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simple technique that allows direct coupling to a Rankine heat pump while preserving all the virtues of the free-piston Stirling engine. Unlike previous work, no effort is made to completely isolate the working fluid of the engine from the heat pump. The FPSHP utilizes a He–CO2 mixture for its working fluid and some intermixing is allowed to occur. The engine will operate well with concentration ratios of CO2 to He of 100/0 to 50/50. On the other hand, the Rankine heat pump will be more strongly affected by the working fluid mixture and consequently a He separator must be employed. Since the state of the CO2 is at one point in the cycle almost entirely liquid, it is relatively easy to separate out the He and return it to the engine thereby keeping the Rankine working fluid almost entirely free of He. The net result is that the engine operates with a He–CO2 mixture while the heat pump operates with an almost pure CO2 cycle. The device detailed here has been sized for residential applications using a ground water source/sink for one end of the heat pump cycle. The CO2 cycle never goes transcritical and therefore COP is maintained at high levels under all load conditions. Pressures also remain relatively low compared to the standard CO2 transcritical cycle. The use of CO2 has a number of strong attractions. It is identified as a ‘natural’ working fluid and as such is seen as relatively benign to the environment. In the application of a ground water heat pump, its operation is always below the critical point which greatly simplifies the implementation of the cycle. For the particular machine discussed here, CO2’s high dissociation temperature (in excess of 1500 K) and transport properties, make it far less detrimental to the performance of the Stirling engine even in relatively high proportions to the He charge. 2 SYSTEM DESCRIPTION The FPSHP system has been configured for use with a ground water source/sink in order to maximize the energy utilization per unit input, as shown in Fig. 1. The system is composed of two major sub-systems: the FPSE and the CO2 heat pump. The FPSE is composed of a combustor and a Stirling engine. The CO2 heat pump is composed of typical Rankine cycle components with the addition of the He separator. A control system not shown in the Fig. 1 is required for load matching between the FPSE output power and compressor power for the Rankine cycle. Total estimated mass of the FPSHP but not including external heat exchangers and auxiliaries is 18 kg. Heat transport to indoor spaces could be by secondary water or air circulation. The energy source for the external combustion system most readily available in residential areas would be natural gas. For a completely autonomous system, additional power could be provided from the FPSE alternator/motor to power the peripheral components such as water circulation pump, indoor fan, burner blower, and electronics. Two 4-way valves are necessary to switch the heating and cooling mode of the heat pump. Rejected heat, from the FPSE, can be recovered and utilized, either by a storage tank to augment domestic hot water production or by direct integration into space heating. Waste heat of combustion is used for super-heating CO2 vapor entering the suction process. This is done in order to increase the heat pump discharge temperature and promote heat transfer in heating mode. The CO2 cycle remains sub-critical for all conditions during the heating mode. Therefore, there are four possible modes of operation: indoor heating, indoor heating plus water heating, water heating, and indoor cooling plus water heating. Since CO2 leakage across the compressor piston to the engine side increases the engine mean pressure, the He–CO2 mixture returns to the compressor by a return device that is activated by pressure difference between engine mean pressure and compressor suction pressure. He out of the mixture returned into the compressor side is separated by the He separator. The He separator located at the end of condensation and sub-cooling process separates He gas and returns it to the engine side. If the CO2 cycle goes to a transcritical cycle, the He separator can be located after a partial expansion with an additional 2PDF Image | HERMETIC GAS FIRED RESIDENTIAL HEAT PUMP
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CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
Heat Pumps CO2 ORC Heat Pump System Platform More Info
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