The Vuilleumier cycle...utilize[s] working gas expansion and compression at three variable volume spaces in order to pump heat from a low to a moderate temperature level.
The interesting characteristic of the Vuilleumier machine is that the induced volume variations are realized without the use of work, but thermally.
In March 2014, the Vuilleumier Cycle was tested in application with updating conventional HVAC (heating, ventilation, and air-conditioning) systems by utilizing the cycle's proposed thermodynamic process of moving heat energy, and having results of increased output efficiencies coupled with a reduced carbon footprint.
[4] This work culminated in the demonstration of the ThermoLift system at Oak Ridge National Laboratory in August, 2018.
The demonstration showed that the ThermoLift technology (TCHP), is able to achieve coefficients of performance (COP) for the cycle that well exceeded the DOE’s target COPs for cold-climate heat pumps (although not at all exceeding Geothermal heat pump efficiencies).