Researchers Demonstrate Quantum-Coupled Thermal to Electric Conversion With Efficiency as High as 40% of Carnot Limit, With Calculated Potential of Up to 90%
Researchers from MIT, with colleagues from IISc in Bangalore, India and HiPi Consulting in Maryland have experimentally demonstrated the conversion of heat to electricity using thermal diodes with efficiency as high as 40% of the Carnot Limit. Their calculations find that this new kind of system could theoretically reach as much as 90% of that ceiling.
By contrast, current solid-state thermoelectric devices only achieve about one-tenth of the Carnot Limit, according to MIT Associate Professor Peter Hagelstein, co-author of a paper on the new concept published 13 November in the Journal of Applied Physics.
The scheme is conceptually simple, wrote Hagelstein and graduate student Dennis Wu in the 2007 Progress Report from MIT’s Research Laboratory of Electronics. In the simplest possible implementation, an electron reservoir on the cold side supplies an electron to a lower state. Coupling with the hot side causes the electron to be promoted to an excited state, and then the electron proceeds to a second electron reservoir at elevated potential. An electrical load connected between the two reservoirs can be driven from the current due to the promoted electrons.
Hagelstein says that with present systems it’s possible to efficiently convert heat into electricity, but with very little power. It’s also possible to get high-throughput power from a less efficient, and therefore larger and more expensive system. “It’s a tradeoff. You either get high efficiency or high throughput,” says Hagelstein. But the team found that using their new system, it would be possible to get both at once, he says.
A key to the improved throughput was reducing the separation between the hot surface and the conversion device. A recent paper by MIT professor Gang Chen reported on an analysis showing that heat transfer could take place between very closely spaced surfaces at a rate that is orders of magnitude higher than predicted by theory. The new report takes that finding a step further, showing how the heat can not only be transferred, but converted into electricity so that it can be harnessed.
Thermal to electric energy conversion with thermophotovoltaics relies on radiation emitted by a hot body, which limits the power per unit area to that of a blackbody. Microgap thermophotovoltaics take advantage of evanescent waves to obtain higher throughput, with the power per unit area limited by the internal blackbody, which is n2 higher. We propose that even higher power per unit area can be achieved by taking advantage of thermal fluctuations in the near-surface electric fields.
For this, we require a converter that couples to dipoles on the hot side, transferring excitation to promote carriers on the cold side which can be used to drive an electrical load. We analyze the simplest implementation of the scheme, in which excitation transfer occurs between matched quantum dots.
Next, we examine thermal to electric conversion with a lossy dielectric (aluminum oxide) hot-side surface layer. We show that the throughput power per unit active area can exceed the n2 blackbody limit with this kind of converter. With the use of small quantum dots, the scheme becomes very efficient theoretically, but will require advances in technology to fabricate.—Wu et al.
A company called MTPV Corp. (for Micron-gap Thermal Photo-Voltaics), founded by MIT alum Robert DiMatteo is already working on the development of “a new technology closely related to the work described in this paper,” Hagelstein says.
DiMatteo says he hopes eventually to commercialize Hagelstein’s new idea. In the meantime, he says the technology now being developed by his company, which he expects to have on the market next year, could produce a tenfold improvement in throughput power over existing photovoltaic devices, while the further advance described in this new paper could make an additional tenfold or greater improvement possible. The work described in this paper “is potentially a major finding,” he says.
While it may take a few years for the necessary technology for building affordable quantum-dot devices to reach commercialization, Hagelstein says, “there’s no reason, in principle, you couldn’t get another order of magnitude or more” improvement in throughput power, as well as an improvement in efficiency.
There’s a gold mine in waste heat, if you could convert it. A lot of heat is generated to go places, and a lot is lost. If you could recover that, your transportation technology is going to work better.—Peter Hagelstein
D. M. Wu, P. L. Hagelstein, P. Chen, K. P. Sinha, and A. Meulenberg (2009) Quantum-coupled single-electron thermal to electric conversion scheme. J. Appl. Phys. 106, 094315; doi: 10.1063/1.3257402
P. L. Hagelstein and Dennis Wu (2007) Thermal to Electric Conversion with a Novel Quantum-Coupled Converter (MIT Research Laboratory of Electronics, Progress Report 2007)