IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C2-A, November 9, 2009
After a temperature change of one of the external flows entering a vessel, the temperatures
of all components which are influenced either by the external heat carrier (external
components such as the water headers) or by the refrigerant and sorbent (internal
components such as the solution heat exchanger) change consecutively. This is, of course,
connected with some heat being stored in all external and internal components which are
involved in heat transfer mechanisms. The temperature change of these components is a
complex three-dimensional heat transfer problem. One simplified approach to modelling it is
the assumption that some components or parts of components follow internal temperatures
and others follow external temperatures. To account for the accordingly different heat
transfer rates the total thermal mass has been divided into an external and an internal part.
The external part follows the mean external temperature ϑ
X
, the internal part follows the
mean internal temperature
T
X
. Also, different heat transfer coefficients and exchange areas
have been assumed for the internal and external parts. The external and internal
components that have been incorporated for thermal storage are listed in Table 1.
Table 1. External and internal components involved in heat transfer.
Heat
Mass
Mass
Mass
Mass
Location
Component
capacity
in E
in C
in G
in A
[kJ/kgK]
[kg]
[kg]
[kg]
[kg]
external
Vessel wall (10% of total weight)
External water header
Water in external parts
Heat exchanger tube bundles
(50%)
0.48
0.48
4.19
0.38
2.0
7.6
8.5
5.7
2.0
7.3
7.4
5.1
1.9
9.8
12.6
8.9
1.9
10.6
11.9
8.0
Heat exchanger tube bundles
(50%)
0.38
5.7
5.1
8.9
8.0
LiBr/water solution (50% each)
3.70
-
-
19.0
19.0
internal
Refrigerant water (50% each)
Solution heat exchanger (50%
4.19
0.38
7.5
-
7.5
-
-
4.1
-
4.1
each)
0.48
-
-
5.0
5.0
Solution pump (50% each)
0.48
10.0
10.0
9.5
9.5
Vessel walls (50% of total weight)
Figures 3 and 4 illustrate the assumptions for the heat transfer from the external heat carrier
to the heat exchanger and further on to solution or refrigerant. For generator and absorber
the tube bundle is divided into two virtual parts, one being at mean external temperature, ϑ ,
and one being at mean internal temperature, T
X
. The mean vessel temperature T
X
is
shown as a virtual crossover temperature between internal and external temperature levels.
X
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