.

Fig. 1: PCMs are used to avoid peak temperatures indoors and thus to save on cooling energy. Conventional night ventilation involves the replacement of warm air in the building with cold night-time air.
© GLASSX, Gaston Wicky

Fig. 2: Temperature profile as a function of the amount of stored heat in the case of storage of sensible heat and latent heat.
© ZAE Bayern
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Storing heat using phase changes

Heat storage is often necessary in order to use energy efficiently. Materials that store latent heat can deliver tailored solutions for many application areas. Optimal processing of the phase change materials in order to provide for effective heat exchange is a critical factor in this type of application. Particularly flexible are the micro-encapsulated PCMs that can be integrated into many building materials and building systems.

Heat storage plays an important role in all cases where the supply of and demand for heating/cooling have to be matched in terms of times and amounts; heat storage is also important when security of supply and stand-alone (off-grid) supply must be ensured. Many “heat sources” such as solar energy or waste heat from industrial processes or power plants can be used in a viable manner by employing heat storage, which makes these sources available when they are needed. Thismeans that heat provision from a heating or solar energy system need not be designed for the maximum demand, but can instead be sized for a given intermediate demand level. Alternatively, the low temperatures available at night can be used for cooling during the day.

Nowadays, demand-oriented heat storage is generally implemented using hot water storage, where the temperature of the stored water is raised to the demand temperature or above it. The heat stored in this manner is referred to as sensible heat, as this is “tangible” storage. The advantage of using water in this process is that water is most often the medium that is required later on – for example, shower water can be taken directly from the storage tank. In addition, water is generally inexpensive. Sensible heat storage also takes place when the tiles on a tiled stove are heated up. These tiles then release their heat over the course of number of hours, even long after the fire itself has already gone out.

Latent storage materials, also called PCMs (phase change materials), store large quantities of heat by means of a phase change – for instance from solid to liquid. Compared to conventional sensible heat storage equipment, PCM storage devices allow for a high energy density with a largely constant operating temperature. The amount of energy required to melt one kilogram of water would result in a temperature increase of around 80 °C in the case of sensible storage. For manymaterials, temperature changes of just a few degrees (10 K) that involve a melting process can achieve heat storage densities which are up to 10 times higher than those achieved with sensible storage. Figure 2 shows: Heat storage is usually associated with an increase in the temperature of the storage material that is proportional to the amount of heat stored (blue curve). Once the phase change temperature has been reached in the case of “latent” (hidden) heat storage, no temperature increase occurs for a period until the storage material has fully melted (red curve). The heat stored is released again during solidification.

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