Phase-change material

A sodium acetate heating pad. When the sodium acetate solution crystallises, it becomes warm.

A video showing a „heating pad“ in action

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.

Characteristics and classification

Latent heat storage can be achieved through liquid–>solid, solid–>liquid, solid–>gas and liquid–>gas phase changes. However, only solid–>liquid and liquid–>solid phase changes are practical for PCMs. Although liquid–gas transitions have a higher heat of transformation than solid–liquid transitions, liquid->gas phase changes are impractical for thermal storage because large volumes or high pressures are required to store the materials in their gas phase. Solid–solid phase changes are typically very slow and have a relatively low heat of transformation.

Initially, solid–liquid PCMs behave like sensible heat storage (SHS) materials; their temperature rises as they absorb heat. Unlike conventional SHS materials, 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 °C.^[1] Within the human comfort range between 20–30 °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 (C_nH_2n+2), carbohydrate and lipid derived. ^[3][4][5]

• Advantages
  • Freeze without much undercooling
  • 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
  • Carbohydrate and lipid based PCMs can be produced from renewable sources
• Disadvantages
  • Low thermal conductivity in their solid state. High heat transfer rates are required during the freezing cycle
  • Volumetric latent heat storage capacity can be low
  • Flammable. This can be partially alleviated by specialist containment
  • 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 high costs

Inorganic

Salt hydrates (M_nH₂O)^[6]

• Advantages
  • High volumetric latent heat storage capacity
  • Availability and low cost
  • Sharp melting point
  • High thermal conductivity
  • High heat of fusion
  • 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

c-inorganic, inorganic-inorganic compounds

• Advantages
  • Eutectics have sharp melting point similar to pure substance
  • Volumetric storage density is slightly above organic compounds
  • Extra water principle can be used to avoid phase change dedragation, involveing dissolving the anhydrous salt during melting to result in a thickening of the liquid material so that it melts to a gel form
• Disadvantages
  • Only limited data is available on thermo-physical properties as the use of these materials are relatively new to thermal storage application

Hygroscopic materials

Many natural building materials are hygroscopic, that is they can absorb (water condenses) and release water (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, large surfaces area allows significant (1–2 °C) heating or cooling in buildings. The corresponding materials are wool insulation, earth/clay render finishes,.

Selection criteria

Thermodynamic properties. The phase change material should possess:^[7]

• 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 supercooling 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
• Availability

