A phase-change material (PCM) is a substance which releases/absorbs sufficient energy at phase transition to provide useful heat or cooling. Generally the transition will be from one of the first two fundamental states of matter - solid and liquid - to the other. The phase transition may also be between non-classical states of matter, such as the conformity of crystals, where the material goes from conforming to one crystalline structure to conforming to another, which may be a higher or lower energy state. The energy required to change matter from a solid phase to a liquid phase is known as the enthalpy of fusion. The enthalpy of fusion does not contribute to a rise in temperature. As such, any heat energy added while the matter is undergoing a phase change will not produce a rise in temperature. The enthalpy of fusion is generally much larger than the specific heat capacity, meaning that a large amount of heat energy can be absorbed while the matter remains isothermic. Ice, for example, requires 333.55 J/g to melt, but water will rise one degree further with the addition of just 4.18 J/g. Water/ice is therefore a very effective phase change material and has been used to store winter cold to cool buildings in summer since at least the time of the Achaemenid Empire. By melting and solidifying at the phase-change temperature (PCT), 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 changes from solid to liquid and vice versa or when the internal structure of the material changes; PCMs are accordingly referred to as latent heat storage (LHS) materials. There are two principal classes of phase-change material: organic (carbon-containing) materials derived either from petroleum, from plants or from animals; and salt hydrates, which generally either use natural salts from the sea or from mineral deposits or are by-products of other processes. A third class is solid to solid phase change. PCMs are used in many different commercial applications where energy storage and/or stable temperatures are required, including, among others, heating pads, cooling for telephone switching boxes, and clothing. By far the biggest potential market is for building heating and cooling. In this application, PCMs hold potential in light of the progressive reduction in the cost of renewable electricity, coupled with the intermittent nature of such electricity. This can result in a mismatch between peak demand and availability of supply. In North America, China, Japan, Australia, Southern Europe and other developed countries with hot summers, peak supply is at midday while peak demand is from around 17:00 to 20:00. This creates opportunities for thermal storage media. There are two common ways that PCMs may be used: first, they can be used passively, where the PCM is located so as to absorb and then release heat due to temperature difference, which is thereby moderated. In such applications, the PCM may be encapsulated, and integrated into the structure of the object or space that is to be conditioned. In some applications, especially when incorporation to textiles is required, phase change materials are micro-encapsulated. Micro-encapsulation allows the material to remain solid, in the form of small bubbles, when the PCM core has melted. Alternatively, the PCM can be contained in a vessel, and heat flow to and from the PCM can be controlled by pumping a heat transfer fluid through a heat exchanger, generally immersed in the PCM within the vessel. In this case the system is a sub-category of "thermal battery" or "TES", thermal energy storage, which encompasses sensible heat storage as well.
Classification of phase-change materials Phase-change materials (PCMs) used for thermal energy storage are commonly classified according to their chemical composition and phase transition behavior. Most reviews distinguish three broad groups – organic, inorganic and eutectic PCMs – and, more recently, composite and microencapsulated PCMs are considered as separate subclasses because they are specifically engineered to overcome drawbacks such as low thermal conductivity, leakage and phase segregation.
Organic PCMs Organic PCMs are mainly based on paraffin waxes (linear alkanes) and non-paraffin organics such as fatty acids, fatty alcohols and polyols. They undergo a solid–liquid phase transition over a relatively narrow temperature range and typically exhibit latent heat values of roughly 150–250 kJ·kg⁻¹ in the building-relevant temperature range (0–65 °C). Organic PCMs are chemically stable, exhibit little or no supercooling and show good cycling stability, which makes them attractive for long-term operation. They are also non-corrosive towards most container materials and can be produced from petrochemical or bio-based feedstocks. However, organic PCMs generally suffer from low thermal conductivity (typically around 0.2 W·m⁻¹·K⁻¹), which limits the rate of heat storage and release unless conductive fillers or fins are added. Paraffins are also flammable, and some fatty-acid based PCMs may emit odors or interact with polymer matrices in composite systems. Their volumetric energy density is lower than that of many inorganic salt hydrates because of their lower density.
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