A phase-change material 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 units. Characteristics and classification
PCMs latent heat storage can be achieved through liquid–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 materials; their temperature rises as they absorb
heat. Unlike conventional SHS, however, when PCMs reach the temperature at which
they change phase 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. 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. = Organic PCMs=
Paraffin and fatty acids2nCOOH) 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
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 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 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= 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 relatively new to
thermal storage application = Hygroscopic materials=
Many natural building materials are hygroscopic, that is they can absorb and
release water. The process is thus: Condensation ΔH0; enthalpy increases
absorbs heat. Whilst this process liberates a small
quantity of energy, large surfaces area allows significant 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: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 of common PCMs Volumetric heat capacity J·m−3·K−1
Thermal inertia = Thermal effusivity J·m−2·K−1·s−1/2
Thermophysical properties of 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. 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 for rail and
road applications 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 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. 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. 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. 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 and other 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 into the relatively low conducting PCM thus
increasing overall or bulk 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 and a
matrix. A thermal composite is not so clearly defined, but could similarly
refer to a matrix 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
Applications 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, treatment of birth asphyxia 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 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.. "Phase Change Materials". Annual Review of Materials Research 39:
25–48. Bibcode:2009AnRMS..39...25R. oi:10.1146/annurev-matsci-082908-145405.