Phase change materials (PCM)
Introduction
A phase change material (PCM) is a substance that releases/absorbs enough energy in a phase transition to provide useful heat or cold. Typically, the transition occurs between one of the first two fundamental states of matter (solid and liquid). Phase transition can also occur between non-classical states of matter, such as crystal conformity, in which the material goes from conforming to one crystal structure to conforming to another, which may be a state of higher or lower energy.
The energy released/absorbed by the phase transition from solid to liquid, or vice versa, the heat of fusion is generally much greater than the sensible heat. Ice, for example, requires 333.55 J/g to melt, but then water will rise one degree higher with the addition of only 4.18 J/g. Water/ice is therefore a very useful phase change material and has been used to store winter cold and cool buildings in summer at least since the time of the Achaemenid Empire.
By melting "Fusion (change of state)") and solidifying at the phase change temperature (TCP), a PCM is capable of storing and releasing large amounts of energy compared to sensible heat storage. Heat is absorbed or released when the material goes from solid to liquid and vice versa or when the internal structure of the material changes; Therefore, PCMs are called latent heat storage (LHS) materials.
There are two main classes of phase change materials: organic (carbon-containing) materials derived from petroleum, plants or animals; and salt hydrates, which generally use natural salts from the sea or mineral deposits or are byproducts of other processes. A third class is the solid-to-solid phase change.
PCMs are used in many different commercial applications where energy storage and/or stable temperatures are required, including, but not limited to, heating pads, cooling for telephone switch boxes, and clothing.
The largest potential market by far is the heating and cooling of buildings. In this area of application, PCMs have potential in light of the progressive reduction in the cost of renewable electricity, coupled with the intermittent nature of such electricity. This can cause a mismatch between demand peaks and supply availability. In North America, China, Japan, Australia, southern Europe and other developed countries with hot summers, peak supply occurs at midday, while peak demand is around 5:00 p.m. and 8:00 p.m. This creates opportunities for thermal storage media.
Solid-liquid phase change materials are typically encapsulated for installation in the final application, to contain them in a liquid state. In some applications, especially when incorporation into textiles is required, phase change materials are microencapsulated. Microencapsulation allows the material to remain solid, in the form of small bubbles, when the PCM core has melted.