Air-conditioning buildings with salt hydrates
In many office buildings, it is just as important to supply cooling in summer as heating in winter. Researchers have therefore developed salt hydrates that are suitable as storage material in chilled ceilings and centralised cold storage systems. The special advantage: Through a phase change, they store cooling energy with a high energy density and at the temperature level needed for air conditioning. This enables the required cooling to be provided very efficiently.
Salt hydrates absorb large amounts of heat and thus cool their environment when they are heated above their melting point. They then release this heat when solidified. These so-called phase change materials (PCMs) only change their temperature to an insignificant extent when they store or release the energy. Salt hydrates can be produced with melting points in the temperature range between 0 and 130 °C and are generally comparatively inexpensive. This makes them suitable for many storage tasks when heating, cooling and air-conditioning. However, their high storage capacity is limited to the narrow temperature range in which the phase changes from solid to liquid. Therefore different application areas require precisely adapted PCMs.
Until now there have been no salt hydrate-based phase change materials with optimum melting temperatures available for chilled ceilings and centralised cold storage systems. In addition, many salt hydrates tend to sub-cool. This means that they do not release the stored heat at the temperature at which it was stored, but only at significantly lower temperatures. For this reason, scientists in the PC-Cools_V project have developed two new salt hydrates with narrow melting ranges that are respectively around 14 and 20 degrees Celsius. During the course of the work, the researchers managed to significantly improve the cycle stability of the new salt hydrates and reduce the sub-cooling.
The researchers determined the optimum melting point of the salt hydrates mathematically in a system study. For this purpose, they simulated the distribution of cooling energy in a typical office building. Based on the results they adapted the target temperatures for developing the salt hydrates: from initially 21 to 20 degrees Celsius and from 15 to between 13 and 14 degrees Celsius.
While the salt hydrate that begins to melt at 20 degrees Celsius has been optimised for room-integrated storage systems with an active recooling capability, i.e. for chilled ceiling systems, the other salt hydrate that melts at 14 degrees Celsius is suitable for centralised cold storage systems. The high storage temperatures compared with conventional cold storage systems offer the advantage that renewable cooling sources and chillers can be used more efficiently.
Encapsulation and carrier structure
The macro-encapsulations made of aluminium from the Rubitherm Technologies project partner are also suitable for the new salt hydrates. This is confirmed by long-term corrosion tests. However, the researchers have also developed new macro-encapsulations that are potentially less expensive and variable in shape. Here the PCM is not located directly in a macro-encapsulation, but in a carrier matrix made of calcium silicate. This is infiltrated with the PCM under low pressure and coated with a high-barrier film. Initial tests have been highly promising: the PCM exceeds more than 80 percent of the total volume of the module cores. The melting behaviour of the previously investigated salt hydrates is only insignificantly influenced by the carrier matrix. However they tend to sub-cool slightly more.
Mesocapsules a few millimetres in size can also be produced on the laboratory scale using calcium silicate granules filled with salt hydrate. For this purpose, the Promat project partner has developed a method that requires only one process step for producing non-coated, salt hydrate-filled calcium silicate granules with diameters between 1 and 10 millimetres.
ZAE Bayern identified coating materials that, thanks to their excellent vapour density, enable the granules to achieve a theoretical lifetime of about 10 years. However, initial attempts to coat the granules mechanically still leave open whether meso-encapsulated salt hydrates will be able to prove themselves technically and economically.
Application test for the newly developed salt hydrates for chilled ceiling applications
The newly developed PCM for room-integrated storage systems was dynamically characterised in a chilled ceiling panel with a special measuring device under known and controllable boundary conditions. It was then subsequently installed in an office room in the Energy Efficiency Centre. By monitoring it under real conditions, the researchers are comparing it with the existing PCM chilled ceiling, whose storage material melts at 23 degrees Celsius. The initial results promise a significant increase in the passive cooling performance achieved by the newly developed PCM.