The efficiency of solar systems, in particular photovoltaic panels, is generally low. The output of the P.V. module is adversely affected by their surface rise in temperature. This increase is associated with the absorbed sunlight that is converted into heat, resulting in reduced power output, energy efficiency, performance and life of the panel. The use of cooling techniques can offer a potential solution to avoid excessive heating of P.V. panels and to red. The efficiency of solar systems, in particular photovoltaic panels, is generally low. The output of the P.V. module is adversely affected by their surface rise in temperature. This increase is associated with the absorbed sunlight that is converted into heat, resulting in reduced power output, energy efficiency, performance and life of the panel. The use of cooling techniques can offer a potential solution to avoid excessive heating of P.V. panels and to reduce cell temperature. This paper presents details of various feasible cooling methods, including novel and advanced solutions for P.V. panels and indicates future trends of research. Different features and capability about each cooling techniques are presented, to provide better insight and valuable guidelines for researchers who intend to study, improve or optimise any type of cooling techniques of P·V. modules.••Module temperatureP.V. moduleCooling methodsPCMIn this industrial world, people live in an energy-intensive and consumer-led environment. This has contributed to the rapid downfall of fossil fuels, which is the primary basis of electricity production. It is therefore highly necessary to find sustainable sources in order to reduce our reliance on fossil fuels. Solar is the commonly used non-conventional energy available worldwide. Sun radiation is the source of all types of renewable energy. It can be converted directly or indirectly into electrical energy either by means of photovoltaic (P·V.) or thermal collectors respectively. The solar thermal system efficiencies range between 40 and 60% while P.V. has efficiencies between 10 and 20% [1,2]. Solar cells use an only visible range of wavelengths from 380 to 700 nm(nanometres) to generate electricity. Longer wavelengths of more than 700 nm do not have sufficient energy to build electron-hole pairs [3,4]. Shorter wavelengths of radiation such as X-rays do have high photon energies, but the high-energy photons could potentially damage the photovoltaic cell through ionisation processes. Outside of the visible wavelengths, the undesired radiant energies from the Sun are subsequently converted to heat, causing the solar cell temperature to increase.The electrical power from the solar cells is increased by reducing the operating temperature [,, ]. In addition, if the lifetime of P.V. is also extended, th. 2.1. Effect of solar irradianceThe short circuit (ISC) current is affected by the amount of photons absorbed by the semiconductor material and is thus related to the light intensity. The conversion efficiency is therefore fairly constant in such a way that the power output is usually associated with the irradiance, but the efficiency is reduced if the cell temperature rises (Fig. 1). The open-circuit voltage (VOC) varies only marginally with the light intensity.2.2. Effect of ambient temperatureThe VOC decreases so much with the rise in temperature of the panel above 25 °C but short-circuits current, Isc, increases only marginally (Fig. 2). The temperature effect on P.V. performance is identified as the temperature coefficient. The net result is a reduction in power output with temperature rise. The percentage of temperature coefficient indicates a shift in output as it rises or falls against the normal conditions of 25° Celsius. For illustration, if the temperature coefficient for a specific panel is −0.5%, the maximum power for every 10 °C increase will be reduced by 0.5%.Fig. 2. Characteristics of a solar P·V.: Effect of temperature.The nominal operating cell temperature (NOCT) an. 3.1. Need for coolingThe change in surface temperature is influenced by external climate variables such as sunlight, wind velocity, moisture, atmospheric temperature and concentrated dust. Improvement of efficiency can be accomplished by reducing the operating temperature as it is more problematic to modify other parameters involved. Of example, in the construction of photovoltaic panels on the building facades, which are vertical and non-directional surfaces, solar radiation is an uncontrollable parameter. To make photovoltaics more efficient, by avoiding the issue of temperature rise, a variety of cooling techniques have been carried out and have been reviewed in a variety of various literature.3.2. Classification of cooling techniquesScientists are working on cooling systems for reducing solar cell operating temperatures, which are known as active and passive cooling systems. The appropriate cooling of the P.V. array tends to reduce the loss of output and increases the reliability of the P.V. module. Passive cooling and active methods of cooling are employed to improve performance of P·V. modules.Active cooling requires a coolant, like air or water, which typically involves fan or pump powe.
