Phase-change material
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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.
Applications
Phase change materials have been employed in, for example:
- Thermal energy storage
- Conditioning of buildings, such as 'ice-storage'
- Waste heat recovery
- Off peak power utilization
- Heat pump systems
- Space applications
- Laptop computer cooling
- Coolsuits
- Telecom shelters
- Cooling drinks.
Behavior
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 perform like conventional storage materials; their temperature rises as they absorb heat. Unlike conventional storage materials, however, when PCMs reach the temperature at which they change phase (their melting temperature) they absorb large amounts of heat without a significant rise in temperature. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat. 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.
Technology
PCMs can be broadly grouped into two categories; ”Organic Compounds“ (such as waxes, vegetable extract, polyethylene glycol) and “Salt-based Products” (such as Sodium sulfate). 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.
Eutectic salts 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.
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 utilising 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.
Phase change materials perform best in small containers, which therefore 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. Steel and polyethylene are common packaging materials.
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.
Micro-encapsulated phase change materials
Microencapsulated PCMs 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).
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.
Links
- Laptop cooler