Modelling and experimental analysis of the desorber of a gas-fired absorption heat pump.

Gas-fired ammonia/water absorption heat pumps offer a great potential concerning the reduction of energy consumption and CO2 emissions in the field of domestic heating. A critical component of those systems is the desorber, which generates refrigerant vapor by partly evaporating aqueous ammonia solution. Since the vapor pressure of water is not negligible compared to the one of ammonia, purification of the refrigerant from remaining water is realized by means of rectification. The mass flow rate and the ammonia mass fraction of the refrigerant have significant influence on the evaporator performance and hence on the heat pump’s overall efficiency. Within this work, the desorber of a small-scale absorption heat pump was analyzed with focus on the rectification column. The goal was to gain a deeper understanding concerning the factors, which have a relevant influence on the refrigerant’s mass flow rate and ammonia mass fraction. A rate-based modelling approach was used, which is characterized by the assumption of thermodynamic equilibrium at the liquid-vapor interface, while the heat and mass fluxes across the interface are determined by using estimated transfer coefficients. The predictions of this model were compared with experimental data from a prototype. Furthermore, the model was used to examine the influence of the operating conditions and the column’s design on the purification performance by means of parameter variations.