Thermophotonics (often abbreviated as TPX) is a concept for generating usable power from heat which shares some features of thermophotovoltaic(TPV) power generation. A TPX system consists of a light-emitting diode (LED) (though other types of emitters are conceivable), a photovoltaic (PV) cell, an optical coupling between the two, and an electronic control circuit. The LED is heated to a temperature higher than the PV temperature by an external heat source. If no power is applied to the LED, the system functions much like a very inefficient TPV system, but if a forward bias is applied at some fraction of the bandgap potential, an increased number of electron-hole pairs (EHPs) will be thermally excited to the bandgap energy. These EHPs can then recombine radiatively so that the LED emits light at a rate higher than the thermal radiation rate ("superthermal" emission). This light is then delivered to the cooler PV cell over the optical coupling and converted to electricity.
The control circuit presents a load to the PV cell (presumably at the maximum power point) and converts this voltage to a voltage level that can be used to sustain the bias of the emitter. Provided that the conversion efficiencies of electricity to light and light to electricity are sufficiently high, the power harnessed from the PV cell can exceed the power going into the bias circuit, and this small fraction of excess power (originating from the heat difference) can be utilized. It is thus in some sense a photonic heat engine.
The advantage to this process is claimed to be a higher possible efficiency than a TPV system and a lower useful operating temperature. If the PV cell is not cooler than the LED, then the thermal radiation from the PV cell balances out the heat energy extracted from the LED, which is how the device adheres to the Carnot limit on the efficiency of heat engines.
Possible applications of thermophotonic generators include solar thermal electricity generation and utilization of waste heat. TPX systems may have the potential to generate power with useful levels of output at temperatures where only thermoelectric systems are now practical, but with higher efficiency.
Thermophotonics was first publicly proposed by solar photovoltaic researcher Martin Green in 2000. To date, no TPX device is known to have been demonstrated, apparently because of the difficulty in operating emitters at high temperature with sufficient efficiency. However, both LEDs and PV cells have been improving substantially in efficiency and operating temperature due to extensive research efforts in both technologies. Much of the physics involved is closely related to the physics of optical cooling in LEDs, and TPX can be viewed as its reverse process.
Green, Martin A. (5-2000). ""Third Generation Photovoltaics: Advanced Structures Capable of High Efficiency at Low Cost". Proceedings of the 16th European Photovoltaic Solar Energy Conference. Glasgow, Scotland. pp. 51.
Harder, Nils-P.; Martin A. Green (2003). "Thermophotonics". Semiconductor Science and Technology 18 (5): S270–S278. doi:10.1088/0268-1242/18/5/319.