Experimental investigations of an absorption heat pump prototype with intermediate process for residential district heating

Despite being efficient means of district heating, conventional single-effect absorption heat pumps suffer significant performance deterioration at low ambient temperatures. To solve this problem, an ammonia-water absorption heat pump prototype with intermediate process is designed and built. It utilizes both low-grade heat from the ambient and exhaust heat from natural gas combustion, through the evaporator and intermediate evaporator, respectively. To perform comparison study, the prototype also has the single-effect mode. Experimental results indicate that when the evaporation temperature is 0 °C, the prototype with intermediate process can provide 30 kW heating capacity to heat the water in sequence through the rectifier, condenser and absorber, from 34.24 to 55.09 °C, and the coefficient of performance and primary energy efficiency are 1.66 and 1.28, respectively. When evaporation temperatures reduce to −5, −10 and −15 °C, the coefficient of performances are 1.51, 1.40 and 1.28, respectively, reaching 77%–90% of the simulation values under corresponding working conditions. Compared to the conventional single-effect system, the prototype with intermediate process performs better at lower evaporation temperature, with the coefficient of performance improved by 11%. By replacing the original heat exchangers, the proposed system can be easily applied to the existing heat networks based on gas-fired boilers to improve energy efficiency, and the favorable experimental performance proves that it is a more efficient way of residential district heating, especially in cold regions.