Civil Engineering Reference
In-Depth Information
Table 2
Comparison of PCM types (Adapted from Rathod and Banerjee
2013
)
Organic
Inorganic
Eutectics
Paraffins
Fatty acids
Salt
hydrates
Metals
Formula
C
n
H
2n+2
(n = 12-38)
CH
3
(CH
2
)
COOH
AB
nH
2
O-
-
Melting
point
-12-71 8C
7.8-187 8C
11-120 8C
30-96 8C
4-93 8C
Melting
enthalpy
190-260 kJ/kg
130-250 kJ/kg
100-200 kJ/
kg
25-90 kJ/
kg
100-230 kJ/
kg
Cost
Expensive
2 to 3 times more
expensive than
paraffins
Low cost
Costly
Costly
• a small volume variation during solidification,
• a high thermal conductivity, and
• availability and abundance.
Paraffins are a mixture of pure alkanes that have quite a wide range of the
phase-change temperature. These paraffins also have low thermal conductivity
compared to inorganic materials, and therefore. the choice of those which can be
used for practical solar applications are very limited.
Commercial paraffin waxes are cheap with moderate thermal storage densities
(*200 kJ/kg or 150 MJ/m
3
) and a wide range of melting temperatures. They
undergo negligible subcooling and are chemically inert and stable with no phase
segregation. However, they have low thermal conductivity (*0.2 W/m 8C), which
limits their applications.
The main limitation of salt hydrates is their chemical instability when they are
heated, as at elevated temperatures they degrade, losing some water content every
heating cycle. Furthermore, some salts are chemically aggressive towards struc-
tural materials, and they have low heat conductivity. Finally, salt hydrates have a
relatively high degree of subcooling.
Salt hydrates are attractive materials for use in thermal energy storage due to
their high volumetric storage density (*350 MJ/m
3
), relatively high thermal
conductivity compared to organic materials (*0.5 W/m C), and moderate costs
compared to paraffin waxes, with few exceptions.
According to Cabeza et al. (
2011
), the PCM to be used in the design of a
thermal storage system should have desirable thermophysical, kinetic and chem-
ical properties and desired economics as listed below:
1. Thermophysical properties
• Melting temperature in the desired operating temperature range: to assure
storage and extraction of heat in an application with a fixed temperature range.
• High latent heat of fusion per unit volume: to achieve high storage density
compared to sensible storage.
