Demonstration building for a textile solar collector Michael Karwath1, Judith Sarsour1, Rosemarie Wagner1 1

Wagner Tragwerke Sterneckerstrasse 16A D – 70563 Stuttgart, Germany.

Abstract Typical for single layer membrane structures is the less isolation against heat and cold. A demonstration building is build during summer 2012 to show the possibilities of textiles for energy harvesting. A membrane structure is developed based on a translucent multi layer, double curved and pretensioned membrane structure with high thermal isolation properties. The demonstration building shows a solution of textile materials as solar collector. The structural task is the development of a multi layer, double curved membrane with layers of different stiffness, load carrying behaviour and environmental properties with in a height of max. 30 cm. The main aspects are the position of the load carrying layer, the different shapes of equilibrium for each layer depending on the material and the influence of the different temperatures of the single layers to the load carrying behaviour of total package. Rosemarie Wagner, [email protected] Keywords: Multilayered membranes, thermal active membranes, textile solar collector, thermal properties of membranes

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Introduction

The target of the presented design project is tied together the theoretical knowledge of the fields of textile materials, structural design of membranes, mechanics of enclosed gas volumes and thermodynamics of the heated gas flow, with the technological experience concerning energy technology, supply technology, connecting methods, detailing, cutting pattern and practical know-how with regard to manufacturing new solar collectors made of textile materials.

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The actuality of the design lies in developing unconventional ways to the use of solar energy, minimizing the use of fossil fuels for heating and cooling of buildings. The presented design is also suitable to rearm existing buildings with a lightweight second skin increasing the thermal isolation and having the potential of harvesting solar energy. In the presentation a textile building is developed with all necessary structural components including a layer for the air flow which can heat up by solar radiation to a temperature of max. 140° C. The textile solar collector is connected to an energy storing system based on a chemical-physical process. For harvesting solar energy the air flow needed to be highly protected against the loss of temperature through the solar collector and the pipes to the storage system. The main tasks are preventing the solar radiation from the collector into the interior of the building and avoiding cooling out on the out side of the solar collector. The design of the demonstration building is a cooperation of several local companies of Baden-Württemberg and ITV Denkendorf as project coordinator. ITV Denkendorf has developed on the solar-thermal functions of the polar bear's fur spacing textiles which combines the flexibility of a fibre-reinforced textile material with translucent thermal insulation properties. The knitted spacing fabric is able to let the sunlight though to a textile absorber which transfers visible and UV-light into hot air. The hot air is guided in canals thought the textile solar collector to energy storage system, figure 1. The stored energy is used to heat the inner space of the building during the cold seasons. To demonstrate the total process of energy harvesting, cooling down the inner space during summer time and heating the inner space in the winter time the building requires also for the wall elements, floor and windows good thermal isolation properties. sun light

Transparent thermal isolation

Structural membrane

Heat pipe

Thermal isolation Textile absorber

Thermal isolation

Gas pump

Thermo-chemical tank

Figure 1. Proposal of the demonstration building for harvesting solar engery

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Material Properties

The design of a multi-layer membrane is presented which is used for harvesting solar energy and requires a higher thermal resistance than usual membranes in the textile architecture such as coated fabrics or foils made of polymers. The multilayer system for harvesting of solar energy is called subsequently functional membrane. The length of solar collector needed to be at least 5 m for reaching the maximum of temperature in the air flowing through the spacer fabric. The length is improved by tests of the ITV Denkendorf. For demonstrating the capability of the textiles a free span of app. 6 m is chosen on the south side of the building. The surface is double curved in relation to the necessary pretension stress of the membrane avoiding fluttering under wind or snow and water sags. This includes that all layers are made of flexible membranes, should have the same curvature and being pretensioned. Depending on the layout of the functional membrane the load carrying membranes needed to withstand temperatures up to 140 °C. The load carrying capacity of coated fabrics such as polyester and glass fibre as well as ETFE foils is tested up to the temperature of 150°C. The unidirectional tensile strength of the polyester and glass fabrics lies between 60 kN/m and 100 kN/m and allows easily the span of 6 m. Differences between polyester and glass fabrics are the temperature expansion and temperature resistance. Usual polyester fabrics have an improved load-carrying capacity and suitability up to a temperature of 70°C. If polyester fabrics are used as load bearing elements, it should be made sure that the maximum temperature on the membrane surface is less than 70°C. A Glass fabric has a higher temperature resistance, the stiffness and unidirectional tensile strength is showing only little reduces up to temperatures of 150°C as result of tests carried out by the ITV Denkendorf. The absorber membrane is chosen as silicon coated glass fibre fabric. The upper layer of the spacer fabric is an ETFE foil to get as much solar radiation as possible to the absorber fabric. The values of the load-carrying capacity and suitability of ETFE foil is given up to a temperature of 70 °C on the foil surface. If temperatures increases to more than 70°C in the functional membrane and the foils have, in addition, a load carrying function, tests are carried out to get values for the unidirectional tensile strength and seam strength under higher temperatures as well as the change of stiffness. It turned out that the ETFE foil requires a special treatment to avoid wrinkles in the foils after the first cycle of hot air flow and cooling down again. The task needed to be solved is how high the loss of pretension stress is if the membranes are heated up to a surface temperature of 140° C. The critical load case is a sudden thunder storm in summer time. The sun heats up the ETFE and the silicon/glass fibre fabric and the material is still hot and more elongated if the wind acts on the structure. To grasp the influence of the temperature expansion on the load bearing behaviour numerical calculations are carried out. The result of the case studies is to avoid to high loads on the materials used for the solar collector. Even for the silicon coated glass fabric the pretension stress is lost if the temperature increase of more than 40°C and the membrane can only be used to carry the dead load of he system.

