Design considerations for dynamically operated large-scale ammonia heat pumps.

Abstract

Large-scale heat pumps (HPs) are being implemented to provide heat for district heating (DH) systems from renewable heat sources. They are further expected to play a key role in integrated energy systems as flexible loads that can be used to balance the electricity grid. However, most large-scale HPs installed are designed for base load operation and are thus not optimized with regard to dynamic behaviour. Within this work, the dynamic behaviour of different HP configurations was assessed with regard to performance during load change. For this purpose, a two-stage and a one-stage ammonia HP were modelled using Modelica. Further, different adaptions of the two-stage HP to avoid condensation in the suction line were assessed. Fast ramping resulted in oscillation of the liquid level in the receivers. It would thus be necessary to design receivers carefully and include suction traps to avoid entrainment of droplets into the suction lines. The results indicated that condensation in the low-pressure suction line was a limiting factor for how fast the HPs may ramp down. This could be avoided by preheating of the suction line. In that case, the HPs under assessment were able to regulate between 100 % and 40 % load in less than five minutes, and they would thus be capable of providing secondary regulation power.