Determining the Effect of Photovoltaic Module Surface Temperature on Generation Efficiency

It is imperative to consider the environmental impact of energy production and its cost in deciding how to meet future energy needs. In this regard, it is possible to harness the power of the sun by using photovoltaic (PV) cells. However, when the temperature of a PV cell increases, its generation efficiency is negatively affected. The open-circuit voltage of PV modules is the most sensitive parameter to temperature changes. As the temperature rises, this parameter decreases, and the short-circuit current increases. The circuit’s resistance also rises as the electrons’ speed is reduced. Temperature also affects the lifespan of PV cells, increasing the rate of thermal decay in their materials. On the other hand, when solar radiation is absorbed at lower temperatures, the system’s efficiency, power capacity, and useful life increase. PV module surface temperatures can be reduced in a variety of ways, e.g., the surface can be cooled using water. This work studied hybrid PV-thermal modules under the climate conditions of the Hatay province (Turkey) in order to assess the effect of water cooling on their generation efficiency. The results allow stating that up to 52.6% more electricity can be generated by cooling the module’s surface. Additionally, it was found that, in order for PV modules to perform efficiently in Hatay’s climate, they must operate at a maximum surface temperature of 55 °C.


Introduction
Energy, a key factor for human socio-economic and sustainable development, as well as for improving quality of life, is one of the most important resources in sustaining our lives (Meinshausen et al., 2009).Although the world has been seriously harmed as a result of the misuse of natural resources, there is an ongoing search for ways to obtain clean and sustainable energy (Zanlorenzi et al., 2018), as the majority of today's energy comes from fossil fuels and nuclear sources.
Solar generation is popular, allowing the energy provided by the sun to be used in different ways by converting it into electrical power (Tiwari et al., 2006).Solar cells operate on the photovoltaic (PV) principle for power generation, i.e., they generate electricity when they receive light.When solar radiation in the form of photons hits the surface of semiconductor materials such as silicon, electrons are released from atoms, entailing an electric voltage.Solar energy is free and available in abundance.Many applications based on this resource have been developed, including different types of concentrated solar thermal power technologies, solar PV generation, hybrid solar PV and thermal techniques, solar desalination, solar hydrogen production, solar-assisted heat pump technologies, etc. (Wang et al., 2021).
It is the type of PV employed that has the most bearing on electrical performance.Power generation from solar cells ranges from 6 to 20% depending on weather conditions and the type of cells used in a typical PV module.The remaining incident solar radiation is transformed into heat, which raises the temperature of the PV module and reduces its efficiency (Dubey et al., 2013).As PV cells have spectrum-dependent characteristics, ineffective solar radiation increases the cell temperature after it is absorbed, causing a decrease in photoelectric conversion efficiency.Solar PV-thermal (PV-T) technologies employing beam splitting can solve this problem to a certain degree (Wang et al., 2023).PV-T collectors can be used to gather this heat from flowing water or air beneath the PV module.
Increases in intrinsic carrier concentrations at higher temperatures lead to increases in the dark saturation current of the p-n junction, which reduces PV cell efficiency.Due to excessive doping, the intrinsic carrier concentration rises as a result of a reduced band gap.The open-circuit voltage drops linearly as the dark saturation current rises, resulting in a temperature change of 2.3 mV/°C for silicon at 300 K, as stated by Peng et al. (2017).
At temperatures ranging from 20 to 100 °C, it is estimated that the reduction in the solar cell's band gap increases the short circuit current by 0.1%.Despite this increase in current, reductions in the open-circuit voltage cause a noticeable decrease in the maximum electrical power, as seen in the characteristic curves of PV modules at different operating temperatures (Figure 1) (Andreev et al., 1997).A study by Radziemska (2003) determined that the power and voltage produced by monocrystalline silicon solar cells and the maximum power vary at different temperatures (Figure 2).One of the advantages of using a solar PV and thermal hybrid system is that it simultaneously generates electricity and thermal energy from the sun.The use of PV-T systems leads to the production of both useful heat energy and electrical power.The solar PV cells currently on the market have a comparatively low efficiency (less than 15%).After PV conversion, more than 80% of the solar energy absorbed is lost as heat into the environment.Cooling PV systems is important, as their efficiency decreases when the temperature goes above a certain point.In order to boost performance, it is critical to keep PV modules cool (Rawat and Dhiran, 2017).
While the short-circuit current of PV cells increases with temperature, the open-circuit voltage decreases.The resistance that reduces the speed of electrons in the circuit also increases with temperature.Therefore, high operating temperatures have a negative impact on power and efficiency in PV systems.With an effect on the thermal degradation of the module's material, temperature also reduces the effective efficiency of the cells.Therefore, lowering the operating surface temperature is an effective way to increase the efficiency of a PV module and reduce its thermal degradation rate.This can be achieved by cooling the module and lowering the heat stored in the PV cells during operation.There are several methods for reducing PV module surface temperatures, including water cooling systems.
The goal of this study is to determine how the efficiency of PV modules changes when cooled via the liquid fluid approach in a PV/T system in the province of Hatay.We expect our results to aid in determining the surface temperature values for the most efficient operation of PV systems, as well as in identifying the efficiency losses regarding electricity production that are caused by temperature in similar hot climate conditions.It is also stated in the studies mentioned above that the panel surface temperature affects the electricity generation efficiency of PV systems.This study aims to determine the extent of this effect and the efficient operating temperature ranges to be used in similar regions.In this vein, a PV-T experimental setup, which can be operated with or without water cooling, is used to take measurements.

