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Space-energy technology redesigned to make power stations more efficient

March 2007

Satellite-powering technology that was abandoned decades ago has been reinvented with the goal of helping conventional power stations convert heat to electricity more efficiently and reduce the use of fossil fuels. A Stanford University study in the February edition of the journal Nano Energy presents a novel heat-to-electricity conversion device that uses graphene instead of metal, making it almost seven times more efficient than previous technologies.

The researchers behind the study, led by Stanford Electrical Engineering Professor Roger Howe, say the use of new materials could reignite the field of thermionics, a method for converting heat to electricity first developed in the 1950s for the space program.

In the United States, more than 80 percent of electricity comes from large fossil-fuel power plants that use mechanical heat engines and turbines based on a 19th century technology. A thermionic energy convertor can convert heat to electricity more efficiently without the need for big, expensive equipment through the phenomenon of thermionic emission.

GCEP investigator Roger Howe (left) and Ph.D. candidate Hongyuan Yuan
Stanford Professor Roger Howe (left) and graduate student Hongyuan Yuan.
Credit: GCEP/Stanford Univrsity

A thermionic energy convertor is composed of two electrodes, an emitter and a collector, separated by a small vacuum gap. The device generates elecricity by transfering electrons through the vacuum from the hot emitter to the cooler collector. The Stanford team tested a prototype converter using a single sheet of carbon atoms – graphene – instead of tungsten metal as the collector material. They found that graphene improved the efficiency of the converter, making it 6.7 times more efficient at converting heat into electricity at 1,800 degrees Fahrenheit (1,000 degrees Celsius).

“This technology is very exciting," said study lead author Hongyuan Yuan, a doctoral student at Stanford. "With improvement in the efficiency, we expect to see an enormous market for it. Thermionic energy convertors could not only help make power stations more efficient, and therefore have a lower environmental impact, but they could be also applied in distributed systems like solar cells. In the future, we envisage it being possible to generate 1 to 2 kilowatts of electricity from water boilers, which could partially power your house.”

Existing thermionic energy technology faces two obstacles: A high loss of energy at the anode surface, which leads to reduced output voltage, and high electrical barriers against electrons moving in the gap between the collector and the emitter, which results in reduced output current. For the first time, the new prototype tackles both of these problems simultaneously. The findings of the study reveal an electronic efficiency in energy conversion of 9.8 percent – by far the highest efficiency at 1,800 F (1,000 C).

The technology is not yet ready for use in power stations or people’s homes, since the prototype is designed to work in a vacuum chamber. The researchers are now developing a vacuum-packaged thermionic energy convertor to test the reliability and efficiency of the technology in real-world applications.

“This prototype is just the first step – there is a lot more to do,” said Yuan. “But our results so far are promising and reflect a happy marriage between modern materials science and an old-fashioned energy technology, which provides a route for re-sparking the field of thermionic energy conversion.”

Other Stanford co-authors are recent Ph.D. graduate student Daniel Riley, Zhi-Xun Shen, a professor of applied physics and at SLAC National Laboratory, Piero Pianetta, a professor (research) of photon science at SLAC, and Nicholas Melosh, an associate professor of materials science and engineering.

This work was supported by the Stanford Global Climate and Energy Project and Bosch Energy Network.

Adapted from a news release from the Elsevier publishing company.

Schematic of the thermionic energy convertor prototype with a graphene collector. Right: Photograph of the thermionic prototype during operation in the Stanford lab.
Left: Schematic of the thermionic energy convertor prototype with a graphene collector. Right: Photograph of the thermionic prototype during operation in the Stanford lab. © Elsevier


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