Environmental Engineering Reference
In-Depth Information
TABLE 7.2
Physical Properties of Latent Storage Materials
Average
Heat
Conduc-
tivity
Volume-
Specific
Heat
Capacity
Average
Heat
Capacity
Average
Density
Media
Cost
Media
Cost
PhaseChange
Storage
Medium
Temperature
(°C)
(kg/m
3
)
(W/mK)
(kJ/kgK)
(kWh
t
/m
3
)
($/kg)
($/kWh
t
)
NaNo
3
308
2,257
0.5
200
125
0.20
3.6
KNO
3
333
2,110
0.5
267
156
0.30
4.1
KOH
380
2,044
0.5
150
85
1.00
24.0
Salt-ceramic
(NaCo
3
-
BaCO3-MgO)
500-850
2,600
5.0
420
300
2.00
17.0
NaCl
802
2,160
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
2,290
2.0
236
150
0.60
9.1
Source:
Hermann, U., Geyer, M., and Kearney, D. 2002.
Overview of Thermal Storage Systems
.
With permission.
high volumetric energy densities for small temperature swings, making
them good latent heat storage materials.
The primary advantage of phase change materials is their abil-
ity to store energy at reduced temperatures and in reduced quantities.
Furthermore, their high heat of fusion and other thermal properties allow
freezing with little supercooling, no segregation, chemical stability, and a
sharp melting point. One advantage is that some PCMs have low thermal
conductivity in the solid state, making high heat transfer rates necessary
during the freezing cycle. They are often flammable, requiring the addi-
tion of safety features to the design of storage vessels. In addition, latent
heat is much more difficult to transfer than sensible heat. Table 7.2 shows
the material properties of several common media used for latent heat
storage. A TES designer should choose materials based on these specific
characteristics.
Thermochemical Energy
Thermochemical energy is stored as the bond energy of a chemical com-
pound. During a thermochemical reaction, atomic bonds are broken
through a reversible chemical reaction and are catalyzed by an increase in
temperature—which allows the energy to be stored. After thermochemical
separation occurs, the constituents are stored apart until the combination
reaction is desired. Recombination of the bonds between atoms releases the
stored thermochemical energy.
