The advantage of a PCM is the use of the latent heat
which is available during the phase change process. A smaller amount of
the heat storage capacity (depending on the temperature difference) consists
of sensible heat.
The specific heat capacity of latent heat paraffins is about 2,1 kJ/(kg·K). Their melt enthalpy lies between 120 and 160 kJ/kg, which is very high for organic materials. The combination of these two values results in an excellent energy storage density. Consequently, latent heat paraffins/waxes offer four to five times higher heat capacity by volume or mass, than water at low operating temperature differences.
An efficient input and output of heat energy, for example into a latent heat storage unit, requires a high thermal conductivity coefficient. Other applications, such as food transport, have no such requirement.
Like nearly all other organic materials, latent heat paraffins have a
low heat conductivity. Although this is seemingly a disadvantage (in food
transport systems for example, this is an advantage), it can be compensated
for by large heat transfer surface areas of the heat storage material.
This is achieved in the case of many of our bound PCM forms, where a large
surface area of thin layers of PCM is created.
Every material changes its density and thus its volume
when it goes through phase change from solid to liquid. This can be problematic
in certain applications. Consequently, if used in a closed container,
provision must be made for the volume expansion of pure PCM's, in order
to avoid excess pressure.
If the melting point of a material is found well above it's solidification point, supercooling is observed. During phase change the entire heat transported should be available at the same temperature for both, melting and solidification. In contrast to many other known PCM's, RUBITHERM PCM's show little to none supercooling.
During the "lifespan" of a PCM, it is submitted
to an immense number of thermal energy charging and discharging cycles.
Ideally, the thermodynamic properties of the PCM should not be affected,
i.e. neither the heat storage capacity nor the melting and congealing
temperatures should change over the product's life time.
Unlike many other latent heat storage materials, RUBITHERM
PCM's are long-lasting and stable throughout phase change cycles. This
is because there is no chemical reaction during the thermal energy storage
process, not within the material itself, nor with the heat transport medium,
nor with the construction materials used in the application construction.
Melting and solidification are purely physical processes, which is the
reason why the heat storage capacity, remains at a high level throughout
the unit's working life.
If the temperature within a system is higher than
foreseen, overheating of the PCM could occur. Over short periods, the
result is simply that additional sensible heat is stored within the system.
If overheating takes place over a longer time period, there could also
be a negative effect on the PCM itself.
Another important feature of a PCM is its lack of reactivity to other materials. The PCM should ideally cause neither corrosion nor other negative effects within a storage unit.
Latent heat paraffins are chemically inert to nearly all materials. This
very low chemical reactivity is the origin of the term "paraffin",
derived from the Latin expression "parum affinis" effectively
meaning, nearly chemically inert. Consequently, no corrosion problems
occur within paraffin containers. In fact, paraffins are used as corrosion
prevention materials (cavity conservation and varnish protection) in the
automotive industry.
Nowadays,
customers also consider the ecological aspects of a product to be an important
factor.
Latent heat waxes are ecologically harmless, having no adverse affects on plants, animals or micro-organisms. They are classified as non water endangering substances (if melting above 27 °C) and are 100 % recyclable. They are furthermore neither toxic nor dangerous to health.
Refined latent heat paraffins meet the purity requirements of the German (BgVV) and American (FDA) regulations for materials in contact with food, and are thus used, for example, as cheese coatings, in chewing gums and as additives for foodstuff packaging materials. In the cosmetic and pharmaceutical industries they serve as a base material for creams and ointments.
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Glossary: • The
Latent Heat Storage Process |