Experimental investigation on the performance of a transcritical CO2 heat pump with multi-ejector expansion system

Carbon dioxide is becoming an interesting option also for HVAC heating appliances due to its eco-friendly characteristics. The incurrence of transcritical cycles makes the use of ejectors attractive to improve performance. Part load conditions and varied ambient temperatures imply the use of variable geometry systems of appropriate design to keep their efficiency high. This paper investigates experimentally a multi-ejector air-to-water CO2 heat pump for heating needs, measuring the performance of the system and of each component under partial and full load conditions. A sensitivity analysis was run varying, separately or in a combined way, the following parameters or operating conditions: ejector area ratio in a range of 33.1-86.6%; compressor frequency in the range of 30-60 Hz; ambient temperature in the range of −15-12 °C according to the standard UNI EN 14511/2011. The results show the possibility to reach an optimum of the COP by varying the ejector area, once the other parameters are fixed, as a consequence of the ejector regulation on the pressures at the inlet and outlet of the compressor.