was raised higher than PV with PCM [96]. Following other researchers, Pascal Henry Biwole et al. [87], Khanna et al. [97], and other researchers also reexamined the M. J. Huang system design to achieve effective heat transfer between PV and PCM to enhance the electrical efficiency of the PV module.

PV-PCM Metal Scrap
Other than fins (as for thermal conductivity enhancer), Subarna Maiti et al. [98] used a metal scrap as composting material to improve the heat transfer; advantageously, since metal scrap is a low-cost material, it is easy to incorporate with PCM and is as effective as thermal distribution through the inter-fin design as shown in Figure 8a. This study takes 5.5 kg of paraffin wax as PCM by solving the energy balance equation. The performed indoor experimental result showed that 23 °C of average TPV reduction is monitored for a maximum of 3 h. After a continuous 3 h of experimentation, PCM completely turns to liquid resulting in an increase in the value of TPV, 62 °C, which is lower than the value compared to the TPV value of PV without PCM.

PV-PCM Nano-Compounds
Thermal distribution fin and metal scraps are a well-known thermal conductivity enhancer; however, less volume occupancy and improper mixture with PCM encouraged [99] to blend the copper and graphite powders with PCM resulting in a composite PCM.

PV-PCM Nano-Compounds
Thermal distribution fin and metal scraps are a well-known thermal conductivity enhancer; however, less volume occupancy and improper mixture with PCM encouraged [99] to blend the copper and graphite powders with PCM resulting in a composite PCM. The results showed that the composite PCM reduced 5.6 • C and 2.9 • C of T PV compared to PV without PCM and pure PCM, respectively.

PV-PCM Graphite
Following nanomaterial, Karthikeyan et al. found that expandable graphite (EG) as a thermal enhancer has advantages such as being readily available on the market, low cost, lightweight, and low density. At first, EG is heated by an electric furnace for a 1 min duration of 800 • C to expand its physical dimension with a volume of about 200-300 times and higher porosity as shown in Figure 8b. Compositing high pores of expanded graphite with PCM creates improper mixing texture in the mixture. Therefore, to attain the perfect blend, expanded graphite is compressed where the unwanted pores are sealed, and the bulk volume density of the expanded graphite is reduced. After compression, expanded graphite porous foam is impregnated in the liquid PCM for absorption [49]. EG is an effective thermal conductivity enhancer in terms of proper texture composition, ease of handling, and EG will not settle on the bottom surface like other nano compounds and powder materials.

PV-PCM Metal Foam
Following EG, metal foam is a better option to enhance the K PCM than other thermal enhancers [100]. Abdulmunen R. Abdulmunen et al. [101] showed that impregnating Al foam with paraffin cools the PV module to 39.58 • C; although it is not practically viable for mid/large scale systems. However, metal foam is expensive, heavy weight (Figure 8c), not readily available in the local market, and requires additional care on the mounting structure.
Overall, it is concluded that composite PCM enhances the electrical power and efficiency; although it increased the total weight of the PCM container. Integrating a heavy PCM container with a PV module could damage the physical structure of the PV module. However, adding PCM to the PV module for cooling purposes is an external investment cost. Therefore, to rectify the temperature loss, investing more into thermal conductivity enhancers weakens the system performance economically.
Further, it is necessary to consider the waste heat to useful energy by incorporating the thermal collector with PCM. This hybrid technique will minimize the conventional low thermal application such as hot water for bathing, cooking, cleaning vegetables and vessels, hot air for room heating, and other thermal applications.

Overview of PCM-Thermal Collector Based PV Module Cooling
The above two subsections explained the PCM influence on cooling the PV module only to increase the electrical efficiency. It is noted that under certain conditions, heat energy from the PCM can be utilized for low-temperature applications such as warm water for cooking, cleaning vegetables and fruits, cloth washing, and indoor space warming [102]. Converting waste energy into useful heat energy will reduce the conventional thermal energy load, which directly minimizes the traditional building load. Moreover, adopting this hybrid technology could minimize the total amount of the PCM that is usually used for PV module cooling. This hybrid technique removes the heat from PCM, whereas

Overview of PCM-Thermal Collector Based PV Module Cooling
The above two subsections explained the PCM influence on cooling the PV module only to increase the electrical efficiency. It is noted that under certain conditions, heat energy from the PCM can be utilized for low-temperature applications such as warm water for cooking, cleaning vegetables and fruits, cloth washing, and indoor space warming [102].
Converting waste energy into useful heat energy will reduce the conventional thermal energy load, which directly minimizes the traditional building load. Moreover, adopting this hybrid technology could minimize the total amount of the PCM that is usually used for PV module cooling. This hybrid technique removes the heat from PCM, whereas the heat transfer rate between PV and PCM is increased in daytime hours and for next-day operations, PCM continues without any interruption or reduction in performance.

PVT-PCM-Air
Hagar Elarga et al. [103] performed a numerical simulation for three different locations namely Venice, Helsinki, and Abu Dhabi using RT42 (Venice/Helsinki) and RT55 organic PCMs (Abu Dhabi). The PV module and PCM together were placed in between the glass called cavity construction or double skin façades, as shown in Figure 9a. During the daytime, outdoor air is pumped into the double-skin façades to flow on both the PV module front and back surface of the PCM to remove the heat. The PV module without PCM using cavity construction increases the indoor air temperature by a rise in T PV . Interestingly, the PCM-assisted system stabilized the indoor air temperature and is well suited for human comfort that minimizes the conventional building heating load. Air is abundant in nature, although the specific heat capacity of air is lower than water, making air an inefficient material compared to water. However, air is widely used to cool the PV module where the building heating load is required.

PVT-PCM-Water
Secondly, water-assisted thermal collectors with PCMs are widely used as they have a more efficient storage facility and direct usage for several thermal applications than air. Ankita Gaur et al. [104] developed a PCM-assisted thermal collector using selective coated rectangular copper water channels along with a wetted absorber plate and PCM container. Thermal energy from the PV module is transferred to the water channel and then the wetted absorber plate and PCM receive the heat. A 50 mm thickness of glass wool is placed on the bottom surface of the PCM to act as an insulator to prevent heat loss, as shown in Figure 9b. During the daytime, water is heated by both the PV module and PCM, and at nighttime PCM alone assists in heating the water; whenever hot water is required, a 24 W DC pump assists in flowing the water in a rectangular channel. This system design effectively enhances the electrical efficiency; the maximum T PV reduced for summer and winter is 15 • C and 5 • C, respectively.

