Experimental and model-based analysis of an R600 high-temperature heat pump in sub-critical and trans-critical operation

Optimizing the operation and control of high-temperature heat pumps (HTHPs) enables to maximise the potential of this technology for waste heat recovery. Certain cycle configurations, such as flooded evaporators or low-pressure accumulators, require a control of the refrigerant subcooling or more specifically of the high pressure. The investigated R600 HTHP enabling sub- and trans-critical operation utilizes a single-stage cycle with low-pressure accumulator and internal heat exchanger. Since the achievable Coefficients of Performance (COPs) and heating capacities are strongly depending on the high pressure, a control strategy for this operating parameter is evaluated. The approach to utilize the temperature difference between refrigerant outlet and water inlet at the condenser as control variable for the high pressure control is experimentally tested. Using this control strategy, the COP and heating capacity of the HTHP are analysed in a broad range of operating conditions covering heat source temperatures from 40 to 60 °C and heat sink outlet temperatures between 110 and 160 °C, enabling a COP between 4.4 and 3.0 for 60°C heat source inlet temperature. The introduced simulation model predicts the dynamic response of the high pressure to EXV position changes with a maximum deviation of approx. 2 bar and +6% regarding refrigerant mass flow.