Experimental characterization of a CO2 heat pump with ejector for building heating applications

In the context of growing environmental concerns, natural refrigerants, particularly CO₂, have gained renewed interest. While CO₂ performs especially well in refrigeration applications, its use in heating systems remains relatively limited. To improve the efficiency of CO₂-based systems, complementary technologies such as two-phase ejectors are integrated to help recover expansion losses. This study focuses on an experimental characterization of a transcritical CO₂ heat pump integrated with an ejector. Various operating conditions, representing typical heating applications in buildings, were tested to assess heat pump performance and ejector behavior.

Results show that the ejector heat pump achieved a COP of 1.9 to 3.4 under the tested conditions, consistently outperforming the basic heat pump. While both systems showed lower COP at higher water inlet temperatures, the ejector’s pressure lift increased with water temperature, improving performance by 9 % at 25 °C and 18 % at 40 °C. This makes the ejector particularly beneficial for high-temperature applications. Its primary advantage lies in reducing the compressor’s compression ratio, further boosting overall system efficiency and heating capacity. Finally, a novel high-pressure control strategy was evaluated, utilizing valves arranged in series and parallel with the ejector, and demonstrated effective pressure control but introduced losses that limited overall gains.