Thermophysical properties

Common PCMs

Material	Organic PCM	Melting point oC	Heat of fusion kJ·kg−1	Heat of fusion MJ·m−3	Specific heat, cp solid kJ·kg−1·K−1	Specific heat, cp liquid kJ·kg−1·K−1	Density, ρ solid kg·m−3	Density, ρ liquid kg·m−3	Thermal conductivity, k solid W·m−1·K−1	VHC solid kJ·m−3·K−1	VHC liquid kJ·m−3·K−1	Thermal effusivity, e solid J·m−2·K−1·s−1/2	Cost USD·kg−1
Water	No	5000000000000000000♠0	7002333600000000000♠333.6	7002319800000000000♠319.8	7000205009999999999♠2.05	7000418600000000000♠4.186	7002917000000000000♠917	7003100000000000000♠1,000	7000191000000000000♠1.6[8]-2.22[9]	7003188000000000000♠1,880	7003418600000000000♠4,186	7003189000000000000♠1,890	6997312500000000000♠0.003125[10]
Sodium sulfate (Na2SO4·10H2O)	No	7001324000000000000♠32.4	7002252000000000000♠252										0.05 [11]
NaCl·Na2SO4·10H2O	No	7001180000000000000♠18	7002286000000000000♠286										0.05 [11]
Lauric acid	Yes[12][13]	7001442000000000000♠44.2[14]	7002211600000000000♠211.6	7002197700000000000♠197.7	7000176000000000000♠1.76	7000227000000000000♠2.27	7003100700000000000♠1,007	7002862000000000000♠862		7003177200000000000♠1,772	7003195700000000000♠1,957		7000160000000000000♠1.6 [15][16]
TME(63%w/w)+H2O(37%w/w)	Yes[12][13]	7001298000000000000♠29.8	7002218000000000000♠218.0	7002240900000000000♠240.9	7000275000000000000♠2.75	7000358000000000000♠3.58	7003112000000000000♠1,120	7003109000000000000♠1,090		7003308000000000000♠3,080	7003390200000000000♠3,902
Mn(NO3)2·6H2O+MnCl2·4H2O(4%w/w)	No[17][18]	7001200000000000000♠15–25	7002125900000000000♠125.9	7002221800000000000♠221.8	7000234000000000000♠2.34	7000277990000099999♠2.78	7003179500000000000♠1,795	7003172800000000000♠1,728		7003420000000000000♠4,200	7003480400000000000♠4,804
Na2SiO3·5H2O	No[17][18]	7001722000000000000♠72.20	7002267000000000000♠267.0	7002364500000000000♠364.5	7000383000000000000♠3.83	7000457000000000000♠4.57	7003145000000000000♠1,450	7003128000000000000♠1,280	6999115500000000000♠0.103−0.128[19]	7003555400000000000♠5,554	7003585000000000000♠5,850	7002801000000000000♠801	7000803999999999999♠8.04[20]
Aluminium	No	7002660320000000000♠660.32	7002396900000000000♠396.9	7003100720000000000♠1,007.2	6999896900000000000♠0.8969		7003270000000000000♠2,700	7003237500000000000♠2,375	7002237000000000000♠237[21][22]	7003242200000000000♠2,422	?	7004239600000000000♠23,960	7000204626000000000♠2.04626[23]
Copper	No	7003108462000099999♠1,084.62	7002208700000000000♠208.7	7003176950000000000♠1,769.5	6999384600000000000♠0.3846		7003894000000000000♠8,940	7003802000000000000♠8,020	7002401000000000000♠401[24]	7003343800000000000♠3,438	?	7004371300000000000♠37,130	7000681256000000000♠6.81256[25]
Gold	No	7003106418000000000♠1,064.18	7001637200000000000♠63.72	7003116630000000000♠1,166.3	6999129000000000000♠0.129		7004193000000000000♠19,300	7004173100000000000♠17,310	7002318000000000000♠318[26]	7003249100000000000♠2,491		7004281400000000000♠28,140	7004342978000000000♠34,297.8[25]
Iron	No	7003153800000000000♠1,538	7002247300000000000♠247.3	7003183660000000000♠1,836.6	6999449500000000000♠0.4495		7003787400000000000♠7,874	7003698000000000000♠6,980	7001804000000000000♠80.4[27]	7003353900000000000♠3,539		7004168700000000000♠16,870	6999324790000000000♠0.3248[28]
Lead	No	7002327450000000000♠327.46	7001230200000000000♠23.02	7002253200000000000♠253.2	6999128600000099999♠0.1286		7004113400000000000♠11,340	7004106600000000000♠10,660	7001353009999900000♠35.3[29]	7003145900000000000♠1,459		7003718000000000000♠7,180	7000211510000000000♠2.1151[25]
