Effect of Evaporative Cooling on Photovoltaic Module Performance

The photovoltaic module (PV) consists of many photovoltaic cells made of silicon that lose their properties with an increased temperature. Increasing photovoltaic cell temperature represents an intrinsic problem that causes a drop in the open-circuit voltage of the PV module, thus affecting its performance. The present work investigates using evaporating cooling as a passive cooling technique to absorb the generated heat from the PV module and lower its temperature by cotton wicks immersed in the water (CWIWs) attached to the backside photovoltaic module. The CWIWs decrease air dry temperature and increase humidity and then produce cool air to help cool PV modules. The results showed dropped PV module temperature by about 22% attributed to providing appropriate cooling produced by the moist condition of the cotton bristles immersed by water exposed to the wind. The output power generation and efficiency have increment by 16.3 W and 53%, respectively. The electrical exergy was equivalent to output power under the evaporating cooling, while entropy generation dropped about 14% with increased the PV module temperature.


Introduction
The increase in human activities in the industry, agriculture, and even the scope of domestic activities fields is contemporary problems due to their dependence on fossil fuels as a significant energy source. Fluctuations in oil prices, shortage of energy supplies, and the environmental pollution issue are intrinsic problems that have encouraged many researchers and governments to adopt renewable energy as an alternative to meet energy supplies and reduce pollution (Alktranee and Bencs 2021). Solar energy is a renewable energy branch that provides electrical and thermal energy. Photovoltaic (PV) panels are one of the solar energy devices used to generate electrical energy, which converts sunlight to electricity (Saqaff et al. 2019;Zubeer et al. 2017). The PV module converts about 15-20% of the sunlight to electrical energy, and the rest convert to heat that influences the PV cell performance. Direct exposure of the PV module to high intensity of solar radiation with increased ambient temperatures during operation leads to overheating PV cells and decreasing the PV modules performance (Browne et al. 2016). The PV module efficiency mainly depends on the intensity of the solar radiation, the ambient temperature, and the PV cell material made of silicon. The rising PV cell temperatures lose their properties, which causes the degradation of the PV module work (Abed et al. 2021). In countries with hot weather, especially in the summer season, the climatic conditions represent the major affecting factor the operating of the PV modules temperature that causes an intrinsic problem such as damage or reduction of the PV module productivity (Odeh and Behnia 2009). The efficiency improvement of the PV modules is an important issue that can achieve by dropping their temperature.
Therefore, cooling the PV modules is important for maintaining the working of the PV modules. Various cooling techniques have been used to enhance the PV module conversion efficiency and keep the performance of the PV panels with rising temperatures (Nasrin et al. 2018). The efficiency of the modern PV cells reaches 40-45% achieving through used cooling techniques (Rozsas et al. 2021). The cooling techniques used with PV modules are passive and active cooling techniques; both methods help remove the heat from PV panels (Essalaimeh et al. 2013). Passive cooling has been widely applied recently, especially in the past 2 years, which contributed to improving the PV modules performance. Figure 1 shows an increase in the number of passive cooling applications. Evaporative cooling is an effective cooling technique in hot climates, and its cost is relatively low. Decreasing air dry temperature and increasing humidity produce cooler air because of the transformation of the heat and mass between air and water at constant enthalpy. The water evaporates, causing phase equilibrium with the water vapor in the surrounding air; in other words, it changes the state from a liquid state to a vapor state when the temperature is less than the boiling point. Thereby, water evaporates lead to absorb heat from the surroundings and then produces a cooling effect that can help cool PV modules, representing the principle of evaporating cooling (Farmahini-Farahani and Pasdarshahri 2011;Haidara et al. 2018). Hasan et al. (2019) investigated the effect of evaporative cooling consisting of pin fins with a moist wood wool pad on PV module performance. The pin fins were arranged uniformly to transfer heat from the PV module to air and to increase the exchange of heat of the PV module.
A moist wool wood pad was attached on the backside to ensure the touch with the backside surface of the pin fins, and the water naturally dropped to provide a moist condition for the pad. The results indicated decreased the average PV module temperature of 61 to 45 °C due to the prevailing moist condition at the rear of the PV module, thereby enhancing the output power and efficiency of the PV module by 32.7% and 31.7%, respectively. An experiment conducted by Haidara et al. (2018) used evaporated cooling involved wetting a piece of cloth placed at the backside of the PV module. The piece of cloth is supplied water by rubber pipes attached at the back surface of the PV module. The water flow to the piece of cloth is controlled manually, and the wet cloth is directly exposed to the surrounding air without a fan. The cooling technique achieved a lower in the PV module temperature by more than 20 °C and then an increment of output power by about 14% compared to the PV module without cooling. Another technique consists of a piece of cloth placed inside an inclined duct attached to the backside of the PV module; water flows on a piece of cloth with air blown by a fan inside the duct in the direction of water flow. Passing the air on the wetting cloth surface inside the duct causes water to evaporate, absorbing the heat from the PV module due to cooling the air, thus decreasing the PV module temperature and enhancing the electricity production (Haidar et al. 2016). A study has proposed passive cooling, consisting of the heat sink with fins made of aluminum and copper fins placed at the backside of the PV modules. The study indicates that increased fins with a copper base gave better cooling capacity and reduced the PV module temperature by about 10.2 °C, positively reflecting the efficiency (ArifinZ et al. 2020).
Fins integration with cotton wicks is supplied with fluid as a passive cooling technique used to control the temperature of PV modules during their operation.
The results show a drop in the PV module temperature to 12%, which causes the increased electrical yield to 14%. Creating a continuous cooling environment has helped to decrease the PV module temperature (Chandrasekar and Senthilkumar 2016). Evaporative cooling by synthetic clay was used as a passive cooling with PV modules at rising temperatures due to the absorption of solar radiance. A layer of synthetic clay was placed on the copper sheet, allowing a thin film of water to evaporate. This layer was attached to the backside of the PV module. It was observed lowering the PV temperature, which increased the output power to 19.1% (Alami 2014). Cooling by desiccants is one of the passive cooling techniques used to face the challenge of increasing the PV module temperature. The technique is represented by integrating the desiccants material on the PV module to adsorb water from humidity air when the PV modules are cooling at night. During the day, as the temperature rises, the water starts to passive evaporation with an increase in the PV module temperature that removes the excess heat and cooling the PV module. This technique contributes to lowering the temperature of the PV module to 30 °C due to evaporative cooling (Simpson et al. 2017). Passive cooling, represented by evaporating cooling, is one of the optimization techniques applied that help to sustain the PV module performance in hot climate conditions. The current work investigates the effect of evaporating cooling generated by the use CWIWs on thermal behavior, output power, efficiency, and exergy of the PV module under hot climate conditions to develop a sustainable vision of using such technique and compare it with the PV module without cooling. The CWIWs are attached to the backside of the PV module; water is supplied to cotton wicks from top to bottom by gravity which helps the effective absorption of cotton and reduces water consumption. Exposure of the wetted cotton wicks to the surrounding air decreases dry air temperature and increases humidity, producing evaporating cooling that can exploit it to absorb the rising heat from the backside of the PV module. The technique contributes by creating a continuous cooling environment with less water consumption and better performance under hot conditions than in other studies. Limited studies were conducted on such techniques with different methods and operating conditions than the current work.

