Phase-change material

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The phase change material ( PCM ) is a substance with high fusion heat which, melts and hardens to a certain temperature, is able to store and release large amounts of energy.. The heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCM is classified as a latent heat storage unit (LHS).

Video Phase-change material

Characteristics and classification

Latent heat storage can be achieved through liquid-> solid, solid-> liquid, solid-> gas- and liquid-> gas-phase. However, only the solid -> liquid and liquid -> phase changes are practical for PCM. Although gas-liquid transitions have higher heat of transformation than liquid-solid transitions, liquid-phase changes are not practical for heat storage because large volumes or high pressures are required to store materials in their gas phase. Solid-phase changes are usually very slow and have relatively low heat transformations.

Initially, solid-liquid PCM behaves like a sensible heat storage (SHS); their temperatures rise while absorbing heat. Unlike conventional SHS materials, however, when PCM reaches the temperature at which they change the phase (melting temperature) they absorb large amounts of heat at almost constant temperature. PCM continues to absorb heat without significant temperature rise until all materials are converted to liquid phase. As the ambient temperature around the liquid material falls, PCM hardens, releasing latent stored heat. A large number of PCMs are available in various temperatures ranging from -5 to 190 ° C. In 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 such as water, rock or stone.

Organic PCM

Bio-Based, or Paraffin (C _n ), or lowered carbohydrates and fats.

• Advantages
  • Freeze without much undercooling
  • The ability to melt by itself
  • Self nucleating properties
  • Compatibility with conventional construction materials
  • Without segregation
  • Chemically stable
  • High fusion heat
  • Safe and not reactive
  • Can be recycled
  • PCM-based carbohydrates and lipids can be generated from renewable sources
• Losses
  • Low thermal conductivity is in solid state. A high degree of heat transfer is required during the freezing cycle. Nano composites were found to result in an increase in effective thermal conductivity of up to 216%.
  • The volumetric latent storage capacity can be low
  • Flammable. This can be partially offset by a detention specialist, or by incorporating an eco-friendly fire barrier.

Inorganic

Hydrate salt (M _n H ₂ O)

• Advantages
  • High volumetric latent storage capacity
  • Availability and low cost
  • Smooth melting point
  • High thermal conductivity
  • High fusion heat
  • Non-flammable
• Losses
  • Irregular melting and phase separation when cycling can cause significant losses in latent heat enthalpy.
  • Corrosive to many other materials, such as metal.
  • Volume change is very high
  • Super cooling is a major problem in dense-liquid transitions
  • Nucleation agents are needed and they often become dysfunctional after recurring cycling

Inorganic Eutectics

Inorganic-inorganic c-inorganic compounds

• Advantages
  • Some inorganic eutectics have a sharp melting point similar to pure substance.
  • Volumetric storage density is slightly above the organic compound.
  • Additional water principles may be used to avoid phase change degradation, which involves dissolving anhydrous salts during fusion to produce a thickening of a liquid material thereby melting to gel form; however, this can lead to a large decrease in latent heat.
• Losses
  • They still have the same disadvantages as inorganic PCM, such as reducing thermal performance during cycling, corrosion, high volume change, and super high cooling.
  • Sharp crystals can form when solid salt PCR hydrates, potentially causing leakage in case of macro encapsulation.
  • Limited data is available on thermo-physical properties because the use of these materials is limited compared to organic PCM.

Hygroscopic Material

Many natural building materials are hygroscopic, ie they can absorb (water condenses) and release water (water evaporates). The process is thus:

• Condensation (gas to liquid)? H & lt; 0; reduced enthalpy (exothermic process) emits heat.
• Evaporation (liquid to gas)? H & gt; 0; Increased enthalpy (endothermic process) absorbs heat (or cools).

While this process frees up a small amount of energy, the large surface area allows significant heating or cooling (1-2 Â ° C) inside the building. Suitable materials are wool insulation, earth rendering/clay finish.

