Efficiency limits of concentrating spectral-splitting hybrid photovoltaic-thermal (PV-T) solar collectors and systems

Spectral splitting is an approach to the design of hybrid photovoltaic-thermal (PVT) collectors that promises significant performance benefits. However, the ultimate efficiency limits, optimal PV cell materials and optical filters of spectral-splitting PVT (SSPVT) collectors remain unclear, with a lack of consensus in the literature. We develop an idealized model of SSPVT collectors and use this to determine their electrical and thermal efficiency limits, and to uncover how these limits can be approached through the selection of optimal PV cell materials and spectral-splitting filters. Assuming that thermal losses can be minimized, the efficiency limit, optimal PV material and optimal filter all depend strongly on a coefficient w, which quantifies the value of the delivered thermal energy relative to that of the generated electricity. The total (electrical plus thermal) efficiency limit of SSPVT collectors increases at higher w and at higher optical concentrations. The optimal spectral-splitting filter is defined by sharp lower- and upper-bound energies; the former always coincides with the bandgap of the cell, whereas the latter decreases at higher w. The total effective efficiency limit of SSPVT collectors is over 20% higher than those of either standalone PV modules or standalone ST collectors when w is in the range from 0.35 to 0.50 and up to 30% higher at w approximate to 0.4. This study provides a method for identifying the efficiency limits of ideal SSPVT collectors and reports these limits, along with guidance for selecting optimal PV materials and spectral-splitting filters under different conditions and in different applications. Optimizing the performance of hybrid photovoltaic-thermal collectors Scientists have developed a method for predicting and optimizing the performance of hybrid photovoltaic-thermal (PVT) collectors. Photovoltaic (PV) cells use only a portion of the solar spectrum to generate electricity, limiting the efficiency of single-junction cells to 20-30%. PVT collectors improve on this overall solar utilization by recovering unused thermal energy and delivering a useful thermal output. Spectral splitting is a promising approach for improving the performance of PVT collectors by exploiting the spectral sensitivity of PV cells. Christos Markides and Gan Huang from Imperial College London, in collaboration with Kai Wang from Zhejiang University, have developed a comprehensive framework for modelling spectral-splitting PVT collectors, which they have used to determine the efficiency limits of such collectors and how these limits can be approached through the selection of optimal PV cell materials and spectral-splitting optical filters.