Phase change materials
See also: Advanced phase change materials.
Phase change materials (PCM) are generally materials with large specific latent heat capacity. Phase change materials used in buildings will typically melt and solidify within a range of 18-30ºC and are normally contained in the building fabric.
They can be used in the built environment to reduce internal temperature change by storing latent heat in the solid-liquid or liquid-gas phase change of a material (although the liquid-gas phase change is actually impractical due to the large volumes or high pressures experienced in the gas state). Phase change materials are able to store 5 to 14 times more thermal energy per unit volume than conventional thermal storage materials. Heat is absorbed and released almost isothermally and is used to reduce the energy consumed by conventional heating and cooling systems by reducing peak loads.
Heat = (Troom – Tair) . Cair . m
- Troom = Temperature of room air (ºC)
- Tair = Temperature of air within PCM unit (ºC)
- Cair = Specific heat of air (1000 J / kgK)
- m = Mass of air moving through the PCM unit per second (kg/s)
Heat = (Troom – Tair) . 1.2 . Q
Q = air flow rate (l/s)
Since phase change materials transform between solid-liquid in thermal cycling, encapsulation is the obvious storage choice. However, the early development of macroencapsulation with large-volume containment failed due to the poor thermal conductivity of most phase change materials. Phase change materials tend to solidify at the edges of the containers preventing effective heat transfer.
Microencapsulation on the other hand showed no such problems. Phase change materials can be incorporated into construction materials, such as concrete, easily and economically. By coating a microscopic-sized phase change material with a protective coating, the particles can be suspended within a continuous phase such as water.
Phase change materials perform best in small containers and so are usually divided into cells. The cells are shallow to reduce static head (based on the principle of shallow container geometry). The packaging material should be a good conductor of heat and 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 to prevent the material drying out and should resist leakage and corrosion.
Phase change materials have been used in tropical regions in telecom shelters. Here, they protect high-value equipment shelters by keeping the indoor air temperature below the maximum permissible by absorbing heat generated by power-hungry equipment such as base station subsystems. In remote locations, phase change materials can minimise the need to use diesel generators in the event of power failure to conventional cooling systems. Across thousands of telecom sites this can translate into very significant savings. Phase change materials are also being used in the thermal regulation of electronics.
Some phase change materials are suspended in water, and are relatively nontoxic. Others are hydrocarbons or other flammable materials, or are toxic. As such, phase change materials must be selected and used very carefully in accordance with fire and building codes and sound engineering practices. It may not be wise to use flammable phase change materials within residential or other regularly occupied buildings because of the increased fire risk as well as the dangers of flamespread, smoke, and the potential for explosion when held in containers.
This article was created by --Buro Happold.
 Related articles on Designing Buildings Wiki
- Advanced phase change materials.
- Cool roofs.
- Dynamic thermal modelling of closed loop geothermal heat pump systems.
- Evaporative cooling.
- Heat transfer.
- Phase change.
- The thermal behaviour of spaces enclosed by fabric membranes.
- Thermal comfort.
- Thermal labyrinths.
- Transparent insulation materials.
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