Cooling Technique Used
Different cooling techniques have been used to improve the cooling performance by reducing as much heat as possible generated in the PV module, such as phase change material (PCM) and fin heat sink. In the present work, two PV modules have been used; the first PV modules were equipped with CWIWs as a cooling, while the second is used as a reference without cooling for comparison between them during operation. Using the current technique contributes to generating the evaporating cooling by exposure of the wetted cotton wicks to the surrounding air, which decreases dry air temperature and increases humidity, producing evaporating cooling that can exploit it to absorb the rising heat from the backside of the PV module. The experiment used cotton wicks (CWs) which are commonly used for lanterns or lamps that are locally available. The CWs have a high absorbent ability, which helps in the transmission and distribution of water to all CWs arranged at the backside of the PV module and is characterized by a suitable heat transfer coefficient and lower thermal conductivity, as shown in Table 1. The appropriate cost and ease of use made its application feasible compared to other techniques. The internal structure of the cotton wicks consists of four small sub-cotton wicks collected together and covered with a cotton cover. The CWs were arranged as serpentine forms and attached to the backside of the PV module. The arrangements of cotton wicks avoided any space between them and were fixed by thermal silicon, as shown in Fig. 2. At the top edge of the PV module are eight ends of CWs immersed in the bottom of two plastic bottles full of water; this immersion method allows absorbing and transferring of the water from top to bottom of the PV module. The gravity-free flow of water helps transmit the water to the CWs without extra power. Thus, this method helps to distribute the water entirely from cotton wicks throughout the PV module. Thereby, the technique helps to create continuous cooling with less water consumption due to the effective absorption of cotton wicks.

