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IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
Internally, in addition to the tube bundles, the solution contributes by 50% of its mass in
generator and absorber. In analogy, the refrigerant is assumed to have a 50% internal
contribution in both evaporator and condenser. The mass of solution heat exchanger and
solution pump has an internal effect on both generator and absorber heat transfer and is
assumed to contribute by 50% of its weight on both. The vessel wall now contributes
internally by 50% of its weight due to the larger surface area involved in heat transfer mainly
in the sumps.
Enthalpy balances
Taking internal and external thermal storage as in Figures 3 and 4 into account, equations
(6) to (9) can be refined and the external and internal enthalpy balances for the vessels can
now be expressed. The external enthalpy balance of the vessels describes that the heat
which is given off (generator) or taken on (absorber) by the heat carrier (left-hand side of
equation (25) or (26)) partly crosses the heat exchanger (first term on the right-hand side,
grey arrows in Figures 3 and 4) and reaches the mean vessel temperature, partly is used to
heat up parts of the vessel which are more or less at external temperature (second term on
right-hand side). For generator and evaporator, the external enthalpy balance reads
ϑX
, i
−ϑX
, i−1
m& X , w
⋅ c
p,
w
⋅ ( ϑX
, in
−ϑX
, out
) = ( UA) ext,
X
⋅ ( ϑ
X , i
− TX
, i
) + ( M ⋅ c
p
) ⋅
(25)
ext,
X
∆t
In equation (25), index X denotes the different vessels of generator and evaporator (X = G,
E). For absorber and condenser the external enthalpy balance reads
ϑX
, i
−ϑX
, i−1
m& X , w
⋅ c
p,
w
⋅( ϑX
, in
−ϑX
, out
) = −( UA) ext,
X
⋅ ( ϑX
, i
−TX
, i
) − ( M ⋅c
p
) ⋅
(26)
ext,
X
∆t
Index X in equation (26) denotes absorber and condenser (X = A, C).
The internal enthalpy balances for all vessels read as follows.
T E,
i − T E,
i−1
( r − c ⋅ ( T C,
i − T E,
i
) + ( M ⋅ c ) ⋅
= ( UA ⋅ ( T − T E i )
& (27)
∆t
m
v, A,
i
⋅
( pc)
p,
v
p
)
E
E,int
E,
i ,
,int
m&
=
v,
G,
i
⋅ ( r(
pc)
+ l(
pc)
+ c
p,
w,
i
⋅ ( T G,
i − T A,
i
) + Q&
SHX , i
+ ( M ⋅ c
p
)
( UA) ⋅ ( T − T G,
i )
G,int
G,
i
G,int
T
⋅
G,
i
− T
∆t
G,
i−1
(28)
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