Laboratory investigation of control strategies of a cascade heat pump system with intermediate water loop
A water-loop, cascaded heat pump system has been designed targeting the renovation of buildings originally heated by a central gas boiler. The project included the control development, laboratory tests and installation and monitoring of the system at a demonstration site. The system uses the existing water distribution loop to connect a central to room heat pumps: central heat pumps provide low-temperature heat to the water-loop, which supplies booster heat pumps covering domestic hot water, space heating and cooling demands. The control of all the components is crucial for the effective operation of the whole system. The paper presents the laboratory characterization of the system, while in operation under four different rule-based controls. In the first case, we considered the same control implemented on board of the system installed at the demonstration site, while the others explore potential improvements. The system has been set up in three adjacent climatic chambers: one hosted the external unit (representing the outdoor ambient), one the water-loop and the storages (technical room) and one the internal units (indoor ambient). The tests were conducted by producing dynamic air temperature and humidity in the chambers, by emulating domestic hot water space draws, hence allowing to test the system under a range of real-like operating conditions. The results demonstrated the control strategies effectiveness: the importance of the control of the source heat pump which COP affects the cascade COP and the importance of the control of the water loop that affects the thermal losses and the auxiliary consumptions. From the base control the system COP has been improved by 2% to 80% with the different controls. The cascade system has been compared with an air-to-water heat pump that feeds fan-coils or radiators. The cascade system COP is 40% higher than the single stage unit with fan-coil.