Hot water supply using a transcritical carbon dioxide heat pump : a thesis presented in fulfilment of the requirement for the degree of Master of Engineering at Massey University, Palmerston North, New Zealand
In New Zealand (NZ) a typical household uses between 160-330 I of hot water per day at 50 to 60°C. Most hot water systems are electrically heated. Heat pumps using carbon dioxide (CO₂) in the transcritical heat pump cycle offer high potential for energy savings. The use of CO₂ also offers further benefits such high volumetric heating capacity, reduced environmental impact, good availability and low costs. The objective of this project was to design, build and test a hot water supply system (HWSS) using a CO₂ heat pump. The main components of the HWSS were the heat pump, a stratified hot water storage cylinder (HWC), a water pump and a control system. The heat pump design was based on a prototype Dorin CO₂ compressor which was available. Key features were use of a vented spiral tube-in-tube heat exchanger for the gas cooler, use of a low pressure receiver incorporating an internal heat exchanger after the evaporator and the use of a back-pressure regulator as the expansion valve. The heat pump had a nominal design heating capacity of 8.1 kW with a COP of 3.9 at 0°C/34.8 bar.a evaporation temperature/pressure and 100 bar.a discharge pressure when heating water from 15°C to 60°C. The prototype heat pump performance was measured for a range of operating conditions including 0°C/33.8 bar.g to 15°C/49.8 bar.g evaporation temperatures/pressures, 18 to 30°C cold water inlet temperature, 40 to 60°C hot water outlet temperature and 90 to 120 bar.g discharge pressures. Liquid refrigerant and/or oil carry over caused by limited LPR separation capacity and/or oil foaming in the LPR was apparent for some trials but could not be completely eliminated. The compressor isentropic and volumetric efficiencies were about 30% lower than stated by the manufacturer. Possible reasons were mechanical and/or compressor oil related problems. The gas cooler was marginal in capacity especially when the heat pump operated at high evaporation pressure conditions. The measured heat pump heating capacity at the design conditions was 5.3 kW at a COP of 2.6. The heat pump COP was not sensitive to the discharge pressure across a wide range of operating conditions, so constant discharge pressure control was adopted. Overall best heat pump efficiency for 60°C hot water was achieved at 105 bar.g discharge pressure. At these discharge conditions the heating capacity and COP ranged from 4.8 kW and 2.2 at 0°C/33.8 bar.g evaporation temperature/pressure and 30°C cold water inlet temperature to 8.7 kW and 3.9 at 15°C/49.8 bar.g evaporation and 18°C water inlet respectively. A mathematical model of the HWSS was developed. The model parameters were determined from a small set of separate trials. The overall agreement between measured and the predicted HWSS performance was good. The HWSS performance was predicted for conditions likely to occur in a one or two family home. The biggest efficiency losses were HWC standing losses to the ambient air. The heat pump operated with close to the maximum COP of 2.75 because the water inlet temperature seldom rose above 25°C. There was potential for efficiency improvements if the short on/off intervals caused by the relatively small HWC relative to the heating capacity of the heat pump could be avoided. Overall, the investigation has shown that the CO₂ heat pump combined with a stratified HWC can provide a very efficient HWSS. The heat pump prototype performance was competitive with conventional heat pumps but there was significant potential for efficiency improvements due to the poor compressor performance. However, the availability and costs of heat pump components and the poor compressor performance constrain the commercial implementation.