Performance analysis of a free-piston Vuilleumier heat pump with dwell-based motion

Vuilleumier heat-driven heat pumps (VHPs) provide potential energy savings of 30-50% over traditional combustion heating systems. Nearly all prior VHP studies in the literature utilize a sinusoidal displacer motion. Free-piston-based Vuilleumier heat pumps, however, can operate with non-sinusoidal displacer motions as well. This work presents the analysis of a dwell-based displacer motion in which only one displacer moves at a time. A thermodynamic model that incorporates mass and energy conservation for all major VHP components is solved numerically for the heat flow, coefficient of performance (COP), and the pressure and temperature during one complete cycle. Model results are compared to experimental from the literature for two different VHP designs, with good agreement found in both cases. Further analysis of the dwell-based motion reveals that it provides 28% higher heat output with a slightly reduced COP for the same machine geometry operating in sinusoidal motion. Partial-load situations are found to be obtainable by increasing the dwell time with little impact on the COP. For free-piston VHPs a multi-motion strategy is suggested that may provide optimal performance across a range of load conditions.