Dynamic modeling of an innovative gas-driven absorption heat pump for residential applications: Transient Start-Up analysis

This paper deals with transient modelling of an innovative small-capacity, gas-driven, single effect ammonia water absorption heat pump prototype for residential and light commercial applications. Three transient mathematical models, with increasing complexity and suitable for the different heat and mass transfer processes occurring in absorption heat pumps employing binary fluids, are described. Then, the proposed models are applied to absorption heat pump components providing a detailed transient model of the absorption heat pump, by following a modular approach. To achieve this goal, numerical models of specific components as the solution pump are also described. Furthermore, the innovations introduced to obtain a compact and effective direct fired generator are shown. The overall heat pump numerical model has been experimentally validated with respect to the transient operation of the prototype during the start-up phase, showing reasonable agreement between experimental and calculated results. Moreover, the effect of different initial conditions, such as the charge of ammonia and water, are investigated. Furthermore, the influence of expansion valves characteristics is considered, showing the importance of a detailed model for this component. Results show that expansion valve characteristics and initial conditions can strongly influence the dynamic behavior of pressures and temperatures during the start-up phase, whereas a good agreement is generally obtained between calculated and experimental data at nearly steady state conditions.