# Selection of a Photovoltaic Panel Cooling Technique Using Multi-Criteria Decision Analysis
## 1 Introduction  
The recovery of valuable electrical power with minor alterations in the heat dissipation process of photovoltaic (PV) panels by implementing diverse passive or active panel cooling procedures are reported over several years of photovoltaic research. A steep decline in cell net power output or efficiency is a consequence of elevated panel temperature. A linear relationship between efficiency and temperature was established when the temperature goes above the nominal operating cell temperature [1]. The maximum power output of a PV panel is a product of its open circuit voltage, short circuit current and the filling factor. An increase in panel temperature increases the short circuit current and decreases the open circuit voltage as well as the filling factor. The overall effect is a decrease inthe power output of the PV panel. Only a minor fraction of the incident solar radiation is extracted as useful PV power, and the rest is converted to heat, because of which the cell temperature rises. The power output from the panel decreases from 0.25% to 5% for every \({}^{\circ}\)C temperature elevation. Especially for PV systems with concentrators, where high levels of solar radiation fall on the panel, the temperature effect reduces efficiency as well as the life span of the panel in spite of higher power output levels [2; 3; 4; 5]. Thin film solar panels are less affected by a temperature rise compared to silicon-based panels. Their temperature coefficient, which is the percentage reduction in efficiency for a unit \({}^{\circ}\)C increase in temperature, is \(-0.0984\%\) compared to \(-0.109\%\) for mono-crystalline panels and \(-0.124\%\) for polycrystalline panels [6]. Several cooling mechanisms have already been proposed, which are in the laboratory stage and in the field, apart from the development of new cooling techniques [7]. It has been shown that a considerable quantity of power can be gained, up to a total of 5%, by the utilization of cooling systems. Panel cooling techniques with simultaneous heat usage for heating requirements were proposed early on in PV exploitation [8]. Although cooling evidently gives higher electrical output, it requires an extra arrangement that can remove heat from the panel and deliver it elsewhere. Notably, the construction and maintenance of such a cooling technique can be expensive, and there is a possibility that the cost of system maintenance could outweigh the benefits of the improved electrical yield. Cooling techniques adopted different physical approaches using sensible and latent heat storage and dissipation or heat convection techniques using different mediums, and these were tested in order to study the reduction in panel operating temperature. These techniques can be classified as either passive methods, where no external energy is used, or active methods, where extra energy is used to circulate the cooling fluid. Passive techniques include the application of fins or extended surfaces to promote heat transfer, the application of phase change material for absorbing the heat produced in the panel and using heat pipe cooling and convection by natural circulation water or air cooling. Active techniques involve the circulation of air or water on the panel surfaces with and without the help of fins with the hot fluid being used for other applications, as given in Figure 1.  
Figure 1: Classification of different PV panel cooling methods. Research on the passive cooling of PV panels has utilized a variety of principles such as air passive cooling, water passive cooling, conductive cooling, heat pipe or thermosiphon cooling and phase change cooling. Conductive cooling involves heat transfer by conduction through the panel material or fin material and finally disposal by convection to air or water. A relative increase in electric efficiency of 9% has been gained via the usage of passive cooling with a heat sink, and the depth of the air-flow channel beneath the PV cells was found to have a significant influence on passive cooling for larger PV surfaces [9].