Solar Energy System of the Month:

Building with the Sun: Solar Façades Insulate with Sunlight

The idea is amazingly easy - implementation is simply astonishing: combs made out of cellulose that are manufactured from cardboard or wastepaper are the foundation of solar heat insulation. The glazed solar comb air collector is mounted on the outer walls and takes up the solar energy impinging upon the walls in winter. The development of "ESA-Energiesysteme (Energy Systems) Aschauer Ltd." in Linz, Austria, unites the characteristics of traditional thermal insulation with the storage of solar energy in the wall.

The difference in temperature between indoors and outdoors is basically offset. The walls turn into a warm shell of the building. During the winter time an average temperature difference of approximately 18° Celsius is reached between the comb and the outer wall. Where no heat is lost, there has not to be produced any heat. Furthermore the appealingly integrated façades in residential and commercial buildings are a feast for the eyes: colors and material give the architect a wide scope for creative design. The individual solutions, from one family houses to housing developments, show solar architecture at its finest.

The passive house "Pree" near Linz, Austria: all the outer walls and the underside of the pitched roof are glazed. The average k-Value of all solar walls is around 0.08 W/m²K.

passive house "Pree"
ESA-Energiesysteme Aschauer Vertriebs Ltd.

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.


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.

Graphic: ESA-Energiesysteme Aschauer Vertriebs Ltd
ESA-Energiesysteme Aschauer Vertriebs Ltd.

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.

"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)

The Fent one family house
The Fent one family house, Switzerland.
Photo: ESA

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.

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.

Mounting of wall elements (Passive House "Pree"
Photo: ESA.

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.

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.




Office building by VOEST MCE
Office building by VOEST MCE, Linz, Austria.
Photo: ESA.

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.

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.

Housing development "Balance"

Housing development "Balance" near Zurich, Switzerland.
Photo: ESA.

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

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.

European Solar Award
European Solar Award 1995 for the solar façades.

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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