Thermoradiative Diode Technology: Applications, Projects, and Possibilities of Mid-Infrared Radiative Heat for Power

  The concept of a thermoradiative cell to harvest energy from infrared radiation to the thermal sink of deep space was first described in 2014-2015. A thermoradiative cell is basically a photovoltaic cell that is run in the thermodynamically reversed direction. According to Geoffrey A. Landis of NASA’s John Glenn Research Center (2022):

“It is based on the concept that an ideal photovoltaic cell is a heat engine, operating on a temperature difference between photons (e.g., from the sun) as a high temperature source, and the external environment as a low temperature sink. Since an ideal heat engine will operate when the high and low temperature sides of the engine were reversed, Strandberg showed that a device identical in structure to a photovoltaic cell would operate with sources reversed. Thus, the thermoradiative cell has heat as the energy input and photons as the waste heat output (figure 1). The cells radiate heat to a lower temperature, which he assumed to be the low temperature of deep space.”

“A photovoltaic cell absorbs light and produces electrical power. In the process, of course, since thermodynamics demands that no energy converter can be a hundred percent efficient, it also produces waste heat. We can therefore think of a solar cell as a thermodynamic heat engine that converts sunlight (at an effective temperature of 6000 K, the temperature of the sun) into electrical power, and rejecting waste heat on the “cold” side, typically at room temperature, around 300 K. But, thermodynamically, a heat engine is reversable: if you switch the hot side and the cold site, it will still produce power. Thermodynamically, then, it should be possible to heat the photovoltaic device, to make it emit (infrared) light, and in the process produce electrical power. The concept sounds absurd [5]; but nevertheless it is based on sound physical principles.”

     This technology can be used for terrestrial energy, converting waste heat into energy, and for converting heat from isotope or nuclear power sources to electricity for spacecraft. Its use in space is deemed the most promising due to the presence of the heat sink of deep space. Using these devices on planet surfaces will result in significant efficiency losses. However, they can be used on moons or asteroids without atmospheres. Thermoradiative arrays can operate at high heat source temperatures between 300K and 1000-1500K. They are high-temp, low band gap vs. solar PV cells which are low-temp, high band gap. A thermoradiative device consists of a p-n diode, where its surface area is open to space (or, generally, any cold-temperature radiative heat sink). Like PV cells thermoradiative cells have no moving parts. Below is a list of research needs for this immature technology. Thermoradiative conversion may become as good as or better than thermoelectric and thermophotovoltaic conversion.

     Thermoradiative diodes emit photons in the mid-infrared range of the spectrum. This emitted infrared energy can produce electricity. Here on Earth the applications would be very limited to small power needs like charging small devices with low power needs. Diodes made of mercury cadmium telluride (HgCdTe) have been tested for such uses. Another potential application is supplemental power for satellites in low-Earth orbit, which typically cycles between 45 minutes of sun exposure and 45 minutes of darkness. In terms of understanding physics, thermoradiative technology fills in a gap in our knowledge:

“In plots of current against voltage for optoelectronic devices, light-emitting diodes (LEDs) occupy the first quadrant (positive voltage; positive current), solar cells the second (positive voltage; negative current), and light detectors the third (negative voltage; negative current). But the fourth quadrant (negative voltage; positive current) is empty. That’s where thermoradiative diodes will fit in.”

     As I understand it, the early experiments only produced a small amount of power barely detectable, and with a very low efficiency of about 1.8%. However, researchers think they can get it closer to theoretical limits which can mean that it can be used to produce about 10% of the power of an equivalent solar PV cell, or basically a 10% power increase. That is not much on Earth but it can be significant in space.

     A December 2024 paper in Nature evaluated the power outputs of thermoradiative diodes under different terrestrial atmospheric conditions. They concluded that the power output varies with humidity, but the ideal bandgap remains around 0.094 eV.

    

 

References:

 

The ‘solar cells in reverse’ that can generate power at night. Nature Portfolio. The ‘solar cells in reverse’ that can generate power at night

Evaluating potential power output of terrestrial thermoradiative diodes with atmospheric modeling. Jamie A. Harrison, Phoebe M. Pearce, Fei Yang, Michael P. Nielsen, Helen E. Brindley, and Nicholas J. Ekins-Daukes. iScience. Volume 27, Issue 12, 20 December 2024, 111346. Evaluating potential power output of terrestrial thermoradiative diodes with atmospheric modeling - ScienceDirect

Semiconductor thermoradiative power conversion. Michael P. Nielsen, Andreas Pusch, Phoebe M. Pearce, Muhammad H. Sazzad, Peter J. Reece, Martin A. Green & Nicholas J. Ekins-Daukes. Nature Photonics volume 18, pages1137–1146 (2024). Semiconductor thermoradiative power conversion | Nature Photonics

Thermoradiative Power Conversion from HgCdTe Photodiodes and Their Current–Voltage Characteristics. Michael P. Nielsen, Andreas Pusch, Muhammad H. Sazzad, Phoebe M. Pearce, Peter J. Reece, and Nicholas J. Ekins-Daukes. ACS Photonics. Vol 9. Issue 5. May 9, 2022. Thermoradiative Power Conversion from HgCdTe Photodiodes and Their Current–Voltage Characteristics | ACS Photonics

Harvesting renewable energy from Earth’s mid-infrared emissions. Steven J. Byrnes, Romain Blanchard, and Federico Capasso. PNAS. February 3, 2014. Harvesting renewable energy from Earth’s mid-infrared emissions | PNAS

Thermoradiative Arrays: a New Technology for Conversion of Heat into Electrical Power. Geoffrey A. Landis. NASA. 2022. Thermoradiative arrays

Bizarre night-time solar cell generates power in a backwards process. Loz Blain. New Atlas. May 18, 2022. Bizarre night-time solar cell generates power in a backwards process