1. Introduction
Solar PV, together with wind energy, is fast becoming a mainstream and competitive source of power production. Although accounting for only 4.5% of total electricity generation in 2015, they are expected to represent 58% of total electricity production by 2050 [
1]. Electricity generation from solar radiation is achieved through photovoltaic (PV) cells or concentrated solar power plants (CSP). This solar radiation can be used for electricity generation or heat production (space heating, hot water supply).
PV cells absorb 80% of the incident solar radiation and depending on the PV module material, a small part of this solar radiation (only 15 to 20%) is converted into electrical energy while the remaining part is converted into heat [
2]. Manufacturers claim that the available photovoltaic modules have an efficiency from 6 to 16% [
3]. However, this claimed efficiency is measured at 25 °C, and they have not considered the PV module temperature rise during their working conditions. The overheating temperature of the module is due mainly to high solar radiation and high ambient temperatures [
4]. PV modules during sunny days can reach temperatures of 35 °C above ambient temperature. This temperature increment strongly influences the performance and electrical efficiency of the PV system, which can lead to power losses from 0.40%/°C at standard test conditions [
5] to 0.65%/°C [
6], and increase the ageing of the module. Typical efficiencies for different PV module materials can be found in
Table 1.
To inhibit the temperature rise in PV modules, several authors have proposed different cooling techniques using air (natural or forced circulation), water (water cooling system or heat pipes), thermoelectric systems, or Phase Change Materials; some of these methods are passive while some others are active. PV modules can also be combined with solar thermal (PV/T) to deliver heat and electricity into a single module. Some studies have shown an increase in electrical efficiency by 5% [
9]. PV/T technology is mainly used in domestic and industrial applications for heating air or water as well as electricity generation [
10]. In those cases, water or air are mainly used as the heat transfer fluid. Air type PV/T collectors are used for drying, space heating, and ventilation, whereas water types are used to removing the heat from the PV module. Water types are more effective than air types because the fluid temperature variation is narrower. Already some researchers have pointed out higher thermal efficiencies, 50 to 70% for water heating and 17–51% for air heating [
11]. These types of PV/T collectors are mainly used in thermal/heat pump systems, water desalination, solar cooling, or solar greenhouse [
2]. However, since 2010 the study of PCMs and nanofluid to increase PV module’s efficiency has increased [
12]. PCMs are materials that store thermal energy through a phase change, the solid/liquid phase change being the most used. These materials are used for thermal energy storage and also thermal management applications as they can charge/discharge at an almost constant temperature and have high energy density (small footprint) [
13] despite suffering poor thermal conductivity. In recent years, innovative passive cooling methods have been presented, compared to PV/T, as they do not require additional power consumption, work at a higher operating temperature to supply useful heat, and are a more complex system with a higher initial investment [
14]. Abd-Elhady et al. [
15] proposed drilling through holes in the PV module to allow the hot layer of air under the module to rise, creating natural flows that cool down the module. The temperature of the PV module decreased with the increased number of through-holes until an optimum number of holes was reached. The increase of the through-holes diameter reached a maximum cooling effect on the PV modules, above which less cooling occurred. Also, PCM has been proposed as a potential solution, although further cost-effective studies need to be conducted [
16,
17,
18]. Several researchers have proposed the use of thin layers attached to the PV modules, similar to the research carried out by Stropnik et al. [
17] which achieved an increase of the electrical power by 9.2% under experimental conditions. Su et al. [
19] introduced a PCM layer to an air-cooled system, improving its efficiency by 10.7% compared with the PV module with no PCM. Also, other researchers proposed the use of microencapsulated phase change material (MEPCM) [
20]. A MEPCM layer attached to a water-surface PV module resulted in a 2.1% relative efficiency improvement compared with the one without MEPCM [
21]. Hasan et al. [
19] used different melting temperature PCMs to evaluate the performance of each PCM in four different systems. They found that the salt hydrate PCM (CaCl
2) achieved the highest temperature reduction in most of the insulations. The results showed that the thermal conductivity of the PCM container had a strong impact on low thermal conductivity PCMs performance.
Most PCMs have low thermal conductivity, which strongly affects their heat transfer rate during the charging/discharging process and limits their application, as several researchers have stated in their work. Different strategies to overcome this challenge are currently under study. Huang et al. [
22] studied the thermal behavior of PV modules with and without PCM experimentally and by simulation. The system consists of a vertical southeast-oriented PV/PCM system using real ambient temperature and insolation conditions in South East England. The improvement in the thermal performance achieved using metal fins in the PCM container was significant as they enabled a more uniform temperature distribution within the PV/PCM module. The PCM and fins delayed the temperature increment maintaining the operating temperature of the PV cell at a much lower level for extended hours. It was observed that after the PCM melting process, the rate of PV heat extraction decreased, which produced a rapid increase in the module temperature. Khanna et al. [
16] focused on optimizing a finned PV/PCM module to achieve the required cooling under different solar radiation; different lengths, thicknesses, and spacing between fins were used. An alternative to increase the thermal conductivity is the use of metallic foams, which was evaluated by Klemm et al. [
23]. According to the simulation results, a storage unit consisting of a PCM-filled metallic fibre structure represents an adequate mean for passive thermal management of PV modules in given ambient conditions. The system was able to decrease 20 K of the PCM storage module. However, the configuration has to be validated experimentally under real conditions, and the volume reduction has to be considered. Other researchers used a PV/PCM system with form-stable paraffin/Expanded Graphite (EG) to improve the uniformity of the temperature distribution of the PV modules and thus improve their power output [
18]. The PCM/EG helped to control the temperature and the temperature distribution of the PV modules. The output power achieved was above that of the conventional PV module for 230 min, with a maximum increment of 11.50% and an average increment of 7.28% under the experimental conditions. Others, such as Kumar et al. [
24], used nanoPCMs to increase the efficiency. The authors achieve a PV panel electrical performance enhancement of up to 4.3%. The prototype studied consisted of a combined PCM mixture of calcium carbonate, copper nanoparticles, and SiC in a ratio of 7:2:1.
Among the techniques for cooling systems mentioned above, PCMs are the most promising and effective cooling technique for photovoltaic due to their higher energy density per unit volume [
25,
26]. The use of PCM for PV modules cooling shows higher heat transfer rates than both forced air circulation and forced water circulation, a higher heat absorption due to the latent heat, and an isothermal heat removal [
27]. Moreover, there is no electricity consumption, no noise, and no maintenance cost. However, the PCM has a higher cost than natural and forced air circulation; some PCMs are toxic, have fire safety issues, are strongly corrosive, and are considered disposable after their life cycle is complete. The research regarding this technology needs to move forward, offer solutions to unresolved problems, and understand the potential barriers to practical application.
Additionally, the geographic location of the PV modules, no matter the system, has a direct impact on the intensity of solar radiation and wind speed, together with humidity conditions, dust in the air and/or pollution, factors that determine the PV module performance and output fluctuation [
12]. Although the reported studies showed a considerable enhancement of the PV module’s performance, the experimental results were mainly conducted in lab conditions, where the solar radiation and ambient temperature were fixed at values of 1000 W/m
2 and 25 °C, respectively. These tests make it difficult to predict the actual amount of PCM needed for real applications. Therefore, systems must be investigated at a designated location [
28]. Studies have shown that common assumptions about the UK, such as not receiving enough sunshine and not being viable to install PV, were wrong; some findings have shown that a significant proportion of a house’s electrical needs could be obtained more than 40% on average [
29]. Another aspect that is sometimes overseen is the container dimensions. Typically, a rectangular-shaped PCM container is considered both in modeling and experimental systems. Novel PCM container shapes, different from the usual rectangular solid container filled with the phase change material at the backside of the PV panel, should be considered. Nizetic et al. [
30] proposed a new configuration, where several small containers filled with the PCM material were attached to the PV panel. The number of PCM materials was approximately 47% less and the container material, aluminium, was 36% less when compared with a full PCM container. Both configurations performed better than the PV panel without PCM. Although there were periods where the full PCM configuration had the highest power output, the overall performance considering long periods of time for the small container configuration, was better. The authors relate to that outcome due to the more effective thermal management of the small containers owing to less effective heat transfer from the full PCM container strategy.
In this study, passive cooling of PV systems using PCMs was investigated where three different PCM candidates were selected (RT42, RT31, and RT25) based on the average ambient temperature, and a polycrystalline PV module was used. The optimization of the PV module considered different parameters such as ambient temperature, daily solar radiation, PCM type, and its melting temperature, and PCM container shape and size. The parameters were assessed and compared with the system without PCM. This work aims not only to assess the performance of the novel PV/PCM system but also to determine the optimum PCM container parameters (shape/geometry, depth, length, and insulation), the PCM type, and the combined effect on the PV module surface temperature, efficiency and power output using real solar radiation and ambient temperature data. Computational Fluid Dynamic CFD was implemented using Ansys Fluent V18.2 [
31], and the dynamic heat transfer, fluid flow, melting/solidification, and other PV/PCM system parameters were studied.
4. Result and Discussion
The current work investigates the potential of using PCMs to enhance the performance of PV modules where a PCM container is attached to the bottom surface of the PV module. The study aims to optimise the system variables: PCM container (shape, height/depth and length), insulation and PCM type, which contains a sufficient amount of PCM to meet the cooling load, by studying their effect on the operating temperature of the PV module. The PV module temperature was predicted using CFD modelling with Ansys Fluent V18.2 software to assess its performance. A published experimental work was used to validate the developed model. The dynamic power output from the PV module and its conversion efficiency with respect to the operating temperature was calculated using well-known empirical equations.
Three different PCMs (RT42, RT31 and RT25) were chosen based on the average ambient temperature (~30 °C) of the studied case. The melting temperature of these PCMs was approximately 5 °C less, equal to and 10 °C higher than the average ambient temperature, to determine the effect of the selected phase change temperature on the container size, insulation, PV temperature, conversion efficiency, and power output.
Figure 5a,b show the PV temperature of the RT42 PV/PCM system with different container heights, with and without insulation, versus the daytime. In both cases, the temperature of the PV-only system is included for comparison purposes. The results showed that the insufficient container heights (20, 40, 50, and 60 mm) led to an increase in the PV/PCM module temperature even higher than the conventional system (PV-only) at certain times. The increase was higher with insulation, as shown in
Figure 5b; this is due to the increasing PCM temperature when it completely melts and releases its heat, becoming higher than the ambient temperature. Thus, the conventional PV system showed a lower temperature when the heat could be easily separated from the back of the PV module. Without insulation, the optimum tank height was 70 mm while it was 80 mm with insulation; this means that having insulation in the PCM container when RT42 is used increases the required PCM amount by 14%, in addition to its cost.
RT31 and RT25 showed a similar trend to RT42, with and without insulation, as shown in
Figure 6 and
Figure 7. However, the average PV module temperature using RT31 and RT25 with the optimal PCM height was around 37 °C and 32 °C, respectively, which were lower than that of using RT42 (43 °C). Thus, RT31 and RT25 provide a significant reduction in the PV temperature at peak times by 23 °C and 28 °C, respectively, compared with the PV-only system when it was 17 °C after RT42 was used. The optimum tank height for RT31 was 110 mm when no insulation was used and 120 mm using insulation. When RT25 is used, the optimal heights were 120 and 125 mm, respectively.
Figure 8 shows the comparison of RT42, RT31, and RT25 in terms of the PCM container size. The figure demonstrates that when RT31 and RT25 were used, the required amount of PCM was 56% and 72% higher than that of RT42.
Regarding the effect of the tank shape on the PV module temperature, four different tank geometries (cases) were considered, as shown in
Figure 2. In all these cases, no insulation was used, and RT42 was selected as the PCM material. When the PV module temperature becomes higher than the melting temperature of the PCM, the melting process starts, and the density change occurs. This density change forces the liquid phase to move to the top side of the PCM container, leading to a nonuniform temperature distribution in the PV module. As mentioned above, this temperature gradient is highly dependent on the tank depth, shape, and length.
