| dc.description.abstract | Currently, there is a lack of monoblock CO2 heat pumps specifically designed for domestic hot water (DHW) production with low heating capacities (2 kW). In light of this market gap, this work focuses on the development and performance evaluation of a monoblock CO2 heat pump for DHW production. Such a heat pump has the potential to provide a sustainable and efficient solution to meet the increasing demand for DHW while reducing carbon emissions.
The objective is to investigate the possibility of converting an existing heat pump water heater (HPWH) system, currently using R-134a refrigerant, to a CO2-based system while maintaining comparable performance, size, and cost. The project includes the design and modelling of various components such as the gas cooler, internal heat exchanger (IHE), expansion device, evaporator, liquid separator, and oil return management. An innovative control logic according to the approach temperature in the gas cooler is also investigated.
Simulation models of the heat pump have been developed in Modelica using Dymola as a simulation environment and the TIL-Suite library. Two simulation models have been developed and compared: one with a control logic based on the temperature approach of the gas cooler and one with fixed pressure to assess their performance. The results demonstrate that the approach temperature control logic effectively maintains the desired approach temperature. However, after a certain period, the approach temperature increases due to the limitations of the fixed high-side pressure set by the compressor's envelope. The isentropic efficiencies of the compressor stages are also evaluated, with the first stage exhibiting higher efficiency compared to the second stage.
The coefficient of performance (COP) of the cycle is significantly influenced by the compressor performance, and it is observed that the COP improvement with the temperature approach control logic is more significant at higher ambient temperatures. The relationship between COP and return water temperature is examined, emphasizing the importance of stratification in the tank for transcritical cycles. The heating capacity of the system decreases over time and with increasing return water temperature. The control logic with the temperature approach shows an improvement in overall COP compared to simulations with fixed high-side pressure. Furthermore, the CO2 transcritical cycle, compared to the previous R-134a system shows faster heating time with comparable COP.
Overall, the results demonstrate the potential of \co2-based heat pump systems for domestic hot water production and the effectiveness of the temperature approach control logic in enhancing performance. The findings highlight the potential of \co2-based heat pump systems for efficient domestic hot water production while reducing environmental impact. The temperature approach control logic presents a promising approach to maintaining desired temperatures, and further optimization efforts can unlock even greater performance improvements. The potential cost increase for a CO2 system is approximately 15\%, but industrialization can lead to cost reductions. | |