**1. Introduction and Background Review**

In pursuit of green technology innovations, the energy industry demonstrates a focus on long-term sustainability renewable energy generation. The goal is to generate and transfer power to major domestic and industrial customers by 2030. It is concerned with a number of factors, for example, energy generation, public awareness, demand, and the risk at which local and national transmission occurs. The manufacturers of solar panel modules are more interested in improving the efficiency of solar panels; for this, they need meticulous alternative selection and assessment. There are challenges and opportunities in solar modules [1,2] influenced by ambient temperature, solar radiation intensity, the solar panels' surface temperature, dust, and shading, among other factors that may be overcome by adopting a suitable cooling and cleaning system. Solar panel cooling approaches [3,4] use several physical ways employing various flow media to minimize the solar panels' surface temperature. These approaches are categorized as either passive (no external energy is needed) or active (additional energy is utilized to circulate the cooling fluid). Passive approaches [5,6] include the use of fins or expanded surfaces to facilitate heat transfer, the use of phase change material to absorb heat produced in the panel, the use of heat pipe cooling, and convection via natural circulation water or air cooling. Active approaches [7,8] involve the circulation of air or water over the panel surfaces, both with and without the assistance of fins. There are several factors to consider when comparing active and passive solar panel cooling systems. However, the comparative ease of operation depends on the specific cooling system being used, as well as factors such as the size and location of the system; the required level of maintenance; the effective

**Citation:** Rehman, A.U. Solar Panel Cooling System Evaluation: Visual PROMETHEE Multi-Criteria Decision-Making Approach. *Sustainability* **2023**, *15*, 12947. https://doi.org/10.3390/ su151712947

Academic Editor: Oz Sahin

Received: 31 July 2023 Revised: 22 August 2023 Accepted: 24 August 2023 Published: 28 August 2023

**Copyright:** © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

performance, particularly in hot and humid environments; and the cost when evaluating different solar panel cooling systems. For example, studies [7,8] have shown that water spraying can reduce the temperature of solar panels by up to 23 ◦C and significantly increase their electrical efficiency. Temperature impact affects efficiency and panel life span despite greater energy generation [9,10]. Likewise, researchers [5,11,12] have documented the recovery of useful electrical power with considerable changes in the heat dissipation process of solar panels by using various passive or active panel cooling processes. Although cooling clearly increases renewable energy production [5], it necessitates an additional structure that can extract heat from the panel and distribute it elsewhere. Notably, the design and maintenance [13] of a cooling strategy can be expensive, and the cost of system maintenance may outweigh the benefits of increased power generation. When compared to silicon-based panels [14], thin-film solar panels [15] are less influenced by a rise in temperature. Also, the effectiveness of solar panel cooling systems may vary depending on various factors such as the climate, panel design, and the type of cooling approach used. Recently, studies [16] have shown that the use of nano-fluids improves the heat transfer coefficient, solar panel power, and system performance. Past studies [17,18] have also proposed the use of phase change material cooling and microchannel heat sink cooling. However, there is no conclusive evidence to suggest which solar panel cooling strategy is the most effective, as it depends on various factors [4]. It is obvious that selecting the best solar panel cooling system necessitates the use of mathematical tools to analyze the alternatives. As a result, multi-criteria decision analysis is the ideal tool and can be used well in certain scenarios. The PROMETHEE (Preference Ranking Organization METHod for Enrichment Evaluations) decision-making method compares options based on a set of criteria [19]. There are several studies that have applied the PROMETHEE in various ways, such as in the selection of the most appropriate variant of the solar water supply system [20], ranking sites for solar farms [21,22], evaluating the effectiveness of integrated shading devices for office buildings [23], and for decision making in solar plant locations [24], as it allows for the consideration of multiple criteria, and can help decision makers to assess energy technologies [25]. Similarly, it is evident that researchers justified the importance of reliability information to evaluate solar panel selection [26]. On the other hand, strategies are developed to select an efficient solar panel, and a comprehensive comparative analysis is presented [27].

Multiple qualitative and quantitative metrics were used to evaluate, assess, and rate the various solar cooling systems. For example, Mardani et al. [28] reviewed multi-criteria approaches in sustainable and renewable energy system problems. Wang et al. [29] used multi-criteria decision analysis as a key tool to evaluate renewable energy technologies in households. Seker and Kahraman [30] proposed a socio-economic evaluation model for sustainable solar panels by integrating the analytic hierarchy process and multiplicative multi-objective ratio analysis method. Similarly, Krysiak and Kluczek [31] assessed the sustainable development of photovoltaic modules using a multi-criteria decision-making method based on the analytic hierarchy process [32]. Similarly, a multi-attribute decisionmaking approach based on intuitionistic fuzzy logic is adopted to select and assess solar panels [33].

