Optimizing control of two-stage ammonia heat pump for fast regulation of power uptake

Large-scale heat pumps are expected to be able to help in balancing the power grid by providing ancillary services. The most common type of heat pumps supplying district heating in Denmark is the two-stage ammonia heat pump. The objective of this study was to assess, how the control strategy influences the dynamic behaviour of the heat pump and to investigate whether the heat pump could supply primary frequency regulation for the electricity grid. For this purpose, a dynamic model of this heat pump type with variable speed piston compressors was implemented in Modelica and validated against experimental data from a heat pump in Copenhagen, Denmark. The results showed that the best suited control structure for fast regulation included a direct control of the power uptake, evaporation pressure control, source outlet temperature control, and preheating of the suction line. It allowed regulation of the heat pump power input from 250 kW to 175 kW in 85 s and from 250 kW to 100 kW in 140 s without the risk of condensation in the low-stage suction line. This means primary frequency regulation, which requires ramping rates below 150 s, could be supplied with the assessed large-scale ammonia heat pump.