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HERMETIC GAS FIRED RESIDENTIAL HEAT PUMP ( hermetic-gas-fired-residential-heat-pump )

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HERMETIC GAS FIRED RESIDENTIAL HEAT PUMP David M. Berchowitz, Director and Yong-Rak Kwon, Senior Engineer Global Cooling BV, Ijsselburcht 3, 6825 BS Arnhem, The Netherlands Tel: 31-(0)26-3653431, Fax: 31-(0)26-3653549, E-mail: info@globalcooling.com ABSTRACT 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. Key Words: heat pump, free-piston, Stirling, gas-fired, natural working fluid, CO2 cycle 1 INTRODUCTION It is well known that fuel driven heat pumps allow far greater energy utilization. This is measured by the primary energy ratio (PER), which is defined as the ratio of useful energy provided by the device divided by the original generated energy needed for input. For example, the PER of an electric heater is just the fraction of energy originally consumed as fuel at the central electric generating facility that is made available as electricity at the point of use. Usually about 0.38. An electric heat pump has a much higher PER since it adds energy taken from the environment to the input to provide the heat output. In this case the PER may be above 2.0 for a ground water heat pump. A fuel driven heat pump has the highest potential PER in that by utilizing the primary energy source on site, it is able in theory to capture all the heat of conversion to provide that in addition to the pumped heat. The PER in this case may approach 2.5 with the additional advantage that primary energy is generally cheaper on a per unit energy cost basis. Gas fired heat pumps using the Stirling as the prime mover has been pursued many times before. Efforts include a Duplex Stirling arrangement by Sunpower in 1983 (Penswick and Urieli 1984), a free- piston Stirling engine hydraulically driving a Rankine heat pump by Mechanical Technology Inc. (Marusak and Ackermann 1985), a second and third effort by Sunpower using an inertial drive and magnetic coupling to a Rankine heat pump (Wood et al. 2000, Chen and McEntee 1993) and various efforts in Japan (MITI 1986) and Europe (Lundqvist 1993). All these efforts identified the obvious advantages of the Stirling engine being its high part load efficiency coupled with a potential for high reliability and long life. However, in every case, these efforts failed in their approach to coupling the Stirling engine to the heat pump in a practical cost-effective manner. The concept laid out here offers a 1



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