IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
Figure 6. Evolution of evaporator heat flow for internally coupled thermal mass only.
Figure 5 shows the external heat flow from the chilled water, Q E,ext , and the heat flow into the
external thermal mass, Q st,E,ext . The difference between both is the heat flow into the
refrigerant. In the first seconds after the step, the temperature of the evaporator rises
somewhat due to the increased absorber pressure as previously explained. The external
heat flow drops, some heat is stored in the external mass. At 67s after the step (elapsing of
c 1 ) a sharp negative peak can be seen in the heat flow which is stored in the external mass:
the evaporator cools down because more absorption takes place as the change in solution
mass fraction reaches the absorber. Consequently the external heat flow into the evaporator
increases with a steep gradient. In parallel to this process, solution with the initial weak
concentration from the absorber reaches the generator at 61s after the step (elapsing of c 2 ).
Therefore the equilibrium concentration in the generator sump decreases due to the mixing.
This slightly weaker solution leaves the generator sump and reaches the absorber with a
time difference of 6s (∆c 1-2 ). It increases the absorber pressure and decreases the
evaporator heat flow, visible in Figure 5 at 128s after the step where the evaporator heat flow
plateaus at constant level. At approx. 200s after the step new stronger solution reaches the
absorber from the generator, the absorber pressure decreases and the evaporator heat flow
increases again. The gradient is, however, less steep than the first time due to the increased
mixing in both generator and absorber sump and the corresponding equalisation of solution
mass fraction. After this second increase another plateau can be observed in the evaporator
heat flow at approx. 260s after the step. This process of alternating solution concentration
changes is repeated a few more times with less and less intensity until at 500 seconds after
the initial step the heat flow out of the thermal mass has stopped almost completely. A new
steady-state is reached at 1200s after the initial step.
Figure 6 shows a similar evolution but with allocation of the thermal mass to the internal
evaporator temperature. The behaviour is very similar, but there is an influence on the time
to reach a new steady-state and also on the course of the heat flow after the step. Figure 6
shows that allocating the thermal mass to the internal parts results in a slower response with
approx. 1400s to reach the new steady-state. Even the cooling down of the evaporator takes
about 1000 seconds. The influence of the solution transport delay is much stronger, as the
internal mass now includes the solution, the solution heat exchanger, and the solution pump
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