U.S. Army Aims for a World First with a Stirling Engine to Harvest Heat and Drive an Autonomous Car
With a new battery system and a Stirling engine originally designed to power homes future uncrewed vehicles could go months between charges.
The U.S. Army claims to have developed a large-scale Stirling engine — a system for generating electricity based on the temperature differential between two surfaces — capable of powering an uncrewed ground vehicle for months at a time.
Named for Robert Stirling, a Stirling engine converts heat energy into mechanical energy. Work on the first Stirling engines began in the 19th century as a competitor to the steam engine, but difficulties in scaling the technology up meant it was typically limited to low-power applications — giving it the status of "domestic motor" and leaving the technology to be all-but abandoned following the development of electric motors.
In recent years, the Stirling engine has become of interest once again - particularly in the military, where modified Stirling engines can be found acting as generators running on otherwise-wasted heat or cooling by-products. It's here the U.S. Army's interest in the technology begins: The conversion of a Stirling generator originally developed to bring energy to the home for use on an uncrewed autonomous ground vehicle for long-term deployment.
"This all has to be done in a way that integrates well with the vehicle," explains lead researcher Justin Shumaker, "is reliable, has enough headroom to operate on very hot days and uses very little electricity for the pumps and fans. One day, autonomous robots will outperform Soldiers at certain tasks and do so without the requirement of eating, sleeping or resting. This has the greatest potential benefit to the Soldier, removing them from harm’s way."
While the Army has yet to publish any detailed technical documents about the technology, it has confirmed that the test vehicle is based on a Clearpath Robotics Warthog all-terrain uncrewed ground vehicle stripped down to its chassis and wheels and then built back up again through a lithium-titanate battery system and DC-DC power converter with a cooling loop to keep the Stirling cycle intact.
"In order for a Stirling engine to operate," Shumaker explains, "there needs to be a temperature difference between the hot and cold ends. The cooling loop removes heat from the cold end to keep it cold."
More information on the project, which is to be delivered as a proof-of-concept in the autumn, can be found on the U.S. Army website; Shumaker also discussed the project late last year in a U.S. Army podcast.