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Researchers Demonstrate Quantum-Coupled Thermal to Electric Conversion With Efficiency as High as 40% of Carnot Limit, With Calculated Potential of Up to 90%

18 November 2009

Hagelstein
Basic scheme of the quantum-coupled converter. Shaded boxes indicate electron reservoirs. Arrows represent couplings. Letter U represents the electrostatic interaction while letter V represents the tunneling. Source: Hagelstein, 2007. Click to enlarge.

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

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November 18, 2009 in Materials, Vehicle Systems, Waste Heat Recovery | Permalink | Comments (14) | TrackBack (0)

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Economic direct conversion of heat to electricity would would be important.

Some have claimed that quantum well diodes can be used to exceed Carnot's limit which are always set well under 100% in most energy convertors to account for multiple losses.

If that is possible, energy transformation, (from heat to electricity) could eventually be achieved at 60+% efficency.

If this level of efficency can be achieved it would put an end to all (most) current ICE. However, if up to 60% of ICE waste heat could be efficiently recovered, the ICE inherent very low efficiency could be increased from about 30% to close to 50%.

A direct heat to electricty convertor, at 60+% efficiency (if possible) would be a better solution.

There is so much wasted heat in our transportation, electric plants, etc, that this is very exciting. If it wasn't coming out of MIT i would be very skeptical.
Any time you see "orders of magnitude" you gotta wonder.

It would be nice to see them explain this more in engineering terms. Will this work on cars? steam power plants?

"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."

This very process is the key to all manner of ubiquitous energy that permeates the universe. Hopefully they will be careful not to open this bag too wide.

Don't get too excited yet, it's experimental.

It sounds like it is equivalent to the Peltier Effect (normal thermocouples which I can relate to) but with the "wires" replace by thermally poor but electrically good conductors.

If the thermal conduction is very low - high efficiency.
But if the electrically conductive part is not so good - low power.
"But the team found that using their new system, it would be possible to get both at once”

Ahhh, I hope it comes along faster than the fuel cell.

Just an anecdote, but I walked in front of an armored truck that carries cash to the bank the other day, the heat coming off the radiator was immense. If you could harness even 1/3 of that heat you could probably double the mileage.

yes don't get too excited, quantum effect can only be obtained in nanostructure monocristal and using extremely high purity material. (quantum wells are routinely used for diodes lasers) Usually material using these effects are grown using Chemical Vapor Deposition (CVD) with growth rate like 1 microns/hour. So to make thermolectric cooler 1cm2 ok, but don't count on this for massive production of energy.

@HarveyD

What the thermodynamics rules say is that you can't get WORK from heat at more than the Carnot efficiency. And most of the time, people convert heat to work before converting work to electricity.

But of course, the rules concerning the conversion of heat to something other than work are different, so it wouldn't suprise me at all if this quantum effect could achieve 60%+ efficiency.

The Carnot cycle is based on the isothermal and adiabetic expansion and compression of an ideal gas. The efficiency of the cycle depends on the difference in absolute temperatures of a heat source and heat sink divided by the temperature of the heat sink. The Sterling engine is a device that comes closest to achieving the theoretical Carnot efficiency.

Fuel cell efficiency as well as this latest invention have nothing to do with the Carnot cycle since gas expansion and compression are not involved in the underlying process.

I think the Carnot limit simply bounds the amount of work that can be generated from any type of engine using two heat reservoirs, regardless of the type of engine.

This is a heat pump used in reverse in fact so the Carnot limit has to applied with care.

I think they use Carnot as a basis for comparison. This is not a heat engine, so that analysis does not apply here.

Exceeding the Carnot limit is impossible.  If you could do it, you could use a Carnot-cycle heat pump to supply heat to the over-Carnot engine and create a perpetual motion machine.

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