Calculation-Based Assessment of Selected Operating Conditions and a Proposed Supervisory Control Concept for a Transcritical R744 Heat Pump

Abstract

This paper presents a calculation-based assessment of selected operating conditions and proposes a supervisory control concept for a small transcritical R744 heat pump designed for the utilization of low-temperature waste heat. The unit has been constructed, but its complete operating envelope and proposed control structure have not yet been experimentally validated. The presented results were obtained using an in-house steady-state thermodynamic model. At the nominal operating point, defined by an evaporation temperature of −10 °C, a gas-cooler outlet temperature of 35 °C, a compressor frequency of 50 Hz and an active ejector-nozzle area of 14%, the calculated gas-cooler capacity was 6.0 kW, the evaporator capacity was 3.8 kW, the compressor electrical input was 2.12 kW, the heating COP was 2.81 and the discharge pressure was 92.46 bar. Across the evaluated operating combinations, the mean COP increased from approximately 2.40 to 3.30 as the evaporation temperature increased from −12 °C to −5 °C. The control-oriented interpretation identifies the high-side pressure, gas-cooler outlet temperature, heat-source conditions, compressor frequency and ejector setting as relevant inputs to the proposed supervisory control strategy. A priority-based supervisory structure is therefore proposed, giving precedence to high-pressure protection and heat-source protection, followed by capacity control and efficiency optimization within the admissible operating region.