This device, driven by a radioisotope heat source, will allow an order of magnitude increase in mass specific power (~30 vs ~3 W/kg) and a three orders of magnitude decrease in volume (~0.2 vs ~212 L) as compared to a conventional multi-mission radioisotope thermal generator (MMRTG).
This technology will allow a proliferation of small versatile spacecraft with power requirements not met by photovoltaic arrays or bulky, inefficient MMRTG systems. This will directly enable small-sat missions to the outer planets as well as operations in permanent shadow such as polar lunar craters. This study will investigate the thermodynamics and feasibility of the development of a radioisotope enabled thermoradiative power source focusing on system size, weight, power (SWaP) as well as materials growth of identified materials including InAsSb or InPSb by metalorganic vapor phase epitaxy. We will analyze a thermoradiative converter to power a cubesat (or fleet of cubesats) that can ride along with a Flagship Uranus mission, doing such tasks as serving as information relay for atmospheric probes, and getting a parallax view of the planet and moons.
This study is being led by Stephen Polly, Researcher/Engineer II, Nanopower Research Laboratory, Golisano Institute for Sustainability at the Rochester Institute of Technology. He received an Intelligence Community (IC) Postdoctoral Fellowship where he research doped carbon nanotube (CNT) thermoelectric devices which included developing software for numerical physics-based models, lab development of thermoelectric characterization techniques, and CNT manufacturing methods.
Brett Bellmore Proposes Details on this Technology Which Were Not Included in NASA Press Release
This uses an isotopic heat source to produce IR radiation for a band gap-matched IR photo cell. Which for thermodynamic reasons has to be kept substantially cooler, of course.
It has the potential to run at better efficiency than thermoelectric power conversion, which is part of why the power density is higher. The volumetric power is increased due to the heat rejection being at a higher temperature, thermal radiation being a function of T^4.
The NASA blurb had practically no information. But I’d be shocked if they’re not exploiting photonic crystals to improve the efficiency by causing the IR to be more concentrated in an efficient band for the photo-cells.
Related Background Research
Thermoradiative cells are also based on the physical principles that pave the ground for photovoltaics. A fundamental difference exists, because thermoradiative cells are supposed to be heated to temperatures higher than the ambient temperature during operation. The concept involves converting part of the heat that is supplied to keep the cell at a constant temperature to electricity. A sketch of this is shown below. The thermoradiative cell is thus a type of emissive energy harvester.
Ideal thermoradiative cells are assumed (in look forward from 2015) to possess the following properties:
(I) The main part of the cell consists of an active material that has an energy gap over which electrons can only be excited by photons. De-excitations
over the energy gap are only allowed if the excess energy is emitted as a photon.
(II) The cell is a perfect absorber and emitter of photons with energy larger than the energy gap. The absorptivity for photons with energy lower than the
energy gap is zero.
(III) Electrons can be inserted to electron states above the energy gap by an ideal contact. The ideal contact ensures lossless transport of electrons between these states and an external electric circuit. Similarly, another ideal contact can extract electrons from electron states below the
energy gap.
(IV) Electrons can move inside the thermoradiative cell without loss of energy.
(V) The back side of the device should be a perfect reflector.
The properties above are similar to those assumed for an ideal photovoltaic cell and Stephen Polly prior research is on ideal photovoltaic cells.
A new method to produce electricity from heat called thermoradiative energy conversion is analyzed. The method is based on sustaining a difference in the chemical potential for electron populations above and below an energy gap and let this difference drive a current through an electric circuit. The difference in chemical potential originates from an imbalance in the excitation and de-excitation of electrons across the energy gap. The method has similarities to thermophotovoltaics and conventional photovoltaics. While photovoltaic cells absorb thermal radiation from a body with higher temperature than the cell itself, thermoradiative cells are hot during operation and emit a net outflow of photons to colder surroundings. A thermoradiative cell with an energy gap of 0.25 eV at a temperature of 500 K in surroundings at 300 K is found to have a theoretical efficiency limit of 33.2%. For a high-temperature thermoradiative cell with an energy gap of 0.4 eV, a theoretical efficiency close to 50% is found while the cell produces 1000 W/m2 has a temperature of 1000 K and is placed in surroundings with a temperature of 300 K. Some aspects related to the practical implementation of the concept are discussed and some challenges are addressed. It is, for example, obvious that there is an upper boundary for the temperature under which solid state devices can work properly over time. No conclusions are drawn with regard to such practical boundaries, because the work is aimed at establishing upper limits for ideal thermoradiative devices.
