ElsevierRedon Climent, A.; Navarro-Peris, E.; M. Pitarch; José Gonzálvez-Maciá; José M. Corberán (2014). Analysis and optimization of subcritical two-stage vapor injection heat pump systems. Applied Energy. 124:231-240. Analysis and optimization of subcritical two-stage vapor injection heat pump systems AbstractTwo of the major problems of heat pump systems working in extreme conditions are the loss of efficiency of the system and the high compressor discharge temperatures which can even impede the correct performance of the machine. One possibility in order to overcome these issues is to perform the compression in two stages. In this frame the use of vapor injection two stage cycles represent an economic and effective solution. The results of using this kind of cycles are the reduction of the pressure ratio of each compression stage, the reduction of the discharge temperature due to the cooling effect provided by the intermediate cooling and the increase of theoretical cycle efficiency. This study analyzes two vapor injection cycles, in terms of the injection conditions for three different refrigerants (R407C, R290 and R22). The work considers the efficiencies of each component and their sensibility to the working conditions. From all the analysis, guidelines for the optimum design and control of such systems are obtained in terms of capacity, COP and discharge temperature criteria.Keywords: Heat pump, vapor injection, efficiency, simulation, Two-stage.

Subcritical systems working with a subcooler have been proved as a good solution for sanitary hot water production, where the high water temperature lifts (usually from 10ºC to 60ºC) make it possible to produce a high degree of subcooling. This paper presents the experimental results obtained from a new heat pump prototype working with Propane (R290) as refrigerant, With the proposed configuration, the subcooling is made inside the condenser (separate subcooler heat exchanger is not used), and can be controlled independently at any point. The obtained results have shown that COP depends strongly on subcooling. In nominal condition (inlet/outlet water temperature at evaporator is 20ºC/15ºC and the water inlet/outlet temperature in the heat sink is 10ºC and 60ºC), the optimum subcooling is 42K with a heating COP of 5.35, which is about 25% higher than the same cycle working without subcooling.

Experimental study of a subcritical heat pump booster for sanitary hot water production using a subcooler in order to enhance the efficiency of the system with a natural refrigerant (R290).

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ElsevierPitarch, M.; Navarro-Peris, E.; Corberán, JM. (2017). Evaluation of different heat pump systems for sanitary hot water production using natural refrigerants. Applied Energy. 190:911-919. AbstractHeat pumps that work with a high degree of subcooling in subcritical systems have shown a significant margin of improvement when working with sanitary hot water applications. Recently, two different approaches to overcome the high degree of subcooling have been presented in the literature: with a subcooler (separate from the condenser) and by making all the subcooling in the condenser. In this paper, a comparative evaluation between both alternatives is presented, and the obtained results are compared with a representative solution already available on the market using natural refrigerants for this application. The results of this analysis have shown that in a system with subcooling in the condenser, it is possible to obtain a COP comparable to that of transcritical CO2 heat pump water heaters. Furthermore, the system with subcooling has been demonstrated experimentally as being capable of producing water up to 90ºC and has shown a COP up to 20% higher than some CO2 commercial products (catalogue data reference).