The motivation to evaluate solar panel cooling systems is to improve solar panel efficiency and output. When solar panels get too hot, their efficiency drops, resulting in less energy output. The temperature of the solar panels may be adjusted by integrating cooling devices, resulting in enhanced efficiency and output. Existing research has mostly focused on the installation of cooling systems in residential buildings and their influence on energy efficiency. However, research evaluating the effectiveness of various cooling technologies and their effects on the long-term durability of solar panels is lacking. Furthermore, research has been performed in certain geographic locations, and further studies that outrank cooling systems are needed.

The goal of this research is to present a multi-criteria decision-making approach that is both quantitative and qualitative in nature for selecting solar panel cooling systems, to assist decision makers in determining the best viable choice, and to visualize the relationship between the different solar panel cooling systems and performance measures under consideration. The proposed approach provides substantial support in comparing and ranking solar panel cooling systems, as well as their validation and evaluation through sensitivity analysis. Six different scenarios, depending on the subjective and objective importance given to each performance measure, are possible choices.

This paper is divided into eight sections. The Section 1 provides an introduction; the Section 2 presents various attributes, performance measures, and alternative solar panel cooling systems; and the Section 3 presents alternative solar panel cooling systems. Section 4 presents the adopted MCDM approach steps. Sections 5 and 6 then present the implementation of the proposed approach and sensitivity analysis using subjective and objective criterion weights, respectively. Section 7 discusses management benefits, while Section 8 concludes the paper with conclusions and future study directions.

### **2. Attributes and Performance Measures**

There are several benefits of cooling solar panels, which include improving panel overall efficiency, reducing energy consumption, and extending the panel lifespan. By lowering the temperature of the panels, either through passive cooling techniques or more advanced cooling systems [4], the electrical conversion efficiency of the panels can be increased, resulting in higher energy production [34]. Additionally, reducing the temperature of the panels can also help to reduce the wear and tear on the system, leading to a longer lifespan for the panels and a higher return on investment [35]. Furthermore, by reducing energy consumption through more efficient cooling mechanisms, the environmental footprint of the solar system can be minimized, making it a more sustainable option for generating renewable energy [36]. It is decided to make use of a systematic multi-criteria analysis approach to identify the competitiveness of each alternative. The multiple attributes opted to evaluate alternative solar panel cooling systems are as follows: cooling effectiveness, energy efficiency, environmental impact, durability, noise, panel size and weight, and or cost as decision-making attributes (refer to Figure 1). Decision makers opt for multiple combinations of attributes to be evaluated in order to select the most suitable panel cooling system to improve overall efficiency.

As represented in Figure 1, the adopted performance measures to evaluate cooling techniques are briefly described below.

Energy Efficiency (PM01): Energy efficiency is crucial for solar panel cooling since it lowers the amount of energy used for cooling and raises the overall effectiveness of the solar panel system. The surface temperature of the solar panel can be managed by utilizing more energy-efficient cooling techniques, which helps to maximize the electrical conversion efficiency of the solar panel. This can result in considerable drops in energy usage while raising the solar system's output of energy [37]. In order to increase the effectiveness of solar panels, a number of researchers have used cutting-edge cooling techniques. The electrical efficiency is a function of the cell temperature [23], and it is mathematically estimated by researchers.

Cooling cost (PM02): The cost of cooling solar panels plays an important role in determining the overall cost-effectiveness and return on investment of the solar system [38]. While advanced cooling techniques may improve the electrical conversion efficiency and extend the lifespan of the solar panels, additional supplies and installation charges may also be required, raising the overall cost of the system. Therefore, it is important to consider the costs of cooling technologies and weigh them against the potential benefits in terms of energy production and system longevity [39].

Reliability factor (PM03): Reliability refers to the ability of components of the cooling system to operate without failure or malfunction over time [40]. If the cooling system fails, the temperature of the solar panels can rise, which can have detrimental effects on the energy output and lifespan of the system [41]. Thus, it is crucial for the cooling system to be reliable. Additionally, a reliable cooling system can help to minimize maintenance and

repair costs, as well as reduce downtime, ensuring that the solar system operates over its expected lifetime with minimal interruptions [2,30].

**Figure 1.** Attributes, their performance measures, and alternatives opted for in evaluation of solar panel cooling systems.