Instruments Used
Temperatures, voltage, and current were measured by a data logger type (AT mega 2560) multi-channel Arduino, consisting of 17 thermocouples distributed at different places on the PV modules, as shown in Fig. 2. Thermocouples used were K-Type, measuring from − 200 to 1350 °C, with a resolution of 0.25 and length of 2 m. The thermocouples were distributed at different places on the surface and backside to investigate the thermal behavior of the PV module. Measuring the temperature of the PV module surface helps to gain knowledge of the temperature distribution and the variation range at 4 points on the PV module surface. Thus, distributed thermocouples at a specific position of the PV module surface to avoid the shadowing and prevent the cable of thermocouples from obstructing the solar radiation from falling on the surface of the PV module in case of place the thermocouples are in the middle. The data logger involved two voltage sensors with a measuring range up to 25 V and a resolution of 0.02445 V, and two current sensor type ACS712 with 0.04 accuracy, and measures ranging up to 30 A. Two lamps, 18 V, were connected between the data logger sensors and the PV modules as a load to measure the output voltage and current through the experiment period. The data logger is programmed to record the temperature, voltage, and current values and save the data in 8 GB memory for each 10 min during the experiment days. Another device was used for measuring solar radiation type (Model SM206) with a 0.1 W/m 2 resolution, and the measurement was done manually for 30 min during the experiment period.

Photovoltaic Module Used and Experiment Conditions
Two PV module types, polycrystalline 50 W, were used with dimensions of 0.65 m × 0.55 m on a flat roof. The height of PV modules from the ground and the tilt angle allow great air to flow on the backside of PV modules. The first PV modules were equipped with CWIWs, while the second was used as a reference without cooling for comparison between them during operation. The output power of the PV modules (P PV ) has been calculated depending on the average values of current (I PV ) and voltage (V PV ) from Eq. (1). Thus, to estimate the module efficiency, the PV module area (A) and the average values of the solar radiation (S) are considered, as seen in Eq.
(2) (Chandrasekar and Senthilkumar 2016). The experiments were conducted according to climatic conditions of Basra City, Iraq, from 20 to 29 of August 2021. August is characterized by a sunny month with high temperatures and low humidity on most days with moderate winds. There was no significant variation in the temperature recorded throughout the experiments, and the weather was sunny during the experiment days. The solar radiation values were obtained throughout the experiment days. Figure 3 shows the average solar radiation recorded. At the beginning of the early morning, the solar radiation values increase until a higher value between 12:10 and 13 PM; after that, it

Exergy Analysis
Exergy is the maximum work extracting from a quantity of available energy from any system (Sudhakar and Srivastava 2014). Therefore, exergy analysis is important to assess the real performance of PV modules (Yazdanpanahi et al. 2015). Exergy analysis deals with the conservation of mass, energy principles and the second law of thermodynamics, thereby effectively helping enhance energy resources and determine the systems' efficiencies and the losses (Dincer and Rosen 2007). Evaluation of the exergy requires knowledge of the amount of a control volume and its input and output in the PV module, as shown in Fig. 4; in this experiment, the exergy received from the sun represents the input exergy that can calculate from Eq. (4) (Jeter 1981).
(1) P PV = V PV × I PV where T a is the ambient temperature and T sun is the equivalent surface temperature of the sun, which is considered 5770 K (Maadi et al. 2017). Thus, the output exergy calculates from Eq. (5): In the current work, it is impossible to conserve the thermal energy generated because CWIWs absorb it and then dissipate it into the air atmosphere, but it can calculate. Thus, electrical exergy is only the practical output exergy produced. According to Chow et al. (2009) the, electrical energy and electrical exergy are equivalent and defined in Eq. (6): The thermal exergy is calculated from Eq. (7) (Bayrak et al. 2017;Dufe and Beckman 2013), and Q is the heat emitted to the surrounding, which calculates from Eq. (8): where (h = 5.7 + 3.8u) represents the convective heat transfer coefficient in (W/m 2 K), because of the flowing air around the surface of the PV module, and u is the wind speed m/s 2 (Safarian et al. 2020). Thus, the exergy efficiency of the PV module calculates by An increase in the PV cells temperature affects heat transfer between the PV cells and the ambient in the entropy generation. Thereby, evaluating the entropy generation and exergy loss in the PV module is important. Calculating the lost exergy of the PV module is useful to compare the effect of the evaporating cooling with the reference PV module; the lost exergy in the system can calculate from Eq. (9). Calculating the exergy loss helps to assess the entropy generation to evaluate the irreversibility of the PV module with and without evaporating cooling; the entropy generation determines by the relations below ).

