Phase Change Material

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A PCM
A phase change material (PCM) is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heat storage (LHS) units.
Contents
1 Characteristics and classification
1.1 Organic PCMs
1.2 Inorganic
1.3 Eutectics
2 Selection Criteria
3 Thermo-physical properties of selected PCMs
4 Technology, Development and Encapsulation
5 Thermal composites
6 Applications
7 Fire and safety issues
8 Links
9 References
//
Characteristics and classification
PCMs latent heat storage can be achieved through solid-solid, solid-liquid, solid-gas and liquid-gas phase change. However, the only phase change used for PCMs is the solid-liquid change. Liquid-gas phase changes are not practical for use as thermal storage due to the large volumes or high pressures required to store the materials when in their gas phase. Liquid-gas transitions do have a higher heat of transformation than solid-liquid transitions. Solid-solid phase changes are typically very slow and have a rather low heat of transformation.
Initially, the solid-liquid PCMs behave like sensible heat storage (SHS) materials; their temperature rises as they absorb heat. Unlike conventional SHS, however, when PCMs reach the temperature at which they change phase (their melting temperature) they absorb large amounts of heat at an almost constant temperature. The PCM continues to absorb heat without a significant raise in temperature until all the material is transformed to the liquid phase. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat. A large number of PCMs are available in any required temperature range from -5 up to 190 oC [1]. Within the human comfort range of 20 to 30鐧�, some PCMs are very effective. They store 5 to 14 times more heat per unit volume than conventional storage materials such as water, masonry, or rock.[citation needed]
Organic PCMs
Paraffin (CnH2n+2) and Fatty acids (CH3(CH2)2nCOOH)
Advantages
Availability in a large temperature range
Freeze without much super cooling
Ability to melt congruently
Self nucleating properties
Compatibility with conventional material of construction
No segregation
Chemically stable
High heat of fusion
Safe and non-reactive
Recyclable
Disadvantages
Low thermal conductivity in their solid state. High heat transfer rates are required during the freezing cycle
Volumetric latent heat storage capacity is low
Flammable. This can be easily alleviated by a proper container
Due to cost consideration only technical grade paraffins may be used which are essentially paraffin mixture and are completely refined of oil
Inorganic
Salt hydrates (MnH2O)
Advantages
High volumetric latent heat storage capacity
Low cost and easy availability
Sharp melting point
High thermal conductivity
High heat of fusion
Low volume change
Non-flammable
Disadvantages
Change of volume is very high
Super cooling is major problem in solid-liquid transition
Nucleating agents are needed and they often become inoperative after repeated cycling
Eutectics
Organic-organic, organic-inorganic, inorganic-inorganic compounds
Advantages
Eutectics have sharp melting point similar to pure substance
Volumetric storage density is slightly above organic compounds
Disadvantages
Only limited data is available on thermo-physical properties as the use of these materials are very new to thermal storage application
Selection Criteria
Thermodynamic properties, The phase change material should possess [2]
Melting temperature in the desired operating temperature range
High latent heat of fusion per unit volume
High specific heat, high density and high thermal conductivity
Small Volume changes on phase transformation and small vapor pressure at operating temperatures to reduce the containment problem
Congruent melting
Kinetic properties
High nucleation rate to avoid super cooling of the liquid phase
High rate of crystal growth, so that the system can meet demands of heat recovery from the storage system
Chemical properties
Chemical stability
Complete reversible freeze/melt cycle
No degradation after a large number of freeze/melt cycle
Non-corrosiveness, non-toxic, non-flammable and non-explosive materials
Economic properties
Low cost...(and so on)

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