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7.2.4.3 The Expansion Valve The relative exergy loss for the expansion valve during operation in the SH mode was nearly twice as high as in the combined mode and the DHW mode. This was mainly a result of the 10 to 25 K higher CO2 outlet tempe- rature from the tripartite gas cooler. However, at higher inlet water tempe- ratures for the DHW preheating gas cooler unit, the relative expansion loss will be significant even in the combined mode and the DHW mode. In addition to an optimum gas cooler design resulting in a minimum temperature approach, Lorentzen (1994) stated that methods of current interest for reducing the expansion loss in CO2 heat pump and air condi- tioning systems include multiple compression and expansion, recovery of expansion work by means of an expansion turbine and installation of a suction gas heat exchanger (internal heat exchanger). From an economical and technical point of view, only the latter alternative represents a viable option in residential CO2 heat pump systems. A suction gas heat exchanger increases the temperature of the suction and discharge gas of the compressor, reduces the CO2 mass flow rate due to lower vapour density at the compressor inlet and lessens the optimum high-side pressure. As a consequence, there will be a drop in both the gas cooler heating capacity and the compressor power input. The CO2 heat pump simulation model (ref. Section 6.1) was used to calculate the COP of the prototype CO2 heat pump unit during operation in the SH mode at 35/30oC supply/return temperatures and varying suction gas temperatures. The simulations showed that increasing superheat had a negligible effect on the COP (<1%). Consequently, for integrated brine-to-water CO2 heat pumps, a suction gas heat exchanger should only be installed as long as the system is equipped with a low-pressure receiver (LPR), and it is considered as necessary to evaporate the liquid droplets in the suction line. Reference is made to Appendix A2.2, Methods of Controlling the High- Side Pressure, for further details on LPR systems. 7.2.4.4 The Space Heating System The exergy loss for the space heating system can only be reduced by lowering the average temperature level during heat rejection. This implies that low-temperature heat distribution systems, such as hydronic floor heating systems, fan-coils and convectors, should be used instead of radia- tors and other high-temperature systems (ref. Section 2.3, Hydronic Heat Distribution Systems). 7 – Discussion and Analysis 223PDF Image | Residential CO2 Heat Pump System for Combined
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