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43
ZEB
annual report 2014
STORING ENERGY IN THE FAÇADE
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LAGRING AV ENERGI I FASADEN
The window of the future will make
smart use of light and solar energy
Francesco Goia (NTNU)
Solar energy plays an important role in
buildings, providing daylight and heat through
windows and transparent components in
general. These building elements have
therefore a crucial impact on the energy
performance of buildings, affecting passive
use of solar gains, cooling and daylighting
availability.
One particular window technology, that
may represent improvement development
of present-day systems, is based on the
incorporation of a Phase Change Material
(PCM) layer into a transparent system.
PCMs are materials characterized by an
elevate energy storage potential occurring
when the PCM undergoes a phase change. In
practise, when exposed to heat, the material
starts melting and collects large amount of
(thermal) energy, that it is later realised when
it solidifies. PCMs are therefore used to store
energy within the building when this is in
excess and to make it available when there
is a lack of it. Because of the transparency/
translucency of certain PCMs, it is possible
to create a building component that exhibits
a high energy storage density together with
a certain degree of transparency to solar
radiation. Therefore, this class of technology
transforms the façade into a thermal/solar
energy storage element while still allowing the
exploitation of daylight.
Within the framework of a wide research
activity on the effectiveness of this concept,
a dedicated investigation on the optical
properties of window systems with PCMs
was necessary to fully understand the
behaviour of these components when it
comes to interaction with solar radiation
and transmittance of light. In fact, although
some research activities on this topic have
been carried out in the past, very little data
concerning the optical properties of such
materials can be found in the literature making
it difficult to carry out reliable simulations of
these systems.
In order to deepen the understanding of
the optical behaviour of PCM windows,
different samples (characterized by several
thicknesses of PCMs and different PCMs)
were tested by means of alarge integrating
sphere equipment, a so far not common test
bed for assessment of optical properties in
glazing systems. This apparatus is necessary
because, when in solid state, PCMs are
translucent and transmitted electromagnetic
radiation is scattered, making it impossible
to obtain correct values for the main optical
properties when conventional detection
systems are used.