PVT-PCM-Nanofluid
Ali H Al-Walei et al. [105] used high conductivity nanofluid to flow inside a copper tube to enhance the heat transfer and was found to be the best replacement for water. A computational fluid dynamic (CFD) simulation was performed to optimize the flow rate and diameter of the tube. The simulation result showed that an increase in tube diameter and flow rate enhances the heat transfer [106]; however, a flow rate higher than 0.175 kg/s produced mechanical vibration and noise that likely could damage the system. Following Ali, Mohammad Sardarabadi et al. [107] used deionized water and ZnO/water nanofluid to flow in a copper tube thermal collector to remove the heat from PCM. In this study, 2 kg of paraffin wax was used to remove heat from the PV module and enhance the electrical and thermal efficiency, as shown in Figure 9c. It is noted that deionized water and ZnO without PCM reduced the T PV maximum of 10 • C and 11 • C, respectively, whereas the same system with PCM reduced the T PV maximum of 17 • C.
Overall, it is noted that adding nanofluid into the PCM and working fluid greatly enhanced the heat transfer rate that favors increasing both the system's electrical and thermal efficiency. Technically, nanofluid is a great innovation for cooling the PV module but it is not economically feasible. The amount of electrical energy that has been demolished by the excess rise in T PV will be cheaper than the incorporation budget of nanofluid. As mentioned earlier, nanofluid is an effective material but is not suitable for cooling the PV module considering the economic aspect; the whole point in cooling the PV module is to increase the revenue of the solar power plant. However, nanofluid can be used for the PV module cooling process when the cost of the nanofluid becomes cheaper. Based on the literature review, most of the research is conducted in cooling the PV module rather than considering whether it is economically feasible for a large-scale system. Further, researchers recommend that low-cost and easily available nanofluid can cool the PV module.

Recent Trend in PCM Based Active Cooling Technology
As mentioned earlier, PCM-based active cooling technology favors cooling the PV module and reduces the existing thermal load in residential and mid-commercial buildings. Integrating the thermal collector inside or above the PCM container cools the PV module effectively when the collector's surface area is more-unless an uneven cooling performance is achieved.
On other hand, PCM-based active cooling methods minimize the thermal resistance by employing the fluid in the channel. Low thermal conductivity of PCM often creates resistance in PCM; both charging and discharging periods using active fluid flow helps to maintain the higher temperature difference between PCM and the PV layer. When there is a high-temperature difference, thermal resistance in PCM is low and heat energy from the PV module is transferred to the PCM at a higher rate. This technique is often considered hybrid by utilizing the heat from PCM into a useful thermal load. This hybrid technique can be widely adopted for low and mid-sized power plants especially when the thermal load and electrical load are consumed at the same level, which is where hybrid technology will benefit.
In recent years, active cooling methods have gained attention among researchers as compared to pure PCM and composite PCM. Table 4 shows some of the recently published PCM-assisted active cooling technology (PVT).

Recent Trend in PCM Based Active Cooling Technology
As mentioned earlier, PCM-based active cooling technology favors cooling the PV module and reduces the existing thermal load in residential and mid-commercial buildings. Integrating the thermal collector inside or above the PCM container cools the PV module effectively when the collector's surface area is more-unless an uneven cooling performance is achieved.
On other hand, PCM-based active cooling methods minimize the thermal resistance by employing the fluid in the channel. Low thermal conductivity of PCM often creates resistance in PCM; both charging and discharging periods using active fluid flow helps to maintain the higher temperature difference between PCM and the PV layer. When there is a high-temperature difference, thermal resistance in PCM is low and heat energy from the PV module is transferred to the PCM at a higher rate. This technique is often considered hybrid by utilizing the heat from PCM into a useful thermal load. This hybrid technique can be widely adopted for low and mid-sized power plants especially when the thermal load and electrical load are consumed at the same level, which is where hybrid technology will benefit.
In recent years, active cooling methods have gained attention among researchers as compared to pure PCM and composite PCM. Table 4 shows some of the recently published PCM-assisted active cooling technology (PVT).

Author Thermal Collector Heat Transfer Fluid Methodology
Ahmadi et al. [112] Ahmad i et al. [112] Water • In this study, PS-CNT polyHIPE foam is used as encapsulating material for paraffin wax. • Laboratory type 5.5 × 5.5 cm 2 solar cell is embedde with PCM layer and following that aluminum water channel in place.

•
Experiments are performed in both passive and active techniques to make an effective comparison.
Kazem ian et al. [113] EG/water • Three system configurations are examined namely PV-PCM without the thermal collector, PV-PCM with a serpentine thermal collector, and glazed PV-PCM with serpentine thermal collector.

•
In this study, EG is added with water to avoid working fluid freezing.

•
Working fluid flows in the heat exchanger in a serpentine flow with a mass flow rate of 20-60 L/hr. Basalik e et al. [114] Water and Al2O3 • n-octadecane and eutectic capric-palmitic as PCM are used in this simulation.

•
To ensure the PCM and heat pipe physical contact 10-layer inflation was applied. • Grid independent study was approached with fiv different mesh sizes to find the resistance in heat transfer.

•
The thermal collector is made up of a copper tube impregnated in the PCM container. Once the working fluid removes the heat from PCM, the car radiator acts as a secondary heat exchange to remove the heat from the nano-fluid.

Water
• In this study, PS-CNT polyHIPE foam is used as encapsulating material for paraffin wax. • Laboratory type 5.5 × 5.5 cm 2 solar cell is embedded with PCM layer and following that aluminum water channel in place.

•
Water flow was maintained at 0.2, 0.6, and 1 L/minute. • Experiments are performed in both passive and active techniques to make an effective comparison.
Kazemian et al. [113] Ahmad i et al. [112] Water • In this study, PS-CNT polyHIPE foam is used as encapsulating material for paraffin wax. • Laboratory type 5.5 × 5.5 cm 2 solar cell is embedded with PCM layer and following that aluminum water channel in place.

•
Experiments are performed in both passive and active techniques to make an effective comparison.
Kazem ian et al. [113] EG/water • Three system configurations are examined namely, PV-PCM without the thermal collector, PV-PCM with a serpentine thermal collector, and glazed PV-PCM with a serpentine thermal collector.

•
In this study, EG is added with water to avoid working fluid freezing.

•
Working fluid flows in the heat exchanger in a serpentine flow with a mass flow rate of 20-60 L/hr. Basalik e et al. [114] Water and Al2O3 • n-octadecane and eutectic capric-palmitic as PCMs are used in this simulation.

•
To ensure the PCM and heat pipe physical contact, 10-layer inflation was applied. • Grid independent study was approached with five different mesh sizes to find the resistance in heat transfer.

•
The thermal collector is made up of a copper tube impregnated in the PCM container. Ahmad i et al. [112] Water • In this study, PS-CNT polyHIPE foam is used as encapsulating material for paraffin wax. • Laboratory type 5.5 × 5.5 cm 2 solar cell is embedde with PCM layer and following that aluminum water channel in place.

•
Experiments are performed in both passive and active techniques to make an effective comparison.
Kazem ian et al. [113] EG/water • Three system configurations are examined namely PV-PCM without the thermal collector, PV-PCM with a serpentine thermal collector, and glazed PV-PCM with serpentine thermal collector.

•
In this study, EG is added with water to avoid working fluid freezing.

