The Potential of Thermophotovoltaic (TPV) Cells for Converting Heat into Electricity: New Higher Conversion Efficiency Announced

 

         According to Wikipedia: “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 admitted from the hot object.” The problem with TPV systems is that they work at lower temperatures, have lower output voltages than solar PV, and tend to have lower conversion efficiencies. Lower conversion efficiencies mean they are not economic in most scenarios. However, they do have niche uses and new research is suggesting that efficiency improvements to get them on par or better than lithium-ion battery efficiencies and costs are possible. The niche uses of TPV include powering spacecraft, collection of waste-heat from sources like steam turbines, off-grid co-generation or combined heat and power, and as a form of thermal energy storage. It is for the latter use, as a ‘thermal battery,’ that new research suggests it could one day compete with lithium-ion batteries if conversion efficiency could be increased sufficiently and at scale. 

     Different TPV system designs exist. Radioisotope thermoelectric generators (RTGs) power conventional spacecraft using radiation from a radioactive material to heat a block of material which is converted into electricity using a thermocouple. However, thermocouples are very inefficient, and the use of TPV cells could increase the efficiency of RTGs. TPVs as the basis of a thermal storage system involves using off-peak time electricity to use resistance heating to heat a block of carbon to very high temperature. The block is surrounded by TPV cells which are surrounded by a reflector and insulation. When the system is not collecting heat. i.e., charging, the photons are reflected back to the carbon block to keep it warm and able to provide power as needed.

     The earliest TPV systems were built in the late 1950’s and the 1960’s. 30% efficiency was reached in 1980. In 2022 MIT and NREL announced that they had achieved nearly 41% efficiency with a TPV system, and they think it could be tweaked to achieve up to 50% efficiency. According to Diederik van der Hoeven in biobasedpress.eu (I think he explains it a bit better than the paper in Nature)  

 

“In the new device, both the emitter and the TPV have been changed. Previous thermal battery setups heated the emitters to about 1400°C. This maximized their brightness in the wavelength range for which TPVs were optimized. The new device has a temperature 1000°C higher; tungsten then emits more photons at higher energies, which could improve the energy conversion. But in order to catch that energy, the team had to rework the TPVs as well.”

 

He also notes challenges and the potential future implications, including much cheaper storage than current:

 

“The TPVs are made from III-V semiconductors, more expensive than the silicon used in rooftop solar cells. But other parts of a thermal batteries, including graphite, are cheap. The team also created ceramic pumps that can handle the ultra-high-temperature liquid metals needed to carry heat around an industrial scale heat energy storage setup.”

 

“There is much commercial interest for this technology. Researchers estimate that thermal batteries could store electricity for $10 per kilowatt-hour of capacity, less than one-tenth the cost of grid-scale lithium-ion batteries. And it could store electricity for a longer time than batteries, even for many days at a time. Moreover, thermal batteries are modular. They do not have to be constructed at a massive scale. They could also provide electricity for a small village. That makes thermal batteries unusually flexible. To be continued, therefore.”

 

The researchers in their Nature paper conclude:

 

“These cells can be integrated into a TPV system for thermal energy grid storage to enable dispatchable renewable energy. This creates a pathway for thermal energy grid storage to reach sufficiently high efficiency and sufficiently low cost to enable decarbonization of the electricity grid.”

 

Source: Thermophotovoltaic efficiency of 40. Alina LaPotin, Kevin L. Schulte, Myles A. Steiner, Kyle Buznitsky, Colin C. Kelsall, Daniel J. Friedman, Eric J. Tervo, Ryan M. France, Michelle R. Young, Andrew Rohskopf, Shomik Verma, Evelyn N. Wang & Asegun Henry. Nature. 604, 287–291 (2022). April 13, 2022. doi.org/10.1038/s41586-022-04473-y. s41586-022-04473-y.pdf

Since this research has come out, other researchers have proposed ways to increase efficiency further. Researchers in Spain have been employing bifacial TPV cells with mirrors to reflect back photons:

"The key behind such high efficiency is the inclusion of a highly efficient mirror in the rear of the TPV cell that turns back to the thermal emitter the outband energy photons. Efficiencies over 50% could be theoretically attainable by approaching a mirror reflectance of 100%."

The authors note that these bifacial TPV cells could enable high-efficiency low-cost TPV systems for power generation from thermal storage in an extended range of heat source temperatures.

     If the issues can be worked out with this storage technology and it could be scaled up it could be a game changer for decarbonization. However, it is uncertain if or when that could occur, only that it is more likely to occur than it was.

 

  

References

Thermophotovoltaic energy conversion. Wikipedia. Thermophotovoltaic energy conversion - Wikipedia

MIT’s new heat engine beats a steam turbine in efficiency. Big Think. May 30, 2022. MIT's new heat engine beats a steam turbine in efficiency - Big Think

‘Thermal batteries’ could efficiently store wind and solar power in a renewable grid. Science.org. Robert F. Service. April 13, 2022. ‘Thermal batteries’ could efficiently store wind and solar power in a renewable grid | Science | AAAS

Thermal batteries could back up green power. Robert F. Service. Science. Vol. 376 Issue 6590. Thermal batteries could back up green power (science.org)

Bifacial Thermophotovoltaic Energy Conversion. A. Datas. American Chemical Society. ACS Photonics. February 9, 2023. Bifacial Thermophotovoltaic Energy Conversion | ACS Photonics

Thermal batteries that store solar and wind power. Diederik van der Hoeven. Bio Based Press. April 30, 2022. Thermal batteries that store solar and wind power - Bio Based Press

Thermophotovoltaic efficiency of 40. Alina LaPotin, Kevin L. Schulte, Myles A. Steiner, Kyle Buznitsky, Colin C. Kelsall, Daniel J. Friedman, Eric J. Tervo, Ryan M. France, Michelle R. Young, Andrew Rohskopf, Shomik Verma, Evelyn N. Wang & Asegun Henry. Nature. 604, 287–291 (2022). April 13, 2022. doi.org/10.1038/s41586-022-04473-y. s41586-022-04473-y.pdf