Heat pumps provide cooling in summers and heating in winters. It is inevitable that the capacity and COP of the heat pumps degrade significantly in the case of high ambient temperatures in summers and low ambient temperatures in winters, when the maximum capacity is desired. Refrigerant vapor-injection technique has been well justified to improve the performance of systems in refrigeration applications, however, it has not received much attention for air conditioning applications, particularly with air-conditioning for hot climates and heat pumps for cold climates. The performance degradation of conventional residential equipment at extreme weather conditions warrants further investigation of the vapor-injection technique.

This dissertation is focused on the experimental and theoretical investigations of a two-stage heat pump system with an innovative vapor-injected scroll compressor. Unlike other research, a heat pump system without a liquid receiver has been studied in this research. A 3-ton R410A heat pump equipped with a conventional scroll compressor has been built, and tested to serve as a baseline. The heat pump has been modified to be a two-stage system with the cycle options of flash tank and internal heat exchanger configurations, and been tested under the same ambient conditions to the baseline. Both compressors have the same displacement volume. The operating options of the two-stage system have been compared, and analyzed. The vapor-injection effects on the subcomponents of the system have been addressed.

The vapor-injected compressor has been modeled using compressor-mapping method. A simulation model of the two-stage system has been built using VapCyc and CoilDesigner software packages developed by CEEE, and been validated using the experimental data. The model is able to predict the system performance with ±5% of deviation to the experimental results for most performance variables.

The results show that the vapor-injection technique can effectively increase the system performance. A cooling capacity gain of around 14% with 4% COP improvement at ambient 46.1°C, about 30% heating capacity improvement with 20% COP gain at -17.8°C and about 7% HSPF improvement in U.S. Department of Energy's northern Region 4 climate have been found for the vapor-injected heat pump system as compared to the conventional system.