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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 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

freezing cycle

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2. Volumetric latent heat storage capacity is low

3. Flammable. This can be easily alleviated by a proper container 

4. To obtain reliable phase change points, most manufacturers use technical grade paraffins

which are essentially paraffin mixture(s) and are completely refined of oil, resulting in highcosts

Inorganic

Salt hydrates (MnH2O)

Advantages

1. High volumetric latent heat storage capacity

2. Low cost and easy availability

3. Sharp melting point

4. High thermal conductivity

5. High heat of fusion

6. Non-flammable

Disadvantages

1. Change of volume is very high

2. Super cooling is major problem in solid-liquid transition

3. Nucleating agents are needed and they often become inoperative after repeated cycling

Eutectics

Organic-organic, organic-inorganic, inorganic-inorganic compounds

Advantages

1. Eutectics have sharp melting point similar to pure substance

2. Volumetric storage density is slightly above organic compounds

Disadvantages

1. Only limited data is available on thermo-physical properties as the use of these materials are

very new to thermal storage application

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Hygroscopic materials

Many natural building materials are hygroscopic, that is they can absorb water (water condenses) and

release (water evaporates). The process is thus : Condensation (gas to liquid) ΔH<0; enthalpy decreases

(exothermic process) gives off heat. Vaporization (liquid to gas) ΔH>0; enthalpy increases (endothermic

process) absorbs heat (or cools).

Whilst this process liberates a small quantity of energy, due to the large surfaces areas possible

significant +/- 1 to 2 degree C heating or cooling can be achieved in buildings. For example wool

insulation, earth/clay render finishes.

Selection Criteria

Thermodynamic properties, The phase change material should possess [3]

1. Melting temperature in the desired operating temperature range

2. High latent heat of fusion per unit volume

3. High specific heat, high density and high thermal conductivity

4. Small Volume changes on phase transformation and small vapor pressure at operating

temperatures to reduce the containment problem

5. Congruent melting

Kinetic properties

1. High nucleation rate to avoid super cooling of the liquid phase

2. High rate of crystal growth, so that the system can meet demands of heat recovery from the

storage system

Chemical properties

1. Chemical stability

2. Complete reversible freeze/melt cycle

3. No degradation after a large number of freeze/melt cycle

4. Non-corrosiveness, non-toxic, non-flammable and non-explosive materials

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Economic properties

1. Low cost

2. Large-scale availabilities

]Thermo-physical properties of selected PCMs

Material(s)

Org anic PC  M 

Melting

point

oC

Heat of fusionkJ·kg−1

Heat of fusion

MJ·m−3

c

 

 p

 solid kJ·kg−

1·K −1

c

 

 p

liquid kJ·kg−

1·K −1

ρ soli 

d kg·m−3

ρliquid kg·m−3

k  solid 

W·m−1·K −1

VHC solid kJ·m− 3

 

·K −1

VHCliquid kJ·m−

3·K −1

e solid 

J·m−2  ·K −1·s−1/2

CostUSD·kg−1

Water No 0333.

6

319.

82.05 4.186 917

1,0

00

1.6[4]-

2.22[5] 1,880 4,186 1,8900.003

125[6]

Lauric acidYes[

7][8]

44.2[

9]

211.

6

197.

71.76 2.27

1,0

07862 ? 1,772 1,957 ?

1.6 [10]

[11]

TME(63%w/w)

+H2O(37%w/w)

Yes[

7][8] 29.8218.

0

240.

92.75 3.58

1,1

20

1,0

90? 3,080 3,902 ? ?

Mn(NO3)2·6H2O+MnCl2·4H2O(4%w/w)

 No[1

2][13]

15 -

25

125.

9

221.

82.34 2.78

1,7

95

1,7

28? 4,200 4,804 ? ?

Na2SiO3·5H2O(pentahydrate)

 No[1

2][13] 48267.

0364.

53.83 4.57

1,450

1,280

.

