## 1 Introduction  
### PCM, PCM Composite Materials, and Encapsulated PCM  
Due to the increasing use of renewable energy sources, specifically sun and wind, energy storage will play an even more crucial role in the future to be able to match availability of energy with demand. Thermal energy storage, often also called heat and cold storage depending on the type of application, has already today a significant share of energy storage. And its importance will further grow with increasing use of solar heat or heating or cooling by use of intermittent electricity from sun and wind. Basically, three effects can be used for thermal energy storage: raising the temperature of a material as it is, changing its phase, or changing its composition. Just raising the temperature is the most frequently used, in technical applications as well as in everyday life. The associated heat is also called sensible heat because the temperature change associated with it can be sensed. For thermal energy storage, solids or liquids are used because gases store comparatively little thermal energy. When using a phase change, usually used is the phase change between solid and liquid, less often between different solid phases. In comparison to a phase change involving a gas phase, these not only allow storage of a significant amount of thermal energy in a small volume at low pressure but also in a narrow temperature range. If the phase change is even at a single temperature, the associated heat is called latent heat as it cannot be felt without temperature change. Materials able to store a significant amount of thermal energy, respectively, heat, when changing phase, are called Phase Change Materials (PCMs). It is also common to call them latent heat storage materials, even though many materials used as PCMs--e.g., many mixtures--change phase in a temperature range. The ability to store a significant amount of heat in a small temperature range by using a phase change leads to two main fields of use [1]: heat storage with high storage density (per mass or volume) in a small temperature interval and passive temperature stabilization (temperature control). PCMs are today used in a wide range of commercial applications [2]: in buildings for space heating and cooling as well as for domestic hot water; in logistics, specifically in the cold chain; for heating or cooling the human body, including in medical applications, in mobility, in electronics; as well as in industrial processes. And more applications have been developed, e.g., for the energy system. The most common PCM is water, with a solid-liquid phase change at 0 \({}^{\circ}\)C. For other temperatures, other materials must be used, like paraffins, fatty acids, alcohols, and salts, as a pure material or as a mixture, e.g., salt hydrates. Organic PCMs are also sometimes called bio-based if produced from biological sources. To work properly for thermal energy, respectively, for heat storage, many PCMs also contain additives, e.g., a nucleator to reduce supercooling or a gelling or thickening material to reduce phase separation. PCMs are also sometimes combined with other materials to combine material-specific advantages, thus becoming a PCM composite material. For example, PCMs are combined with graphite or metal foams, combining the ability of PCMs for heat storage and the ability of graphite or metal foams for heat transfer. Also, PCMs can be combined with polymer foams to give them a stable shape/form even if they are a liquid. In applications, encapsulated PCMs are often used to avoid loss of the PCM, specifically if it is a liquid, to avoid contact with the ambient, and to keep the shape/form as defined surface for heat exchange. Compared to a composite material where the PCM is combined with another material within its interior, in an encapsulation, the combination is with a material at the boundary, the PCM surface. Encapsulation also serves as a well-defined package size, and thus the encapsulated PCM acts as a heat storage module or object.