Materials
Today, hybrid PV-T solar systems are one of the most popular methods for cooling PV modules.These hybrid systems consist of solar PV modules combined with a cooling system.To cool the surface of the PV modules, water or air is pumped over them, boosting their efficiency.
In the experimental setup, (except PV modules) parts such as a pump circulating the cooling water in the cooling system, a water tank, a solar radiation meter, temperature sensors, data recorders, current and voltage recorders, a charge controller, a light bulb, a mini meteorological station, PE pipe components were used (Figure 3).A PV/T module was used, whose features are presented below (Table 1, Figure 4).

Source: Authors
The PV-T module was cooled using water, which was in turn cooled with a compressor cooling system.An evaporator in the form of a spiral copper pipe immersed in the tank was entrusted with water cooling.The cooling system was managed with a digital temperature controller that can be set to a minimum temperature of 18 °C (Figure 3, no.5, 7, 8, 9).
The measurement involved a PWM charge controller; a 12 V, 100 Ah gel-type battery to measure the power produced by the system; and a mini meteorological station to record climate data including wind speed and direction, temperature (ambient, perceived, and dew), and relative and absolute pressure.These data were measured in 5 min intervals (Figure 3, no.3).

Method
The electricity generation efficiency of the PV/T module was determined in two ways: with and without cooling.To this effect, as shown in Figure 5, the current and voltage values produced by the module were measured in 5 min intervals by connecting two multimeters.In both experiments, in addition to these measurements, module surface temperature, solar radiation levels, and climate parameters were measured.Measurements were taken between 10:00 a.m. and 3:00 p.m., when the sun was at its most intense.
The water cooled via the compressor system was used to reduce the temperature of the PV/T module.With the help of a 12/24 V DC pump, water at a temperature of around 20 °C was pushed through the PV/T module and discharged back into the water tank, providing perpetual circulation.
Moreover, for the sake of comparison, the current, voltage, and surface temperature values of the PV module were measured in the experimental setup.The power produced by the solar modules was calculated using Equations ( 1) and ( 2), aiming to determine the efficiency of the modules.The power consumed by the cooling water system was not considered.The results of these measurements were graphed.This includes the evolution of the surface temperature of the PV-T module with and without cooling as well as the module's efficiency, aiming to determine their relationship.The temperature changes were calculated by taking the differences between the two measurement points, and the efficiency change was determined by calculating the percentage of the difference between the two measurement points.

