Functional materials for energy-efficient buildings
EPJ Web of Conferences
Functional materials for energy-efficient buildings
H.-P. Ebert 0 1
0 Am Galgenberg 97 , 97074 Wu ̈rzburg , Germany
1 ZAE Bayern, Bavarian Center for Applied Energy Research
Summary. — The substantial improving of the energy efficiency is essential to
meet the ambitious energy goals of the EU. About 40% of the European energy
consumption belongs to the building sector. Therefore the reduction of the
energy demand of the existing building stock is one of the key measures to deliver a
substantial contribution to reduce CO2-emissions of our society. Buildings of the
future have to be efficient in respect to energy consumption for construction and
operation. Current research activities are focused on the development of functional
materials with outstanding thermal and optical properties to provide, for example,
slim thermally superinsulated facades, highly integrated heat storage systems or
adaptive building components. In this context it is important to consider buildings
as entities which fulfill energy and comfort claims as well as aesthetic aspects of a
1. – Thermal insulation
Thermal insulation of buildings is one of the most effective ways to save energy
resources for heating and cooling and providing comfortable temperatures in living and
working rooms. The physical principle behind these efforts which were empirically
optimised with time and passed down generations is to generate a volume of still air within
a porous structure and to avoid convection effects. Thus the thermal conductivity of
still air, i.e. 0.026 Wm−1 K−1 at ambient conditions comes significantly into effect and
provides a reasonable thermal insulation. Thermal insulation materials or systems which
show effective thermal conductivity values far below the conductivity value of still air
at ambient conditions are known as so called superinsulations. An overview of thermal
insulation materials are depicted in fig. 1.
Within a porous insulation material heat is transported by three different mechanisms:
conductive heat transfer via the solid backbone, heat conduction within the gas phase
and radiative heat transfer (cf. fig. 2). Convection, i.e. the transport of energy by free or
forced convective gas flow, does not occur in thermal insulations because of the limited
free space, the relatively low temperature and nonexistent pressure differences in building
The thermal insulation properties of an insulation material are determined by the total
effective thermal conductivity λeff , which is a temperature-dependent material property
and is defined by Fourier’s law. The total effective thermal conductivity could be
described in a good approximation by the sum of the solid thermal conductivity, λs, the
thermal conductivity of the gas within a given porous structure, λg, and the radiative
thermal conductivity, λr, which reflects the involved heat transfer mechanisms [
λeff (T, pg) = λs(T ) + λg(T, pg) + λr(T ),
with T the temperature and pg the gas pressure.
In fig. 3 the gas pressure-dependent thermal conductivity of typical insulation
materials are depicted. For materials like glass fibres and foams with large pores in comparison
to the mean free path of the gas molecules (for nitrogen at ambient conditions about
70 nm), the saturation region where the diffusive heat transfer occurs can be clearly
observed for gas pressures above 10 mbar. Here the thermal conductivity is nearly
independent of gas pressure. For microporous materials, like precipitated and fumed silica
with pore sizes in the range of the mean free path of nitrogen, the thermal conductivity
of the pore gas is already reduced and a total effective thermal conductivity below or in
the range of the thermal conductivity of still air could be seen. The thermal conductivity
values for the gas pressure independent regime at low gas pressures indicate the sum of
the solid thermal and radiative conductivity, λs + λr.
From fig. 3 also two obvious directions for the realisation of superinsulation could be
recognised. Firstly, the possibility to evacuate porous insulations would lead to effective
thermal insulation systems with thermal conductivity values about 10 times lower as they
are known for conventional insulation materials. This effect leads to the development of
vacuum insulation panels for building applications [
]. Secondly, the reduction of pore
size below 1 μm would have the effect that even at ambient conditions thermal insulation
products would have thermal conductivity values below those of still air.
