Improved thermophotovoltaic system performance with a novel metamaterial thermal emitter combined with a passive radiative cooling approach

Thermophotovoltaic (TPV) energy conversion systems have gained great interest in recent years as they can directly convert thermal energy into electricity using photovoltaic (PV) cells. The increase of the radiative power and the power conversion efficiency in TPV systems is mainly due to two key factors. The first one deals with the nature of the selective emitter. A new high temperature selective emitter should be designed to have a high emissivity (absorptivity) above the bandgap energy (Eg) and a low emissivity below Eg of the PV cells. The second key factor is related to the cooling mechanism of the PV cell as the conversion efficiency of the latter degrades with increasing temperature. In this work, we first propose and investigate a new efficient metamaterial thermal emitter, to reach a desired emissivity in the wavelength range 0.8−1.72μm. Secondly, to minimize the operating temperature of the TPV system, a passive radiative cooling approach is suggested in which an efficient double-layer coating deposited on the substrate of PV cells is elaborated. It has a high emissivity in the atmospheric transparency windows 8−13μm and low absorptivity in the solar spectrum 0.3−8μm. We show that the high-temperature metamaterial design can achieve high emissivity, over 0.97 in the wavelength range, 0.8−1.72μm, while reducing the sub-band loss of the PV cells. At 1400K, we demonstrate that adding the proposed cooling design to the substrate of a GaSb PV cell panel with an area ratio between the passive radiative cooler and the PV cell panel of γ=20, can improve the efficiency of the latter by 2% and drop its operating temperature by 44K.