IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
left graph) and the evaporator temperature as well as the chilled water outlet temperature
decreases (T E in graph 3 of Figure 1 and t 18 in graph 3).
Summarising Figures 1 and 2, the model predicts the performance of the absorption chiller
after an input temperature change with very good consistency and logic. However, there is a
small inaccuracy in the model due to the way the solution heat exchanger is being modelled
[1]. With the assumed transport delay in the solution circuit, no changes of absorber and
evaporator parameters should occur during the first 2s after the temperature step. There is,
however, a very small change in the equilibrium mass fraction of the absorber at time interval
201, barely visible in Figure 1 (x wA in graph 4). This also results in small changes of the
vapour mass flow from evaporator to absorber (m vA in graph 6), the evaporator equilibrium
temperature (T E in graph 3) and the absorber pressure (p A in graph 5).
The reason is as follows. In time interval 201, the increase of strong solution mass flow and
equilibrium temperature in the generator results in an immediate increase of the solution heat
exchanger heat flow as well which, in reality, would be dampened due to the thermal mass of
the solution heat exchanger. Therefore, more heat has to be dissipated to the cooling water
in the absorber, as visible in Figure 2 (bottom right graph). In consequence, the absorber
pressure rises and less vapour can be absorbed, resulting in an equilibrium concentration
change in the absorber and a temperature rise in the evaporator. The order of magnitude of
these deviations is however very small and can be neglected for the overall model
performance. The concentration changes by a value of 0.0002 kg/kg, the equilibrium
temperature in the evaporator changes by 0.017K. This results in a vapour mass flow change
of 2 10 -5 kg/s.
Sensitivity analysis
The sensitivity of the model on its dynamic terms is important information for the model user
as well as for developers of chillers. In the following, the influence of thermal storage and
solution transport delay on the overall model performance will be investigated. Characterising
parameters for the thermal storage are the values of internal and external heat capacity,
( Mc p
) X , int
and
( Mc p
constants c 1 and c 2 .
) X , ext
. The transport delay is characterised by the transport delay
Thermal storage variations
To investigate the influence of thermal storage variations, three different cases have been
simulated. In the first case, the actual thermal capacity of the absorption chiller on which the
model is based is used. See Table 1 for details. The second case assumes 50% of the actual
thermal capacity and in the third case no thermal capacity is assumed. In all cases, the
external capacity equals the internal capacity and the actual solution transport delays are
utilized, i.e. 67s and 61s for c 1 and c 2 , respectively. Again a hot water temperature step from
75°C to 85°C at 200 s after simulation start has be en used as the model input for all cases.
Figure 3 shows the results, expressed by the heat flow from the external heat transfer fluid
(the water to be chilled) into the evaporator.
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