11 PCM-Metal Foam Composite Systems for Solar Energy Storage
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Energy is transferred to the system by heating it from one side. This may be directly
heated by incoming solar radiation or more typically heated by the heat transfer
fluid circulated from the collector. Multiple metal foam-PCM composite systems are
needed to limit the size of each storage unit.
11.2.3 Materials
Various materials have been proposed and tested for the metal foam structure as well
as for the PCM. Aluminum and copper are the most widely tested material for metal
foam as they offer excellent thermal conductivity (Hong and Herling 2006; Siahpush
et al. 2008; Wang et al. 2016; Mancin et al. 2015; Xiao et al. 2013). Graphite foams,
although not technically metal, have also been tested (Ji et al. 2014; Lafdi et al.
2008; Zhang et al. 2012; Zhao et al. 2014). For the PCM, the choice of material
depends on the temperature range of application as well as its suitability for use with
a metallic foam. The PCM must not react with the metal foam structure or corrode
the surface of the foam. Typical materials which can be used are different types of
organic compounds such as paraffin.
11.3 Important Design Parameters for a PCM-Metal Foam
System
The performance of a composite metal-foam PCM system depends on various factors.
The important factors are: thermo-physical properties of the foam material and PCM,
the foam structure, foam porosity and overall storage unit size and aspect ratio. Each
of these is discussed in the following sub-sections.
11.3.1 Thermo-physical Properties of Foam Material
The properties of the foam material play a vital role in the performance of the storage
system. The foam material should have very high thermal conductivity and high
specific heat for better heat transfer and sensible energy storage characteristics. Low
density of the material is desirable for reducing the overall weight of the system.
However, low density also leads to lower sensible heat storage and thus reduces
the energy storage capacity of the system. Low thermal coefficient of expansion
is necessary so that the foam structure does not change much during the charging
or discharging process. Relatively good mechanical properties such as strength are
211
Energy is transferred to the system by heating it from one side. This may be directly
heated by incoming solar radiation or more typically heated by the heat transfer
fluid circulated from the collector. Multiple metal foam-PCM composite systems are
needed to limit the size of each storage unit.
11.2.3 Materials
Various materials have been proposed and tested for the metal foam structure as well
as for the PCM. Aluminum and copper are the most widely tested material for metal
foam as they offer excellent thermal conductivity (Hong and Herling 2006; Siahpush
et al. 2008; Wang et al. 2016; Mancin et al. 2015; Xiao et al. 2013). Graphite foams,
although not technically metal, have also been tested (Ji et al. 2014; Lafdi et al.
2008; Zhang et al. 2012; Zhao et al. 2014). For the PCM, the choice of material
depends on the temperature range of application as well as its suitability for use with
a metallic foam. The PCM must not react with the metal foam structure or corrode
the surface of the foam. Typical materials which can be used are different types of
organic compounds such as paraffin.
11.3 Important Design Parameters for a PCM-Metal Foam
System
The performance of a composite metal-foam PCM system depends on various factors.
The important factors are: thermo-physical properties of the foam material and PCM,
the foam structure, foam porosity and overall storage unit size and aspect ratio. Each
of these is discussed in the following sub-sections.
11.3.1 Thermo-physical Properties of Foam Material
The properties of the foam material play a vital role in the performance of the storage
system. The foam material should have very high thermal conductivity and high
specific heat for better heat transfer and sensible energy storage characteristics. Low
density of the material is desirable for reducing the overall weight of the system.
However, low density also leads to lower sensible heat storage and thus reduces
the energy storage capacity of the system. Low thermal coefficient of expansion
is necessary so that the foam structure does not change much during the charging
or discharging process. Relatively good mechanical properties such as strength are