Solid-solid PCM material

A group of specialized PCMs undergoing solid/solid phase transitions with absorption and release of large amounts of heat. These materials change their crystal structure from one lattice configuration to another at a fixed and well-defined temperature, and the transformation may involve latent heats comparable to the most effective solid/liquid PCM. These materials are useful because, unlike solid/liquid PCM, they do not require nucleation to prevent supercooling. In addition, since this is a solid/solid phase change, no changes are seen in the PCM display, and there are no problems associated with handling liquids, such as containment, leakage potential, etc. The current temperature range of solid-solid PCM solutions ranges from -50 Â ° C (-58Â ° F) to 175 Â ° C (347 Â ° F).

Maps Phase-change material

Selection criteria

Thermodynamic properties. Phase change materials must have:

• The melting temperature in the desired operating temperature range
• High latent fusion heat per unit volume
• High specific heat, high density and high thermal conductivity
• Small volume changes in phase transformations and small steam pressure at operating temperature to reduce containment problems
• Liquid congruent
• Kinetic properties
• High nucleation level to avoid liquid phase cooling
• The high rate of crystal growth, so the system can meet the heat recovery request of the storage system
• Chemical properties
• Chemical stability
• Complete freezing/thawing cycle
• There is no degradation after a large number of freezing/melting cycles
• Non-corrosive, non-toxic, non-flammable, and non-explosive material
• Economic property
• Low cost
• Availability

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Thermophysical properties

General PCM

Kapasitas panas volumetrik (VHC) JÂ · m ^-3 Â · K ^-1

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

Thermal inertia (I) = Efektivitas termal (e) JÂ · m ^-2 Â · K ^-1 Â · s ^-1/2

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

PCM yang tersedia secara komersial

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

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Technology, development and encapsulation

The most commonly used PCMs are salt hydrates, fatty acids and esters, and various paraffins (such as octane). Recently also ionic liquids were investigated as new PCMs.

Since most organic solutions are water free, they can be exposed to air, but all salt-based PCM solutions must be packed to prevent evaporation or water absorption. Both types offer certain advantages and disadvantages and if they are applied correctly, some losses become advantages for certain applications.

They have been used since the late 19th century as a medium for thermal storage applications. They have been used in various applications such as refrigerated transport for rail and road applications and their physical properties, therefore, are well known.

In contrast to ice storage systems, PCM systems can be used with conventional water chillers for either new or alternative retrofit applications. Positive phase change in temperature allows centrifugal cooling and absorption as well as conventional reciprocating and screw chiller systems or even lower ambient conditions using cooling towers or dry coolers for charging TES systems.

The temperature range offered by PCM technology provides a new horizon for building services and cooling engineers on medium and high temperature energy storage applications. The scope of this thermal energy application is the area of ​​solar heating, hot water, heating rejection, ie cooling towers and cooling thermal energy cooling application cooling applications.

Because PCM changes between the solid-liquid in the thermal cycle, naturally encapsulated becomes a clear storage option.

• PCM Encapsulation
  • Macro-encapsulation: The early development of macro encapsulation with large volume storage failed due to poor thermal conductivity of most PCMs. PCM tends to solidify at the edge of the container preventing effective heat transfer.
  • Micro encapsulation: Micro-micro-encapsulation on the other hand shows no such problem. This allows the PCM to be incorporated into construction materials, such as concrete, easy and economical. Micro-encapsulated PCMs also provide portable heat storage systems. By coating the microscopic PCM with a protective layer, the particles can be suspended in a continuous phase such as water. This system can be considered as pulp phase change ( PCS ).
  • Molecular encapsulation is another technology, developed by Dupont de Nemours that allows very high PCM concentrations in a polymer compound. This allows storage capacity up to 515 kJ/m ² for 5 mm boards (103 MJ/m ³ ). Molecular-encapsulation allows drilling and cutting of materials without PCM leakage.

