Experimentally-based modelling of a gas-fired absorption heat pump with a refrigerant-cooled flue gas heat exchanger

Gas-fired ammonia/water absorption heat pumps (GAHPs) for heating applications have a significantly higher efficiency compared to commonly used condensing boilers, thus a GAHP can substantially contribute to the reduction of greenhouse gas emissions in the building sector.
Within the scope of this work a GAHP with a nominal heating capacity of 18 kW was equipped with a standard water-cooled flue gas heat exchanger and a novel refrigerant-cooled flue gas heat exchanger. Especially in retro-fit applications, the temperature in the heating system is too high to condensate water vapour via a water-cooled flue gas heat exchanger which can be overcome by a refrigerant-cooled one. A simulation model based on mass, species and energy balances as well as assumptions derived from experimental results of a GAHP with both flue gas heat exchanger types was set-up using the software Engineering Equation Solver (EES). The results have shown an efficiency of the GAHP of up to 1.9 and an increase in efficiency of up to 9 % by using a refrigerant-cooled instead of a water-cooled flue gas heat exchanger.