The Principle: Inversion of the Heat Current
The lower winter sun will be transformed into heat when it penetrates a glass plate in the cellulose comb: the temperature on the exterior of the wall increases substantially. The
absorbed heat leads to a delay in an increase in the temperature of the wall, which is used as heat storage. The solar façade inverts the heat flow of the building: whereas a
conventional house looses heat, that has to be replaced by the heating system, the solar façade obtains energy from sunlight and eases the burden on the heating system. It is not
surprising, that the "intelligent" façades are being used in passive house construction. The solar façade uses the power of the sun, just like the transparent thermal insulation (TWD -
transparente Wärmedämmung). The façade is, however, in contrast to TWD not an energy maximizing system (panel heating). Instead, it creates a warm zone in the area of the outer wall and
transfers the house, so to say, into a warmer climate zone.
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Diagram of a solar façade: in traditional buildings, an exterior wall area temperature difference of about 20 degrees (measured in Kelvin) has to be equalized by
the heating system during the heating period.
A temperature level of circa 18 degrees (measured in Celsius) can be maintained in the solar comb. The heating system has to merely equalize a temperature
difference of 2°, in order to maintain the room temperature at 20° (Celsius). Since there is almost no temperature drop between the housing space and the surrounding area, there
is little heat loss through the wall. This can be reduced by one tenth through this utilization.
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Graphic:
ESA-Energiesysteme Aschauer Vertriebs Ltd.
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The house will not, however, get too hot in the summer, because the high location of the sun and the steep incoming rays do not penetrate the solar comb deep enough. The heat is
transformed on the surface of the comb, whereby a thermal conditional circulation develops in the ventilation slot behind it. The façade independently dissipates off the superfluous
heat. In contrast to transparent heat insulation systems, the solar façade does not need an additional mechanical shading devise, which saves money.
Good k-Values from East to West
As the installation of other collectors the installation of solar façades makes sense even if they cannot be orientated exactly to the south. The thermal optimization of the building
shell is more important. The solar façade in passive houses is even mounted on the north side and produces a considerable improvement of the k-value just by the utilization of diffuse
sunlight.
The k-value indicates how much heat energy is flowing through a square meter of a building component (for example: walls, windows, doors, roofs). It is measured at a temperature
difference of 1 degree Kelvin, the formula is then watt per square meter Kelvin, in short: W/m²K. The lower the k-value is (recently often named the Environmental Protection Value
"Umweltschutzwert" or the U-value), the lower the so-called transmission losses are - those that go outside. The k-Value is dependent on the construction of the entire building shell.
The solar comb made out of cellulose indicates a heat conductance of 0.08 W/m²K. 5 centimeters correspond to the insulation value of 2.5 centimeters of traditional material. Indeed, the
comb does not only insulate, it also absorbs solar energy. The effective k-value also takes this into account. Values of 0 W/m²K are possible in lightweight construction and southern
orientation by means of the solar energy yield and approximately 0.10 W/m²K on the north side. The manufacturer calculates the exact value with a simulation program for construction
projects.
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"The ESA Solar Façade depicts a new heat insulation system. It indicates a reversal of the trend in high insulation technology integrating the active incidental energy into the
heat balance system of the building. It does not use this energy however like the already-known transparent heat insulation as a heating source, but rather constructs a heated air
cushion in order to minimize heat loss."
(Guiseppe Fent, Architect and Building Owner)
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The Fent one family house, Switzerland.
Photo: ESA
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Insulation Is Not Everything,
But Without Insulation, Everything Is Nothing
Especially well done passive house concepts prove how important well thought out, consequent heat insulation is. An entire avoidance of heating, however, is not sufficient in our
latitudes. That is also true for the solar façades. Indeed, the contribution of solar combs to the thermal stability of the building supports a more efficient utilization of the
remaining energy-saving factors, from thermal solar heating systems to ventilation up to the use of the waste heat from people and electric appliances. Even low temperature energy
becomes useful: when the solar heating system supplies only 25° Celsius water on cloudy days, low-temperature floor heating can be supplied, which causes a room temperature of about 20°
Celsius in a solar insulated house. The efficiency rate of ventilation and solar heating systems can be considerably raised by the use of the previously unused energy.
