Thermophotovoltaic (TPV) energy conversion is a direct conversion process from heat to electricity via photons. A basic thermophotovoltaic system consists of a hot object emitting thermal radiation and a photovoltaic cell similar to a solar cell but tuned to the spectrum being emitted from the hot object.
As TPV systems generally work at lower temperatures than solar cells, their efficiencies tend to be low. Offsetting this through the use of multi-junction cells based on non-silicon materials is common, but generally very expensive. This currently limits TPV to niche roles like spacecraft power and waste heat collection from larger systems like steam turbines.
General concept PV Typical photovoltaics work by creating a p–n junction near the front surface of a thin semiconductor material. When photons above the bandgap energy of the material hit atoms within the bulk lower layer, below the junction, an electron is photoexcited and becomes free of its atom. The junction creates an electric field that accelerates the electron forward within the cell until it passes the junction and is free to move to the thin electrodes patterned on the surface. Connecting a wire from the front to the rear allows the electrons to flow back into the bulk and complete the circuit.
Photons with less energy than the bandgap do not eject electrons. Photons with energy above the bandgap will eject higher-energy electrons which tend to thermalize within the material and lose their extra energy as heat. If the cell's bandgap is raised, the electrons that are emitted will have higher energy when they reach the junction and thus result in a higher voltage, but this will reduce the number of electrons emitted as more photons will be below the bandgap energy and thus generate a lower current. As electrical power is the product of voltage and current, there is a sweet spot where the total output is maximized.
Terrestrial solar radiation is typically characterized by a standard known as Air Mass 1.5, or AM1.5. This is very close to 1,000 W of energy per square meter at an apparent temperature of 5780 K. At this temperature, about half of all the energy reaching the surface is in the infrared. Based on this temperature, energy production is maximized when the bandgap is about 1.4 eV, in the near infrared. This just happens to be very close to the bandgap in doped silicon, at 1.1 eV, which makes solar PV inexpensive to produce.
This means that all of the energy in the infrared and lower, about half of AM1.5, goes to waste. There has been continuing research into cells that are made of several different layers, each with a different bandgap, and thus tuned to a different part of the solar spectrum. , cells with overall efficiencies in the range of 40% are commercially available, although they are extremely expensive and have not seen widespread use outside of specific roles like powering spacecraft, where cost is not a significant consideration.
TPV
The same process of photoemission can be used to produce electricity from any spectrum, although the number of semiconductor materials that will have just the right bandgap for an arbitrary hot object is limited. Instead, semiconductors that have tuneable bandgaps are needed. It is also difficult to produce solar-like thermal output; an oxyacetylene torch is about 3400 K (~3126 °C), and more common commercial heat sources like coal and natural gas burn at much lower temperatures around 900 °C to about 1300 °C. This further limits the suitable materials. In the case of TPV most research has focused on gallium antimonide (GaSb), although germanium (Ge) is also suitable.
Another problem with lower-temperature sources is that their energy is more spread out, according to Wien's displacement law. While one can make a practical solar cell with a single bandgap tuned to the peak of the spectrum and just ignore the losses in the IR region, doing the same with a lower temperature source will lose much more of the potential energy and result in very low overall efficiency. This means TPV systems almost always use multi-junction cells in order to reach reasonable double-digit efficiencies. Current research in the area aims at increasing system efficiencies while keeping the system cost low, but even then their roles tend to be niches similar to those of multi-junction solar cells.
Actual designs TPV systems generally consist of a heat source, an emitter, and a waste heat rejection system. The TPV cells are placed between the emitter, often a block of metal or similar, and the cooling system, often a passive radiator. PV systems in general operate at lower efficiency as the temperature increases, and in TPV systems, keeping the photovoltaic cool is a significant challenge.
This contrasts with a somewhat related concept, the "thermoradiative" or "negative emission" cells, in which the photodiode is on the hot side of the heat engine. Systems have also been proposed that use a thermoradiative device as an emitter in a TPV system, theoretically allowing power to be extracted from both a hot photodiode and a cold photodiode.
Applications RTGs Conventional radioisotope thermoelectric generators (RTGs) used to power spacecraft use a radioactive material whose radiation is used to heat a block of material and then converted to electricity using a thermocouple. Thermocouples are very inefficient and their replacement with TPV could offer significant improvements in efficiency and thus require a smaller and lighter RTG for any given mission. Experimental systems developed by Emcore (a multi-junction solar cell provider), Creare, Oak Ridge and NASA's Glenn Research Center demonstrated 15 to 20% efficiency. A similar concept was developed by the University of Houston which reached 30% efficiency, a 3 to 4-fold improvement over existing systems.
Thermoelectric storage Another area of active research is using TPV as the basis of a thermal storage system. In this concept, electricity being generated in off-peak times is used to heat a large block of material, typically carbon or a phase-change material. The material is surrounded by TPV cells which are in turn backed by a reflector and insulation. During storage, the TPV cells are turned off and the photons pass through them and reflect back into the high-temperature source. When power is needed, the TPV is connected to a load.
