Near-Field Thermophotovoltaic Energy Conversion: Progress and Opportunities

Direct conversion of heat to electricity via solid-state approaches is a rapidly advancing area of research owing to its prospective use in recovering waste heat and in energy storage and utilization. In particu-lar, thermophotovoltaic (TPV) approaches that take advantage of near-field (NF) evanescent modes are receiving increased attention due to their potential for increased power output. This review examines the latest progress in theoretical and experimental explorations of near-field thermophotovoltaic (NF TPV) systems. We begin by introducing thermal radiation, its use in TPV systems, and how these systems have developed over time. Next, we describe the fundamentals of NF radiative heat transfer, as well as the operating principles and models used to evaluate NF TPV systems. We discuss theoretical stud-ies, based on fluctuational electrodynamics, that have predicted large enhancements in radiative energy transfer and consequently large electrical power outputs in NF TPV systems. Subsequently, we summa-rize recent advances in the experimental exploration of NF TPV systems and describe how researchers have recently developed unique experimental platforms to explore the principles of NF TPVs and have achieved significant enhancements in power output at reasonable conversion efficiencies. Additionally, we discuss related thermoradiative and thermophotonic energy conversion systems that provide interest-ing new avenues for photonic energy conversion. These systems, while theoretically promising, still await experimental validation in realistic material systems. Finally, we present remarks on the future potential of NF TPV systems and opportunities for improving NF TPV performance, and discuss the challenges that must be overcome to achieve large-scale NF TPV devices.