Due to ever more strict requirements on the global warming potential, there is a shift in the kinds of refrigerants that are being used in heat pump systems. Particularly, in recent years mildly flammable and flammable refrigerants are being introduced. The resulting safety requirements form an additional driver towards reduction in refrigerant charge. On the other hand, the performance of heat pump systems is continually being improved, with ever increasing operation areas and coefficients of performance. High COPs are critical not only in full load design conditions, but also in off-design part load conditions. Currently, the refrigerant charge is optimized by testing on physical prototypes and needs to be redone when there are big design changes. As this takes a significant amount of resources, reliable predictions of the impact of design choices on the refrigerant charge would contribute to faster development and better systems.
The goal of this thesis is to develop a simulation model, which can predict the performance of a heat pump system over a wide variety of operation conditions, taking into account a fixed refrigerant charge. The impact of different design choices is evaluated, in order to provide good performance over a wide variety of operation conditions, while minimizing the required refrigerant charge. Particularly, the impact of the ratio of the heat exchanger volumes, the type of expansion valve control and the design of the charge storing components such as liquid receivers and suction accumulators is taken into account.