Carbon emission (PM04): The carbon footprint of the downstream processing and manufacturing of solar cooling systems is highlighted by researchers [17,42,43], indicating that any cooling system's environmental impact is an important consideration. Additionally, government policies promote solar energy modules due to their low-carbon emission profile. Therefore, employing more energy-efficient cooling systems in solar modules is an important task. Carbon emissions of the entire system can be reduced, making it a more sustainable option for generating renewable energy [44].

Ergonomic factor (PM05): In the development of solar cooling systems, the primary focus is on technology feasibility, sustainability, and energy efficiency. However, ergonomics could indirectly play a role in selecting solar cooling techniques by ensuring the safety and comfort of individuals involved in the installation, operation, and maintenance of the solar system [45]. By designing these cooling systems to be ergonomically efficient, risks associated with injury, fatigue, and discomfort could be reduced, ultimately improving the overall safety and performance of the system.

Panel temperature dropping (PM06): The operating temperature of a solar panel has a significant impact on its energy output and lifespan, and excessive temperatures can negatively affect overall efficiency and performance [35,36]. Therefore, reducing the operating temperature of the solar module is one of the primary approaches to increasing power generation [46]. Various cooling techniques are being explored to decrease the operating temperature of solar panels and increase their efficiency. The drop in the operating

temperature achieved through these cooling approaches can lead to a significant increase in power output, ranging from 20% to 30%, depending on the cooling approach adopted.

Panel size and shape (PM07): The solar cooling system's effectiveness is often measured in terms of the decrease in operating temperature of the solar panels, and hence, the area of the panel indirectly impacts the cooling requirements [35]. Larger solar modules have more surface area that is exposed to the sun and absorb more energy, resulting in higher operating temperatures. Therefore, larger solar modules may potentially require more powerful cooling systems to maintain their operating temperatures within the desired range. However, the efficiency of solar panels also varies with their size, efficiency, and technology, which may be important factors in selecting a cooling technique [47,48].

Degradation resistance (PM08): It is evident that [49] there are multiple reasons why degradation, i.e., corrosion, coating formation, and scaling, can occur in solar cooling systems. For example, exposure to different environmental factors, including humidity, saltwater, and other pollutants, can lead to corrosion and scaling. Similarly, chemical reactions between the cooling medium and the solar panel materials can also cause corrosion and coating formation. For example, copper in the system can form copper salts that can affect the solar panel's performance. The materials with lower resistance to corrosion are more likely to corrode and form coatings. Incorrect system design or installation can also contribute to corrosion and coating formation. For example, using dissimilar metals in a cooling system can lead to galvanic corrosion.

Thermal decomposition (PM09): One of the potential challenges [16,50–52] that could arise in a solar cooling system is thermal decomposition, which means physical degradation of heat transfer fluids. Heat transfer fluids are used [53] in some cooling systems to improve heat transfer and reduce the operating temperature of solar panels. However, the fluids can suffer physical and thermal decomposition and degradation over time, which reduces their effectiveness. Some of the factors that can cause physical degradation in heat transfer fluids include the fluid's properties, such as viscosity and thermal stability, the operating temperature range, and the type of solar panels used. Additionally, exposure to ultraviolet radiation, oxygen, and other environmental factors can also contribute to fluid degradation. If the fluid is not replaced or maintained regularly, its ability to transfer heat effectively can decrease, which can negatively affect the solar panel's performance [39–41]. Therefore, it is crucial to ensure that the heat transfer fluid used in a solar cooling system is selected carefully and maintenance and replacement schedules are adhered to in order to maintain the system's optimal performance.

Leakage issues (PM10): Leakage issues are a significant challenge in solar cooling systems that use heat transfer fluids, as these fluids can leak through damaged piping or seals. The loss of fluid from the system not only reduces its effectiveness in cooling solar panels but can also potentially damage other components in the system. Moreover, the leakage of fluids can create safety concerns, as they may pose a risk to the environment or human health. Therefore, it is crucial to consider potential leakage issues when designing, installing, and maintaining solar cooling systems and take appropriate measures to prevent or mitigate them [1,3,8].

Impact of any equipment failure (PM11): Electric equipment failure/or any power supply failure can impact the overall performance and reliability of the system, which can indirectly affect the cooling system's ability to maintain the desired operating temperatures of the solar panels [54]. If the electric/non-electric equipment fails, it may lead to a complete shutdown of the system, which can have a domino effect on other components, including the cooling system. Additionally, if the cooling system is not designed or installed correctly [55,56], it may be more susceptible to failures caused by electric equipment failures, such as a power surge or overload, which may impact its ability to cool the solar panels effectively. Thus, the impact of any equipment failure on a solar cooling system depends on various factors, including the type and severity of the failure, the system's design and installation, and the overall maintenance and operation of the system [3,44].