Lithium	No	7002180540000000000♠180.54	7002432200000000000♠432.2	7002226000000000000♠226.0	7000358160000000000♠3.5816		7002534000000000000♠534	7002512000000000000♠512	7001848000000000000♠84.8[30]	7003191300000000000♠1,913		7004127400000000000♠12,740	7001622164000000000♠62.2164[31]
Silver	No	7002961780000000000♠961.78	7002104600000000000♠104.6	7003103580000000000♠1,035.8	6999235000000000000♠0.235		7004104900000000000♠10,490	7003932000000000000♠9,320	7002429000000000000♠429[32]	7003246500000000000♠2,465		7004325200000000000♠32,520	7002492524000000000♠492.524[25]
Titanium	No	7003166800000000000♠1,668	7002295600000000000♠295.6	7003127350000000000♠1,273.5	6999523500000099999♠0.5235		7003450600000000000♠4,506	7003411000000000000♠4,110	7001219000000000000♠21.9[33]	7003235900000000000♠2,359		7003719000000000000♠7,190	7000804690000000000♠8.0469[34]
Zinc	No	7002419539999900000♠419.53	7002112000000000000♠112.0	7002767500000000000♠767.5	6999389600000000000♠0.3896		7003714000000000000♠7,140	7003657000000000000♠6,570	7002116000000000000♠116[35]	7003278200000000000♠2,782		7004179600000000000♠17,960	7000215735000000000♠2.15735[25]
	No	7002310000000000000♠310	7002174000000000000♠174										[36]
	No	7002282000000000000♠282	7002212000000000000♠212										[36]
NaOH	No	7002318000000000000♠318	7002158000000000000♠158										[36]
	No	7002337000000000000♠337	7002116000000000000♠116										[36]
KOH	No	7002360000000000000♠360	7002167000000000000♠167										[36]
NaOH/ (7.2%)	No	7002283000000000000♠283	7002340000000000000♠340										[36]
NaCl(26.8%)/NaOH	No	7002370000000000000♠370	7002370000000000000♠370										[36]
NaCl/KCL(32.4%)/LiCl(32.8%)	No	7002346000000000000♠346	7002281000000000000♠281										[36]
NaCl(5.7%) / (85.5%) /	No	7002287000000000000♠287	7002176000000000000♠176										[36]
NaCl / (5.0%)	No	7002284000000000000♠284	7002171000000000000♠171										[36]
NaCl(5.0%)/	No	7002282000000000000♠282	7002212000000000000♠212										[36]
NaCl(42.5%)/KCl(20.5)/	No	7002389000000000000♠385-393	7002410000000000000♠410										[36]
	No	7002290000000000000♠290	7002170000000000000♠170										[36]
/KCl(4.5%)	No	7002320000000000000♠320	7002150000000000000♠150										[36]
/KBr(4.7%)/KCl(7.3%)	No	7002342000000000000♠342	7002140000000000000♠140										[36]
Paraffin 14-Carbons [37]	Yes	5.5	228
Paraffin 15-Carbons [37]	Yes	10	205
Paraffin 16-Carbons [37]	Yes	16.7	237.1
Paraffin 17-Carbons [37]	Yes	21.7	213
Paraffin 18-Carbons [37]	Yes	28	244
Paraffin 19-Carbons [37]	Yes	32	222
Paraffin 20-Carbons [37]	Yes	36.7	246
Paraffin 21-Carbons [37]	Yes	40.2	200
Paraffin 22-Carbons [37]	Yes	44	249
Paraffin 23-Carbons [37]	Yes	47.5	232
Paraffin 24-Carbons [37]	Yes	50.6	255
Paraffin 25-Carbons [37]	Yes	49.4	238
Paraffin 26-Carbons [37]	Yes	56.3	256
Paraffin 27-Carbons [37]	Yes	58.8	236
Paraffin 28-Carbons [37]	Yes	61.6	253
Paraffin 29-Carbons [37]	Yes	63.4	240
Paraffin 30-Carbons [37]	Yes	65.4	251
Paraffin 31-Carbons [37]	Yes	68	242
Paraffin 32-Carbons [37]	Yes	69.5	170
Paraffin 33-Carbons [37]	Yes	73.9	268
Paraffin 34-Carbons [37]	Yes	75.9	269
Formic acid [37]	Yes	7.8	247
Caprilic acid [37]	Yes	16.3	149
Glycerin [37]	Yes	17.9	198.7
p-Lattic acid [37]	Yes	26	184
Methyl palmitate [37]	Yes	29	205
Camphenilone [37]	Yes	39	205
Docasyl bromide [37]	Yes	40	201
Caprylone [37]	Yes	40	259
Phenol [37]	Yes	41	120
Heptadecanone [37]	Yes	41	201
1-Cyclohexylooctadecane [37]	Yes	41	218
4-Heptadacanone [37]	Yes	41	197
p-Joluidine [37]	Yes	43.3	167
Cyanamide [37]	Yes	44	209