Data Reduction
Uncertainty analysis is performed for the reliability of the experimental measurements and to confirm the precision of recorded data during the experimental measurements and then estimating the uncertainties when measuring the PV module's output power and efficiency. Thus, the voltage, current, and solar radiation are the measurement errors considered during the experimental measurements. The uncertainty analysis was conducted using Coleman and Steele method (Coleman and Steele 2009). Estimating uncertainties was related to output power data and PV modules' efficiency. The output power ( P PV ) is calculated from Eq. (1) depending on the PV module voltage ( V PV ) and current ( I PV ). The efficiency of the PV module (ɳ) is calculated from Eq. (2) depending on the incident solar radiation data (S) and PV module area (A). Therefore, both Eqs. (12) and (13) were applied to measure the quantities like voltage, current, area of the PV module, and solar radiation. The uncertainties of measurements indicated ± 0.051% for output power and ± 0.29% for efficiency.

The Thermal Behavior of the PV Modules
Investigating the PV module's thermal behaviour is necessary to understand the effect of temperature on the PV module performance. Thus, the surface and backside of the PV module's temperature were measured by four (T 1 , T 2 , T 3 , and T 4 ) thermocouples distributed in different places on the surface. The other four (T 5 , T 6 , T 7 , and T 8 ) thermocouples are on the backside of the PV modules. The cotton wicks were immersed in the water, connecting the loads between PV modules and the data logger before the experiment started 2 h. Thus, after less than 2 h passed, the water covered all the cotton wicks attached to the backside of the PV module. Depending on the average temperature recorded for each 10 min for the PV modules and throughout the experiment periods, it could notice that the variation in the temperature on the surface and backside of the PV module recorded by thermocouples was very slight, as shown in Fig. 5. Therefore, it adopted the average temperature for all experiment days and the average ambient temperature to evaluate the PV modules performance under similar operating conditions. The temperature of the PV module surface was higher than the backside due to being directly exposed to intense solar radiation. Thereby, the increase obtained by the ambient temperature is related to the intensity of solar radiation. The PV module with CWIWs cooling showed thermal behavior closer to the ambient temperature throughout the experiment period due to evaporating cooling of CWs. The airflow on the wet surface of the cotton wicks has a major role enhance the evaporation process due to a convection mass transfer and decreasing the PV module temperature, which enhances the rate of evaporation cooling. The highest temperature recorded of the PV module with the CWIWs was 46.2 °C at 12:20 PM, while the ambient temperature was 43.8 °C. In contrast, it increased the average PV module temperature without cooling to 56.4 °C simultaneously and during the experiment, as shown in Fig. 6. Rising the temperature at this level causes degradation of the PV cells performance and drops the energy yield discussed in the next section. The wet CWIWs exposed to wind have provided adequate cooling that effectively reduces the PV module temperature to 10.2 °C, positively reflecting the PV module performance. The flowing of the air over the wetted surface of the cotton wicks causes water evaporates and cools the air that absorbs the heat from the PV module by forced convection and transfers it to air, which the productivity of the PV module enhances.

Performance of the PV Modules with and without CWIWs
The performance of the PV modules depends on the intensity of solar radiation and ambient temperature, where increasing the PV cell temperature influence the conversion efficiency of the PV modules. The PV module without cooling causes an increase in the temperature, which influences the output voltage and current and thus causes to a dropped output power of the PV module. The maximum output voltage and current obtained from the PV module without cooling were 8.9 V and 1.6 A, respectively, at 12:20 PM. Figure 7 shows the maximum output power of the PV module without cooling was 14.2 W, which refers to the influence of an increase in the temperature on the PV module performance, and that was clear by dropping the efficiency to 3.4%, which confirmed the deterioration performance due to rising temperature. On another side, the effective absorption and transferring of the water from top to bottom of the PV module by natural flow helped achieve a good cooling rate throughout the experiment.
The airflow on the wet surface of the cotton wicks has a major role enhance the evaporation process due to a convection mass transfer and decreasing the PV module temperature. Thus, the tilt angle of the set PV module has allowed easy wind-run-off in the backside of the PV modules. Used CWIWs improved the output power of the PV module to 30.5 W due to an increase in the output voltage values and current to 16.3 V and 1.96 A, respectively. Because the cotton bristles immersed in water and exposed to the wind have provided continuous cooling improved, the power yield with an increment of about 16.3 W compared without cooling. Figure 8 shows the maximum efficiency of the PV module with CWIWs being 7.25% compared to the PV module without cooling. The decrease in the PV module temperature and improved performance is attributed to cotton wicks immersed in water from the top to the bottom. The cotton wick immerses helped the distribution of water entirely on the backside of the PV module, which gave continuous cooling throughout the experiment. Compared to other studies used different evaporation cooling techniques conducted by (Chandrasekar and Senthilkumar 2016;Haidar et al. 2016;Haidar et al. 2021), the mechanism of evaporative cooling used has enhanced the performance of the PV module better, which makes it more reliable for application. Table 2 shows the PV module performance compared with previous studies that used similar cooling methods to