Long life radioisotope-based direct-conversion nuclear batteries created by coupling of radioactive sources to a semiconductor element were found to be prospective for use as long-living power supplies of different purposes. The sources of highly ionizing charged particles emitted by radioisotopes are of particular interest due to a high energy density deposition in the solid state. At the high flux of the ionizing particles from isotope source a radiation damage of the converter occurs. A utilization of wide-band or liquid semiconductors partly resolves this problem. Moreover, as higher band gap of semiconductor is, as better conversion efficiency is achievable. Recently, a few types of direct-conversion devises have been reported including evaluation of α-voltaic unit based on diamond coupled to 241Am source.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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Do any of the commenters here have the knowledge to be able to show how many of these 30W/kg TRCs would be required to power one of the standard Tesla cars ?
The problem here is power density, not energy density. Radioisotopes store tremendous times more energy than batteries, but release at a considerably slower rate that we can’t easily control. A tesla model s plaid has a peak power draw of 1020 hp, or ~750 kW. At 30W/Kg, that would require 25,000 Kg. Not very feasible. Tesla’s semi has a similar power draw to that, and has ~10,000 lb of batteries and ~45,000 lb of usable haul capacity, which is about 25,000 Kg. So it would be able to haul a driver and pretty much nothing else.
Essentially, this uses an isotopic heat source to produce IR radiation for a band gap matched IR photo cell. Which for thermodynamic reasons has to be kept substantially cooler, of course.
It has the potential to run at better efficiency than thermoelectric power conversion, which is part of why the power density is higher. The volumetric power is increased due to the heat rejection being at a higher temperature, thermal radiation being a function of T^4.
The NASA blurb had practically no information. But I’d be shocked if they’re not exploiting photonic crystals to improve the efficiency by causing the IR to be more concentrated in an efficient band for the photo-cells.
Ideally, you would want a large spacecraft bus with a conventional reactor—-and a lot of these cubes with the TRCs facing inwards to keep it fresh…then dispensed along the way.
I’d want a super JIMO sent to Planet Nine with about a dozen of these shoved overboard as a network with the main spacecraft bus serving as a relay.
more details given at: https://ttu-ir.tdl.org/bitstream/handle/2346/87261/ICES-2021-337.pdf?sequence=1
It appears that they are relying on the 3K background temperature of outer space to be the heat sink. so if your house a a readily available 3K heatsink, it’d work for you as well. 🙂
One order of magnitude improved power to weight, and 3 orders of magnitude improved power to volume. So it has 2 orders of magnitude greater weight to volume?
Were the old designs made of polystyrene foam or the new ones made of compressed tungsten?
Runs at a higher temperature, so it can reject heat from smaller radiators. Or so I gather.
When can I get one to power my house and charge my car?
30W/kg, so a 100kg system for 3kW electric plus enough hot water output to heat most homes. If the system is 20% efficient at making electricity, it’ll put out 12kW of waste energy mostly as heat.
Add a battery to cover peak loads.
Even with the radiation risk, probably safer than having gas in the house…
It would be nice, but I have a sneaking suspicion that those rely on being in vacuum in order to radiate efficiently… I wish there was a description of the mechanism in the writeup Brian linked to, but unfortunately it’s just a blurb.
Radiation shouldn’t work materially better in a vacuum compared to air. But the heat sink available from convection through air would be much much better. And if you do have it hooked up to water to act as a heater that would be orders of magnitude better than radiating to space.
So I would expect it to work far better on earth than in space.
Also, I guess that a 3kW, 100 kg system should displace 100 kg of Li batteries on your Tesla and give you a 1 million km range between needing recharges.
Maybe you’re right! Although reading what I can find about thermoradiative cells, I find that it’s a kind of “reverse solar cell”, where power is generated by radiating heat away into a cold reservoir, so convection might not help? Or perhaps the concept here requires having a really cold sink like one can easily find in the outer Solar System.
https://aip.scitation.org/doi/10.1063/1.4907392
Huh. And here I’d thought it was another name for thermo-photo-voltaics. Well, learning new things is nice.
I wonder if you can couple one of these things to an IR photo-voltaic cell operating at a lower temperature?
Thermoradiative power cells is a bit ambitious for the red faced NASA crowd who don’t know enough physics to land a booster rocket on a floating barge.
they could, but didn’t like the concept within capitalism and its difficulties with security of employment (?)
Certain type of crystal growth, please explain the method power is extracted.
Right. Too few details to make this anything more than thermodynamic agitprop.
Now I have to look up “epitaxy,you could have defined it.
noice.