18 May 2017

O.4.5.4 Dynamic modelling of an air-cooled LiBr-H2O absorption chiller based on heat and mass transfer empirical correlations


The paper describes the numerical modelling of a direct air-cooled single-effect absorption chiller and validates the obtained results by means of experimental data obtained from an own-designed and commissioned small capacity absorption chiller. This direct air-cooled single-effect absorption machine is conceived for low temperature driven, such as solar cooling or waste heat. For performing numerical simulations, a modular object-oriented simulation platform is used (NEST platform tool), which allows the linking between different components (solar collectors, pump, valves, heat exchangers, absorption chiller, etc.). This modelling put in relevance valuable information for further regulation and control protocols. In this numerical platform each component is an object of an arbitrary complexity, which can be either an empirical-based lumped model (e. g. heat exchangers, valves, pumps, etc.) or a detailed 3D CFD calculation if necessary. A lumped parametric dynamic model based on mass, momentum and energy balances, applied to the internal components of the absorption machine (absorber, generator, condenser, evaporator and solution heat exchanger) has been implemented. Thermal and mass storage in each one of the components are taken into account in the transient evaluation and pressure losses in the solution heat exchanger are evaluated by means of a resistance coefficient. The model depends on heat and mass transfer coefficients, which has been implemented depending on basic empirical correlations. The aim of this paper is to improve the available numerical modelling approaches by analyzing the heat & mass transfer phenomena based on previous experiences in falling film absorption. As a first step the model is validated against the results of a previous laboratory prototype. Finally, the performance of a prototype demonstration 7 kW air-cooled LiBr-H2O absorption chiller is calculated.