Phase-change material

src: www.climatetechwiki.org

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.

Video Phase-change material

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. 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

Bio-Based, or Paraffin (C_nH_2n+2), or carbohydrate and lipid derived.

• 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. Nano composites were found to yield an effective thermal conductivity increase up to 216%.
  • Volumetric latent heat storage capacity can be low
  • Flammable. This can be partially alleviated by specialist containment, or by incorporating environmentally friendly fire retardants.

Inorganic

Salt hydrates (M_nH₂O)

• Advantages
  • High volumetric latent heat storage capacity
  • Availability and low cost
  • Sharp melting point
  • High thermal conductivity
  • High heat of fusion
  • Non-flammable
• Disadvantages
  • Incongruous melting and phase separation upon cycling which can cause a significant loss in latent heat enthalpy.
  • Corrosive to many other materials, such as metals.
  • 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

Inorganic Eutectics

c-inorganic, inorganic-inorganic compounds

• Advantages
  • Some inorganic 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 degradation, involving dissolving the anhydrous salt during melting to result in a thickening of the liquid material so that it melts to a gel form; however, this can cause huge reduction in latent heat.
• Disadvantages
  • They still have the same disadvantages as inorganic PCMs, such as reduced thermal performance upon cycling, corrosivity, high volume change, and high supercooling.
  • Sharp crystals may form when the salt hydrate PCM solidifies, potentially causing leaks in cases of macro-encapsulation.
  • Limited data is available on thermo-physical properties as the use of these materials is limited compared to organic PCMs.

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) ?H<0; enthalpy decreases (exothermic process) gives off heat.
• Vaporization (liquid to gas) ?H>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,.

Solid-solid PCM materials

A specialised group of PCMs that undergo a solid/solid phase transition with the associated absorption and release of large amounts of heat. These materials change their crystalline structure from one lattice configuration to another at a fixed and well-defined temperature, and the transformation can involve latent heats comparable to the most effective solid/liquid PCMs. Such materials are useful because, unlike solid/liquid PCMs, they do not require nucleation to prevent supercooling. Additionally, because it is a solid/solid phase change, there is no visible change in the appearance of the PCM, and there are no problems associated with handling liquids, e.g. containment, potential leakage, etc. Currently the temperature range of solid-solid PCM solutions spans from -50 °C (-58 °F) up to +175 °C (347 °F).

Maps Phase-change material

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

src: www.strategyr.com

Thermophysical properties

Common PCMs

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

${\displaystyle \mathrm {VHC} =\rho c_{p}}$

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

${\displaystyle I={\sqrt {k\rho c_{p}}}=e={(k\rho c_{p})}^{\frac {1}{2}}}$

Commercially available PCMs

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

src: www.andores.com

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 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 (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.

src: hq.imm.cnr.it

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.

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Applications

Applications 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
• 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.

src: www.euro-technologies.eu

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.

src: etsprojects.com.au

See also

• Heat pipe

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References

src: www.coldchaintech.com

Sources

• PHASE CHANGE MATERIAL (PCM) BASED ENERGY STORAGE MATERIALS AND GLOBAL APPLICATION EXAMPLES

Zafer URE M.Sc., C.Eng. MASHRAE HVAC Applications

• Phase Change Material Based Passive Cooling Systems Design Principal and Global Application Examples

Zafer URE M.Sc., C.Eng. MASHRAE Passive Cooling Application

src: steemitimages.com

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. 
• Phase Change Matters (industry blog)

Source of the article : Wikipedia