•
Working fluid flows in the heat exchanger in a serpentine flow with a mass flow rate of 20-60 L/hr. Basalik e et al. [114] Water and Al2O3 • n-octadecane and eutectic capric-palmitic as PCM are used in this simulation.

•
To ensure the PCM and heat pipe physical contact 10-layer inflation was applied. • Grid independent study was approached with five different mesh sizes to find the resistance in heat transfer.

•
The thermal collector is made up of a copper tube impregnated in the PCM container. Once the working fluid removes the heat from PCM, the car radiator acts as a secondary heat exchange to remove the heat from the nano-fluid.
Water and Al 2 O 3 • n-octadecane and eutectic capric-palmitic as PCMs are used in this simulation.

•
To ensure the PCM and heat pipe physical contact, 10-layer inflation was applied. • Grid independent study was approached with five different mesh sizes to find the resistance in heat transfer.
Hasan et al. [115] 2 Ahmad i et al. [112] Water • In this study, PS-CNT polyHIPE foam is used as encapsulating material for paraffin wax. • Laboratory type 5.5 × 5.5 cm 2 solar cell is embedde with PCM layer and following that aluminum water channel in place.

•
Experiments are performed in both passive and active techniques to make an effective comparison.
Kazem ian et al. [113] EG/water • Three system configurations are examined namely PV-PCM without the thermal collector, PV-PCM with a serpentine thermal collector, and glazed PV-PCM with serpentine thermal collector.

•
In this study, EG is added with water to avoid working fluid freezing.

•
Working fluid flows in the heat exchanger in a serpentine flow with a mass flow rate of 20-60 L/hr. Basalik e et al. [114] Water and Al2O3 • n-octadecane and eutectic capric-palmitic as PCM are used in this simulation.

•
To ensure the PCM and heat pipe physical contact 10-layer inflation was applied. • Grid independent study was approached with fiv different mesh sizes to find the resistance in heat transfer.

•
The thermal collector is made up of a copper tube impregnated in the PCM container.

•
Graphene nano fluid flows inside the copper tube with a mass flow rate of 0.33-0.67 kg/s.

•
Once the working fluid removes the heat from PCM, the car radiator acts as a secondary heat exchange to remove the heat from the nano-fluid. Once the working fluid removes the heat from PCM, the car radiator acts as a secondary heat exchanger to remove the heat from the nano-fluid. In this study separate thermal energy storage t is placed to store the recovered heat from the PV mo ule.

•
Five different melting temperature of the PCM examined to find the relationship of PCM melting te perature with a reduction in TPV.

PCM Tmelt
As mentioned earlier, PCM Tmelt is an essential parameter in cooling the PV mod experimenting with inappropriate Tmelt of PCM causes a reduction in the performanc the system [79,82]. Adeel Waqas et al. [83] performed a numerical simulation to optim the PCM Tmelt for hot climatic conditions. Several PCMs were examined to find the P Tmelt (30 °C, 35 °C, 40 °C, and 44 °C); among these, RT44 extracted high amounts of ther energy from the PV module. This optimized PCM (RT44) reduced 28 °C of TPV at the t of peak daytime hours. From this finding, Waqas concluded that the Tamb plays an portant role in the operation of the PCM. Following Waqas, Arici et al. [118] performed a numerical simulation for two dif ent locations to find the PCM Tmelt and thicknesses for an entire year. Arici also show that Tamb plays an important role in cooling the PV module. From this finding, it is c that selecting a single Tmelt of PCM will not be effective for an entire year and also, PCM thickness varies for each month as listed in Table 4. On the other hand, changing PCM for each month is not practical; further, it is recommended to split the seasons (su mer and winter) and choose two PCMs for an entire year cooling process or cascade P will be an option; low Tmelt of PCM can be placed as the first layer and the second la will be high Tmelt, whereas it can serve the cooling effect for both summer and winter as shown in Figure 10. On the other hand, Karthikeyan et al. revealed that PCM Tmelt be selected without performing lex simulation tools such as CFD/numerical simulation only analyzing the experimental local ambient temperature of the ambient temperatur Figure 11. It is found that PCM operation in cooling the PV module is majorly classif into two categories: ineffective TPV reduction and effective TPV reduction. When the lected PCM Tmelt is equal to Tamb, less than Tamb, and 6 °C higher than average, Tamb cau ineffective TPV reduction. These conditions mostly absorb heat from the surroundings ther than absorbing from the PV module. Secondly, if the PCM failed to reach the Tme the effective daytime hours, Hm will not be utilized effectively, and the resulting nega In the daytime, 50 g of mint leaves are dried using the recovered heat from the PV module.
Yang et al. [117] Ciftci et al. [116] Air In the daytime, 50 g of mint leaves are dried using the recovered heat from the PV module.
Yang et al. [117] Water As mentioned earlier, PCM T melt is an essential parameter in cooling the PV module; experimenting with inappropriate T melt of PCM causes a reduction in the performance of the system [79,82]. Adeel Waqas et al. [83] performed a numerical simulation to optimize the PCM T melt for hot climatic conditions. Several PCMs were examined to find the PCM T melt (30 • C, 35 • C, 40 • C, and 44 • C); among these, RT44 extracted high amounts of thermal energy from the PV module. This optimized PCM (RT44) reduced 28 • C of T PV at the time of peak daytime hours. From this finding, Waqas concluded that the T amb plays an important role in the operation of the PCM. Following Waqas, Arici et al. [118] performed a numerical simulation for two different locations to find the PCM T melt and thicknesses for an entire year. Arici also showed that T amb plays an important role in cooling the PV module. From this finding, it is clear that selecting a single T melt of PCM will not be effective for an entire year and also, the PCM thickness varies for each month as listed in Table 4. On the other hand, changing the PCM for each month is not practical; further, it is recommended to split the seasons (summer and winter) and choose two PCMs for an entire year cooling process or cascade PCM will be an option; low T melt of PCM can be placed as the first layer and the second layer will be high T melt , whereas it can serve the cooling effect for both summer and winter [54] as shown in Figure 10. On the other hand, Karthikeyan et al. revealed that PCM T melt can be selected without performing lex simulation tools such as CFD/numerical simulation by only analyzing the experimental local ambient temperature of the ambient temperature in Figure 11. It is found that PCM operation in cooling the PV module is majorly classified into two categories: ineffective T PV reduction and effective T PV reduction. When the selected PCM T melt is equal to T amb , less than T amb , and 6 • C higher than average, T amb causes ineffective T PV reduction. These conditions mostly absorb heat from the surroundings rather than absorbing from the PV module. Secondly, if the PCM failed to reach the T melt in the effective daytime hours, H m will not be utilized effectively, and the resulting negative impact will reflect in cooling the PV module as listed in Table 5. To achieve effective cooling, PCM T melt must be 3-6 • C higher than T amb . Table 5. Optimization of PCM T melt and thickness for Ankara and Mersin [118]. impact will reflect in cooling the PV module as listed in Table 5. To achieve effective cooling, PCM Tmelt must be 3-6 °C higher than Tamb.   Thongtha et al. [79] experimented with controlled indoor climatic conditions to study the nature of paraffin wax which melts at 59 • C. It was found that 2-6% of cooling is achieved due to the higher melting temperature of PCM. Yuan et al. [119] stated that T amb plays a crucial role in cooling the PV module. Interestingly, when the T melt is equal to T amb , a moderate or less cooling effect is achieved. PCM struggles to remove the heat from the PV module because PCM starts to absorb from T amb as well. On the other hand, Elavarasan et al. [50] revealed that examining with low T amb causes a non-favorable cooling effect because the PCM turns to liquid before the peak daytime especially in summer. With a rise in solar radiation, T amb also increases, resulting in PCM interacting with the surroundings that cools the PV module. Within an hour of experimentation, PCM influenced the PV module to gain a higher temperature than the unmodified PV module. Overall, this study showed the negative impact on the modified PV module due to inappropriate PCM selection. However, the same PCM works perfectly in winter because of the lower T amb [75,76]. Anna Machniewicz et al. [120] conducted a simulation for climatic conditions in Poland using different PCMs (RT10HC, RT15HC, RT18HC, RT25HC, and RT35HC PCM) to find a suitable PCM for winter and summer. In winter, RT10HC enhanced the cooling effect and the same PCM for summer failed in cooling the PV module effectively. In summer, RT18HC and RT25HC showed an extraordinary cooling process. Following that, Hendricks et al. [84] revealed that an uneven cooling effect is noticed. In Utrecht, PCM starts to cool the PV module at 8 a.m., and the for Malaga PCM cools the PV module at 10 a.m. This analysis showed that PCM must be selected for each location using the meteorological data unless the system delivers a negative impact on the cooling process. Thongtha et al. [79] experimented with controlled indoor climatic conditions to study the nature of paraffin wax which melts at 59 °C. It was found that 2-6% of cooling is achieved due to the higher melting temperature of PCM. Yuan et al. [119] stated that Tamb plays a crucial role in cooling the PV module. Interestingly, when the Tmelt is equal to Tamb, a moderate or less cooling effect is achieved. PCM struggles to remove the heat from the PV module because PCM starts to absorb from Tamb as well. On the other hand, Elavarasan et al. [50] revealed that examining with low Tamb causes a non-favorable cooling effect because the PCM turns to liquid before the peak daytime especially in summer. With a rise in solar radiation, Tamb also increases, resulting in PCM interacting with the surroundings that cools the PV module. Within an hour of experimentation, PCM influenced the PV