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Design of the Functional Membrane

The lost of pretension under higher temperatures for any type of membrane material results in a system of multi layer membranes of an outer and inner load carrying membrane and the functional membrane in between. For the layout of the functional membranes several solutions are developed, tested and evaluated concerning the load carrying behaviour, the reached temperature and the air flow. The functional layer for harvesting energy is a black spacing polyester fabric for the air flow and a black silicon coated glass fibre fabric as absorber. The layers above the spacer fabric are to increase the thermal isolation with the requirement of a having a very high transparency and a very low heat radiation. The layers below are to prevent heating up the inner space, the black silicon/glass fibre fabric is large radiator and the heat should be kept in the spacing fabric. At least 20 different solutions of the layer set up had been tested in a test equipment of the ITV Denkendorf with the aim to increase the air temperature in the spacing fabric under radiation of 400 W/m² to 800 W/m². The air flow is given between 0 m/s and 0,6 m/s. Three of the tested layout are shown in figure 2 and the differences are the number of load carrying layers, the connection layers and the number of layers with air. Layout 1

Layout 2

Layout 3

Figure 2. Layout of the layers for the textile solar collecto The design of the functional membrane is a result of a process concerning load carrying behaviour under temperature up to 150 °C, thermal properties of the materials, available materials on the market, manufacturing of the membranes, gas tightness of the channels for the air flow, air tight connection to the pipes at the bottom and top of the functional membranes, condensation of water on the membranes and construction on the outer and inner side of the membranes. The layout of the different membranes is given from top to the bottom as follow. To earn very high profits from the solar radiation, the ETFE foils which are permeable for 99% of the visible light and ultraviolet rays are suited. The disadvantage of ETFE foil is the less strength and the top layer is reinforced by additional steel cables. The size and the distance of the steel cable are defined by carrying the external loads such as snow and wind. ETFE foils permit a maximum span of 1,0 m in mechanically pretensioned membrane structures and for the location of the demonstration building in the surrounding of Stuttgart, Germany.

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The distance to the next ETFE foil is given to avoid convection of the air in the gap on the one side and to prevent contact under snow on the other side. The air volume between the two layers of ETFE is on all sides open reducing the load transfer from the upper layer to all layers underneath. The second advantage is that condensation water between the two layers can be drain. To avoid heat radiation from the spacer fabric towards the surrounding and to get as much solar energy as possible the black spacer fabric is covered by two layers of ETFE foils designed as cushions with a closed air volume. The cushions are mechanically and air tight clamped onto the black silicon coated glass fibre fabric. The silicon coated glass fabric has a low reduction of strength and stiffness up to a temperature of 150 °C. This membrane is able to carry the dead of the ETFE foils with the clamping system as the upper thermal isolation and the dead weight of the spacer fabric and the dead load of the material itself even under the temperature of 150°C. The thermal isolation underneath the silicon coated glass fibre fabric is carried by cable net. Between cable net and thermal isolation a PE foil is used as water vapour barrier and prevents the penetration of water vapour into the thermal isolation. The used thermal isolation is flexible, stiff enough to be a mattress under the area of the solar collector and with a heat resistant up to 200 °C. The isolation is lying on the PE-foil as water vapour barrier which is unable to carry the dead load of the thermal isolation. A cable net is connected to the structure of the wall elements is chosen as supporting structure for the thermal isolation. Depending on the possibilities to build the textile solar collector in one year special material developments are impossible to develop such as low-E coating or spacer fabric laminated between high transparent foils or fabric with a high temperature resistance The result of the layout of the functional membrane is a design with materials available already on the market and is solved in three layers of tensioned surfaces with different properties, a very stiff top layer with ETFE foil reinforced by cables, a layer of silicon coated glass fabric for the air flow and a very flexible cable net carrying the necessary thermal isolation on the inside of the roof.

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Design of the Structure

The demonstration building is located on the campus of the ITV Denkendorf and design is guided by the following aspects. The inner space has to be large enough to store the energy supply system with all necessary equipment. The structure including the floor system is deployable, all parts of the building such as roof, wall, floor and windows have a high thermal isolation and the building has still the appearance of free shaped membrane structure. Depending on the location and the costs the structure of the demonstration building covers a floor area of 13,9 m x 12,1 m and the building has a height of 5,9 m. The requirement to be deployable leads to steel structure for the floor which can be put on any kind of ground. The main structure for carrying the membranes are two steel arches crossing each other and can stand on their on. On the south side of building all membranes are supported to six v-shaped struts, stabilized by links parallel to the arches for holding the functional membrane.

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The membrane and cable net on the north side of the building are pulled down to the steel structure of the floor. The horizontal components of the tension forces of membranes and cable nets are set in equilibrium by the steel structure of the floor. Only a concrete foundation of the steel arches is necessary. The edges of the steel structure are hold down by 4 ground anchors screwed into the soil. The wooden floor carries the thermal isolation and the wall elements. The steel framework for the wall elements is also the support structure of the cable net. The advantage of the cable net is on the north side of the building a security against vandalism. The gap on top of the two steel arches is covered by transparent polycarbonate plates and closed to the inside by highly translucent and flexible thermal isolations.

Figure 3. steel structure 3D Model (left) and on site (right)

Figure 4. outer layer (ETFE with cables) and inner layer (cable net)

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Figure 5. final construction

Acknowledgement

The paper presents part of the work within research project “Eisbärbauten” founded by the Ministry of Environment, Nature Conservation and Transport of the Federal State of Baden-Württemberg and the European Commission with in the EFREProgram 2007 – 20133. The project partners are ITV Denkendorf, Arnold Isolierungen, Labor Blum, TAO Technologies GmbH, TinniT GmbH and Wagner Tragwerke.

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