# Selection of a Photovoltaic Panel Cooling Technique Using Multi-Criteria Decision Analysis
## 1 Introduction
It has been shown that, for a length-to-depth ratio of 0.085, the PV module cools by 5-6 \({}^{\circ}\)C when compared with a PV module without any cooling arrangement [10]. Numerical studies on heat-sink-based PV panel cooling with different variations in configuration and geometry achieved a temperature reduction of about 10 \({}^{\circ}\)C and the corresponding increase in power output [11]. Since the temperature difference of a panel with the environment rises with the increase in solar radiation intensity, passive flow channels can have the reverse effect on PV module cooling. Hence, optimization of the cooling mechanism is essential. In addition, a fairly stable PV-T panel temperature was obtained with water-cooled heat pipes when the peak insolation rate was 800 W/m\({}^{2}\). The surface of the panel was around 1.0 m\({}^{2}\), while the total amount of circulating water was around 200 L. Heat pipe cooling uses phase change cooling together with convection of the cooling medium, which is usually a liquid that evaporates at the temperature at which the panel is to be maintained. Inside the heat pipe, the evaporating liquid evaporates and expands taking up the heat produced by the panel. The vapor then condenses and releases heat into the surroundings. In the case of heat pipes, the return of the liquid to the evaporator occurs through capillary tubes or wicks. This is adopted when the condenser region is below the evaporator region. In some cases, the condenser region is above the evaporator region and the liquid returns by gravity, in which case, they are called thermosiphon heat pipes. A relation between the number of heat pipes and panel electrical efficiency has been reported [12]. A heat pipe was used to cool down a 0.0625 m\({}^{2}\) panel and obtain an increase in efficiency of 2.6% and a decrease in temperature of 4.7 \({}^{\circ}\)C with a maximum increase in power yield of 8.4 [13]. Tests were conducted on different types of phase change material for cooling panels, and one such study concluded that up to a 10.26 \({}^{\circ}\)C reduction in temperature can be obtained resulting in an efficiency improvement of 3.73% [14]. Paraffin graphite in the form of panels was used with a thickness of 15 mm covered with an aluminum sheet at the back and was tested for cooling the PV panel. It was found that panel efficiency improved when the temperature was more than the phase change temperature, but the result was counterproductive due to the heat dissipation being stopped at a lower panel temperature [15]. Thermosiphon heat pipe cooling methods were adopted and reported by several researchers. Thermosiphon heat pipes were used to cool PV panels and a temperature reduction from 20 to 34% was reported [16]. Thermosiphon systems were tested by using a copper sheet and tubing with a water capacity of 80 L and reported an increase in the relative efficiency of 19% [17]. Thermosiphon heat pipes have the additional advantage of easy manufacturability because of the absence of the wick structure. The working liquid is evaporated when heat from the photovoltaic panel is conducted into the metallic pipe from the surface of the panel. The vapor moves up to the condensation region where it is exposed to cooler temperatures from the external environment. The condensed liquid then returns back to the evaporator region by gravity. Heat removal from the PV module using forced convection of the cooling medium or extracting heat from the cooling medium is generally referred to as active cooling. Usually, water or air is used as the cooling medium. Power consumption is incurred mainly to drive the fans or pumps, which are designed to overcome the friction within flow conduits arranged across the panel. Active-cooled PV panels deliver more power due to the output enhancement achieved by lower temperatures. The heat removed can be used elsewhere. The percentage of energy available for in-house consumption is important.