مدل شبیه سازی پویا برای عملکرد چیلر جذبی گذرا. قسمت اول مدل

This paper is the first of two which presents the development of a dynamic model for single-effect LiBr/water absorption chillers. The model is based on external and internal steady-state enthalpy balances for each main component. Dynamic behaviour is implemented via mass storage terms in the absorber and generator, thermal heat storage terms in all vessels and a delay time in the solution cycle. A special feature is that the thermal capacity is partly connected to external and partly to internal process temperatures. In this paper, the model is presented in detail. For verification, the model has been compared to experimental data. The dynamic agreement between experiment and simulation is very good with dynamic deviations around 10 s. General functionality of the model and a more detailed comparison with experimental data are presented in Part II of this paper.

The dynamic model of an absorption chiller allows the simulation of its transient behaviour for changing input conditions or design parameters. This is important because absorption chillers usually have a high thermal mass, consisting of their internal heat exchangers, the absorbing solution and the externally supplied heat transfer media. The dynamics of an absorption chiller are therefore rather slow compared to similar capacity compression chillers. The time to achieve a new steady-state with all parameters after a change of input conditions is about 15 min for the chiller presented in this paper. If the chiller is implemented in a complex heat supply/cooling demand system, e.g. a solar thermal or waste-heat driven system, the simulation of the chiller is usually being done using steady-state models. They simulate the chiller assuming constant operating conditions and allow the determination of internal and external cycle parameters, such as heat exchanger sizes, pump flow rates, temperatures and heat flows. However, steady-state models do not provide time-dependent information on the thermal behaviour of absorption chillers and are therefore not suitable for transient system simulations. In contrast, the model presented in this work allows the simulation of the dynamic chiller behaviour. It extends the range of applicable models for transient system simulations, where the time constants of the chiller significantly influence the system performance.

In this paper a dynamic model for absorption chillers has been presented. It is based on internal energy balances as well as mass balances. Dynamic behaviour is implemented via eight thermal and two mass storage terms as well as by two delay times. The heat transfer has been divided into two parts: one which transfers heat from the external heat carrier to the exchanger material and the second from there on to the refrigerant or solution side, or vice versa. In this way the thermal mass of the vessel could be introduced easily. The agreement between experiment and simulation is very good with deviations of 10 s for the generator. The total time to achieve a new steady-state after an input temperature step amounts to approximately 15 min. Compared to this, the present dynamic deviations are in the magnitude of approximately 1%. More information about the simulation results, comparison with experiments and sensitivity checks will be presented in Part II of this paper (Kohlenbach and Ziegler, 2007).