cold climate heat pump, vapor injection, CCHP, air-source heat pump, ASHP
Building heating requirements increase with decreasing ambient temperature, while the coefficient of performance of air-source heat pumps (ASHPs) shows the opposite trend. Additionally, heating capacity decreases with ambient temperature, which leads to the utilization of inefficient electric reheat below the design point. Increasing the capacity and COP at lower ambient temperatures is important for improving the market penetration of heat pumps in climates having significant operating time at low ambient temperature. Simulation studies previously showed that compressor vapor injection leads to an increase of COP under exactly those conditions. Furthermore, reduced capacity degradation towards smaller ambient temperatures was predicted. The work presented in this paper shows experimental results obtained from a commercially available 5-ton heat pump that was retrofitted with a two-port vapor injected scroll compressor. The injection ports within the two compression pathways are located in the fixed scroll with different distance from the suction chamber. The vapor for the two injection pressure levels was generated using two vapor separators in a cascade configuration. This configuration made it necessary to not only control the superheat but also the liquid levels in the separators and subcooling of the refrigerant leaving the condenser. Baseline performance data of the heat pump without vapor injection was obtained and compared with that for the vapor injection and other system configurations. For the baseline, the injection lines to the compression pockets were plugged within the fixed scroll to reduce dead volume and re-expansion losses. Also, the vapor-separator section was shut off and bypassed. In the second step, the plugs were removed and a staged expansion process was performed using the separator section. The generated vapor from each separator was injected into the respective compressor port causing an intercooling effect on the compression process. With identical compressor speed, a 28% improvement in capacity was achieved at the 8.33°C (47 F) design point, when compared to the baseline without vapor injection. When the baseline and vapor injected system capacity were matched by adjusting compressor speed, the COP increased by up to 6% at -8.33°C (17 F). Preliminary results of a bin-type analysis of the experimental data predicts an improvement in HSPF of 6% for Minneapolis and nearly 7% for AHRI climate region 5. The benefit is mainly caused by a reduction of the auxiliary electric heater’s runtime.