103−.1

28[14]

5,554 5,850 8018.04[15

]

Aluminium, pure No 660.32

396.9

1,007.2

0.8969 ? 2,700

2,375

237[16]

[17] 2,422 ? 23,960 2.04626[18]

Copper, pure No1,084.62

208.7

1,769.5

0.3846 ?8,940

8,020

401[19] 3,438 ? 37,1306.81256[20]

Gold, pure No1,06

4.18

63.7

2

1,16

6.30.129 ?

19,

300

17,

310318[21] 2,491 ? 28,140

34,29

7.8[20]

Iron, pure No1,53

8

247.

3

1,83

6.60.4495 ?

7,8

74

6,9

8080.4[22] 3,539 ? 16,870

0.324

8[23]

Lead, pure No327.

46

23.0

2

253.

20.1286 ?

11,

340

10,

66035.3[24] 1,459 ? 7,180

2.115

1[20]

Lithium, pure No180.

54

432.

2

226.

0

3.5816 ? 534 512 84.8[25] 1,913 ? 12,74062.21

64[26]

Silver, pure No961.

78

104.

6

1,03

5.80.235 ?

10,

490

9,3

20429[27] 2,465 ? 32,520

492.5

24[20]

Titanium, pure No1,66

8

295.

6

1,27

3.50.5235 ?

4,5

06

4,1

1021.9[28] 2,359 ? 7,190

8.046

9[29]

Zinc, pure No419.

53

112.

0

767.

50.3896 ?

7,1

40

6,5

70116[30] 2,782 ? 17,960

2.157

35[20]

Volumetric heat capacity (VHC) J·m−3·K−1

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Thermal Inertia (I) = Thermal effusivity (e) J·m−2·K−1·s−1/2

Technology, Development and Encapsulation

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.

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Micro-encapsulation: Micro-encapsulation on the other hand showed no such problem.

It allows the PCMs to be incorporated into construction materials, such as concrete, 

easily and economically. Micro-encapsulated PCMs also provide a portable heat

storage system. By coating a microscopic sized PCM with a protective coating, the

particles can be suspended within a continuous phase such as water. This system can

be considered a phase change slurry(PCS).

Molecular-encapsulation is another technology, developed by Dupont de Nemours that

allows a very high concentration of PCM within a polymer compound. It allows storage

capacity up to 515 kJ/m2  for a 5 mm board (103 MJ/m3 ). Molecular-encapsulation

allows drilling and cutting through the material without any PCM leakage.

As phase change materials perform best in small containers, therefore they are usually divided

in cells. The cells are shallow to reduce static head - based on the principle of shallow

container geometry. The packaging material should conduct heat well; and it should be durable

enough to withstand frequent changes in the storage material's volume as phase changes

occur. It should also restrict the passage of water through the walls, so the materials will not

dry out (or water-out, if the material is hygroscopic). Packaging must also resist leakage

and corrosion. Common packaging materials showing chemical compatibility with room

temperature PCMs include stainless steel, polypropylene and polyolefin.

Currently, phase change materials (PCMs) are very widely used in tropical regions in telecom

shelters. They protect the high-value equipment in the shelter by keeping the indoor air 

temperature below the maximum permissible by absorbing heat generated by power-hungry

equipment such as a Base Station Subsystem. In case of a power failure to conventional

cooling systems, PCMs minimize use of diesel generators, and this can translate into

enormous savings across thousands of telecom sites in tropics.

Thermal composites

Thermal-composites is a term given to combinations of phase change materials (PCMs) and

other (usually solid) structures. A simple example is a copper-mesh immersed in a paraffin-

wax. The copper-mesh within parraffin-wax can be considered a composite material, dubbed a

thermal-composite. Such hybrid materials are created to achieve specific overall or bulk

properties.

Thermal conductivity is a common property which is targeted for maximisation by creating

thermal composites. In this case the basic idea is to increase thermal conductivity by adding a

highly conducting solid (such as the copper-mesh) into the relatively low conducting PCM thus

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increasing overall or bulk (thermal) conductivity. If the PCM is required to flow, the solid must

be porous, such as a mesh.

