PCMs in cooling ceilings and heat stores
Flexible use: PCMs in cooling ceilings and heat stores

While regeneration of the PCMs in air-driven systems is limited by the temperatures of the outside air, active PCM systems such as the PCM cooling ceilings with water flow studied in the project (fig. 4) are much more flexible to use and allow PCMs with lower melting points and thus greater cooling capacity to be used. Ideally, cooling is passive and performed by the PCMs during the day, while active operation is only used for regeneration at night.

PCM systems for heating buildings can be used in combination with renewable energy sources such as photovoltaics (PV) and heat pumps for power-to-heat application. If PCMs are integrated into building components such as screed (fig. 5), the PCM loading volume must be sufficiently high to enable significant storage effects.

PCM stores for heating buildings allow much greater storage densities than water tanks alone, which reduces the space requirements for these stores. The modular PCM heat stores for residential buildings that were studied in the project (fig. 6) use targeted supercooling of the PCMs to store heat for several days with virtually no heat loss. The project managed to achieve a reliability of 80 % for the supercooled melt.

Putting findings into practice

The project showed that PCM systems can function with extremely high energy efficiency, provided they are realized and operated by qualified experts. It also made clear that PCM systems can be economical under certain conditions.

In order to achieve both aspects, appropriately optimized control strategies must be implemented and the PCM systems must be put into operation in a rational way, ideally with a subsequent monitoring phase. Those involved in the project were able to further develop the industrial partners’ PCM systems in a practical way and implement them as reference systems in the form of pilot projects. Furthermore, they laid the foundation for the development of new, cost-effective PCM encapsulations.

Essential: Information from planners and users

However, it also became clear that in practice PCM systems are not always operated in an optimal manner. Often the control system or the operating parameters are not adjusted for the individual PCM system, even in commercial installations. This can even cause excess energy to be consumed. For example, in training rooms equipped with a PCM cooling ceiling system, the users set the thermostats to temperatures of around 22 °C in almost all cases, which is below the phase change range of the PCMs used. This meant that the PCM system’s passive cooling could not take effect, because the active cooling switched on beforehand. However, the unused PCMs were actively regenerated every night regardless.

These results indicate a substantial need for information and coordination on the part of planners, tradespeople, building operators, and users. This applies in particular to the design, installation, and operation of PCM systems. This coordination is especially difficult with systems that require input from the users to be included. Often the users are not known during the planning and building phases, or might change later. Further research is needed into developing suitable control strategies. The considerable potential for energy savings from PCM systems can only be realized once both of these obstacles have been eliminated.