As the best-performing phase change materials in small containers, therefore they are usually divided into cells. The shallow cells to reduce the static head - based on the principle of shallow container geometry. Packing materials should heat up well; and must be durable enough to withstand the frequent changes in storage volume due to phase changes occurring. It should also restrict the waterway through the wall, so the material will not dry (or water-out, if the material is hygroscopic). Packaging should also be resistant to leakage and corrosion. Common packaging materials that exhibit chemical compatibility with room temperature PCM include stainless steels, polypropylene and polyolefins.

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Thermal composite

Thermal composite is a term given for a combination of phase change materials (PCM) and other structures (usually solid). A simple example is the copper-mesh moistened in paraffin-wax. Copper-mesh in parraffin-wax can be considered as composite material, dubbed thermal composite. The hybrid material is created to achieve a certain overall or bulk nature.

Thermal conductivity is a common property that is targeted to maximize by creating thermal composites. In this case the basic idea is to increase thermal conductivity by adding high performance solids (such as copper-mesh) to a relatively low performing PCM thereby increasing overall or bulk (thermal) conductivity. If PCM is required to flow, the solid must be porous, such as mesh.

Solid composites such as fiber-glass or kevlar-pre-preg for aerospace industries typically refer to fibers (kevlar or glass) and matrices (glue compacted to hold fibers and provide compressive strength). The thermal composite is not so clearly defined, but may also refer to the matrix (solid) and PCM which is of course usually liquid and/or solid depending on the conditions. They are also meant to find minor elements on earth.

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Apps

Phase change material applications include, but are not limited to:

• Thermal energy storage
• Cooking solar power
• Cold Energy Battery
• Building conditioning, like 'ice storage'
• Heat cooling and electric engine
• Cooler: food, drink, coffee, wine, dairy products, green house
• Medical applications: blood transport, operating table, hot-cold therapy, birth asphyxia treatment
• Cooling the human body under clothes or big costumes.
• Waste heat recovery
• Unused power usage: Heating and Cooling
• Heat pump system
• Passive storage in a building/bioclimatic architecture (HDPE, paraffin)
• Smooths peaks of exothermic temperatures in chemical reactions
• Solar power plant
• The spacecraft's thermal system
• Thermal comfort in vehicle
• Thermal protection of electronic devices
• Food thermal protection: transportation, hotel trade, ice cream, etc.
• Textile used in clothing
• Computer cooling
• Turbine Inlet Cooler with heat energy storage
• Telecommunications in the tropics. They protect high-value equipment at the shelter by keeping the indoor air temperature below the maximum permitted by absorbing heat generated by power-hungry equipment such as the Base Station Subsystem. In the case of power failure for conventional cooling systems, PCM minimizes the use of diesel generators, and this can be translated into substantial savings across thousands of telecom sites in the tropics.

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Fire and security issues

Some phase change materials are suspended in water, and are relatively non-toxic. Others are hydrocarbons or other flammable, or toxic substances. Thus, PCM must be chosen and applied with extreme caution, in accordance with fire and building codes and sound engineering practices. Due to the increased risk of fire, fire, smoke, explosion potential when stored in containers, and liabilities, it may be wise not to use flammable PCM in residential or other buildings that are regularly occupied. Phase change materials are also used in electronic thermal regulation.

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See also

• Heat pipe

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References

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Source

• PHANGE CHANGE MATERIAL (PCM) BASED ON ENERGY STORAGE AND EXAMPLE OF GLOBAL APPLICATIONS

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

• Phase Change Material Based on Main Passive Cooling System Design and Global Application Example

Zafer URE M.Sc., C.Eng. MASHRAE Aplikasi Pendingin Pasif

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Bacaan lebih lanjut

• Raoux, S. (2009). "Bahan Perubahan Fase". Review Tahunan Penelitian Material . 39 : 25-48. Bibcode: 2009AnRMS..39... 25R. doi: 10.1146/annurev-matsci-082908-145405.
• Perubahan Tahap Matters (blog industri)

Source of the article : Wikipedia