Laboratory samples of meso-encapsulated salt hydrates: The
encapsulation material is furnished with a UV tracer. This enables
visual control of the coating quality under UV light.
© ZAE-Bayern
Latent heat storage systems
20.04.2017
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.
Laboratory sample of the meso-encapsulated
salt hydrates
© ZAE-Bayern
Chilled ceiling panel with the newly developed,
macro-encapsulated salt hydrates in the
opened measuring apparatus for dynamic
thermal characterisation
© ZAE-Bayern
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
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.
Energy Storage research initiative
The PC-Cools_V project was funded by the German Federal Ministry for Economic Affairs and Energy as part of
the German federal government's Energy Storage research initiative. The Heat Storage research area is in
particular focussing on latent heat and chemical storage systems as well as on projects relating to renewable
energies. More information about the research initiative can be found on the forschung-energiespeicher.info web
portal.
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