مدل شبیه سازی پویا برای عملکرد چیلر جذبی گذرا. بخش دوم: نتایج عددی و تایید تجربی

This paper is the second paper out of two which present the development of a dynamic model for single-effect LiBr/water absorption chillers. The first part describes the model in detail with respect to the heat and mass balances as well as the dynamic terms. This second part presents a more detailed investigation of the model performance, including performance analysis, sensitivity checks and a comparison to experimental data. General model functionality is demonstrated. A sensitivity analysis gives results which agree very well to fundamental expectations: it shows that an increase in both external and internal thermal mass results in a slower response to the step change but also in smaller heat flow oscillations during the transient period. Also, the thermal mass has been found to influence the heat flow transients more significantly if allocated internally. The time shift in the solution cycle has been found to influence both the time to reach steady-state and the transients and oscillations of the heat flow. A smaller time shift leads to significantly faster response. A comparison with experimental data shows that the dynamic agreement between experiment and simulation is very good with dynamic temperature deviations between 10 and 25 s. The total time to achieve a new steady-state in hot water temperature after a 10 K input temperature step amounts to approximately 15 min. Compared to this, the present dynamic deviations are in the magnitude of approximately 1–3%.

This paper describes the performance and experimental verification of a dynamic absorption chiller model. In Kohlenbach and Ziegler (2007), the model itself was described with regard to dynamic effects, such as transport delays in the solution circuit, thermal storage and mass storage. In detail, the size of the solution sumps in absorber and generator, the time for the solution to flow from absorber to generator and vice versa and the thermal mass of the main components has been accounted for. As a special feature, the thermal mass of the components has been split into two parts, one which responds to the temperature of the external fluids, and the other which responds to the temperature of the solution and the refrigerant (internal fluids). These are the main parameters which determine the dynamic behaviour of the chiller. This second paper is looking at internal consistency, sensitivity and accuracy of the model. Results of a performance analysis using ideal conditions to prove correct model behaviour are shown. A sensitivity analysis on thermal storage and solution transport delay has been performed to investigate the influence of the dynamic parameters on the chiller performance. Finally, a model verification using experimental results is also given in this paper.

In this paper, the performance of a dynamic model for absorption chillers has been investigated. General model functionality is demonstrated and the thermodynamics have been found to be consistent with reality. A sensitivity analysis has been performed on external and internal heat exchange related thermal mass. The analysis shows that an increase in both external and internal thermal mass results in a slower response to the step change but also in smaller heat flow oscillations during the transient period. Also, the thermal mass has been found to influence the heat flow transients more significantly if allocated internally. A time difference of 200 s for reaching the steady-state (response time) was observed between a complete internal and external allocation of thermal mass. The transport delay in the solution cycle has been found to influence both the response time and the transients of the heat flow. A smaller transport delay leads to significantly faster response. Assuming half the value of the real transport delay in the absorption chiller leads to a 33% reduction of response time. The comparison with experimental data shows that the dynamic agreement between experiment and simulation is very good with dynamic temperature deviations between 10 and 25 s. The total time to achieve a new steady-state after a 10 K input temperature step amounts to approximately 15 min for the experimental chiller. Compared to this, the present dynamic deviations of the model are in the magnitude of approximately 1–3%. Steady-state results are being reproduced with temperature deviations between 0.7 and 3.5 K in the model. Accuracy in this respect, however, was not the aim of the present study. The dynamic simulation model presented in this paper is a useful tool in the overall design process of absorption chillers. Technical changes in the construction of an existing absorption chiller model can be tested quickly and easily by incorporating the design changes in the model. Also, new chiller designs can be tested on their performance without the need to build a prototype. The model also allows the identification of transfer functions and control parameters of absorption chillers without the need to perform experiments. The model has been designed for the Phoenix 10 kW chiller but can easily be adapted to other LiBr/water absorption chillers if the required design data of these chillers are available.