Thermal management of photovoltaics using phase change materials

Abstract

While there are many factors influencing the efficiency of photovoltaic (PV) cells one of the most important ones is temperature. Module temperatures can get significantly higher than the ambient temperature, increases of up to 30°C have been reported in literature. An increase in temperature has an adverse effect on the module’s efficiency, literature has reported a drop of 0.40−0.65%/°C. Therefore, it is not uncommon that during warmsunny days PVmodules operate at a much lower efficiency than advertised by the manufacturer. Different methods of thermal management can be applied. In this research, the effect of phase change materials (PCM) to passively lower the operating temperature of PV panels was studied. During the phase change of a material heat is absorbed as latent heat instead of sensible heat, this does not cause the temperature of the material to increase. Using this process a PCM can keep a relatively stable temperature until it has completely changed its phase. This can be utilised by placing PCM behind a PV module, so that the heat generated in the panel can be absorbed by the PCM while remaining at a stable temperature. The scope of this research was to performmeasurements with a PV-PCMsystem under real weather conditions. Additionaly, a thermalmodel was build using ComsolMultiphysics, to make predictions for the optimal PCM parameters. This thermalmodel was validated with the results from the measurements. Measurements for a free rack situation showed surprisingly a slight increase in temperature of the PVPCM module compared to a reference PV module. Most likely due to convective cooling caused by the wind. And indeed, after installing insulation material at the back of the modules to emulate building integrated photovoltaics (BIPV) a significant decrease was found. For a two week period of measuring a −6.69°C temperature reduction was found, with a peak difference of −21.7°C. This resulted in an energy yield increase of 2.8% compared to the reference panel over this period. From this work, it can be concluded that PCM thermal management is not advisable for free standing modules due to the large influence of convective cooling caused by the wind. However, this method of thermal management can have significant benefits in BIPV situations where convection is not relevant.