Thermodynamic and economic analysis of a gas-fired absorption heat pump for district heating with cascade recovery of flue gas waste heat

Among district heating technologies, gas-fired boiler has difficulty in recovering flue gas waste heat and conventional heat pump has poor performance at low ambient temperature. In the present work, a novel gas-fired absorption heat pump is proposed. High-grade sensible heat and low-grade latent heat of the flue gas is recovered in the solution preheater and intermediate evaporator successively. Moreover intermediate evaporation and absorption processes are introduced, and the intermediate pressure is adjusted according to the ambient temperature, to improve the system adaptability in cold regions. Simulation results show that the coefficient of performance (COP), primary energy efficiency and exergy efficiency are 1.69, 1.39 and 54.5% when the evaporation, supply and return water temperature are -15, 55 and 35 degrees C, respectively. The COP of the proposed system has 13% and 20% improvement compared to those of the single-effect absorption system with and without waste heat recovery. Furthermore, the proposed system has energy saving potential of 11.7% and 39.6%, and payback time of 2.9 and 2.5 years compared to the single-effect absorption heat pump and the gas-fired boiler, respectively. The proposed system is suitable to provide 50 kW close range district heating for a typical urban residential building, especially in cold regions.