1950s energy converter reinvented with graphene

US researchers have updated the process of thermionic energy conversion for the 21st century, using wonder material graphene as a key part of the technology.

Thermal energy is one of the most abundant, cheap and widely used energy sources in the world, but it is harvested using mechanical heat engines and turbines based on antiquated 19th-century technology. Thermionic energy converters (TEC), on the other hand, can convert heat to electricity more efficiently, without the need for big, expensive equipment, through the phenomenon of thermionic emission.

Originally developed in the 1950s for use in space programs, TECs are composed of two electrodes — the emitter and the collector — separated by a small vacuum gap. Unfortunately, the 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.

Now, researchers at Stanford University have tested a prototype TEC made using a single sheet of graphene instead of tungsten as the collector material. Writing in the journal Nano Energy, they revealed that this led to an electronic conversion efficiency of 9.8% at 1000°C, making it 6.7 times more efficient than the previous version of the technology.

According to lead author Dr Hongyuan Yuan, TEC technology is “very exciting”, with the efficiency improvements potentially opening up an “enormous market for it”.

“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,” said Dr Yuan. “In the future, we envisage it being possible to generate 1–2 kW of electricity from water boilers, which could partially power your house.”

The prototype currently works in a vacuum chamber but not in a normal setting, meaning the technology is not yet ready for use in power stations or people’s homes. The researchers are now working on a vacuum-packaged TEC to test the reliability and efficiency of the technology in real applications.

“This prototype is just the first step — there is a lot more to do,” said Dr 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.”

Image caption: A schematic sketch of the thermionic energy converter prototype with a graphene collector.