During the course of several past discussions on energy efficiency, the issue of Stirling engines has arisen. These machines convert temperature gradients into usable kinetic energy which can be used to drive machinery or generate electricity. According to an article in this month’s Scientific American, they have found a new use. NASA is phasing out the radioisotope thermoelectric generators (RTGs) that have been used to power some space missions in favour of the older and non-radioactive technology.
RTGs work by using plutonium 238 decay to heat a thermocouple, which then produces usable current. The Stirling based system still uses plutonium decay for energy, but uses the heat more efficiently. The plutonium-Stirling combination is about 25% efficient at converting heat to electricity, compared to 6-7% for a conventional RTG. A prototype constructed by Lockheed Martin uses two Stirling engines to drive a generator and produce 100 watts of power. The unit that does so is about 1m long and 30cm wide, weighing 20kg – half as much as an RTG.
Extrapolating from space technology to more mundane uses is generally hazardous – for instance, satellites have solar panels with 35% efficiency, but they cost millions of dollars. That said, the technology does demonstrate that Stirling engines have a role to play in increasing efficiency in some circumstances.