Analysis and optimization of subcritical two-stage vapor injection heat pump systems

•Thermodynamical analysis of a two stage compression cycles.•Optimization of the displacement ratio for COP in ideal two stage cycles.•Analysis of the influence of compressor efficiency in real system.•Influence of the charge in the optimum conditions of a two stage cycle.•Control in two stage systems with electronic expansion valves. Two 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. 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. This study analyzes the influence of design parameters and injection conditions for two different configurations of two stage cycles, for four refrigerants (R407C, R290, R22 and R32). Design parameters, such as the displacement ratio, are optimized in terms of COP in ideal conditions for both injection systems. A deeper analysis taking into account the efficiencies of the compressor is done finding that two stage systems could reach improvements of 30% in terms of COP compared with one stage systems and that a bad design of this type of systems could represent a loss of improvement between 6% and 10%. Finally a method to control the system at any operating point in order to make it works in its optimum is done. From all the analysis, guidelines for the optimum design and control of such systems are obtained in terms of capacity, Coefficient of Performance (COP), seasonal performance factor and discharge temperature criteria.