Experimental and Numerical Analysis of a PCM-Supported ...

As the thermal and chemical behaviour of each subgroup is greatly different, the
properties of each subgroup are to be discussed in some detail. However, these
PCM to be used in the design of thermal storage systems should exhibit certain
desirable thermo physical, kinetic, and chemical properties, which are listed in frame
(1). In addition, economic competitiveness and large-scale availability of these
materials should be considered.
2.6.1 Organic Phase Change Materials
Paraffins: Some of the more popular and easy to use products are various paraffin
and these can be made with melting points between – 20 °C and 120 °C [103].
Paraffins are non-toxic, ecologically harmless, and chemically inert to nearly all
materials, and this means that there will be no corrosion in energy storage systems.
Paraffin waxes are normally mineral oil products of type C n H 2n+2 , which are a family
of saturated hydrocarbons with very similar properties. Paraffins between C 5 and C 15
are liquids, and the rest are waxy solids. They consist of a mixture of mostly straightchain
paraffins, or normal alkanes, their melting temperatures are strongly influenced
by the length of the chain of the alkalis, and ranges between 23 and 67 °C [97].
Generally, the longer the average length of the chain, the higher the melting
temperature and heat of fusion [96].
Numerous investigations show that paraffin waxes present many desirable
characteristics of a PCM for heat storage purposes such as:
(i) High heat of fusion, good self-nucleating properties, and no tendency of
phase segregation on melting
(ii) Chemically stable, safe, and non-reactive, good kinetic properties for the
phase transition, therefore only a low or no supercooling effect
(iii) Compatible with all metal containers and easily incorporated into heat
storage systems. Care however should be taken when using plastic
containers as paraffins have a tendency to infiltrate and soften some
plastics [99]
(iv) Easily available commercially from many manufacturers in various grades
with reasonable prices
However, the main drawbacks are equally addressed as:
(i) Low thermal conductivity in solid state, which presents a problem when
high heat transfer rates are required during the freezing cycle. Therefore,
special considerations are required for the heat exchanger in order to
overcome this problem. The most sound solutions to enhance the thermal
conductivity of paraffins are to use finned containers and metallic fillers or
through combination latent/sensible storage systems [100]. Aluminum
honeycombs has been found to improve system performance [86].
(ii) In addition, pure paraffin waxes are very expensive therefore economics
only permit the use of technical quality paraffin waxes, which do not have
20