In experiments involving a different new technology, thermal diodes, Peter Hagelstein (MIT) worked with Yan Kucherov, now a consultant for the Naval Research Laboratory, and coworkers to demonstrate efficiency as high as 40 percent of the Carnot Limit. Moreover, the calculations show that this new kind of system could ultimately reach as much as 90 percent of that ceiling. Current commercial thermoelectric devices only achieve about one-tenth of the Carnot limit.
Hagelstein, Wu and others started from scratch rather than trying to improve the performance of existing devices. They carried out their analysis using a very simple system in which power was generated by a single quantum-dot device — a type of semiconductor in which the electrons and holes, which carry the electrical charges in the device, are very tightly confined in all three dimensions. By controlling all aspects of the device, they hoped to better understand how to design the ideal thermal-to-electric converter.
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 plenty of electrical power — what is known as 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.
A company called MTPV Corp. (for Micron-gap Thermal Photo-Voltaics), founded by Robert DiMatteo SM ’96, MBA ‘06, 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.
DiMatteo says that worldwide, about 60 percent of all the energy produced by burning fuels or generated in powerplants is wasted, mostly as excess heat, and that this technology could “make it possible to reclaim a significant fraction of that wasted energy.”
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.
Why Thermoelectrics Are Huge for Energy Efficiency
Industrial waste heat is 7 quads in the USA. There is more waste heat from power plants and from cars. Applying thermoelectrics to our current power plants would be like adding 10-30 nuclear power plants and 150-375 coal plants and 500-1500 natural gas plants that would not use any more fuel because it would be from more efficient use of existing power plants.
Heat flows in a car and using thermoelectrics to tap the waste heat. The standard combustion engine is about 30% efficient, but regular diesel engines are about 38% efficient. New diesel engine and free piston engines can reach 50% efficiency or more. The energy for cooling can also be reduced using thermoelectrics.
The 30% energy efficiency is triple the efficiency of todays common thermoelectrics and double most advanced systems and would get to range of using solid state thermoelectrics to replace refrigerators [thermoelectrics can help cool as well as convert heat to electricity] and many small car sized engines. Typical conversion systems become less efficient as they are scaled down to small size. This means there is a crossover point: below some power level thermoelectric technology will tend to be more efficient. Increasing ZT will move the crossover point to higher power levels, increasing the range of applications where thermoelectrics compete. Thus the ZT of 3 to compete with current best car size and refrigerator mechanical systems.
Thermoelectric Figure of Merit
The primary criterion for thermoelectric device viability is the figure of merit given by:
which depends on the Seebeck coefficient, S, thermal conductivity, λ, and electrical conductivity, σ.
The New Research Paper
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 n^2 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 n^2 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.