Latent Heat of Fusion
The reason behind selecting the PCM as a cooling element for PV modules is its high energy density and latent heat of fusion [121]. PCMs H m is not temperature-dependent material. It stores a high amount of thermal energy during the PCM melting state without increasing the PCMs temperature while charging compared to sensible heat storage material [122,123]. Selecting a high H m material can store a large amount of thermal energy and directly enhance the cooling effect and cooling durability each day. On the other hand, selecting a high H m reduces PCM thickness, whereas PCM consumption is less and has a higher cooling effect. As mentioned earlier, in PV module cooling using PCM as heat removal, it is necessary to consider the T melt at first, if H m is less means it only shortens the cooling durability [124]. However, this issue can be resolved by increasing the thickness of the PCM container to withstand the cooling effect for a longer period [125].

Density
The density of PCM directly correlates with thermal conduction loss because lowdensity material intakes large volumes in containers, whereas the thickness is increased. Especially, organic PCM owes low density, and integrating them with PV modules slightly increases the thickness of the PCM container. On the other hand, organic PCM's low thermal conductivity directly imposes a thermal conduction barrier with an increase in thickness of the PCM container [126,127]. It was noted that the density of PCM is correlated with the cooling effect; although it is often negligible compared to T melt optimization [49].

Specific Heat Capacity
Specific heat capacity is often not considered basic criteria for selecting the appropriate PCM for PV module cooling due to its low thermal conductivity [54,125]. Several studies state that the specific heat capacity of PCM is not efficient to cool the PV module, especially under hot and humid climatic conditions. Karthikeyan et al. showed that specific heat capacity is useful when the K PCM is enhanced unless it is recommended to proceed with the PCM selection process without considering the specific heat capacity as a parameter [49].

Thermal Conductivity
K PCM is the second essential parameter in cooling the PV module. To enhance the cooling effect, it is necessary to increase the K PCM unless the conduction barrier reduces the heat transfer rate between the PV module and PCM [128][129][130]. Several researchers claim that incorporating a thermal distribution heat sink inside the PCM container helps transfer the thermal energy from the PV module to the PCM's inner surface [93,94,104]. This technique enhanced the electrical efficiency greatly as compared to pure PCM. However, fabricating interfin with the PCM container is difficult and also creates leakage in the PCM container. Following interfins, several studies have been conducted on nano material, metal powder, and metal foam, resulting in higher T PV reductions; although, it not feasible for longer operations. The nano compounds or metal powder degrade the thermal stability of the PCM [130], and metal foam increases the total weight of the PCM requiring additional care to the PV module mounting structure. As mentioned above, all K PCM enhancing techniques lack in certain features mostly in the simplest way to enhance the K PCM without degrading the property or increasing the mass of the PCM container, notably expandable graphite is not increasing the K PCM as compared to metal foam, though it is easy to handle and fabricate. Moreover, it increases the K PCM by up to 16.6 W/m.K [131,132]. It is noted that EG increases the thermal conductivity as compared to several nano compounds, which are non-metallic. Further, the combination of EG and metal powder is recommended to composite with PCM. This combination of metallic powder with EG will not allow it to settle down in the PCM container. Considerably, this technique increases the K PCM to a higher order to reduce the T PV effectively with minimal thermal resistance.

Congruent Melting
Inorganic PCMs are not widely examined due to their incongruence in melting after several hundred thermal cycles. Continuous heating and cooling cycle breaks the salt and hydrates separately; as a result, semi-liquefication and freezing occur and can be noticed in the endothermic and exothermic curves [54]. Several researchers reported that organic PCM, especially paraffin wax, is congruent in melting and is an effective material for PV modules as coolants over several thousand thermal cycles [49].

Discussion of Kinetic Criteria Supercooling
During daylight hours, PCM removes heat energy from the PV module and stores it in the PCM by changing the phase solid to liquid. In most cases, PCM turns to a liquid/mushy state at the end of the day. To conduct the consecutive day PV module cooling, PCM must be solidified in the nighttime otherwise the cooling process will not be effective. This is the reason why the selected PCM must be non-supercooling and congruent in the material. Further, it is necessary to examine the supercooling nature of the PCM before experimenting with the PV module, especially for inorganic PCMs, in which examining the thermal properties (thermal cycling) is not recommended to incorporate with the PV module because the supercooling effect is high in inorganic PCM as compared to others. Other than the natural super cooling effect, an increase in thickness of PCM creates a barrier in discharging the stored heat energy and fails to solidify the PCM. Under this condition, PCM also turns into an ineffective material for cooling the PV module. However, this artificial supercooling effect can be rectified by increasing the K PCM . Notably, this artificial supercooling effect is less in composite PCM and thermal collector-based PCM incorporation with the module [116,117,119].

