Impact of System Control on the Performance of a Water-Loop Heat Pump System: A Detailed Analysis
This work focuses on a cascade heat pump system developed to deliver space heating, cooling and Domestic Hot Water to existing small Multi-Family Houses, as this building typology represents the most common in the EU context. The system analysed integrates centralised air-to-water heat pumps maintaining a water loop’s temperature in a range of 20-35 °C. The water loop acts as a thermal source for room-level water-to-air heat pumps, which provide space heating and cooling to a ten-apartment Multi-Family home. Additionally, a water-to-water heat pump connected to the loop supplies Domestic Hot Water. Dynamic simulation models, validated using monitoring data and laboratory measurements, were used to develop an optimised rule-based control strategy. This adjusts the set temperatures of the storage units and the distribution system to maximise the instantaneous system COP. Hourly and daily evaluations were carried out to identify the effect of system components sizes and dynamic set points on the overall system performance. In particular, we noticed that: increasing the storage volume (500-2000 l) reduces centralised heat pumps ON-OFF cycles. Moreover, implementing a specific climatic control for both the storage tank and the water loop can improve the system's COP up to 20% on specific days. This control aims to align the COP of the two heat pumps, as, in cascade systems, this maximises the system’s COP. In addition, the cascade system's daily COP, calculated in a specific winter week, is compared with the system COP of a reference heat pump system. Lastly, during summer, the combined operation of water-towater heat pump (which extracts thermal energy from the loop) and water-to-air heat pumps (which reject thermal energy to the loop) tends to complement each other. This reduces the use of the centralised heat pumps, which could remain off for up to two consecutive days.