For flat vacuum insulation panels which are applicable for building insulation a
microporous kernel is used which consists of a pressed silica powder in the most cases (cf.
fig. 4). Additionally an infrared opacifier is added to the silica powder to reduce the
radiative heat transfer. The pressed silica core is embedded in a core bag to enable a further
dustless manufacturing of the final VIP. The filled core bag will be enfolded by a
multilayer envelope film. This package will be evacuated down to 1 mbar and sealed within a
vacuum chamber. The advantage of a kernel made of fumed silica which is evacuated at
gas pressure of about 1 mbar could be seen in fig. 3. While for conventional insulation
kernels like glass fibres or foams the effective thermal conductivity would immediately
increase with gas pressure at a gas pressure of 1 mbar, for the microporous fumed silica
only at a gas pressure of about 100 mbar a significant increase could be recognised. This
is a safety margin of 100 mbar considering a typical gas pressure increase of 1 mbar per
year by penetrating nitrogen and water molecules through the high barrier laminate and
the sealing rims.
In fig. 5 a typical design of a high-barrier laminate used for VIP is depicted. The
polyethylene (PE) layer is necessary to enable the thermal welding of the laminate at
VIP rims. The aluminium (Al) layers work as barrier layers against penetrating air gas
molecules, e.g. nitrogen, oxygen and water vapour. It is important to use more than one
thin Al layer in the range of several nanometers, e.g. two or three, to reduce thermal
heat transfer via these layers and therefore a thermal bridge at the VIP rims. The
minimum thickness of the adhesive layer embedded between the two metal layers is also
important. If pin holes, i.e. defects which could not be avoided, occur in the Al layers
the penetrating gas molecules have to diffuse parallel to the pressure gradient until they
hit upon a further pin hole in the next Al layer.
A second possibility to reduce the total thermal conductivity of an insulation material
is to reduce the pore size to such an amount that the mean free path of the gas molecules
is reduced. This is the case for microporous materials. Typical microporous materials
used for thermal insulation in building applications are silica aerogels [
silica, i.e. fumed silica and precipitated silica and mixtures and blends from those. Since
the pure materials are fragile and brittle, often blends with fibres are used to enhance
the mechanical stabilityin products. Additionally also infrared opacifier are added to
reduce the heat transfer by thermal radiation. A picture of silica aerogel as monolith
and granulate is shown in fig. 6. Monolithic silica aerogel is a material which shows
one of the lowest thermal conductivity values of the world at ambient conditions, which
could be in the range of 0.010 Wm−1 K−1. In comparison to this value the effective total
thermal conductivity increases for silica aerogel powder to about 0.013 Wm−1 K−1 and
above 0.02 Wm−1 K−1 for silica aerogel granulate, because more and more large pores
between the particles increase the contribution of the pore gas.
2. – Vacuum insulation glass (VIG)
The walls of modern well insulated buildings nowadays can reach U -values of <
0.2 Wm−2 K−1. Today, the remaining thermal leaks in the fac¸ade of these buildings
are glazings with typical U -values of about 1.0 Wm−2 K−1. One attractive possibility to
essentially improve the insulation properties of a glazing is to suppress the heat transport
due to conduction and convection of the filling gas by evacuating the space between the
glass panes (cf. fig. 7).
Funded by the German Federal Ministry of Economics and Technology within
several R&D projects a consortium of partners from industry and research institutes was
established to investigate the feasibility of producing a vacuum insulation glazing with
outstanding thermal resistance [
]. In a vacuum insulation glazing two glass panes,
connected by an airtight edge seal, are evacuated to a pressure of about 10−4 mbar. One pane
is coated with an infrared-reflecting, low-emissivity layer (emissivity ≈ 0.03) to minimize
thermal transport between the panes. A matrix of spacers is necessary to handle the
mechanical load lasting on the glass panes due to the atmospheric pressure and prevent
them from collapsing.
Actually new sealing techniques are developed which allow for lower fabrication
temperatures and thus enable the implementation of highly efficient low-emissivity coatings.
These sealing techniques include the soldering of metal foils onto the glass panes and the
subsequent welding of the foils in a vacuum chamber forming the needed airtight edge
seal. First samples have already been constructed and the mechanical stability as well
as the tightness of the edge seal is very promising.
