The need to get rid of excess heat creates a major source of inefficiency in everything from computer processor chips to car engines. According to Peter Hagelstein, an associate professor of Electrical Engineering at MIT, existing solid-state devices to convert heat into electricity are not very efficient. Experimenting with thermal diodes, an MIT team has come closer to the theoretical limitations for the efficiency of such a conversion.
Theory says that such energy conversion can never exceed a specific value called the Carnot Limit, based on a 19th-century formula for determining the maximum efficiency that any device can achieve in converting heat into work. Current commercial thermoelectric devices only achieve about one-tenth of that limit, Hagelstein says. In experiments involving thermal diodes, the MIT team demonstrated efficiency as high as 40 percent of the Carnot Limit. The calculations also show that this new kind of system could ultimately reach as much as 90 percent of that ceiling.
Graduate student Dennis Wu and Yan Kucherov, now a consultant for the Naval Research Laboratory, were part of the research team. They carried out their analysis using a 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 is possible to efficiently convert heat into electricity, but with very little power. It’s also possible to get enough electrical power — or high-throughput power — from a less efficient, and therefore larger and more expensive system. The team found that using their new system, it would be possible to achieve both high efficiency and high throughput at once.
A key to the improved throughput was reducing the separation between the hot surface and the conversion device. MIT professor Gang Chen previously found 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 research 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.
Hagelstein says that when this work began around 2002, such devices “clearly could not be built. We started this as purely a theoretical exercise.” Developments since then have brought development much closer to reality, though it may still take a few years for the necessary technology for building affordable quantum-dot devices to reach commercialization.
“There’s a gold mine in waste heat, if you could convert it,” Hagelstein says. The first applications are likely to be in high-value systems such as computer chips, but ultimately it could be useful in a wide range of applications, including transportation. “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.”