Environmental Engineering Reference
In-Depth Information
Table 14.4.2 Materials and fluids applicable for thermal storage (NREL, 2000).
Temperature
Average
Average Volume
Media Media
Average
heat
heat
specific
cost
cost per
Cold Hot
Density
conductivity
capacity
capacity
per kg
kWh th
Storage Medium
[ C]
[ C]
[kg/m 3 ]
[W/mK]
[kJ/kgK]
[kWh th /m 3 ]
[$/kg]
[$/kWh th ]
Solid Media
Sand-rock-mineral oil 200
300
1.7
1.0
1.30
60
0.15
4.2
Reinforced concrete
200
400
2.2
1.5
0.85
100
0.05
1.0
NaCl
200
500
2.16
7.0
0.85
150
0.15
1.5
Cast iron
200
400
7.2
37.0
0.56
160
1.0
32.0
Cast steel
200
700
7.8
40.0
0.60
450
5.0
60.0
Silifica fire bricks
200
700
1.82
1.5
1.00
150
1.0
7.0
Magnesia fire bricks
200
1200 3
5.0
1.15
600
2.0
6.0
Liquid media
Mineral oil
200
300
770
0.12
2.6
55
0.3
4.2
Synthetic oil
250
350
900
0.11
2.3
57
3.0
43.0
Silicone oil
300
400
900
0.10
2.1
52
5.0
80.0
Nitrite salts
250
450
1.825
0.57
1.5
152
1.0
12.0
Nitrate salts
265
565
1.87
0.52
1.6
250
0.7
5.2
Carbonate salts
450
850
2.1
2.0
1.8
430
2.4
11.0
Liquid Sodium
270
530
850
71.0
1.3
80
2.0
21.0
Phase change media
NaNO 3
308
2.257
0.5
200
125
0.2
3.6
KNO 3
333
2.11
0.5
267
156
0.3
4.1
KOH
380
2.044
0.5
150
85
1.00
24.0
Salts-ceramics
NaCO 3 -BaCO 3 /MgO 500-850
2.6
5.0
420
300
2.00
17.0
NaCl
802
2.16
5.0
520
280
0.15
1.2
Na 2 CO 3
854
2.533
2.0
276
194
0.20
2.6
K 2 CO 3
897
2.29
2.0
236
150
0.60
9.1
to crystalline structure). Nowadays, mainly the solid-to-liquid transition has been
applied, and substances used under this technology are called phase change materials
(PCM). Storage systems utilizing PCM can be reduced in size compared to single-phase
sensible heating systems. However, heat transfer design and media selection are more
difficult, and experience with low-temperature salts has shown that the performance
of the materials degrades after a moderate number of freeze-melt cycles. Phase change
materials allow large amounts of energy to be stored in relatively small volumes as a
result of a higher energy density, theoretically resulting in cost reduction in compari-
son with the other storage concepts. For these reasons their application in CSP plants
seems to be attractive, even if the potential reduction of costs and technical feasibility
are still to be demonstrated.
A further possibility is to exploit the chemical heat of reaction of specific media,
obtaining a chemical storage. This type of thermal storage exploits appropriate chem-
ical reactions that are fully reversible. The heat produced is used to promote an
endothermic chemical reaction and, if this reaction is completely reversible, the stored
 
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