Analysis of different control strategies for improved performance at off design operation in a CO2 heat pump water heater

A commercial heat pump water heater using CO2 as refrigerant is considered in this work. A theoretical model was first validated via experimental data, and then tested to evaluate various operations. A fundamental and well-known parameter for optimizing the system performance is controlling the high stage pressure. In this work, a logic control was derived to maximize performance even under off-design conditions. Poor stratification, or improper sizing of the water system may lead to compressor outlet temperature too high, which should be limited to avoid problems with lubricants. Usually, the maximum compressor discharge temperature is 140 °C and, to control this temperature, this work proposes and compares three different control logic, acting on the back pressure valve or on the internal heat exchanger (IHX) with the aim of gaining efficiency and reducing the operating cost.