Characterist ics 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 oftransformation than solid-liquid transitions. Solid-s olid 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 rise 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 o C. [1] Within the human comfort range of 20° to 30°C, 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. [2] Organic PCMs Paraffin (CnH2n+2) and Fatty acids (CH3(CH2)2nCOOH) Advantages 1. Fre eze wit hout mu ch s uper cool ing 2. Abi lit y t o melt con gruent ly 3. Self nucleati ng pr operties 4. Compa tibi lity with conve ntion al mat erial of con stru ction 5. No segreg at ion 6. Chemical ly stable 7. Hi gh heat of fu sion 8. Safe and non-reacti ve 9. Recyclable Disadvantages 1. Low thermal conductiv ity in the ir soli d state. High heat tr ansfer rates are re quired du ring th e freezing cycle
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5/12/2018 Characteristics and Classification of PCM - slidepdf.com
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 requiredto 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 rise 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°C, 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.[2]
Organic PCMs
Paraffin (CnH2n+2) and Fatty acids (CH3(CH2)2nCOOH)
Advantages
1. Freeze without much super cooling
2. Ability to melt congruently
3. Self nucleating properties
4. Compatibility with conventional material of construction
5. No segregation
6. Chemically stable
7. High heat of fusion
8. Safe and non-reactive
9. Recyclable
Disadvantages
1. Low thermal conductivity in their solid state. High heat transfer rates are required during the
The most commonly used PCMs are salt hydrates, fatty acids and esters, and
various paraffins (such as octadecane). Recently also ionic liquids were investigated as novel
PCMs.
As most of the organic solutions are water-free, they can be exposed to air, but all salt based
PCM solutions must be encapsulated to prevent water evaporation or uptake. Both types offer certain advantages and disadvantages and if they are correctly applied some of the
disadvantages becomes an advantage for certain applications.
They have been used since the late 1800s as a medium for the thermal storage applications.
They have been used in such diverse applications as refrigerated transportation for rail and
road applications and their physical properties are, therefore, well-known.[citation needed
]
Unlike the ice storage system, however, the PCM systems can be used with any conventional
water chiller both for a new or alternatively retrofit application. The positive temperature phase
change allows centrifugal and absorption chillers as well as the conventional reciprocating and
screw chiller systems or even lower ambient conditions utilizing a cooling tower or dry cooler
for charging the TES system.
The temperature range offered by the PCM technology provides a new horizon for the building
services and refrigeration engineers regarding medium and high temperature energy storage
applications. The scope of this thermal energy application is wide ranging of solar heating, hot
water, heating rejection, i.e. cooling tower and dry cooler circuitry thermal energy storage
applications.
Since PCMs transform between solid-liquid in thermal cycling, encapsulation[31] naturally
become the obvious storage choice.
Encapsulation of PCMs
Macro-encapsulation: Early development of macro-encapsulation with large volume
containment failed due to the poor thermal conductivity of most PCMs. PCMs tend to
solidify at the edges of the containers preventing effective heat transfer.
Some phase change materials are suspended in water, and are relatively nontoxic. Others are
hydrocarbons or other flammable materials, or are toxic. As such, PCMs must be selected and
applied very carefully, in accordance with fire and building codes and sound engineering
practices. Because of the increased fire risk, flamespread, smoke, potential for explosion when
held in containers, and liability, it may be wise not to use flammable PCMs within residential or other regularly occupied buildings. Phase change materials are also being used in thermal
regulation of electronics.
External links
PCM University
PureTemp Renewable PCM
Council House 2 (CH2) PCM System Explanation
Micro-encapsulated Salt Hydrates
savEnrg PCM
References
1. ^ a b M. Kenisarin and K. Mahkamov, Renewable & Sustainable Energy Reviews 11 (2007)
1913-1965
2. ^ Atul Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Renewable & Sustainable Energy
Reviews 13 (2009) 318-345
3.^ A. Pasupathy, R. Velraj and R.V. Seeniraj, Renewable & Sustainable Energy Reviews 12
(2008) 39-64
4. ^ HyperPhysics, most from Young, Hugh D., University Physics, 7th Ed., Addison Wesley,
1992. Table 15-5. (most data should be at 293 K (20oC;68oF))