IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
It is clearly visible in Figure 4 that an increase in thermal mass results in a slower response
to the step change. Without thermal mass, the heat flow decreases abruptly after the step.
This is equivalent to Figure 2 (bottom left graph) where the evaporator heat flow decreases
for the time period between the step and the elapsing of time constant c 1 . The lower
evaporator heat flow is due to the increased heat flow in the solution heat exchanger just
after the step which in turn results in increased absorber pressure, an equilibrium
concentration change in the absorber, less absorbed vapour and hence a temperature rise in
the evaporator. The evaporator heat flow increases sharply again after time constant c 1 has
elapsed at time interval 267, i.e. after the change in solution mass fraction reaches the
absorber. For the other two cases with higher thermal mass the changes are much less
abrupt and the transients are not as steep.
The second effect visible in Figure 3 is bigger variations during the transient period for less
thermal mass. This is related to the transport delay in the solution circuit. There, less mass
results in more abrupt variations of the heat flow, induced by sharp changes of the solution
parameters after either c 1 or c 2 have elapsed. An increase in both internal and external
thermal mass provides a damping effect on the heat flow oscillations in the chiller.
Regardless of the assumed thermal mass, the model still converges into the same steadystate
value after the step.
For Figures 3 and 4, it has been assumed that the external capacity equals the internal
capacity. This does not give any information on the influence of either of those capacities by
itself and whether the assumption of equal capacities is reasonable. Therefore, another
simulation has been performed assuming 100% capacity either only on the external or on the
internal side. Figures 5 and 6 show the results for external and internal heat capacity only. As
before, a hot water temperature step from 75°C to 8 5°C at 200s after simulation start has
been used as the model input.
Figure 5. Evolution of evaporator heat flow for externally coupled thermal mass only.
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