18 May 2017

O.1.7.2 Comparison of Primary Energy Consumption of Vapor and Non- Vapor Compression Natural Refrigerant Heat Pumps for Domestic Hot Water Applications

The continued phase out of high global warming potential (GWP) refrigerants and the desire to reduce primary
energy consumption in domestic water heating has spurred development in electric transcritical carbon dioxide
(CO2) vapor compression heat pumps and natural gas-fired ammonia-water absorption heat pumps. While both
technologies are potentially more efficient than electric resistance or conventional gas-fired water heaters, there
has not been an extensive comparison of the two technologies for domestic water heating applications. The
objective of the present study is to assess the baseline performance of the two technologies on a primary energy
consumption, emission and operation cost basis. Thermodynamic state point simulations of representative gasfired,
single-effect ammonia-water absorption system and an electric, transcritical CO2 heat pump with suction
line heat exchanger are developed. The models are then used to assess the sensitivity of system performance to
inlet water temperature and ambient conditions. The results show that choosing an “optimal” system is highly
dependent on the local electricity mix and fuel costs, and the relative importance of operational cost versus
greenhouse gas emissions to the user. At the current U.S. average electricity mix and fuel costs, the absorption
heat pump emits less CO2 and is less expensive to operate. However, in locales with less expensive electricity
and/or less carbon producing electricity generation, the electric heat pumps are at an advantage.