Comparative analysis of free cooling of photovoltaics - phase change versus evaporative cooling

Free cooling of PV cells is a common research topic; it lowers the operating temperature of PV cells, resulting in higher electricity production. This article investigated free cooling techniques for the wide range of thermal properties of the PCM layer installed on the backside of the PV cell. Two options were assumed: micro-encapsulated PCM in which heat is transferred by conduction only, and macro-encapsulated PCM in which heat transfer is enhanced by convection. The effect of free cooling with PCM was compared with the evaporative cooling technique. The thermal response of virtual PV cells (vPV) was evaluated with in-situ experiments. One vPV was upgraded with a micro-encapsulated 5.2 mm thick PCM layer, another with an evaporative layer. Multi-parametric approximation models of the PV cell temperature and combined surface heat transfer coefficients were developed. Approximation models of temperatures were used for the determination of overheating hours (OHH) for the reference and evaporatively cooled vPV, while combined surface heat transfer coefficient models were used in the developed numerical model of thermal response of vPV with micro-and macro-encapsulated PCM. The required thermal properties of PCM were determined to provide the same efficacy of free cooling as an evaporative cooling technique for three selected climate conditions (Stockholm, Ljubljana, and Athens). The study shows that the free cooling of PV cells with PCM could be as efficient as evaporative cooling, but only in the hottest and coldest observed climates if the thermal conductivity of PCM is above 1.8 W/mK for micro-and above 1.2 W/mK for macro-encapsulated PCM, and the latent heat capacity is above 250 kJ/kg. In the milder climate, free cooling will not be as efficient as evaporative cooling (for the defined ranges of thermal properties). It was determined that for such a climate, the additional heat transfer resistance of the PCM layer increases overheating of PV cells if the PCM's thermal conductivity is below 0.4 W/mK and the latent heat capacity is below 150 kJ/kg.