Chemical Stability and Decomposition
The low melting temperature of inorganic PCM is moderately stable chemically and thermally; this is the reason most of experiments are performed with commercial PCMs and paraffin [54,133]. Commercial PCM achieves perfect endothermic and exothermic reaction without any restriction in operation; the only drawback in commercial PCM is investment costs. In most cases, PCM integration investment is higher than the temperature loss which makes commercial PCM unsuitable for real-time applications [16]. As mentioned earlier, paraffin wax is widely examined as it is inexpensive, readily available in the market, and easy to handle. Following commercial PCMs and paraffin, it is recommended to examine organic eutectic and fatty acids as coolant materials for PV module cooling purposes. They are inexpensive compared to commercial PCMs; a wide range of melting temperatures is available besides paraffin wax, and a eutectic mixture can be easily prepared to obtain the expected operating temperature for any desired location [8]. On the other hand, fatty acids and organic eutectic PCMs are thermally and chemically stable even after several thousand thermal cycles and are outnumbered in the literature to cool the PV modules which make them alternative PCMs [134][135][136].

Corrosive
In most cases, PCMs are filled in a container that is made up of stainless steel and aluminum. Notably, inorganic PCMs are highly corrosive with metal, especially salt hydrates [137,138]. Aluminum is a higher conducting metal than stainless steel, but the corrosion rate is higher for inorganic PCMs and moderate for other PCMs compared to stainless steel [139,140]. Over time, the corrosion rate increases gradually and destroys the PCM container's mechanical strength and thermal stability [137,141]. Under certain conditions, an increase in corrosion rate leads to liquid PCM leakage. To minimize this loss, PCM must be examined with aluminum or stainless steel to find the corrosion rate before integrating for PV module cooling. Several researchers state that organic PCMs are less corrosive to metal and easy to handle for PV module cooling purposes. Further organic PCMs are recommended for real-time cooling purposes for a longer period of operation.

Toxic
PCM is a non-hazardous and environmentally friendly thermal energy storage material. Benefits of integrating PCM as a coolant material include reducing the T PV without consuming manpower, natural resources, and less maintenance costs [142,143]. However, laboratory-grade gloves and masks are essential safety devices to use while handling the Sustainability 2022, 14,1963 28 of 65 PCM, some PCMs have a strong odor that makes mild headaches especially inorganic salt, without safety measures, creating rashes and itches on human skin. Other than these minor side effects, PCMs are ecofriendly and can be incorporated with PV modules without contaminating nature.

Flammable and Explosive
In most experiments, organic PCMs are involved in cooling the PV module as compared to the inorganic and eutectic mixture. Moreover, locally available paraffin wax plays a major role in PV module cooling than fatty acids. Following that commercial PCM also widely influenced as listed in Table A1. In prior notice, paraffin wax and commercial PCM are flammable starting from 150 • C; however, the PV module operating temperature will not reach 150 • C but accidental electrical failure or a short circuit in the electrical system may cause a fire in PCM. In such a case, it is necessary to maintain the electrical systems in good condition and fire extinguishers must be kept in the power station. The extinguishers should work autonomously to control the fire before it ignites the entire power plant. It is noted that paraffin is flammable under certain conditions but not explosive, with proper precaution and safety measures, paraffin can be incorporated as a cooling system. Inorganic PCM is non-flammable but scarcity in the corrosion and supercooling effect makes salt hydrates unsuitable for cooling purposes. Other than salt hydrates, fatty acids, and fatty acids eutectic mixtures are replacement heat storage materials than paraffin wax.

Discussion of Technical Criteria
Compactness, Reliability and Simplicity PCM is a compact heat battery as compared to other sensible heat storage materials. However, it is heavier and larger as compared to electrical batteries. PCM stores and discharges thermal energy without raising the temperature, making heat batteries reliable for thermal applications. PCMs have been widely adopted as heat removal material over the last two decades because of their simplicity in fabrication, installation, and operational performance.

Discussion of Economic Criteria Large Scale Availability and Low Cost
Economic analysis should be conducted for further commercialization as some of the commercial and research-grade PCMs are highly expensive than the loss that occurs from T PV . Ahmad Hasan et al. [144] experimented with calpric-palmitic and calcium chloride hexahydrate eutectic under Ireland and Pakistan climatic conditions. For Ireland, the PCMintegrated PV module enhanced the power maximum of 10.7 W and 15.8 W, respectively. Economically, PCM increased the revenue, EUR 51.5 and EUR 76, noticeably this financial enhancement is lower than the net cost of the PCM and its construction which was EUR 92 and EUR 98. However, the same PCM under Pakistan climatic conditions enhanced the output power of 22 W and 33.7 W. The same PCM performed well in Pakistan than in Ireland. It can be concluded that PCM integration is economically viable if the reduction in T PV is higher-order and also based on the grid tariff. Table 6 shows the recent studies on the economic feasibility of the PV module cooling technology using PCM as a cooling agent.