Thus, various alternatives for solar panel cooling in solar power plant installations have been represented in Figure 1 and briefly described in the following section.

#### **3. Solar Panel Cooling Systems: Alternatives**

Solar panel cooling systems use several physical methods, such as sensible and latent heat storage and dissipation or heat convection techniques employing different media, and they are evaluated to examine the reduction in panel operating temperature. These systems are classified as having either a passive approach or an active approach (refer to Figure 1). Active cooling systems [3,4] often perform better in terms of temperature reduction than passive cooling systems. Passive cooling [3,4] does not require much energy to operate and instead relies on natural cooling, which takes longer to cool over time.

Finned air cooling (A1) employs a natural heat transfer process in which heat from the solar panel is transmitted to the fins and then travels through convection by natural wind movement [57]. Heat pipe solar cooling systems (A2) use heat pipes to dissipate heat from solar panels, reducing the temperature and increasing their efficiency. The heat pipes work by transferring excess heat from the panels to the cooler end of the system, where it is dissipated into the air or water [58]. This cooling system has been found to be an attractive option in hot climates [59]. Researchers [60] have conducted studies on the effectiveness of heat pipe solar cooling systems.

Phase change materials (A3) have been used to cool solar panels and increase their efficiency, particularly in hot climates. It works by absorbing the excess heat from the panels and storing it until the temperature drops, providing a buffer against temperature fluctuations [17]. When the panels start to heat up, the phase change material melts and absorbs the heat. As the temperature decreases, the phase change materials then solidify and release the stored heat. Thermosiphon cooling systems (A4) involve a closed-loop system containing a working fluid, such as acetone, that undergoes a phase change to cool a solar panel [61]. On the other hand, a thermosiphon with a clay pot cooling system (A5) uses the clay pot as the medium for evaporative cooling to cool a solar panel [61]. Both systems use the same basic principle of thermosiphon cooling, but they differ in the method of heat dissipation. The clay pot version of the system relies on the process of water evaporation using a simple clay pot placed on top of the solar panel, while the standard thermosiphon cooling system makes use of a working fluid to remove heat by undergoing a phase change in a closed-loop system. Both of these systems are effective in cooling solar panels and improving their performance, but the choice between them depends on factors such as the specific application, location, and cost considerations.

A forced air cooling system (A6) for solar panels is a type of cooling system that uses a fan to circulate air over the solar panels to reduce their temperature. The cool air can be produced in a number of ways, including compressor-cooled refrigerant or chilled water. This type of cooling system is often used in sunny areas where the heat from the sun can cause the solar panels to overheat, which can reduce their efficiency and lifespan [62–64]. By cooling the panels with forced air, their temperature can be regulated, which can help to maximize their energy output and improve their overall performance. This type of cooling system is efficient and cost-effective and is often used in residential and commercial solar power systems.

Evaporative cooling systems (A7) [65] and water spray cooling systems (A8) [66] both use water to cool an area, but the methods and efficiencies are different. Evaporative cooling systems rely on the process of evaporation to cool the air, whereas water spray cooling systems function by directing a spray of water into the air to lower air temperature. Evaporative cooling systems are more efficient in low-humidity areas, while water spray cooling systems are more efficient in high-humidity areas. Additionally, evaporative cooling systems are more energy-efficient than water spray systems, while water spray cooling systems are often used in outdoor settings and for cooling large areas. Both systems have unique advantages and limitations, and the choice between the two depends on the specific application. Nano-fluid cooling systems (A9) can be either active or passive. Some

of the researchers studied [16,52,62] active cooling systems that use nano-fluids as a coolant in conjunction with traditional cooling methods such as air or water cooling, while other researchers [67] studied passive cooling systems that rely solely on a nano-fluid coolant to dissipate heat.

However, there is no conclusive evidence to suggest which solar panel cooling strategy is the most effective, as it depends on various factors. Different cooling techniques have been used successfully in a number of cases, making choosing difficult [4]. Thus, it is evident [40,44] that choosing the appropriate solar panel cooling system involves the use of a scientific instrument to evaluate the options. So, multi-criteria decision making (MCDM) is the best instrument and has been utilized well in these circumstances. The step-by-step details of the adopted methodologies are presented in the following section, followed by their application.
