Control strategies for CO2 heat pump water heater at off design operation

CO2 heat pumps for hot water production, using carbon dioxide as a refrigerant, are on the rise due to their potential to reduce greenhouse gas emissions compared to traditional gas boilers and hydrofluorocarbon systems. However, these heat pumps face efficiency challenges, operating in a transcritical mode. Researchers are developing control strategies and solutions like the Internal Heat Exchanger (IHX) to address these issues. Studies in the open literature have focused on CO2 heat pump optimization. Wang et al. [1] identified correlations for optimizing gas cooler pressure based on ambient and water outlet temperatures, while Qi et al. [2] emphasized CO2 temperature at the gas cooler exit as a crucial factor in determining optimal discharge pressure. Efficiency improvement is a central goal for CO2 heat pump designers, with IHX being a promising solution. Kim et al. [3] found that IHX size influences optimal discharge pressure, refrigerant flow rate, and compressor power in water-to-water CO2 heat pumps. Cao et al. [4] explored the impact of IHX on optimal gas cooler pressure, concluding that IHX operation, especially under certain conditions, reduces optimal discharge pressure. They also developed a model accounting for heat exchanger pinch points to assess IHX efficiency's effect on overall system performance. However, it's worth noting that while IHX improves efficiency, it can increase discharge temperatures, particularly with higher return water temperatures at the gas cooler inlet, as observed by Otón-Martinez et al. [5]. In this study, a CO2 water heater heat pump with an IHX is analyzed through a numerical model and validated with experimental data. The research explores three different control strategies with the goal of enhancing performance while managing discharge temperature under off-design conditions.