Eliminating Out-of-Band Loss in Thermophotovoltaic Systems Utilizing Cell-Side Spectral Control

Abstract

Thermophotovoltaic (TPV) generators have emerged as promising heat engines for use in a wide range of emerging energy generation and storage applications. This approach to energy conversion leverages the photovoltaic effect to convert locally emitted thermal radiation (heat) to electrical power. TPVs are positioned to facilitate the growth of intermittent, renewable energy sources because they can deliver power quickly and efficiently in response to sudden changes in energy demand at various scales. Through integration with thermal batteries, TPVs may enable one of the most affordable and energy-dense approaches for grid-scale electricity storage. TPVs are further well-suited for utilization in distributed co-generation, an alternative to centralized power generation that may reduce energy loss associated with waste heat and electricity transmission.

Despite the appeal of TPVs for use in these promising energy generation and storage technologies, TPV conversion efficiencies remain well below their thermodynamic limits. Practical deployment of the technology is therefore predicated on continued advances in performance. The fundamental challenge of thermophotovoltaics pertains to regulation of the radiative heat transfer between the thermal emitter and the photovoltaic cell. Given the moderate temperature of the thermal emitter, only a small fraction (~20%) of power is usable by the photovoltaic cell. The remaining, unusable power must be properly managed to avoid substantial loss. The present work aims to eliminate radiative loss in thermophotovoltaic systems to improve their conversion efficiencies.

This thesis begins with an exhaustive review of the TPV literature that features a thermodynamic framework for meaningful comparison of dissimilar works. This review helps to recognize leading materials and design choices and identifies opportunities for continued improvements. Spectral inefficiencies are shown to persist as the largest loss pathway for TPVs.

The first of three experimental demonstrations herein involves the realization of thin-film InGaAs optical structures through non-destructive epitaxial techniques. This technique enables recovery and subsequent reuse of the expensive crystalline growth substrate for reduced cell costs. Further, optimized dielectric claddings are shown to improve the spectral performance of the optical structures. Specifically, use of a MgF2 rear spacer enables record-high out-of-band reflectance of 96%, an improvement over cells with conventional metallic rear reflectors.

Secondly, this thesis demonstrates a novel cell architecture featuring air pockets buried beneath a InGaAs/InP heterostructure absorber with near-complete (98.5%) reflectance of out-of-band power. This spectral advance enables record-high conversion efficiency of 32% under illumination by a 1455K SiC emitter, representing an 8% efficiency improvement over a control cell.

Lastly, this thesis describes the development of an entirely new approach to spectral management based on partially transparent cells. Optical analysis of a proof-of-concept optical structure decouples radiative loss in the heterostructure absorber and the supporting substrate to show a pathway to a new regime featuring <1% out-of-band absorptance. Beyond improved conversion efficiency, mitigation of radiative loss as presented here may enable expansion of the TPV application space to include cell materials and heat sources previously considered impractical.