Thermophotovoltaic energy conversion
[1] As TPV systems generally work at lower temperatures than solar cells, their efficiencies tend to be low.
This currently limits TPV to niche roles like spacecraft power and waste heat collection from larger systems like steam turbines.
Connecting a wire from the front to the rear allows the electrons to flow back into the bulk and complete the circuit.
As of 2022[update], 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.
[4] 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.
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.
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.
[7] 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.
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.
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.
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.
The absorption of suboptimal wavelengths by the photovoltaic device also contributes inefficiency and has the added effect of heating it, which also decreases efficiency.
[citation needed] 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.
These energies dictate the range of bandgaps that are needed for practical TPV converters (though the peak spectral power is slightly higher).
As emitter temperature increases, black-body radiation shifts to shorter wavelengths, allowing for more efficient absorption by photovoltaic cells.
These oxides emit a narrow band of wavelengths in the near-infrared region, allowing the emission spectra to be tailored to better fit the absorbance characteristics of a particular PV material.
Researchers at Sandia Labs predicted a high-efficiency (34% of light emitted converted to electricity) based on TPV emitter demonstrated using tungsten photonic crystals.
Silicon's commercial availability, low cost, scalability and ease of manufacture makes this material an appealing candidate.
This allows GaSb to respond to light at longer wavelengths than silicon solar cell, enabling higher power densities in conjunction with manmade emission sources.
When phase separation can be avoided, the IQE and fill factor of InGaAsSb approach theoretical limits in wavelength ranges near the bandgap energy.
[30] For this and other low-bandgap materials, high IQE for long wavelengths is hard to achieve due to an increase in Auger recombination.
PbSnSe/PbSrSe quantum well materials, which can be grown by MBE on silicon substrates, have been proposed for low cost TPV device fabrication.
In addition, owing to the PV's proximity to the radiative source, TPVs can generate current densities 300 times that of conventional PVs.
In early 2001, JX Crystals delivered a TPV based battery charger to the US Army that produced 230 W fueled by propane.
Converting spare electricity into heat for high-volume, long-term storage is under research at various companies, who claim that costs could be much lower than lithium-ion batteries.
In the case of solar energy, orbital spacecraft may be better locations for the large and potentially cumbersome concentrators required for practical TPVs.
Traditional PVs do not provide power during winter months and nighttime, while TPVs can utilize alternative fuels to augment solar-only production.
[37] The proposed CHP would utilize a SiC IR emitter operating at 1425 °C and GaSb photocells cooled by boiling coolant.
TPVs silent operation allows them to replace noisy conventional generators (i.e. during "quiet hours" in national park campgrounds).