Results
Current-voltage, solar radiation-module power, and temperature-efficiency graphs for a day of operating the PV-T cooling module.A total of six days was plotted.The data for June 24 th , 2019, are presented in Figures 6, 7, and 8.The results showed that the current ranges from 4,0 to 5,11 A, while the voltage ranges from 11,0 to 12,90 V.It was determined that the highest current and voltage values of the PV-T module are reported between 11:45 a.m. and 1:30 p.m. (Figure 6).
The module surface temperature value ranged from 46,0 to 60,7 °C, and the electricity generation efficiency went from 4,3 to 6,0%.At many measurements points, temperature increases also caused a decrease in efficiency (Figure 11).
Temperature changes between measurement points ranged from -0.6 to 1,2 °C.The efficiency of the module increased when the surface temperature decreased.According to the calculations, the changes in generation efficiency are between -3,2 and +5,29% with regard to surface temperature.
The overall variation ranges for all days of measurement are presented in Table 2.For all measurements, the temperature change interval of the system without cooling was 7,7 °C, and that of the system with cooling was 9,2 °C.In light of this, the efficiency change intervals were 1,12 and 6,99%, respectively.Based on these results, it can be stated that an efficiency increase and decrease of 3,5% is generated by changes in the module surface temperature.
Table 3 presents the descriptive statistics of the module surface temperature and efficiency values for all measurement days in both scenarios (with cooling and without cooling).7).
The module's surface temperature value ranged from 46,0 to 60,7 °C, and its electricity generation efficiency ranged from 4,98 to 5,84%.At many measurement points, temperature increases caused a decrease in efficiency (Figure 8).The temperature between the measurement points went from -1,3 to 2,0 °C, and the electricity generation efficiency of the module increased when surface temperature decreased.
According to the calculations, the module's efficiency varied between -2,63 and +3,69% with regard to variations in surface temperature.For the PV-T module without cooling, the module surface temperature reported values of up to 75,20 °C.With cooling, the lowest value was 33,30 °C.The efficiency of the system without cooling reached a maximum of 6,10% depending on the surface temperature.Moreover, the highest efficiency value was 9,31% with cooling.By comparing both scenarios, an efficiency increase of 52,62% was observed with the use of the cooling system.
Several studies have been carried out in various applications to reduce the surface temperature of PV panels.Some of these are reviewed below.These works have aimed to increase panel efficiency and generation by reducing the surface temperature.
The efficiency increase achieved with our cooling system is similar to that of Akbarzadeh and Wadowski (1996), who measured water cooling in a hybrid PV-T solar system, with the purpose of increasing the output power by almost 50%.They also determined that the module's cooling process prevents the surface temperature of the solar cells from going above 46 °C for a period of 4 h.
As reported by Teo et al. (2012), when a module with no active cooling only exhibits an 8-9% efficiency.However, when it is cooled, the temperature drops significantly, and its efficiency increases to 12-14%.
Abu-Rahmeh (2017) compared three different PV panel cooling methods: water, nanofluid TiO 2 (0.04% by weight), and rectangular aluminum fins.With regard to a noncooled system, this study reported efficiency increases of 5.37, 2,62, and 1,34% for the TiO 2 , water, and fin cooling methods, respectively.In comparison with our study, this work showed that TiO 2 cooling is more effective than using water, while rectangular aluminum fins reported the lowest efficiency improvements.
In the study by Peng et al. (2017), which compared the ice cooling method applied behind the PV panel against a non-cooled system, the highest value reported by the latter was 4,98%.The cooled system, however, reached a 7,32% efficiency, which represents a 47% improvement.Haidara et al. (2018) set out to determine the decrease in panel temperature and the changes in panel efficiency while using an evaporative cooling system that involved wetting a fabric and placing it on the back surface of the PV panel.They stated that a 14% increase in electricity generation efficiency was achieved, in response to a temperature drop of more than 20 °C in the PV panel.
In a study conducted by Gül and Akyüz (2019), the electrical and thermal performances of a hybrid PV-T system were evaluated.These authors observed a thermal efficiency range of 49,9-52,11% in measurements taken at fluid velocities of 0,015, 0,044, and 0,069 kg/s.A comparison of the module's electrical performance with and without cooling showed a 12,9% increase in the electrical output's maximum power point, and the electrical efficiency was calculated as 12%.
In the study carried out by Luboń et al. (2020), which aimed to reduce the module temperature by pouring cold tap water in the form of a water film on the surface of the PV module, a power increase of up to 20,2 W/m 2 was observed.These authors determined that a 20% increase in the total power generation can be obtained by continuously cooling the module's surface.
The effect of the water flow rate on PV panel performance was determined in the study by Govardhanan et al. (2020), used a uniform water flow on the PV panel surface for cooling.It was determined that cooling the PV module increases the output power by 15% when compared to the conventional approach.The cooling water flow rate that yielded the highest PV module power was 5,3 kg/min.
In a study that experimentally evaluated the effect of simultaneous dual-surface cooling on the output performance of a PV module, where both surfaces were cooled using a water flow on the panel surface and an absorbing cotton wick mesh on the rear surface, an improvement of approximately 30,3% in the output power of the panel was obtained.In addition, it was stated that the average efficiency of the noncooled panel was 12,83%, against an average efficiency of 14,36% in the cooled system (Agyekum et al., 2021).
Zubeer and Ali (2022) set out to determine the changes in panel performance as a result of water cooling in concentrator photovoltaic (CPV) panels.They stated that the surface temperature decreased from 64,1 to 365 °C with this approach.This temperature drop increased the panel's electricity generation efficiency from 14,2 to 17%.In addition, in the water-cooled CPV system, the open circuit voltage and the short circuit current increased by 9 and 5,2%, respectively.et al. (2022) employed water passing through PVC pipes placed behind a PV panel for cooling.Their results showed an improvement of 14,1% in the power generation of the cooled PV panel.In addition, they stated that the electrical efficiency of the PV module with cooling reached 19,8% vs. the 17,4% value achieved without cooling.

