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IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
A int
G int
C
= m&
, int v,
G
⋅
( pc)
p,
v
( r + c ⋅ ( T − T
)
Q & (7)
( r
pc
l
pc
c
p w
( TG
TA
)) Q&
( )
+
( )
+ ⋅ −
SHX
= mv,
G
⋅
,
G
Q & &
+
, (8)
( r
pc
l
pc
c
p v
( TG
TE
) c
p w
( TG
TA
) Q&
( )
+
( )
−
,
⋅ − + ⋅ −
SHX
Q & &
+
, (9)
= mv,
A
⋅
,
C
The solution heat exchanger does not exchange the maximum possible heat between its two
flows. The difference between actual and ideal heat transfer can be assumed as a parasitic
heat flow which has to be added to the generator and has to be removed at the absorber. For
a constant heat exchanger effectiveness
η
SHX
it can be calculated as
Q&
SHX
( 1 −η
) ⋅ m&
⋅ c ⋅ ( T − T )
=
SHX sol,
sG p,
sol,
s
G
A
.
(10)
In equation (10),
& is the mass flow rate of the strong solution from generator to
m
sol , sG
absorber [4].
The dynamic performance of the absorption chiller is influenced by various time-dependent
effects, caused by complex heat transfer phenomena in the internal and external heat
exchangers. In order to keep the model simple, not all of the dynamic effects have been
taken into account. Only the three effects with the estimated biggest influence on chiller
performance have been chosen. These include a time delay in the solution transport
between generator and absorber, mass storage in the vessel sumps, and thermal storage in
the external and internal heat exchangers. These dynamic terms are the backbone of the
model and will be discussed in detail.
Dynamic Modelling
Both generator and absorber vessel have been modelled as a serial connection of a tube
bundle heat exchanger and a solution sump. The tube bundle is the active part; the sump is
a storage and mixing device. Solution can accumulate in absorber and generator sump
according to the actual load, however there is also some solution which is always stored on
the tube bundle. This hold-up, usually, is small as compared to the amount of liquid in the
sump because the film is less than half a millimetre thick. Moreover, the bundle should be
wetted all the time with the consequence that the amount of liquid on the bundle will not
change significantly. Therefore the amount of liquid on the bundle is neglected.
Due to the storage effect, we have to distinguish between the solution flow entering the
vessel, the one leaving the bundle and entering the sump, and the one leaving the sump.
Moreover, we have to consider different concentrations at the inlet of the bundle, at the exit
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