Numerical modeling of a concentrated photovoltaic/thermal system which utilizes a PCM and nanofluid spectral splitting

Abstract

Spectral splitting concentrated photovoltaic/thermal systems aim to utilize the full spectrum of solar radiation by thermally decoupling the photovoltaic cells from the thermal components. Liquidabsorptivefilters are a type of spectral splitters which (ideally) transmit only the solar spectrum that can be most efficiently converted to electricity in the photovoltaic cell, while absorbing the rest for a thermal application to achieve a much higher total solar utilization. In this paper we attempt to advance this field by investigating the potential of employing a phase change material and a nanofluid to achieve a concentrated photovoltaic/thermal system with high energetic and exergetic efficiency. The optical properties of nanofluid and phase change material in solid and liquid states were simulated based upon data from the literature. Two different phase change materials, RT25 (paraffin) and S27 (hydrated salt,CaCl2.6H2O) and an Ag/water nanofluid were employed as the spectral filter components, with water as a coolant. After validating the model with available data from the literature, the effect of time, glass type, phase change material type, nanofluid position, and mass flow rate were analyzed with respect to the outlet temperature and the resultant energetic and exergetic efficiencies were calculated and compared against conventional concentrated photovoltaic/thermal and nanofluid-based spectral splitting concentrated photovoltaic/thermal systems. The results revealed that by employing the combination of the phase change material and the nanofluid it was possible to reduce the photovoltaic operation temperature (by up to 30%), increase the outlet temperature (by up to 54%), and obtain a relative improvement over previous systems in terms of the total exergetic efficiency (by up to 14.9%). The S27 leads to better performance than the RT25 due to the difference in their optical properties, which shows the importance of selecting the appropriate phase change material. By locating the phase change material below the nanofluid (instead of above it), the total energy and exergy efficiencies were increased by over 11 and 7%, respectively. The concentration ratio and the liquid filter’s mass flow rate were found to have the most significant impact on the performance of the spectral splitting photovoltaic/thermal system. Overall, this study is significant because it develops a new pathway towards harvesting the full spectrum of incident solar energy.