30 May 2005


The free-piston Stirling engine driven heat pump (FPSHP) is presented as an alternative residential
heat pump technology. In this type of heat pump system the mechanical output of an externally heated
free-piston Stirling engine (FPSE) is directly connected to a Rankine or transcritical cycle heat pump by
way of a common piston assembly. The attractiveness of this system is the economics of operation when
compared to an electrically driven conventional heat pump as well as the low environmental impact of the
system. It is expected that the primary energy ratio for the ground water source FPSHP will be close to
2.15 for heating mode and 3.34 for cooling mode with the inclusion of domestic hot water generation. The
working fluids are dominantly helium (He) gas for the engine and carbon dioxide (CO2) for the heat
pump. Technical concerns for this system include the effects of working medium mixing and the load
stability under various operating conditions. The direct connection of the Stirling engine to the
compressor of the heat pump allows for the working fluids to mix with each other. He separation for the
heat pump is discussed and the effect of the mixing of working fluids on both the heat pump and Stirling
engine is investigated through a demonstrative experiment and simulation. Experimental verification of
the performance due to the mixed working fluid is presented for the heat pump cycle, while simulation
techniques with proper gas mixture properties are used to determine the effect on the Stirling cycle. About
50% by volume of CO2 gas is expected in the working fluid of the free-piston Stirling engine and less
than 1% by volume of He gas is expected in the CO2 heat pump cycle.