A Phase 1 NASA NIAC study will demonstrate the feasibility of a revolutionary power source for missions to the outer planets utilizing a new paradigm in thermal power conversion, the thermoradiative cell (TRC).
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
(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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