Solid composites such as fibre-glass or kevlar-pre-preg for the aerospace industry usually refer 

to a fibre (the kevlar or the glass) and a matrix (the glue which solidifies to hold fibres and

provide compressive strength). A thermal composite is not so clearly defined, but could

similarly refer to a matrix (solid) and the PCM which is of course usually liquid and/or solid

depending on conditions.

Applications

Applications[1][32] of phase change materials include, but are not limited to:

Thermal energy storage

Conditioning of buildings, such as 'ice-storage'

Cooling of heat and electrical engines

Cooling: food, beverages, coffee, wine, milk products, green houses

Medical applications: transportation of blood, operating tables, hot-cold therapies

Waste heat  recovery

Off-peak power utilization: Heating hot water and Cooling

Heat pump systems

Passive storage in bioclimatic building/architecture (HDPE, paraffin)

Smoothing exothermic temperature peaks in chemical reactions

Solar power plants

Spacecraft  thermal systems

Thermal comfort  in vehicles

Thermal protection of electronic devices

Thermal protection of food: transport, hotel trade, ice-cream etc.

Textiles used in clothing

Computer cooling

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Fire and safety issues

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))

5. ^ http://www.engineeringtoolbox.com/ice-thermal-properties-d_576.html

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6. ^ AAP (April 21, 2009). " Melburnians face 60pc water cost rise - MELBURNIANS face paying

up to 60 per cent more for water and sewerage under proposals announced today by the state's

economic regulator.". The Australian. Retrieved 2010-02-24.

7. ^ a b A. Sari et al. Energy Convers. Manage 43 (2002) 2493

8. ^ a b H. Kakuichi et al., IEA annex 10 (1999)

9. ^ Beare-Rogers, J.; Dieffenbacher, A.; Holm, J.V. (2001). "Lexicon of lipid nutrition (IUPAC

Technical Report)". Pure and Applied Chemistry  73 (4): 685–

744. doi:10.1351/pac200173040685.

10.^ " lauric acid Q/MHD002-2006 lauric acid CN ;SHN products". Alibaba.com. Retrieved 2010-02-

24.

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Pricing. Retrieved 2010-03-10.

12.^ a b K. Nagano et al. Appl. Therm. Eng. 23 (2003) 229

13.^ a b Y. Zhang et al. Meas. Sci. Technol 10 (1999) 201

14.^ Kalapathy, Uruthira; Proctor, Andrew; Shultz, John (2002-12-10). "Silicate Thermal Insulation

Material from Rice Hull Ash". Industrial & Engineering Chemistry Research 42 (1): 46–

49. doi:10.1021/ie0203227.

15.^ http://www.sheffield-pottery.com/SODIUM-SILICATE-WATER-GLASS-ONE-PINT-

p/rmsodsilw.htm

16.^ Hukseflux Thermal Sensors

17.^ http://www.goodfellow.com/E/Aluminium.html

18.^ "Aluminum Prices, London Metal Exchange (LME) Aluminum Alloy Prices, COMEX and

Shanghai Aluminum Prices". 23 Feb 2010. Retrieved 2010-02-24.

19.^ http://www.goodfellow.com/E/Copper.html

20.^ a b c  d  e "Metal Prices and News". 23 Feb 2010. Retrieved 2010-02-24.

21.^ http://www.goodfellow.com/E/Gold.html

22.^ http://www.goodfellow.com/E/Iron.html

23.^ "Iron Page". 07 Dec 2007. Retrieved 2010-02-24.

24.^ http://www.goodfellow.com/E/Lead.html

25.^ http://www.goodfellow.com/E/Lithium.html

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26.^ "Historical Price Query". Aug 14, 2009. Retrieved 2010-02-24.

27.^ http://www.goodfellow.com/E/Silver.html

28.^ http://www.goodfellow.com/E/Titanium.html

29.^ "Titanium Page". 28 Dec 2007. Retrieved 2010-02-24.

30.^ http://www.goodfellow.com/E/Zinc.html

31.^ V.V. Tyagi and D. Buddhi, Renewable & Sustainable Energy Reviews 11 (2007) 1146

32.^ A.M. Omer, Renewable & Sustainable Energy Reviews 12 (2008) 1562