Waste heat collection TPV cells have been proposed as auxiliary power conversion devices for capture of otherwise lost heat in other power generation systems, such as steam turbine systems or solar cells.
History Henry Kolm constructed an elementary TPV system at MIT in 1956. However, Pierre Aigrain is widely cited as the inventor based on lectures he gave at MIT between 1960–1961 which, unlike Kolm's system, led to research and development.
In the 1980s, efficiency reached close to 30%.
In 1997 a prototype TPV hybrid car was built, the "Viking 29" (TPV) powered automobile, designed and built by the Vehicle Research Institute (VRI) at Western Washington University.
In 2022, MIT/NREL announced a device with 41% efficiency. The absorber employed multiple III-V semiconductor layers tuned to absorb variously, ultraviolet, visible, and infrared photons. A gold reflector recycled unabsorbed photons. The device operated at 2400 °C, at which temperature the tungsten emitter reaches maximum brightness.
In May 2024, researchers announced a device that achieved 44% efficiency when using silicon-carbide (SiC) as the heat storage material (emitter). At 1,435 °C (2,615 °F) the device radiates thermal photons at various energy levels. The semiconductor captures 20 to 30% of the photons. Additional layers include air and a gold reflector layer.
Details Efficiency The upper limit for efficiency in TPVs (and all systems that convert heat energy to work) is the Carnot efficiency, that of an ideal heat engine. This efficiency is given by:
where Tcell is the temperature of the PV converter. Practical systems can achieve Tcell= ~300 K and Temit= ~1800 K, giving a maximum possible efficiency of ~83%. This assumes the PV converts the radiation into electrical energy without losses, such as thermalization or Joule heating, though in reality the photovoltaic inefficiency is quite significant. In real devices, as of 2021, the maximum demonstrated efficiency in the laboratory was 35% with an emitter temperature of 1,773 K. This is the efficiency in terms of heat input being converted to electrical power. In complete TPV systems, a necessarily lower total system efficiency may be cited including the source of heat, so for example, fuel-based TPV systems may report efficiencies in terms of fuel-energy to electrical energy, in which case 5% is considered a "world record" level of efficiency. Real-world efficiencies are reduced by such effects as heat transfer losses, electrical conversion efficiency (TPV voltage outputs are often quite low), and losses due to active cooling of the PV cell.
Emitters Deviations from perfect absorption and perfect black body behavior lead to light losses. For selective emitters, any light emitted at wavelengths not matched to the bandgap energy of the photovoltaic may not be efficiently converted, reducing efficiency. In particular, emissions associated with phonon resonances are difficult to avoid for wavelengths in the deep infrared, which cannot be practically converted. An ideal emitter would emit no light at wavelengths other than at the bandgap energy, and much TPV research is devoted to developing emitters that better approximate this narrow emission spectrum.
Filters For black body emitters or imperfect selective emitters, filters reflect non-ideal wavelengths back to the emitter. These filters are imperfect. Any light that is absorbed or scattered and not redirected to the emitter or the converter is lost, generally as heat. Conversely, practical filters often reflect a small percentage of light in desired wavelength ranges. Both are inefficiencies. The absorption of suboptimal wavelengths by the photovoltaic device also contributes inefficiency and has the added effect of heating it, which also decreases efficiency.
Converters Even for systems where only light of optimal wavelengths is passed to the photovoltaic converter, inefficiencies associated with non-radiative recombination and Ohmic losses exist. There are also losses from Fresnel reflections at the PV surface, optimal-wavelength light that passes through the cell unabsorbed, and the energy difference between higher-energy photons and the bandgap energy (though this tends to be less significant than with solar PVs). Non-radiative recombination losses tend to become less significant as the light intensity increases, while they increase with increasing temperature, so real systems must consider the intensity produced by a given design and operating temperature.
Geometry In an ideal system, the emitter is surrounded by converters so no light is lost. Realistically, geometries must accommodate the input energy (fuel injection or input light) used to heat the emitter. Additionally, costs have prohibited surrounding the filter with converters. When the emitter reemits light, anything that does not travel to the converters is lost. Mirrors can be used to redirect some of this light back to the emitter; however, the mirrors may have their own losses.
Black body radiation For black body emitters where photon recirculation is achieved via filters, Planck's law states that a black body emits light with a spectrum given by:
where I′ is the light flux of a specific wavelength, λ, given in units of 1 m–3⋅s–1. h is the Planck constant, k is the Boltzmann constant, c is the speed of light, and Temit is the emitter temperature. Thus, the light flux with wavelengths in a specific range can be found by integrating over the range. The peak wavelength is determined by the temperature, Temit based on Wien's displacement law: