IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
Figure 7. Evolution of evaporator heat flow for different transport delay times.
It is visible that the transport delay has a significant influence on the model performance.
Without a transport delay, the new steady-state is reached at approx. 200s after the step.
With a transport delay of 50% (which corresponds to about half a minute) it takes approx.
500s, e.g. more than double the time to reach it. For the assumed 100% of transport delay
(around 1 minute) it takes approx. 900s or more than four times as long.
Also, with 100% transport delay, the transient period is not only longer but also quite different
in its course. After the elapsing of each transport delay constant, a change in heat flow can
be observed in a repeated pattern. The pattern consists of a small change and a larger
change following each other which is due to the repetitive change in solution mass fraction
as previously explained. The changes become less intensive with time and after approx. 12
delay times the new steady-state is reached. In a real absorption chiller the transport delay is
determined by the flow rate of the solution. The model results in Figure 7 show that a
significantly faster return to steady-state can be achieved if the flow rate of the solution is
high and the transport delays are small.
Experimental verification
In Part 1 of this paper, we showed a first comparison of the simulated generator outlet
temperature using experimentally measured inlet and outlet temperatures to measured data
[1]. In this second part the comparison is also done similarly for the evaporator and absorber.
Flow rates and inlet temperatures of the external flows are taken as input data to the
simulation model. All internal parameters, the heat flows, and the outlet temperatures of the
external flows are simulated. Figure 8 shows measured inlet and outlet temperatures of the
chilled water together with the simulated outlet temperature; Figure 9 shows the same for the
cooling water through absorber/condenser. The constant simulation parameters for the
experimental verification can be found in the Appendix.
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