In the first case (Case 1) of the four configurations, the container had a rectangular-shaped cross-section with a height value of 70 mm, while the second, third, and fourth cases (Case 2, Case 3 and Case 4) had container cross-sections shaped like trapezoids, with a variation in height from bottom to top. The bottom heights of Case 2, 3, and 4 were 50, 40 and 30 mm, whereas 90, 100 and 110 mm were the heights of the top side, respectively. In all cases, the tank length was fixed at 1000 mm.
For the first configuration (Case 1),
Figure 9 shows the PV surface temperature gradient along its length at different day times. After 14:50, most of the PCM melted, and the top part’s temperature started increasing and reached its peak at 15:40. with a difference higher than 4 °C.
Figure 9b,c show the temperature and the mass fraction contours of the PV/PCM system at 15:40.
Figure 10a,
Figure 11a, and
Figure 12a show the temperature gradients of PV temperature along with the trapezoid shape containers of Cases 2, 3, and 4, respectively. The PV surface temperature is almost constant in Cases 2 and 3, with a variation of less than 0.5 °C. However, Case 4 shows a considerable reduction in the surface temperature at the bottom side of the container. This container part was affected by the ambient temperature due to the thinness of the PCM layer.
Figure 10b,
Figure 11b, and
Figure 12b show the mass fraction contours of the PV/PCM system at 15:40. It can be seen that the solid part of the PCM in Cases 3 and 4 did not move to the bottom side of the container due to their high viscosity and the low container slope.
Reducing the tank length leads to a lower temperature variation and vice versa. Considering both the PV module temperature and the movement of the PCM inside the container, Case 2 was the best configuration; however, this result is subjective to the PV tilt angle and the ambient temperature.
The PV module efficiency at the operating temperature was calculated using Equation (8).
Figure 13 shows the variation of the PV module efficiency at the operating temperature during the studied daytime for both the PV/PCM and the PV-only system. By comparing the two systems, unlike the conventional system, the PV/PCM system showed no significant variation in the PV efficiency during the daytime. The lowest melting temperature PCM (RT25) showed the highest PV module efficiency. The PV/PCM systems reached an efficiency increase of 10%, 13% and 17% at 13:00 when RT42, RT31, and RT25 were used, respectively, as shown in
Figure 14. This considerable enhancement of the PV/PCM system efficiency resulted in a great increase in the hourly power output, as shown in
Figure 15, where the power output of the 1 m
2 module’s system is presented.
Figure 16 shows the percentage enhancement of the power output of the PV/PCM system compared to the PV-only system. This enhancement reached around 9%, 11.5% and 14.6% at the maximum solar radiation when RT42, RT31, and RT25, respectively, were used. RT42 showed the lowest PV efficiency and power output enhancement compared with RT31 and RT25. However, the output power when using RT31 and RT25 as PCM showed a maximum increase of only 3% and 5.5%, respectively, compared with RT42, as shown in
Figure 17. These results indicate that using PCM with a melting temperature higher than the average ambient temperature significantly reduces the PCM amount without a significant reduction in the total power output.
The rectangular PCM container shows the most inhomogeneous temperature distribution, as seen in
Figure 9a. This inhomogeneous temperature distribution leads to a mismatch loss, an outcome previously mentioned in
Section 2. A PV module with 72 cells was used to estimate the mismatch losses for Case 1 at the daytime hour of 15:40. The module specification is shown in
Table 4 with a landscape orientation. The cells array consists of 10 cells in each row and 6 in each column. The CFD simulation results were used to feed the mathematical model to calculate each row’s output voltage and power and the whole module. The solar radiation was assumed to be 800 W/m
2, and the results are shown in
Table 5. The results show a mismatch loss fraction of 0.42%, which seems insignificant, but considering a large PV plant consisting of several modules, these losses will significantly contribute to reducing the power generation.