IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
Conclusions
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 200s 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 25s. The total time to achieve a new steady-state after a 10K input temperature step
amounts to approximately 15 minutes for the experimental chiller. Compared to this, the
present dynamic deviations of the model are in the magnitude of approximately 1 to 3%.
Steady-state results are being reproduced with temperature deviations between 0.7 and 3.5K
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 10kW chiller but can easily be adapted to other
LiBr/water absorption chillers if the required design data of these chillers are available.
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