3. – Integrated research for energy-efficient buildings
In 2010 ZAE Bayern started the realization of the research and demonstration
building “Energy Efficiency Center” (EEC) in Wu¨rzburg, Germany (cf. fig. 8). The EEC
comprehends the innovative know-how from the involved research partners from science
and industry in order to share experience and exchange ideas [
]. From the very beginning
of the project an interdisciplinary project team, consisting of researchers from the ZAE
Bayern, architects and engineers developed the building concept and new approaches
to integrate innovative technologies in a general concept. The main objectives were the
implementation of energy-efficient cutting-edge technologies, the optimization of their
interaction for maximum energy efficiency, and the demonstration. The involved innovative
technological approaches are lightweight highly insulating facades (with translucent
aerogel modules, vacuum insulations, low-e coatings), textile roof construction (light and
climate management), innovative low exergy heating, cooling and air conditioning
technology with implemented heat and cold storage systems (PCM components, passive infrared
radiation cooling and open adsorption cooling technology), innovative daylighting and
artificial lighting systems and an adaptive high-level control system, which ensures the most
efficient interaction of the smart building technologies with changing environmental
influences. Many of these technologies were investigated within the research initiative EnOB
(Research for energy-optimized construction) of the German Federal Ministry of
Economics and Technology (BMWi). A research server which is part of the high-level control
system enables the conduction of experiments with the implemented building components
of the EEC and manages the data acquisition. The combination of research,
demonstration and dissemination of knowledge in one place will generate the necessary boost for the
fast implementation of energy-efficient technologies in the building sector. Therefore the
EEC is a highly dynamic innovation driver and this approach has the potential to achieve
maximum market impact and public visibility and accelerate innovation processes.
3.1. Membranes and lightweight construction. – The Energy Efficiency Center
manifests its strict orientation towards sustainability and energy avoidance not only during
its life cycle, but already with the choice of structure type and construction method
applied to erect the building. The design parameters of intensive use of solar effects and
especially the utilization of lightweight materials and weight-saving construction
methods became the building’s central ideas. The significant reduction of primary energy
used to produce, transport and install the lightweight elements of the building shell, the
load-bearing structure and the lightweight structures at the inside of around 1/3
compared to similar constructions of more rigid design (except wood constructions) serves as
important basis for the building’s entire efficiency concept. The low-mass construction
elements are influenced by applying specific coatings and substance additions to optimize
the desired characteristics such as heat capacity and behaviour towards thermal
radiation. Examples are the use of titanium dioxide and low-e coatings as well as of so-called
phase-change materials (PCM).
The EEC’s roof is the most significant architectural particularity in terms of
lightweight construction of the building and emphasizes its distinctive character. In
addition to design and structural characteristics, inherent to roof skins made of textile
construction materials, physical and energetic features are the most prominent in the
case of the EEC, which characterize the use of membranes and foils as part of a
multishell roof construction. The optimization of individual functions at each level of the
multi-layer building shell together with the generation of additional effects in the
intermediate layers leads to synergetic effects, which is why multi-layer design is implemented
in general. This fundamental approach fits well with the philosophy of consistently
using lightweight materials. Each shell layer takes on its intended function for which it is
especially selected and optimized.
The textile construction materials fulfil as frame structure the general structural
function of the self-supporting roof as well as of a weather protection against wind, rain and
snow. Moreover, they form the external boundary of the intermediate zone that either
acts as thermally preconditioned heat-insulating layer or, in the summer, is ventilated to
avoid overheating. The fundamental difference of textile construction material compared
to mostly bending-stiff, hard materials is their per se translucent or transparent property.
Unlike glass or transparent/translucent panel-type materials, textile surfaces under
tensile and compressive stresses can be stabilised without heavy and detailed substructures
thus reducing material use for the membrane structures even more.
The heat-insulating layer is provided by different material combinations made of hard
panel-type materials depending on the required features and the installation situation.
In combination with light-transmission properties of the textile materials on top, the
amount of light and thermal radiation transmitted, reflected and absorbed was precisely
adjusted. High-performance insulation materials such as the extremely light aerogel
contribute to the consistent implementation of the lightweight construction principal.
