Design and optimization of spectral beamsplitter for hybrid thermoelectric-photovoltaic concentrated solar energy devices

The design and optimization of a thin-film multilayer spectral beamsplitter for hybrid thermoelectric photovoltaic (TE-PV) solar energy devices are considered. In the configuration being studied the concentrated solar radiation is either transmitted through the beamsplitter onto a thermoelectric generator, or reflected onto a photovoltaic cell. The role of the beamsplitter is to reflect (transmit) those parts of the solar spectrum that can be converted most efficiently in the PV cell (TE generator). Beamsplitters are designed for maximizing the output of the hybrid system taking into account the spectral efficiency of realistic solar cells based on amorphous or microcrystalline silicon, efficiencies of the thermoelectric generator of either 4% or 8%, and the AM1.5 solar spectrum. The beamsplitter is constructed using thin-film layers of SiO2 and Si3N4 deposited on N-BK7 glass. The layer thicknesses are optimized for maximizing the efficiency of the hybrid system. The total achievable efficiency versus the number of layers used in the beamsplitter is studied considering from app. 20 to app. 200 layers. For an amorphous solar cell the relative increase in hybrid system efficiency above the single solar cell efficiency is 21.4%. Designs are made assuming that solar radiation is incident on the beamsplitter at an angle of 45 degrees. However, the efficiency is found to only change slightly if the angle is varied from 35 degrees to 55 degrees. Finally, it is also found that efficient beamsplitters cannot be constructed for the reverse configuration with the TE and PV elements interchanged since efficient reflection of the long-wavelength radiation relevant for the TE generator is not possible without also having reflection of shorter-wavelength radiation that would be converted more efficiently in the PV cell. (C) 2016 Elsevier Ltd. All rights reserved.