Phase Change Materials (PCMs) are simply substances that absorb, store and release large amounts of thermal energy at a steady temperature when transitioning between solid and liquid states (or phases) i.e. melting and freezing.  Probably the best known PCM is ice, which is an extremely efficient PCM but only operates at 0^oC.  Hence other PCMs have been developed which melt/freeze at a variety of temperatures.

PCMs can be thought of as a kind of rechargeable thermal battery to store heat (or cold) in a very compact, efficient and convenient way.

Commercial PCM products are designed to deliver high performance and usability along the value chain and in end applications.  As the market has developed many different products have emerged varying in chemical composition and physical form.