Thermophotovoltaic Cell Design Improvements through Numerical Simulations: Uncertainty Quantification and Geometry Optimization

Two avenues of research on numerical simulations to achieve robust designs in thermophotovoltaic (TPV) energy conversion devices are explored. For the first of these, a recently proposed numerical scheme able to quantify performance changes due to shape perturbations and its application to one-dimensional (1D) gratings is investigated. The second avenue, also based on numerical simulations, focuses on the study of gratings as selective emitters to enhance TPV cell energy conversion efficiency.

In the first part, we present a novel deterministic method capable of calculating statistical moments of transverse electric polarized fields scattered by perfect electric conductor gratings with small surface random perturbations. Based on a first-order shape Taylor expansion, the resulting electric field integral equations are solved via the method of moments with constant hierarchical basis or Haar wavelets. This allows for a sparse tensor approximation, significantly reducing the number of required unknowns and yielding a higher rate of convergence than a dense approximation. Moreover, the proposed approach converges faster than Monte-Carlo simulations with significantly less computational effort. Validation of the proposed approach is performed for several cases, and simulations applied to the calculation and prediction of grating efficiency for realistic grating structures reveal the applicability of the method.

In the second part, we explores the performance potential of gratings based on tungsten/hafnia (W/HfO2) stacks for thermophotovoltaic thermal emitters via numerical simulations. Structures consisting of a W grating over a HfO2 spacer layer and a W substrate are analyzed over a range of geometries. For shallow gratings (W grating thickness much smaller than the grating pitch), an emittance of 99.9% can be achieved for transverse magnetic (TM) polarization, but the transverse electric (TE) performance is appreciably lower. For deep gratings (W grating thickness on the order of the grating pitch), peak emittances of 97.7% and 99.7% for TE and TM polarizations, respectively, are achieved. We find that both surface plasmon polaritons and magnetic polaritons play a crucial role in shaping the emittance for TM radiation. On the other hand, cavity resonances are responsible for the almost perfect emittance in the case of TE polarization. These results suggest that by introducing an HfO2 layer it is possible to reach high emittance for operating temperatures that match the absorption characteristics of GaSb and InGaAs photovoltaic cells.

In addition, tungsten-hafnia (W-HfO2) selective thermal emitters with high hemispherical emittance for thermophotovoltaic (TPV) applications are explored through numerical simulations. Two structures were analyzed: a planar multilayer stack and a grating. In both cases, through suitable design choices high thermal emittance with low directional sensitivity can be obtained. The designs are obtained by optimization of the structures using a genetic algorithm and a suitable cost function, along with simulations of the structures' emittance by using rigorous coupled wave analysis. Calculations show that these optimized structures possess high hemispherical thermal emittance for the wavelength range that matches the optical response of GaSb photovoltaic cells. For each structure, both the output power from the TPV cell and the conversion efficiency are studied as a function of emitter temperature and physical understanding of the optimized structures is developed.