Oil flooded compression, residential heat pump, regeneration technology
The coefficient of performance and heating capacity of conventional air-to-air heat pumps decrease towards lower ambient temperatures. Heat pump systems are increasingly installed in residential homes but whereas they are already widespread in moderate climates, applications in very cold climates are limited. Since the heating load increases at low ambient temperatures, additional auxiliary heating systems are needed, which results in poor seasonal coefficients of performance. Oil-flooded compression is a technology to improve the performance of heat pump systems. This concept seeks to approach an isothermal compression process by injecting oil characterized by a higher specific heat than the refrigerant to absorb heat during the compression process. In a previous study (Yang et al. 2014), a 5-ton (17.6 kW) R410A packaged heat pump retrofitted with an oil injected compressor and regenerator was tested, in which one circuit within the indoor heat exchanger was modified to serve as an oil cooler. Up to 8% COP improvement was achieved for the oil flooded system relative to the baseline system. However, the heating capacity of the oil flooded system was found to be only slightly higher (1.6% to 3.3% improvement) than the baseline system. The potential of the oil flooded compression technology was not fully achieved due to the reduced heat transfer area of the condenser and unpredictable refrigerant flow maldistribution in the evaporator. The work presented in this paper shows the experimental results of a new prototype oil flooded system. This system has a different baseline configuration than the previous oil-flooded system in which the indoor coil face area is kept unchanged while an additional heat exchanger was added as the oil cooler. In addition, a new bigger counter-flow plate heat exchanger with low pressure drops was used as the regenerator. The results show that by injecting oil, the COP of the system was increased by 6.1% and the heating capacity by 5.7%. If the system is compared with a conventional heat pump (without regenerator and without oil injection), the COP improvement ranges from 4% to 15% depending on the oil mass fraction and operating temperature. The improvement in the heating capacity ranges from 0.4% up to 19%.