SUMMARY

The use of natural refrigerants, such as carbon dioxide, has been of recent interest due to concerns about global climate change and the elimination of refrigerants known to contribute to global warming. One application is for the heating of water using a heat pump operating in a transcritical cycle with carbon dioxide as the refrigerant. This presentation will cover the development of a comprehensive system model to predict the performance and guide the design of a carbon dioxide heat pump water heater. Detailed component-level models were developed for the four main system components (gas cooler, evaporator, compressor, and suction line heat exchanger) and were incorporated into the system model to yield accurate system-level performance predictions. The component-level models were validated through experimental testing on a transcritical heat pump test facility. Using the developed system model, four variations of a heat pump water heating system were designed. The use of a suction line heat exchanger, which has the potential to increase system performance, was compared to the performance of a system without such a component. Two water heating schemes were analyzed for both systems with and without the suction line heat exchanger. These two heating schemes include the heating of the water to its final desired temperature in a single pass through the heat pump and a multiple pass design in which the water is recirculated through the heat pump at a higher flow rate. The tradeoffs between performance and component size for these four system combinations were analyzed through parametric simulations that were performed using the developed system model.