In addition to its contribution to room climate, the solar comb also yields a noticeable improvement in noise protection and thus contributes to the "acoustic environmental protection":
the street noise stays outside, and the children can also make a lot of noise, without the neighbors complaining.
The solar façades can be manufactured for lightweight construction as wall elements without thermal bridges that satisfy the high ecological and biological construction requirements.
The wall behind the solar façade in solid construction has to only satisfy static requirements and can therefore be produced cheaper.
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Mounting of wall elements (Passive House "Pree" near Linz, Austria). The up to 6-meter high walls were prefabricated right at the construction site. The
construction costs amount to 2,400 DM per square meter of housing space; the expected heating costs should be around 100 marks per year for the entire habitable space of 135
square meters.
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Photo: ESA.
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Modern Energy Concepts, Ecologically Oriented Architecture
Around 70 buildings were fitted with solar façades in Austria, Germany, and Switzerland between 1994 and 2000: from one family houses to housing developments, from sports halls to
industrial buildings.
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The architects included the façades in modern designs and created, with the glazed façade elements, an attractive contrast to wood and other construction materials that are often
used. A variety of glass design and colorful combs allow the façade elements discreetly harmonize with the entire complex. The variety of possible glass structures, pattern sizes
and façade colors make an individual building style possible, while taking ecological and economical criteria into consideration.
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Office building by VOEST MCE, Linz, Austria.
Photo: ESA.
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In Wallisellen, near Zurich, 13 residential houses were fitted all around with solar façades. The low-energy housing development consists of four- to five-story houses that were
predominately built with lightweight construction. The functional core with the supporting building components is made out of concrete. The already favorable static k-value of the outer
wall (0.17 W/m²K) will be improved even more by solar radiation yields: the effective k-value approaches the 0-Watt limit depending on the orientation. The value even sinks to under
0.11 W/m²K on the north side due to the solar façade.
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The low k-values create the requirements for the heating supply in the building by means of a ventilation system (exhaust air heat pump). It provides, along with the
sound-absorbing façades, an untroubled atmosphere in this residential area burdened with noise from airplanes.
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Housing development "Balance" near Zurich, Switzerland.
Photo: ESA.
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New Solar Façades for Building Stability
More and more building owners are deciding on the energy saving and energy absorbing solar façades. The concrete savings potential depends on the respective building concept. The more
the k-value can be lowered, the higher the operation cost savings. During the renovation of buildings, special components that have been prefabricated for this purpose will be hung in
front of the existing wall and are furnished with an equalizing layer made out of insulation felt. An interesting side effect arises here. The temperature of the masonry is raised by
the solar façade: this prevents heat loss and has a dehumidifying effect on the wall. The living climate will be improved in this way contributing at the same time to building material
upkeep.
An Idea Prevails
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The research and development of the solar façades began about eight years ago at the Linz Energy Institute. They were installed for the first time in 1994, in the one family house
owned by the Aschauer family. The manufacturer received the solar award of EUROSOLAR in 1995 and mounted 1,000 square meters of solar combs. The solar façades were widely
introduced to the market with the founding of the ESA-Vertriebs (Sales) Ltd. The project volume reached the 10,000 square meter limit. Authorized and trained specialists take over
the sales and mounting of the solar façades in Germany, Austria, and Switzerland. The distribution network is currently being expanded.
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European Solar Award 1995 for the solar façades.
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Material and photos: ESA-Energiesysteme (Energy Systems) Aschauer Ltd.
Solarserver Editorial Staff: Rolf Hug
Read more about this topic in the Solar Magazine
"Sonne aktiv und passiv: Solarhäuser ohne Heizkörper", ("The active and passive use of Solar Radiation: Solar Buildings without Radiators")
Book suggestion
Graf, Anton: Das Passivhaus- Wohnen ohne Heizung. ("The passive House - Living without Heating.")
Translation: Mary Meier
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