Analysis of Two-Phase Injection Compressors for Residential Heat Pump Combi Systems

Residential heat pumps for water heating and space conditioning (combi systems) can offer significant benefits to residents and building owners, including improved comfort and potential lower electricity bills. When replacing legacy hydronic based heating systems, higher condensing temperatures than air-to-air heat pumps are required. To increase the adoption potential of these systems, especially in colder climates, increasing system efficiency and extending operating ranges to cover higher temperature lifts will be important. Currently, the most common way to improve system performance, when high temperatures lifts are required, is vapor injection. However, two-phase refrigerant injection can provide improved compressor cooling to vapor injection alone at similar operating conditions. To better understand two-phase injection systems, we develop a heat pump model with a refrigerant injection compressor. The model is validated using a commercially available air-to-water heat pump tested at the in environmental chambers under multiple operating conditions, including two-phase injection. Finally, the validated model is used to evaluate the performance of the heat pump under different scenarios, including water heating at very low ambient temperatures. These results illustrate the potential benefits of developing two-phase injection based heat pumping systems.