Shalaby
The aforementioned studies show changes in panel surface temperature and electricity generation efficiency depending on the method applied.While the panel surface temperature decreases by over 20 °C with cooling applications, increments of up to 50% in power generation have been observed.According to the cooling method, increments in panel electricity generation efficiency ranging from 12 to 47% have been reported.In our study, it was determined that a maximum temperature difference of 13,7 °C is obtained by cooling the panel surface.Moreover, an efficiency increase of 52,6% was achieved in scenarios involving cooling.These data are similar to those obtained in previous studies.
When analyzing the surface temperature and efficiency graphs for the PV-T module in both scenarios, it was observed that decreases in surface temperature entail increased efficiency.By examining the measurements separately, we noticed a partial decrease in efficiency with increasing surface temperature.This parallel increase in efficiency and temperature could be a result of increased solar radiation.Above a certain temperature, the electricity production of PV modules becomes heat loss, reducing their efficiency.
The main reason for this is that, as the temperature of the cell rises, the voltage produced by it decreases, resulting in thermal resistance.
Figure 12 provides a clearer graph of the relationship between the module's surface temperature and electricity generation efficiency throughout the analysis period.By evaluating all measurements together, a relationship between surface temperature and generation was observed (Figure 12).In all measurements taken without the cooling system, we noticed an efficiency mostly below 6%.Furthermore, the panel surface temperature did not fall below 50 °C.With cooling however, the surface temperature remained below 60 °C.For this scenario, the calculated efficiency values increased by up to 9%.It was determined that a panel surface temperature of 45-55 °C is necessary to achieve the highest PV module efficiency in summer, as losses were reported at higher temperatures.
The climate data (ambient temperature, humidity, and wind speed) on the days of the experiment were also recorded.The climate parameters for all days were approximately the same.Only on June 30 th , 2019, was the air humidity very low in comparison.This day showed the highest values, especially regarding solar radiation (888-1 016 W/m 2 ).

Conclusions
This section summarizes the results obtained during our, which aimed to determine the effect of PV panel surface temperature on electricity production efficiency while using a water-cooled PV-T system.
• Solar radiation was observed to be in the range of 600-1 000 W/m 2 throughout the analyzed period.• The highest power generation value was 77,3 W without cooling and 95,7 W with cooling.• The module surface temperature was between 51,6 and 75,2 °C in the system without cooling and between 33,3 and 61,5 °C in the one with cooling.• The module generation efficiency was 6,10% without cooling and 9,30% with cooling.This represents an increase of 52,62%.• According to the scatterplot showing the relationship between module surface temperature and electricity generation efficiency, the most suitable module surface temperature is 45-55 °C, which yields a high efficiency in PV panels operating during the summer months in the Hatay province.This is also valid for regions with similar climate conditions.• Furthermore, air humidity was shown to have a negative impact on direct solar radiation levels.

Figure 1 .
Figure 1.Influence of temperature on a PV module I-V curve Source: Andreev et al. (1997)

Figure 2 .
Figure 2. a) Voltage-output power and b) maximum output powertemperature relationships in a monocrystalline silicon cell at different surface temperatures Source:Radziemska (2003)

Figure 5 .
Figure 5. Connection diagram of the experimental setup's measurement system Source: Authors

Figure 12 .
Figure 12.PV-T module with and without cooling, overall surface temperature-efficiency graph Source: Authors

Table 2 .
Variations in temperature and efficiency in the system with and without cooling Source: Authors

Table 3 .
Temperature and efficiency values with and without cooling