Exploiting light transmission is of fundamental importance for energy savings during
operation of the building. Whereas in residential buildings the energy need mainly
consists of heat production in form of heating energy and warm water, in office buildings the
energy supply for artificial lighting plays an important role. The creation of translucent
ceilings above the underlaying zones of the offices and the interior corridors, which do not
benefit from the natural lighting through the facade, improves the daylight autonomy up
to 100% in some building areas.
3.2. Integrated smart technology . – The approach of a lightweight construction in
combination with a highly insulating envelope has to be coupled with measures to enhance
the thermal mass of the building to avoid extensive cooling/heating loads and control
technologies. Therefore, in the EEC different thermal storage systems are integrated.
The applied low-exergy heating and cooling systems work with low temperature
differences at a minimum temperature level. To provide the cooling for the regeneration of
the building-integrated phase change materials (PCM) in a very energy-efficient way, the
ZAE-developed “Passive Infrared Night Cooling”-system (PINC) is connected to one of
the firefighting water tanks. To support the passive cooling/heating of the PCM-ceilings,
a new developed Liquid Desiccant Cooling system (L-DCS) provides cool or preheated,
dehumidified air to the offices. These three innovative technologies and their interaction
are described beneath.
The EEC has two firefighting water tanks with a volume of 100 m3 each, which are
also used as cold water thermal energy storage (TES). They are both connected to the
buildings water cooling circuits by means of heat exchangers. One tank is cooled by
Fig. 9. – The cooling load is connected to the TES by means of a heat exchanger. The water in
the TES is re-cooled on the rooftop, mainly during nighttime. Heat is released from the water
flowing upon the roof surface by infrared radiation as well as by convection and evaporation.
a conventional compression cooler, the second one is connected to the ZAE-developed
PINC, which supplies the needed cold. A scheme of this system can be found in fig. 9.
This very efficient cooling method is successfully in operation in the existing ZAE
building in Wu¨rzburg since 2000: heat from laboratories (appliance cooling) and offices
(PCM cooling ceilings) is transferred into the TES by means of a heat exchanger. In
order to re-cool the TES, the contained water is pumped onto a seperate area of the
rooftop during nighttime. Since it is a hydraulically open system, the water runs freely
over the slightly sloped rooftop surface and ideally cools down to dew-point temperature.
That means there is a strong dependence between the cooling power density and
the water temperature. Thus higher water temperatures lead to a higher cooling power
density. Assuming typical water temperatures of about 18 ◦C and the climate conditions
in Wu¨rzburg, which is located in the southern part of Germany, a cooling power density
of around 60–120 Wm−2 roof area can be achieved even in summer nights.
The cooled water flows through rain pipes and a filter and is again collected in the
cold water TES. Electricity is only needed to transport the water, so a high COP (> 20)
can be achieved. The cooling cycle typically lasts about 8 hours and yields a reservoir
temperature of around 13–18 ◦C.
As PINC is an open system, water evaporates and the loss has to be compensated
which is automatically achieved by additionally collecting rain water.
3.3. PCM systems. – Due to its lightweight structure, the translucent building
envelope, and the high internal gains, the EEC has an increased heat load. To prevent an
excessive temperature rise on warm and sunny days, phase change materials (PCM) with
a high thermal storage capacity are integrated into the newly developed cooling ceiling
construction as well as into the wallboards. The PCM undergoes a reversible phase
change between the solid and the liquid state at a temperature of about 23 ◦C. During
phase transition a PCM can store a large amount of heat within a narrow temperature
range as all absorbed heat is required to break up the bonds of the crystal lattice. With
the PCM in thermal contact with the room air this leads to a temperature stabilization
effect, so temperature peaks are cut off. The passive temperature stabilization effect
supports the office-cooling, so no active cooling is needed most of the year. To ensure
the thermal effect of the PCM, it is necessary to solidify (i.e. regenerate) the material
during the night. The regeneration of the PCM in the cooling ceilings is realized by
a cooling circuit connected to the firefighting water tank. As the water in the tank is
mainly cooled by “Passive Infrared Night Cooling” this is a very energy-efficient system.