Electrical Exergy and Entropy Generation
Exergy analysis is an important issue in estimating the real performance of the PV module. The evaporating cooling helps to absorb the generated thermal exergy and dissipates into the surrounding air, reducing the PV module temperature. The maximum average PV module temperature without cooling was recorded at 56 °C at 12:20 PM. The evaporating cooling helped absorb the generated thermal exergy and dissipate it into the surrounding air, reducing the PV module temperature. As mentioned above, the output electrical power of the PV module is equal to the exergy electrical. Figure 9 shows the relation of the electrical exergy with temperature; it can observe the increment gradual of the electrical exergy and exergy efficiency with increased temperature until 56 °C. Maximum electrical exergy and exergy efficiency were 29.8 W and 7.2%; it was a slight variation from electrical energy and efficiency as shown in Table 3, which shows the effect of evaporative cooling on the performance of the PV module. Using CWIWs with a PV module reduces the exergy losses due to absorbing the heat generated by PV cells by the CWIWs and dissipating it to the surroundings. Based on the second law of thermodynamics, lowering the exergy losses affect the entropy generation in the PV module because of the irreversibilities. Figure 10 shows the average entropy generation; using CWIWs leads to a decrease in entropy generation due to reducing the exergy loss exergy by about 14% lower  . 9 The effect of temperature on electric exergy and exergy efficiency

The Comparison of Performance with and without Passive Cooling
The PV module that used CWIWs distinguished with high performance along the experiment period by achieving higher power yield and efficiency than the PV module without cooling. Water absorbed by cotton wicks with rising temperature led to created evaporation cooling helps absorb heat from the backside of the PV module. Figure 11 shows the comparison of the PV module performance using CWIWs and without; the PV module temperatures used CWIWs have a noticeable effect that contributed by decreasing about 22% of the PV temperature compared with PV module without cooling. Reduction of the temperature effectively contributes to enhanced productivity of the PV module by incrementing the power yield of the PV module to 16.3 W. Due to the increase in output voltage and current of the PV module, the efficiency enhanced to 7.25% compared to cooling that recorded 3.4% with 14.2 W output power. According to evaporating cooling, the generated heat at the backside of the PV module is absorbed and dissipated into the air surrounding. The electrical energy (output power) under evaporating cooling was the same as the electrical exergy at high temperatures.
Enhancing the PV module performance is attributed to using CWIWs to provide the appropriate cooling, decrease the temperature, and increase the electrical exergy and exergy efficiency. A significant deterioration in the PV module performance has been observed in the PV module without cooling, especially under hot climate operation, thus negatively impacting the power yield and efficiency.

Conclusions
The temperature rising of a PV module has a negative impact on its performance. Among similar optimization techniques used in other studies, the passive cooling applied in the current study showed sustained performance of the PV modules in hot conditions. The evaporating cooling employed by CWIWs contributed to the dissipation of heat generated at the PV module and enhanced their performance. The energy yield, efficiency, and exergy analyses were investigated based on the results of the experiment as follows: 1-The CWIWs technique has good potential cooling to control the heat of the PV module, lower cost, and an easy configuration with less water consumption. 2-The CWIWs attached to the backside of the PV module are effective passive cooling techniques that contributed to reducing the PV module temperature by 22% compared with the PV module without cooling. 3-The moist condition resulting from the cotton bristles immersed in water and exposed to the wind has provided appropriate cooling that enhances efficiency to 7.25% and the power yield increment about 16.3 W. 4-Using CWIWs decreases entropy generation by about 14% due to reducing the lost exergy of the PV module than the PV module without cooling. 5-The electrical energy (output power) was the same as the electrical exergy, and the exergy efficiency as well of the PV module with evaporating cooling was higher than the PV module without cooling. 6-The passive cooling applied in this work enhanced the performance of the PV module higher than in other similar studies, making it more reliable for application. 7-Significant power yield deterioration and the PV modules' efficiency were observed due to the increase in the PV cells temperature to 56.4 °C.
Funding Open access funding provided by University of Miskolc.
Data Availability All data generated or analyzed in this study are available from the authors on request.

Conflict of Interest
The authors declare no competing interests.

Fig. 11 Comparison of PV module performance
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