Graphene Turns Decades-Old Space Energy Technology into Ultra-Efficient Thermionic Generator

The TEC prototype in operation at the Stanford Lab.(Image courtesy of Elsevier.)

Researchers at Stanford University have found a way to take satellite-powering technology from the 1950s and repurpose it to meet the growing need for renewable energy in the 21^st century. Thermionic energy converters (TEC) traditionally used bimetallic junctions to convert heat into electricity, but a new prototype of the technology recently unveiled in the journal Nano Science uses graphene in the place of metal to make it nearly seven times more efficient than the original.

According to the lead researcher, professor Roger Howe, using graphene in the process could breathe new life the field of thermionic energy conversion—by improving electricity production and reducing environmental impacts of the process.

While thermal energy is one of the most abundant, cheap and widely used sources in the world, to date it has been harnessed primarily using mechanical heat engines and turbines that have been made with the same designs since the 19^th century–and as a result, it can only be employed in large power installations.
In contrast, the new TEC prototype has the capacity to convert heat to electricity more efficiently and without the need for large, costly equipment. TECs were originally used in the 1950s on a large scale for space programs, but were not efficient enough to use in industrial energy processes–until recently.

Stanford’s TEC prototype uses two electrodes, the emitter and collector, which are separated by a small vacuum gap. The researchers tested their prototype using a single sheet of graphene in place of tungsten as a collector material. Their results revealed that the new carbon-based collector material improved the efficiency by 6.7 times when converting heat into electricity at 1,000° C (1,832° F).

"TEC technology is very exciting. With improvement in the efficiency, we expect to see an enormous market for it," said Stanford researcher and lead author of the paper, Hongyuan Yuan. "TECs 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 1-2 kilowatts of electricity from water boilers, which could partially power your house."

While it shows much promise, this new TEC technology is not without challenges to overcome. The researchers need to find solutions for a high loss of energy at the anode surface that leads to reduced output voltage, as well as high electrical barriers against electrons moving in the gap between the collector and emitter. The latter results in reduced output current.

The technology is still not ready to be applied to practical uses such as powering homes, as it still works only in a vacuum chamber. But researchers are working on a vacuum packaged TEC that will allow them to test the reliability and efficiency of the generator in real-world situations.

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