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The National Renewable Energy Laboratory (NREL), in partnership with Oak Ridge National Laboratory, under the Clean Energy Manufacturing Analysis Center (CEMAC ) 1 umbrella, assessed the current state of existing and low-global-warming-potential (GWP) refrigerants for major end-use applications, including heating, ventilating, and air conditioning (HVAC) and those outside of the HVAC industry. The project incorporated a market overview and supply chain assessment to determine the production, distribution, consumption, costs, and potential operating efficiency impacts of new and alternative refrigerants entering the market. Market trends and ongoing research are also documented. This work supplements other U.S. Department of Energy efforts to support research activities on refrigerants and their applications, including a refrigerant research and development roadmap (Goetzler et al. 2014) and an outlook into global air-conditioning markets (Goetzler et al. 2016).

Conventional natural gas-fired boilers exhaust flue gas direct to the atmosphere at 150-200°C, which, at such temperatures, contains large amount of energy and results in relatively low thermal efficiency ranging from 70% to 80%. Although condensing boilers for recovering the heat in the flue gas have been developed over the past 40 years, their present market share is still less than 25%. The major reason for this relatively slow acceptance is the limited improvement in the thermal efficiency of condensing boilers. In the condensing boiler, the temperature of the hot water return at the range of 50-60°C, which is used to cool the flue gas, is very close to the dew point of the water vapor in the flue gas. Therefore, the latent heat, the majority of the waste heat in the flue gas, which is contained in the water vapor, cannot be recovered. This paper presents a new approach to improve boiler thermal efficiency by integrating absorption heat pumps with natural gas boilers for waste heat recovery (HRAHP). Three configurations