The regeneration of the PCM in the wallboards is achieved by convection.
To predict the thermal effect of the ceiling-implemented PCM, a thermal building
simulation was carried out. A detailed model of the south-west–oriented office was
implemented which is shown in fig. 10. The transparent fac¸ade areas are triple glazed; in
the opaque elements vacuum insulation panels are integrated. Therefore a high thermal
insulation is achieved by a thin wall construction. The highly insulating but translucent
aerogel-module which is, in combination with the translucent membrane rooftop,
providing natural lighting for the office, is also represented in the simulation model. The cooling
ceiling with an active area of 21 m2 is equipped with 90 modules of macro-encapsulated
salt-hydrate PCM with a mass of about 180 kg and a surface of 12.4 m2. The inner walls
are constructed of gypsum boards and insulation material.
The simulation was run using the test reference year weather data set for Wu¨rzburg,
Germany, with extreme summer conditions (TRY-Region 13 [
As can be seen in fig. 11, there are high solar gains during the simulated days. The
solar heat gains together with internal heat gains caused by electronic devices and the
employees lead to a rise in operative temperature above outdoor temperature. When
comparing the operative temperature curves of the room with and without PCM in the
ceiling, differences in the thermal behavior of the room become obvious. Due to the good
thermal insulation of the building envelope and the low thermal mass of the building,
the rise in operative room temperature is significantly higher for the case “no PCM”
than for the case “with PCM”. The PCM is able to dampen temperature fluctuations by
storing heat during the phase transition. During the working time from 8 am to 6 pm the
temperature “with PCM” rarely exceeds 26 ◦C, so in most cases the temperature stays
in the comfort range and no additional active cooling is required.
Active regeneration is required to reliably solidify the PCM. The simulation shows
that the PCM can be completely regenerated with cold water of 18 ◦C during a
regeneration period between 9 pm and 6 am (see fig. 12). To reduce energy consumption, no
active regeneration is provided during the weekend (see, e.g., day 176 in fig. 12).
Fig. 12. – Temperature of the upper side of the cooling ceiling (PCM-temperature for the case
In the EEC, the regeneration of the PCM will additionally be controlled by the
buildings high-level control system. The control system uses a local weather forecast
to ensure a demand-based regeneration of the PCM. If the weather forecast predicts a
period of cold days for example, the PCM will not be regenerated actively in order to
further reduce the energy consumption.
3.4. Concept of the building automation and monitoring . – The building automation
should enable an effective and efficient control of the installed HVAC (heating, ventilation
and air conditioning), sun protection and lighting systems. It is essential to receive by
means of the building automation system a maximum of efficiency in respect of energy
consumption and operating costs. In addition to the state-of-the-art building control
system an additional high-level controller is implemented. Therefore scientists are able
to have full control over the automation and data acquisition system and to use the EEC
as a huge experimental set up.
Since the EEC was put into full operation in spring 2014, an intense monitoring
process started. Concerning the PCM systems, eight special test rooms are equipped
with different PCM-ceiling and wallboard systems. Four of the test rooms are situated
on the southern side of the building, four rooms on the northern side. On both sides one
test room functions as reference and does not contain any PCM. This setup enables a
very dense data acquisition and a detailed analysis of the system’s performance. In the
future weather and occupancy forecasts will be implemented for an optimal control of
the thermal storage systems (PCM and PINC) of the building.
4. – Conclusions and outlook
The further improvement of energy efficiency plays a vital role for our future energy
systems. The fact that 40% of the European energy consumption belongs to the building
sector underlines the importance of R&D works in this field. It is important to
recognize that in general a building with all its aspects, i.e. envelope, construction, building
technology and automation, has to be understood as a unit, and interdisciplinary
planning and research work on materials, components and systems is essential to realize the
highly-energy-efficient building of the future.
∗ ∗ ∗
The Energy Efficiency Center was supported by the German Federal Ministry of
Economics and Technology because of a decision of the German Bundestag and the
Federal state of Bavaria.
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