2.13 Phase Change Materials
33
(–5 to 66 °C) and have a high heat of fusion per unit weight. They are non-toxic,
non-corrosive, chemically inert and stable below 500 °C [153]. These PCM have
minimal super cooling behaviour, low volume change on melting, low vapour pressure in the melt and in addition reasonable cost. The density of the paraffins ranges
from 700 to 770 kg/m
3 . The most widely used paraffins are hexadecane (C 16 H 34 ),
octadecane (C 18 H 38 ) and eicosane (C 20 H 42 ), along with some of the other paraffinic
hydrocarbons such as heptadecane (C 17 H 36 ) and nonadecane (C 19 H 40 ).
In spite of the many advantages, there are some limitations of paraffins. The
advantages and disadvantages of using paraffin PCM are described below.
Advantages
1. Availability in a large temperature range;
2. Freeze without much super cooling;
3. Ability to melt congruently;
4. Self-nucleating properties;
5. Compatibility with conventional materials of construction;
6. No segregation;
7. Chemically stable;
8. High heat of fusion;
9. Safe and non-reactive and
10. Recyclability.
Disadvantages
1. Low thermal conductivity in solid state;
2. Requirement of high heat transfer rates during the freezing cycle;
3. Low volumetric latent heat storage capacity;
4. Flammability;
5. Chemically purified paraffins are expensive; and
6. Due to cost considerations, only technical grade paraffins are used.
Experimental results have established that laboratory grade paraffin waxes,
tetradecane, hexadecane and their binary mixtures, are excellent candidates as PCM
for cool storage. However, due to the very high cost involved with laboratory grade
materials, technical grade materials must be used for cool storage. Apart from the
paraffin organics, non-paraffin organics are also a common type of PCM available
for textile applications. The non-paraffin organic PCM such as polyethylene glycol
have been applied to fibres and fabrics.
Commercial paraffin waxes are available with a wide range of melting temperatures and are cheap with moderate thermal storage densities (200 kJ/kg or
150 MJ/m
3 ). They are chemically inert and stable with no phase segregation and
undergo negligible sub-cooling. However, their application is limited as they have
low thermal conductivity (0.2 W/m °C), and within a small physical space, they can
exist in all three phases. Metallic fillers, metal matrix structures, finned tubes and
aluminium shavings have been used to improve their thermal conductivity [154]. As
pure paraffin waxes are very costly, commercially only technical grade paraffins are
33
(–5 to 66 °C) and have a high heat of fusion per unit weight. They are non-toxic,
non-corrosive, chemically inert and stable below 500 °C [153]. These PCM have
minimal super cooling behaviour, low volume change on melting, low vapour pressure in the melt and in addition reasonable cost. The density of the paraffins ranges
from 700 to 770 kg/m
3 . The most widely used paraffins are hexadecane (C 16 H 34 ),
octadecane (C 18 H 38 ) and eicosane (C 20 H 42 ), along with some of the other paraffinic
hydrocarbons such as heptadecane (C 17 H 36 ) and nonadecane (C 19 H 40 ).
In spite of the many advantages, there are some limitations of paraffins. The
advantages and disadvantages of using paraffin PCM are described below.
Advantages
1. Availability in a large temperature range;
2. Freeze without much super cooling;
3. Ability to melt congruently;
4. Self-nucleating properties;
5. Compatibility with conventional materials of construction;
6. No segregation;
7. Chemically stable;
8. High heat of fusion;
9. Safe and non-reactive and
10. Recyclability.
Disadvantages
1. Low thermal conductivity in solid state;
2. Requirement of high heat transfer rates during the freezing cycle;
3. Low volumetric latent heat storage capacity;
4. Flammability;
5. Chemically purified paraffins are expensive; and
6. Due to cost considerations, only technical grade paraffins are used.
Experimental results have established that laboratory grade paraffin waxes,
tetradecane, hexadecane and their binary mixtures, are excellent candidates as PCM
for cool storage. However, due to the very high cost involved with laboratory grade
materials, technical grade materials must be used for cool storage. Apart from the
paraffin organics, non-paraffin organics are also a common type of PCM available
for textile applications. The non-paraffin organic PCM such as polyethylene glycol
have been applied to fibres and fabrics.
Commercial paraffin waxes are available with a wide range of melting temperatures and are cheap with moderate thermal storage densities (200 kJ/kg or
150 MJ/m
3 ). They are chemically inert and stable with no phase segregation and
undergo negligible sub-cooling. However, their application is limited as they have
low thermal conductivity (0.2 W/m °C), and within a small physical space, they can
exist in all three phases. Metallic fillers, metal matrix structures, finned tubes and
aluminium shavings have been used to improve their thermal conductivity [154]. As
pure paraffin waxes are very costly, commercially only technical grade paraffins are