The concentric piston-displacer type of free-piston Stirling engine is being developed at Ohio University for use as a gas-fired air conditioner, a long-life isotope powered electric generator, a solar-electric energy converter, a solar water pump, and a simple classroom demonstrator.
A linear alternator-engine combination has demonstrated 8% overall efficiency at a power output of 17 W with 600 K (620°F) heater wall temperature.
A free-cylinder water pump intended for solar energy operation has achieved 6% overall system efficiency using electric heat at 790 K (960°F).
An engine intended for gas-fired air conditioning has reached 1 kW power output and has demonstrated an ability to self-start on application of heat at 370 K (200°F).
Some experiments have been made with condensing working fluids capable of producing power at low heater temperature.
Several design innovations have been made, including a very simple and effective method of preventing piston position creep, and several means of displacer driving.