Analysis of a modified cycle design of a gas-fired absorption heat pump with a new flue gas heat exchanger.

Gas-fired ammonia/water absorption heat pumps (GAHP) can contribute substantially to the reduction of greenhouse gases in the context of heating. In recent years, only small increases of the efficiency could be achieved. For a significant increase a modification of the heat pump cycle is necessary. Common flue gas heat exchangers in GAHPs use the return flow of the heat distribution system to cool the flue gas below the dew point to condense water vapour. Especially in case of retrofit applications, the temperature in the heating system is too high to condense water vapour in the flue gas. Latent heat is thus dissipated into the environment. To use the latent heat under all operating conditions, a refrigerant cooled flue gas heat exchanger, which is installed between the evaporator and the absorber, is investigated within this work. A model based on mass, species, and energy balances was used to compare the common cycle design efficiency with the novel one. Measurements were conducted on an absorption heat pump with a heating capacity of 3 kW. Due to the new cycle design superheated ammonia vapour enters the absorber which leads to the necessity of investigations of potential impacts on the absorption behaviour.