Benefits and Drawbacks of PV Module Cooling Using PCM
Benefits: The main benefit of using raw PCM is the high amount of Hm that can be utilized for an effective PV module cooling process. On the other hand, it can be easily filled in the PCM container/PV module back surface by melting the PCM. As mentioned earlier, numerous heating and cooling cycles of the PCM face volume changes which deteriorate the cooling process; advantageously, using raw PCM as a coolant material can be refiled easily whenever the PCM volume change is noticed. Adding thermal additives with PCM enables the heat transfer capacity between PV and PCM, resulting in a higher cooling effect. It is well known that the solid and liquid specific heat capacity of the PCM is ineffective in cooling the PV module; however, when the PCM is incorporated with thermal additives, it turns out to be an effective material. On the other hand, the enhanced thermal conductivity of the PCM using EG and other lightweight materials maintains the stability of the system, and thermal dissipation from the PCM enhances the cooling effect and consecutive day cooling performance effectively. Other than PCM as coolant material, it stores excess heat from the PV module and utilizes it for various low thermal applications that minimize the conventional thermal load. Secondly, the thermal collector helps to minimize the total amount of PCM usage in cooling the PV module because fluid motion in the PCM chamber greatly maintains the PCM in a mushy state for a longer period than non-thermal collector-based PV-PCM, resulting in higher temperature differences between the PV module and PCM being maintained. The unique benefit in both composite and thermal collector-assisted PCM favors enhancing the electrical efficiency in the early daytime period itself compared to pure PCM.
Drawback: Overall, using PCM as a coolant material increases the overall system weight. It deteriorates the PV module's back surface thermal dissipation, especially during off-peak daytime hours, as PCM is ineffective in cooling the PV module. The main drawback of raw PCM is its low thermal conductivity; notably, organic PCMs are widely performed in cooling the PV module that has a maximum of 0.3 W/m.K. Although PCM is a highenergy-density material, a lack of thermal conductivity increases the heat transfer resistance between the PV module and PCM. Notably, the low thermal conductivity of PCM failed to achieve consecutive cooling processes, especially in hot and humid climatic conditions, PCM failed to discharge stored thermal energy due to high ambient temperature, resulting in the incorrect utilization of H m on a consecutive day. Although adding additives in the PCM enables the heat transfer rate, it is particularly difficult to construct inter-fin-based compositing materials, as they lead to liquid PCM leakage, increasing the system's total weight. Moreover, adding nano compounds in the PCM deteriorates the thermophysical property of the PCM. Although thermal collector-based PCM favors cooling the PV module greater than other techniques, it is not appropriate for large-scale systems. Mainly, the thermal collector with PCM increased the weight of the system and produced mechanical vibration and noise due to an increase in the flow rate of the fluid. Moreover, recovering heat from the large-scale system will not be converted into useful energy considering the safety issues of the powerplant and grid stability.
As mentioned earlier, PCM is an efficient and effective material that can cool the PV module, but it is not convenient for all climatic conditions. Table 7 shows the importance of PCM and recent research activities in cooling the PV module. However, it is necessary to find a suitable PCM according to the experimental location to achieve an effective payback period, because the operation of PCM is mainly correlated with outdoor climatic conditions. The T PV reduction from different researchers with different PV module cooling techniques is listed in Table 7 and represented graphically in Figure 12. It is found that most previous studies were only performed for a short period in a particular location; for example, some experiments were conducted for less than a day and month-selective days in a particular month and season. In most cases, researchers failed to attempt the annual performance of the PCM cooling. Under this condition, it is difficult to recommend these cooling technologies for large-scale solar power plants. Strategically, only two studies were found in the literature: Hasan et al. [56] and Waqas et al. [57] showed an annual simulation for UAE and Islamabad locations, respectively. In the next section, deep an analysis was conducted for the 15 MWp using the NREL meteorological data.
techniques is listed in Table 7 and represented graphically in Figure 12. It is found that most previous studies were only performed for a short period in a particular location; for example, some experiments were conducted for less than a day and month-selective days in a particular month and season. In most cases, researchers failed to attempt the annual performance of the PCM cooling. Under this condition, it is difficult to recommend these cooling technologies for large-scale solar power plants. Strategically, only two studies were found in the literature: Hasan et al. [56] and Waqas et al. [57] showed an annual simulation for UAE and Islamabad locations, respectively. In the next section, deep an analysis was conducted for the 15 MWp using the NREL meteorological data.   An increase in length of interfin enhanced the heat transfer better than a 40 mm fin length as it connects the front and bottom layer of the PCM that restricts mass convection in PCM.    PCM is encapsulated by HDPE and applied for the dual purpose to reduce the T PV as well as heating, ventilation, and air-conditioning purpose.
-~29 [105] PVT-PCM Paraffin wax 50 Malaysia Simulation and Experiment -Indoor Different thermal collector tubes and flow rates are performed using CFD simulation using different solar irradiance.
Thermal conductivity enhanced PCM and nano fluid-based thermal collector favors to reduce the PCM temperature as well as T PV in higher order.

Case Studies on Implementation of PV and PV PCM Temperature in Solar PV System for Two Different Geographical Locations UAE and Islamabad
The resource assessment for a given geography is of primary importance in developing an MW scale solar PV system. To estimate the year around performance, cost, and payback period requires accurate resource data. The NREL (National Renewable Energy Laboratory) is a national laboratory of the U.S. Department of Energy that provides a TMY (typical meteorological data) in an hourly frequency, available in different formats and data from different sources (NSRDB) 1961-1990 data, TMY2 1991-2010, TMY3 and EnergyPlus weather files. The available weather parameters are: These resource data are analyzed to select the PCM material concerning its thermal properties. In this study, two different geographies are considered, namely UAE and Islamabad, measured PV-PCM temperatures are taken from "Hasan et al. [56] and Waqas et al. [57]" for a respective location and compared with reference NREL data to figure out the impact of PV-PCM temperature on performance enhancement. The experiment requires module temperature to study the effectiveness of PCM material for a given geography and period. The Faiman module temperature transposition method has been used to convert ambient temperature to module temperature using Equation (2). The model is adopted in IEC 61853 standard and uses an empirical heat loss factor.
Solar irradiation availability data over a year is the key weather parameter for investment in a solar PV system. Having satellite and ground measured data will provide the necessary knowledge in determining the potential site. Another dominant weather parameter is ambient temperature. The project size-AC/DC ratio will vary based on the ambient temperature availability to compensate for the loss produced from high temperature operating conditions. To understand the distribution of weather parameters in hourly, daily, and monthly granularity, box plots have been used. Monthly, hourly average, maximum, and minimum temperatures. These plots will assist in identifying the right PCM material before testing it on the field conditions, knowing the thermal properties of the PCM. Apart from understanding the maximum and minimum of weather parameters, it is imperative to understand the spread of weather parameters in ranges, in the sense of the distribution from the median and the density of the dataset.

UAE and Islamabad Irradiance
The distribution of irradiation for UAE and Islamabad locations is observed in this study. Over the year, during daytime hours the UAE location on average yields 543.38 W/m 2 with a peak irradiation of 1085.47 W/m 2 and 854.73 W/m 2 in the third quartile meaning 75% of time observed values in this range. Similarly, the Islamabad location yields 495.66 W/m 2 on average with a peak irradiation of 1030.25 W/m 2 . Figure A1 "Month" January has recorded the lowest irradiance on an average of 261.44 W/m 2 with the maximum of 994.4 W/m 2 and the month of May has recorded the highest irradiance of 325.25 W/m 2 with a maximum of 1040.36 W/m 2 . Subsequently, for the Islamabad location, the monthly average of irradiation over the year has been studied, Explicitly the month of May has seen a higher distribution with an average of 334.5 W/m 2 with a maximum and minimum of 1015.4 W/m 2 and 16.12 W/m 2 , respectively. Figure A1 "hour": For instance, hourly distribution of irradiation has to be studied to ensure the active time of PCM's utilization. During the early daytime hour of 07:00, the average irradiation is 250.91 W/m 2 . At a peak hour from 12:00 to 12:59, the average irradiation is 914.40 W/m 2 with a maximum of 1085.7 W/m 2 . Figure A2: For Islamabad location, the average irradiation during a peak hour noon is 789 W/m 2 with a peak of 1030.25 W/m 2 .

