Scientists at the Tokyo Institute of Technology and Sanoh Industrial have developed a very stable battery cell that can directly convert geothermal heat into electricity.

Currently electricity is generated from geothermal systems using water heated by hot rocks a few kilometers below the Earth’s surface. Hot water is either pumped to the surface or water is pumped down, heated and pumped back up. The heat is used to produce steam to drive a turbine. Such systems need high temperatures, over 180°C, to work

The Japanese researchers have made a battery of sensitized thermal cells which are able to generate electricity from heat at temperatures below 100°C without needing any water or steam.

The battery made up of three layers of material sandwiched between two electrodes – an electron transport layer, a semiconducting layer of germanium and a solid electrolyte layer that transports copper ions.

The battery is designed to be buried in the relatively low temperature hot ground such as exist in vast areas of Australia and the United States. The heat excites the electrons in the semiconductor, causing them to move to the electron transport layer which passes them, via the electrode, through an external circuit and eventually back to the other electrode and into the electrolyte. In the electrolyte, oxidation and reduction reactions carry the low-energy electrons back into the semiconductor, starting the cycle over again.

Eventually, the cycle stops working because different types of ions end up in different places. But the researchers have found that turning on an external circuit recharges the battery – meaning that it can supply power “semi-permanently”.

The system still needs further refinement but it has the potential to supply electricity without burning fossil fuels and without the interruptions experienced with solar or wind power.