.

Fig. 3: Material classes that are being investigated and used as PCMs.
© ZAE Bayern

Fig. 4: Example of macrocapsules
© ZAE Bayern

Fig. 4: Example of macrocapsules
© ZAE Bayern

Fig. 4: Example of macrocapsules
© ZAE Bayern

Fig. 5: Microcapsules.
© Fraunhofer ISE, BASF

Fig. 5: Microcapsules.
© Fraunhofer ISE, BASF

Fig. 6: PCM composite materials: mechanically stable, pourable granulate from Rubitherm GmbH
© ZAE Bayern


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Storing heat using phase changes II

Which storage materials are used?

Intensive research work over the last two decades has identified many phase change materials that are suitable for use in latent heat storage and cover a wide temperature range with their melting points (Fig. 3). Various mixtures of water and salts yield eutectic salt solutions with melting points significantly below 0 °C, for example, or salt hydrates with melting points in a temperature range between 5 °C and 130 °C. This results in numerous applications in the areas of heating, cooling and airconditioning. These materials can boast high storage densities and are relatively inexpensive. Paraffins and fatty acids are the main organic materials that are suitable. They generally have lower storage densities and have higher costs relative to salt hydrates. However, they are easier to work with than salt hydrates.

Although the combination of building materials with PCMs appears rather unspectacular at first glance, there are still a number of requirements that have to be fulfilled. The mechanical stability of the PCM materialsmust be ensured, and sufficient fire protection must be in place for paraffins, for example, which are flammable. It is often a good idea to modify the PCMs in order to change their properties. Examples are granulates that can be poured or trickled, or PCM graphite composite materials for heating or cooling applications.

PCMs – encapsulated for controlled amounts

PCMs are suitable for the construction of storage devices with a high storage density and for passive temperature stabilisation based on melting at a constant temperature. As PCMs become liquid during their use, it is generally necessary to encapsulate them in some type of container. For conventional storage devices, this function is fulfilled by the storage tank, whereas in many applications PCMs are employed as independent storage elements in an existing system.

In such cases, the storage containers used for the phase change materials are termed “capsules”. Depending on their size, they are classified as macrocapsules with a diameter of greater than 1 cm, microcapsules with less than 100 mm, or mesocapsules which cover the intermediate range. Figure 4 shows examples of conventional macrocapsules: plastic containers with a flat shape or else as spheres or bags, etc. Any type of material class can be “packaged” using this technology. However, these capsules cannot be used everywhere because of their size.

Microcapsules have to be used in order to add PCMs to other materials such as building materials. The small size of these capsules means that they can be mixed into these building materials during manufacture, and thus these materials can be processed on the building site in the same way as conventional building materials. Further processing during the usage phase is also permitted, as damage to the capsules is unlikely due to their small size. If a few capsules should be damaged, the amount of material released will be negligibly small. Paraffins in microcapsules have been commercially available for around ten years. Micro-encapsulation of salt hydrates and new methods of meso-encapsulation are the subject of intensive research.

When constructing heat storage devices using PCMs, the usually low thermal conductivity of these materials generally makes sophisticated charging and discharging systems necessary. These systems and the surface of the storage device must be designed to cope with changes in the PCM volume which are often substantial. Energy density and capacity density are key criteria here in the selection of suitable materials, but storage losses, costs and safety also play important roles.

Potential applications for PCMs

Most applications of PCMs under the heading of “energy saving” involve the buffering of temperature cycles in buildings. The focus here is avoiding peak temperatures and thus saving on cooling energy. In the case of conventional night ventilation, warm air in the building is replaced by cold night-time air; PCMs can increase the heat capacity of a building and thus store the night-time coolness in the building mass. Storage devices that play a supporting role in building heating represent another important application.

In general, applications for phase change materials in buildings can be classified as follows:

  • PCM integrated into the building structure (wall, ceiling)
  • PCM in other building components (e.g. facade element)
  • PCM in separate heat and cold storage devices

The first two applications are passive systems that automatically release the heat or cold stored. The third system requires active components such as fans and pumps, as well as a control system. However, it has the advantage that the stored heat or cold can be accessed in a targeted manner when it is required. PCMs with various phase change temperatures are used depending on the area of application. Storage temperatures of between 0 °C and 40 °C are favoured in buildings, with the exception of hot water and heating water provision where temperatures of between 50 °C and 60 °C are needed. The integration of PCMs into the building structure is focused on the temperature range of 21 °C to 26 °C.

Ice storage devices have a much higher storage density than cold water storage equipment. They currently represent the state-of-the-art technology in air-conditioning  for buildings and the use of industrial process cold. They are integrated into the cooling system by means of a brine circuit with a pump. The storage devices can be actively controlled in order to charge and unload them and regulate their performance in a targeted manner. Air-carrying heating and cooling systems represent another possible method for active integration.

By contrast, PCMs with no external control are used for passive temperature stabilisation. An example of this is the use ofmacro-encapsulated PCMs in transport boxes for temperature-sensitive goods such as pharmaceuticals and blood plasma. In recent years, PCMs have increasingly found applications in clothing; here, PCMs buffer short-term excess peaks of heat and reduce perspiration or else use stored heat to stop the body getting cold. Micro-encapsulated PCMs are generally combined with the clothing fabric in these applications.

This approach has also been used for a number of years now in passive temperature stabilisation in buildings. The heat stored by PCMs when they melt is a multiple of the heat capacity of building materials such as plaster, wood, cement or stone, which is generally between 0.8 and 1.5 kJ/kg per 1 °C interval. Micro-encapsulated PCMs are generally integrated into the building materials.

Another area of application has resulted from a building technology that is already established: These systems cool buildings using tube heat exchangers which are integrated into the building elements are used to condition the indoor environment – either completely or else as a supportmeasure. These thermo-active building systems (TABSs) can be used in combination with conventional heating systems (radiators) or with natural or mechanical ventilation. They take the place of conventional building cooling in these applications. In the case of purely passive systems or TABSs, a large heat transfer area should be available because of poor heat transfer to the air. This is not necessary for active systems, as even a small amount of air movement considerably improves the heat transfer and thus the performance of the system.

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