Ambient Temperature of UAE and Islamabad
Ambient temperature is an important weather parameter that estimates the generation for a given geography with higher precision, as it directly affects voltage generation from a solar PV system. For any geography understanding, a temperature profile is necessary in terms of the site feasibility study. On average, UAE has recorded 30.5 • C with a maximum and minimum temperature of 47.2 • C and 5.2 • C, respectively. Furthermore, for a location such as Islamabad, the average ambient temperature is 25.5 • C with a maximum and minimum of 48.56 • C and 6.43 • C, respectively. Figure A3 "Month" depicts the highest and lowest temperature recorded during July and December, respectively. For July, the average temperature is 35.09 • C with maximum and minimum temperatures of 47 • C and 26.5 • C, respectively. For December, the maximum and minimum temperatures are 29.7 • C and 18.57 • C, respectively. Figure A4: For Islamabad location, the average and maximum ambient temperatures are 34.12 • C and 48.56°C during May. Figure A3 "hour" hourly ambient temperature distribution depicts that morning 06:00-07:00 recorded a lower temperature and 12:00 to 13:00 recorded the highest temperature over the year. The average temperatures during the hour 07:00 and 13:00 are 25.06°C and 32.8°C, respectively. The maximum and minimum temperatures are 46°C and 17.12°C for the hour 13:00 and 39.4°C and 19°C for the hour 07:00. From Figure A4 for Islamabad, the average and maximum recorded ambient temperatures were 31.2°C and 48.53°C at hour 12:00.

Wind Speed of UAE and Islamabad
Wind speed helps in reducing the module temperature naturally by convective heat transfer. The higher the wind speed the higher the heat dissipation. Choosing a site with a high average wind speed potentially increases the yield not just by dissipating the heat, but also by the deposition of soil on the surface of the panel.
For the UAE location, the average wind speed during the daytime hours over a year is 4.35 m/s, and the maximum wind speed reaches up to 24.2 m/s and 1.41 m/s on average for Islamabad. Figure A5: "Month" depicts during June there was a higher distribution of wind speed. The average wind speed is 4.95 m/s with the maximum and minimum of 15 m/s and 0.1 m/s, respectively. In the 3rd quartile, the recorded wind speed is 6.2 m/s. Figure A6 for Islamabad 2.08 m/s on average with a maximum wind speed of 4.01 m/s during May. Figure A5: "hour" depicts the hours 12:00 and 17:00 carrying a large distribution. From 12:00 to 13:00 over the year the average recorded wind speed is 4.76 m/s with a maximum and minimum of 13.1 m/s and 0.5 m/s. At 17:00 the maximum wind speed of 24.2 m/s with an average of 5.3 m/s. Figure A6: At hour 11:00, average wind speed is 1.11 m/s.

PV Module Temperature of UAE and Islamabad
Module temperature maintaining close to STC is one of the ways to increase the yield. The average module temperature is 33.48°C, the maximum and minimum module temperature is 76.59°C and 4.161°C for UAE and Islamabad the average module temperature over the year is 33.42°C and the maximum temperature is 82.77°C. Figure 13 "Month": On average, January recorded the lowest temperature. From Figure 13 it is clear that the average module temperature is 24.02°C, which is less than the nominal operating cell temperature. These temperatures are sufficient to set up the PCM for the later part of reducing the temperature of the module at a high operating cell temperature. The minimum and maximum temperatures are 10.49°C and 58.06°C, respectively. From Figure 13, the month of July has seen vigorous environmental conditions to run the solar PV system efficiently; these are the months required for an external cooling system to improve the performance of a solar PV system. The average, maximum, and minimum temperatures are 40.96°C, 76.59°C, and 26.17°C, respectively. From Figure 14, the average module temperature is 43.35°C and the maximum temperature is 82.77°C during May. Figure 13 "Hour" for UAE: During the peak daytime hour of 12:00 the average cell temperature is 52.04°C, the maximum and minimum temperatures are 75.02°C and 20.45°C, respectively. Figure 14: for Islamabad location 55.23°C on average and 78.53°C at 12:00.  Figure 13, the month of July has seen vigorous environmental conditions to run the solar PV system efficiently; these are the months required for an external cooling system to improve the performance of a solar PV system. The average, maximum, and minimum temperatures are 40.96 ℃, 76.59 ℃, and 26.17 ℃, respectively. From Figure 14, the average module temperature is 43.35 ℃ and the maximum temperature is 82.77 ℃ during May. Figure 13 "Hour" for UAE: During the peak daytime hour of 12:00 the average cell temperature is 52.04℃, the maximum and minimum temperatures are 75.02 ℃ and 20.45 ℃, respectively. Figure 14: for Islamabad location 55.23 ℃ on average and 78.53 ℃ at 12:00. Figure 13. Hourly, daily, and monthly PV temperature for UAE location. Figure 13. Hourly, daily, and monthly PV temperature for UAE location. That precise hour shows a difference of 10 ℃ in temperature reduction. Similarly, the lowest ones are 57.2 ℃ and 47 ℃ for February. Figure 16. For Islamabad location during May (the hottest month), the PV module temperature is 81.2 ℃, while the PV PCM temperature is 72.1 ℃. Similarly, January has recorded the lowest temperatures. The PV module temperature is 52.1 ℃ while the PV-PCM temperature is 42.3 ℃.

Power Profile of PV and PV-PCM for UAE and Islamabad
To examine the impact of the PV module and PV PCM temperature, an MW scale solar PV system was designed. A 400Wp Mono-crystalline solar panel with a temperature degradation coefficient(β) of −0.36%/℃ is considered. A 15 MW Solar PV system with a

Power Profile of PV and PV-PCM for UAE and Islamabad
To examine the impact of the PV module and PV PCM temperature, an MW scale solar PV system was designed. A 400 Wp Mono-crystalline solar panel with a temperature degradation coefficient(β) of −0.36%/°C is considered. A 15 MW Solar PV system with a 400 Wp panel requires 37500 panels with 20 panels in series and 1875 in parallel. Theoretically, power was calculated for given irradiation, module temperature, reference temperature, temperature degradation coefficient, and capacity using Equation (3) [165,166].
DC losses applied in Equation (4) are explicitly modeled percentages available in PV-watts NREL that were considered. Default percentage for loss parameters such as soiling 2%, mismatch 2%, shadow 3% wiring 2%, light-induced degradation 0.5%, and nameplate rating 1% applied for both UAE and Islamabad locations.
Estimated power (MW) with PV temperature and PV PCM temperature as shown in Figures 17 and 18. For UAE location the enhancement of power generation during peak hours occurred throughout the year, during February and March even for low insolation produced a maximum power because of low operating conditions. In March, the UAE location saw a rise from 12.41 MW to 13.04 MW as a result of using PV PCM. Similarly, for the Islamabad location during March, 11.55 MW of power generation enhanced to 12.13 MW with the advent of PV PCM temperature.
Estimated power (MW) with PV temperature and PV PCM temperature as shown in Figures 17 and 18. For UAE location the enhancement of power generation during peak hours occurred throughout the year, during February and March even for low insolation produced a maximum power because of low operating conditions. In March, the UAE location saw a rise from 12.41 MW to 13.04 MW as a result of using PV PCM. Similarly, for the Islamabad location during March, 11.55 MW of power generation enhanced to 12.13 MW with the advent of PV PCM temperature.