Methyl eicosanate [37]	Yes	45	230
3-Heptadecanone [37]	Yes	48	218
2-Heptadecanone [37]	Yes	48	218
Hydrocinnamic acid [37]	Yes	48	118
Cetyl acid [37]	Yes	49.3	141
α-Nepthylamine [37]	Yes	59	93
Camphene [37]	Yes	50	238
O-Nitroaniline [37]	Yes	50	93
9-Heptadecanone [37]	Yes	51	213
Thymol [37]	Yes	51.5	115
Methyl behenate [37]	Yes	52	234
Diphenyl amine [37]	Yes	52.9	107
p-Dichlorobenzene [37]	Yes	53.1	121
Oxolate [37]	Yes	54.3	178
Hypophosphoric acid [37]	Yes	55	213
O-Xylene dichloride [37]	Yes	55	121
β-Chloroacetic acid [37]	Yes	56	147
Chloroacetic acid [37]	Yes	56	130
Nitro napthalene [37]	Yes	56.7	103
Trimyristin [37]	Yes	33	201
Heptaudecanoic acid [37]	Yes	60.6	189
α-Chloroacetic acid [37]	Yes	61.2	130
Bees wax [37]	Yes	61.8	177
Glyolic acid [37]	Yes	63	109
Glycolic acid [37]	Yes	63	109
p-Bromophenol [37]	Yes	63.5	86
Azobenzene [37]	Yes	67.1	121
Acrylic acid [37]	Yes	68	115
Dinto toluent (2,4) [37]	Yes	70	111
Phenylacetic acid [37]	Yes	76.7	102
Thiosinamine [37]	Yes	77	140
Bromcamphor [37]	Yes	77	174
Durene [37]	Yes	79.3	156
Methly brombenzoate [37]	Yes	81	126
Alpha napthol [37]	Yes	96	163
Glautaric acid [37]	Yes	97.5	156
p-Xylene dichloride [37]	Yes	100	138.7
Catechol [37]	Yes	104.3	207
Quinone [37]	Yes	115	171
Actanilide [37]	Yes	118.9	222
Succinic anhydride [37]	Yes	119	204
Benzoic acid [37]	Yes	121.7	142.8
Stibene [37]	Yes	124	167
Benzamide [37]	Yes	127.2	169.4
Acetic acid [37]	Yes	16.7	184
Polyethylene glycol 600 [37]	Yes	20	146
Capric acid [37]	Yes	36	152
Eladic acid [37]	Yes	47	218
Pentadecanoic acid [37]	Yes	52.5	178
Tristearin [37]	Yes	56	191
Myristic acid [37]	Yes	58	199
Palmatic acid [37]	Yes	55	163
Stearic acid [37]	Yes	69.4	199
Acetamide [37]	Yes	81	241
Methyl fumarate [37]	Yes	102	242

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

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

Commercially available PCMs near room temperature

The above dataset is also available as an Excel spreadsheet from UCLA Engineering

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 19th century as a medium for the thermal storage applications. They have been used in such diverse applications as refrigerated transportation^[88] for rail^[89] and road applications^[90] and their physical properties are, therefore, well known.

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^[91] 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.
  • 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.

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 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. They are also meant to discover minor elements in the earth.

Applications

Phase-change material being employed in the treatment of neonates with birth asphyxia^[92][93]

Applications^[1][94] of phase change materials include, but are not limited to:

• Thermal energy storage
• Solar cooking
• Cold Energy Battery
• 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, treatment of birth asphyxia^[92]
• Human body cooling under bulky clothing or costumes.
• 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
• Turbine Inlet Chilling with thermal energy storage
• Telecom shelters in tropical regions. 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.

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.

See also

• Heat pipe

References

Further reading

• Raoux, S. (2009). "Phase Change Materials". Annual Review of Materials Research. 39: 25–48. Bibcode:2009AnRMS..39...25R. doi:10.1146/annurev-matsci-082908-145405.