Roadmap to Over 50% Heat to Electricity for Cars, Drones and Space Probes

Researchers achieved 29.1 ± 0.4% thermophotovoltaic power conversion efficiency, by reuse of unabsorbed subbandgap photons. They have roadmap to achieve higher efficiencies by separately considering the realistic improvements of material, device, and chamber parameters. With the improvement of these parameters, it is possible to achieve >50% power conversion efficiency using InGaAs photovoltaic cells. A highly efficient thermophotovoltaic heat engine would be an excellent choice for hybrid automobiles, unmanned vehicles, and deep space probes.

Above – An ideal regenerative thermophotovoltaic system formed by a thermal radiation chamber, and power conversion inside the chamber. (A) High-energy (blue) photons from the emitter are converted to carriers in the photovoltaic cell, while low-energy (red) photons are reflected back to the emitter and rethermalized. (B) A highly reflective rear mirror is essential since a photon will need to be reflected many times before emerging in the high-energy tail of the Planck spectrum, for absorption in the semiconductor. Other losses in the photovoltaic cell arise due to poor material quality, as well as thermalization of high-energy carriers.

In an ideal thermophotovoltaic system employing photon reuse, a hot emitter is surrounded by photovoltaic cells lining the walls of the chamber, collecting light from the emitter. For efficient recovery of unused photons, the photovoltaic cells are backed by highly reflective rear mirrors. Such mirrors are needed, in any case, to provide the voltage boost associated with luminescence extraction.

The projected thermophotovoltaic efficiency is shown and it represents a realistic efficiency projection rather than ideal Shockley−Queisser performance. The optimum bandgap increases slightly upon improving the rear reflectivity, to minimize thermalization losses from photons at the high-energy tail of the emitter Planck spectrum. With an optimal bandgap, thermophotovoltaic efficiency can reach as high as over 50%.

Figures show how they stack the material layers and achieve higher bandgaps which are the key to greater efficiency.

Thermal Solar Power conversion utilizes thermal radiation to generate electricity in a photovoltaic cell. On a solar cell, the addition of a highly reflective rear mirror maximizes the extraction of luminescence, which in turn boosts the voltage. This has enabled the creation of record-breaking solar cells. The rear mirror also reflects low-energy photons back into the emitter, recovering the energy. This radically improves thermophotovoltaic efficiency. Therefore, the luminescence extraction rear mirror serves a dual function; boosting the voltage, and reusing the low-energy thermal photons. Owing to the dual functionality of the rear mirror, researchers achieve a thermophotovoltaic efficiency of 29.1% at 1,207 °C, a temperature compatible with furnaces, and a new world record at temperatures below 2,000 °C.


Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking solar efficiency. Now they report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high thermophotovoltaic efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. Therefore, the rear mirror serves a dual function; boosting the voltage and reusing infrared thermal photons. This allows the possibility of a practical over 50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we report a thermophotovoltaic efficiency of 29.1 ± 0.4% at an emitter temperature of 1,207 °C.

PNAS – Ultraefficient thermophotovoltaic power conversion by band-edge spectral filtering

9 thoughts on “Roadmap to Over 50% Heat to Electricity for Cars, Drones and Space Probes”

  1. Yah… we agree. Thing is, if I were Elon, I’d just have the engineers cobble together a 50% “extra pack” in the trunk, suitably hidden, which could be lashed into service under the same circumstance. After all the gigafactories are supposedly producing terawatt-hours of storage a year. Each.

  2. Yeah, obviously it’s not going to be offered as a range extender for the EVs at your local dealership.

    Though… maybe for Elon Musk’s personal vehicle…

    If Elon Musk ever, ever had his personal Tesla vehicle run out of charge and leave him stranded the publicity would probably cost the company, and him, $billions. Paying $millions for a secret RITPV generator to make sure that can’t happen isn’t insane.

    (Risking the radiation panic if he got caught IS insane, but…)

  3. I think the real problems are 

    1) plutonium doesn’t grow on trees
    2) putting ANY qty of it in hands of public … isn’t terribly wise
    3) any breach to housing creates a PR/health mess
    4) the word ‘radiation’ becomes a non-starter
    5) from 1) … ²³⁸Pu is hideously expensive … $1,000,000 a kg or more. 
    6) from 2) … ‘dirty’ bombs, polonium poisonings, yada, yada
    7) from 3) … doesn’t wash away with soap and water
    8) from all … and many permutations therein

    But it is good science fiction.  Now, if one were to propose these new-and-improved radioisotope generators for all the tractors, earth movers, cranes, borers, buggies and ore separators of our upcoming, just-around-the-corner, right-up Mars colonization, well … all (1) … (7+) would be quite forgiven. Embraced.  

    Earthlings by comparison are chronically afraid of their ghosts’ shadows. 

    GoatGuy ✓

  4. This was a computer story, about the surprising computer discovery, several years ago. It was about laser mirrors, but they said it would work for fiber, at angles within the fiber, which would actually be a tube of mirrors. Don’t remember frequency claims, but the mirrors (as I remember!) were not evenly spaced, so the frequency MAY just “go” to the right spacing and work. I’m utterly incompetent to judge this, but it sounds like the sort of thing that could be missed if looking for a good mirror.

  5. I would assume the mirror configurations would be for a somewhat limited frequency spread to make that reasonable…

  6. 50% is a nice long term goal, but what could be done with the current number of 30%?

    I’ll return to the idea of a trickle charger for your Tesla. If the radioisotope thermophotovoltaic (RITPV. Ritpov? Someone come up with a good name, please.) can generate a continuous 2 kWe, that’s 48 kW.h per day. Enough to drive around all year without recharging except for that one trip interstate.

    At 30% efficiency, that means we need just under 7 kW thermal output. About the same as an idling petrol engine, so we know that this thermal load is easily shed by a car without overheating or damaging the surroundings.

    Plutonium-238 has a half-life of 87.7 years, reasonable power density of 0.54 watts per gram,[12] and exceptionally low gamma and neutron radiation levels. To get 7 kW requires 14 kg. That’s pretty small, though you’re going to have trouble with the road worthy approval.

    Note that this is about 3 times the size of the Pu-238 radioisotope generator that crashed into the sea around Fiji during the Apollo-13 crisis.

    Polonium-210 gives 140 W/g, and so you’d only need 100 g of the stuff. But the short half life means you’d need a new battery every year, or even 6 months, and that counteracts the real attraction of a RITPV.

  7. Some computer *found* an arrangement of the dielectric (layered) mirrors that is ~100% reflective at ALL angles. Presume this is well known!

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