Performance Ratio Metric for UAE and Islamabad
Equation (5) was used to calculate the performance ratio (PR) of the solar power plant. The percentage of power increased was validated with the help of performance metrics IEC 61724. The percentage of PR increased is phenomenal for UAE locations for February, March, and April by 4.42%, 4.78%, and 5.39% respectively as shown in Figure 19. For February, PR improved from 78.17% to 82.59%. Similarly, Islamabad location during March, April, and September saw a rise in PR by 4.82%, 5.50%, and 4.82%, respectively, as shown in Figure 20. In April, PR improved from 72.26% to 77.77%.   Figure A7 depicts the power generation (MW) concerning irradiation and module temperatures. A lower module temperature is recorded during December and 11.27 MW is generated for a peak module temperature of 48°C. It is clear that even for low irradiation the output from the system is high, because the peak PV-PCM temperature is low. During July, even for high irradiation, the generation is 11.4 MW and PV-PCM temperature is 64.27°C. Figure A8 Table 8 for UAE's location, the maximum PV temperature for each month has been considered to analyze the peak performance of a PV-PCM. It is observed that PV-PCM temperature is always lower than the PV module temperature, indicating high certainty of improved performance of the PV module over the year. This is evident from Table 8 as the power (MW) improved by 4.36% on average. Similarly, from Table 9, for Islamabad location, PV-PCM temperature shows a lower trend as compared to PV temperature. This is evident with the power (MW) enhancement. The average increase in power output percentage is 4.35%.   Table 10 shows UAE's location average peak PV temperature over the year is 67.23 • C; with the advent of PCM the average peak PV temperature dropped to 56.50 • C (PV-PCM temperature), which is clear in the power output and performance ratio (PR). The PR (%) on a peak PV temperature and PV-PCM temperature has increased from 79.53% to 75.17% on average over the year. While for the Islamabad location, the average peak PV temperature over the year is 67.30 • C and with PV-PCM the PR (%) of the PV system increased from 79.51% to 75.15%. Comprehensively, both the locations have similar weather conditions over the year.

Conclusions and Future Prospects
Several researchers claim that reducing T PV enhances the electrical conversion efficiency of the PV module. Conventional, water, and air-assisted cooling techniques were widely performed, followed by TEG, heatsink, and other techniques. However, effective PV module cooling is questionable for several locations due to resource unavailability. In recent years, PCM has broken the availability of a cooling process that helps control the excess rise in T PV for all locations. PCM is a stationary unit with a minimum lifetime of 5 years and is readily available in the local market.
• PCM can store a high amount of thermal energy within a small quantity, which makes PCM unique as a sensible heat storage material. In such a way, paraffin wax and Rubitherm commercial PCM's are widely used and have achieved higher T PV reduction. However, PCM also lacks several issues that question the performance of the PCM integration. • PCM is a low thermal conducting material that creates a thermal conduction barrier during charging and discharging mode. Several researchers claim that an increase in the thickness of PCM also creates the conduction barrier. Further, thermal conductivity enhancers are used to increase the K PCM . In such a way, interfin plays a major role in PV module cooling techniques than nano compounds and metal-based enhancers. Secondly, non-metal-based thermal enhancers have gained higher attention in the cooling process, especially EG. The main benefit of EG as a thermal enhancer will not increase the weight of the system and is free from corrosion.
• A further increase in the thickness of PCM failed to discharge the entire stored thermal energy in the nighttime that causes to fail the consecutive charging process. To minimize this loss, thermal collectors or heat pipes are attached inside the PCM to remove the thermal energy from the PCM by flowing working fluid inside the tube. Notably, heat from the PCM is utilized for thermal comforts such as heating and ventilation processes. Secondly, the thermal collector minimizes the usage of PCM and conduction barriers. • From this study, it is clear that before experimenting with PCM, numerical or theoretical work has to be carried out to optimize the T melt of PCM and appropriate thickness. Inappropriate PCM T melt postpones or prepones the cooling process that makes PCM ineffective. If the selected PCM T melt is less than the optimal range, the cooling process will start in the early daytime and end before the peak daytime. If the selected PCM T melt is higher than the optimal range, PCM will initiate the cooling process in late peak daytime. In such a case, PCM turns ineffective and creates a negative effect on increasing the thermal resistance and T PV . These two surveys will reduce the negative impact of the PCM integration. EG is recommended as a thermal enhancer rather than interfin because EG will not increase the system's total weight such as a conventional thermal enhancer. Moreover, eutectic PCM played a minor role in the PV module cooling technique because it is not readily available PCM. However, organic eutectic material is thermally stable for more than 2000 thermal cycles. Further, it is recommended to use the effectiveness of eutectic material to cool the PV module and minimize PCM's cost.

•
Case study: NREL resource data associated with experimental values were implemented upon two geographical locations-UAE and Islamabad. Theoretical power output was compared between PV and PV-PCM temperature. Results indicate that throughout the year the PV-PCM outperforms PV module temperature; more specifically, February, March, and April showed an increased electrical output power by 4.42%, 4.78%, and 5.39%, respectively Similarly, Islamabad location during March, April, and September saw a rise in performance by 4.82%, 5.50%, and 4.82%, respectively.

•
Analyzing resource data before any geographical location would help determine a suitable PCM. Having higher insolation, low average temperature, windy conditions, and a module with a good thermal coefficient would ensure higher yield and reduced loss. Economically, this reduces the payback period and cuts the project cost by enabling a reduced AC/DC ratio(sizing).     [195] Eutectic (capric-palmitic acid) 22.5 [196] 173 [196] Ireland 7 24 950 [197] Calcium chloride hexahydrate 29.8 [198] 191 [198] Ireland 10 24 950 [197] Appendix B  [196] 173 [196] Ireland 7 24 950 [197] Calcium chloride hexahydrate 29.8 [198] 191 [198] Ireland 10 24 950 [197] Appendix B Figure A1. Hourly, daily, and monthly plane of array irradiation for UAE location. Figure A1. Hourly, daily, and monthly plane of array irradiation for UAE location.  Figure A2. Hourly, daily, and monthly plane of array irradiation for Islamabad location. Figure A3. Hourly, daily, and monthly ambient temperature for UAE location. Figure A2. Hourly, daily, and monthly plane of array irradiation for Islamabad location.  Figure A2. Hourly, daily, and monthly plane of array irradiation for Islamabad location. Figure A3. Hourly, daily, and monthly ambient temperature for UAE location. Figure A3. Hourly, daily, and monthly ambient temperature for UAE location.  Figure A4. Hourly, daily, and monthly ambient temperature for Islamabad location. Figure A5. Hourly, daily, and monthly wind for UAE location. Figure A4. Hourly, daily, and monthly ambient temperature for Islamabad location.  Figure A4. Hourly, daily, and monthly ambient temperature for Islamabad location. Figure A5. Hourly, daily, and monthly wind for UAE location. Figure A5. Hourly, daily, and monthly wind for UAE location.