Transient behavior assessments of a single-effect ammonia-water absorption heat pump system: Development of an efficient experimentally validated numerical framework

In this study, the long-term transient response of a single-effect ammonia-water gas-fired absorption heat pump prototype is investigated both numerically and experimentally. For the numerical model, heat and mass transfer components are divided into four groups; (a) tubular heat exchangers comprising condenser, evaporator, absorber, solution and refrigerant heat exchangers, (b) tray column heat exchanger comprising gas-fired generator, (c) refrigerant and solution tanks, (d) solution pump and restrictors. Components of groups a-c are modelled based on discretized volumes in which the unsteady mass, species and energy conservation equations are imposed, whereas solution pump and restrictors are idealized as algebraic models. The experimental analysis covers a period of over 10 h, during which various step changes are applied to system inputs like gas power, inlet water temperature, solution and refrigerant flow rates, and brine inlet temperature. The results of the numerical model are compared with the experimental data which show good agreement in both transient and near steady state operation. Results show that, during transient operation average deviations are less than 2.94% and near steady-state conditions are less than 1.70%. The accuracy of the numerical model in predicting both the transient response and the steady-state values of the measurable outputs, makes it a valuable tool to gain insight on the internal dynamics of ammonia-water absorption heat pumps and cut down costs on time-consuming optimization experiments, such as the fine-tuning of initial solution charge, the implementation of a suitable concentration control strategy and the performance optimization at full and partial loads.