IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
[1]. Thus the heat flow transient is less steep than in Figure 5 and the plateaus are less
distinct.
Comparing Figures 5 and 6 it can be observed that allocating the thermal mass to the
external parts as in Figure 5 yields a faster response with regard to achieving a new steadystate.
The dynamic course of Figure 5 clearly shows obvious plateaus created by the
transport delay in the solution circuit. As no internal thermal mass has been allocated, the
solution changes occur quite rapidly after the step and then become weaker and are
dampened with time. The course in Figure 6 is much less pronounced and the gradients of
heat flows are less steep. The model behaviour shown in Figures 5 and 6 demonstrates that
in order to attain a short response time the absorption chiller should be designed with
possibly small internal thermal mass, however this will result in stronger variations during the
transient period. Internally coupled thermal mass is more important than externally coupled
thermal mass.
Transport delay variations
A second analysis has been performed to determine the sensitivity of the model results on
time constants c 1 and c 2 which represent the transport time of the solution from generator to
absorber and vice versa. The time constants have been set to 0, 50 and 100% of their
original values which have been estimated from the design of the experimental plant. Table 2
shows the time constants used. For these simulations, the same hot water inlet step from
75°C to 85°C as before has been applied. Also, no t hermal mass has been assumed. Figure
7 shows the results, again expressed using the external heat flow to the evaporator.
Table 2. Assumed transport delays for the model.
Transport delay
c 1 c 2
100% 67 61
50% 33 30
0% 2 1
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