thermophotovoltaics

Thermophotovoltaics is a technology that converts thermal radiation into electrical power by coupling a hot emitter to a photovoltaic cell. The basic components are a high-temperature thermal emitter, which radiates photons, and a photovoltaic receiver that converts part of that radiation into electricity. The emitter and the cell are often separated by a vacuum or air gap to minimize conductive losses, and optical elements or selective emitters are used to tailor the spectrum of emitted light.

Operation: The hot emitter, typically at thousands of kelvin, emits photons. Only photons with energy above the PV cell’s bandgap contribute to current; lower-energy photons are not absorbed. To maximize efficiency, the spectrum is engineered so that most radiative power is above the bandgap, using selective emitters, filters, and photonic structures. In near-field TPV, extremely close gaps enable tunneling of high-k photons, boosting radiative transfer beyond the far-field limit.

Performance and limits: TPV efficiency depends on emitter temperature, PV bandgap, radiative coupling, and losses from non-radiative recombination, heat in the cell, and parasitic absorption. Theoretical limits involve detailed balance calculations that account for spectral selectivity and thermal management. Real devices face material stability at high temperatures, optical losses, and challenges in maintaining close optical gaps.

Materials and architectures: Common PV materials include InGaAs, GaSb, and Ge for near- to mid-infrared operation. Emitters often use tungsten or specialized high-temperature coatings; selective emitters, photonic-crystal or metamaterial surfaces, and multilayer filters are used to shape emission. Devices may incorporate back mirrors or optical cavities to increase photon flux toward the cell.

Applications and challenges: TPV is explored for waste-heat recovery, distributed power generation, and space power systems. Major challenges include achieving high efficiency at practical temperatures, ensuring long-term thermal and optical stability, reducing parasitic losses, and scaling fabrication for reliable, cost-effective devices.