Royal Society Publishing

Bionics in textiles: flexible and translucent thermal insulations for solar thermal applications

Thomas Stegmaier, Michael Linke, Heinrich Planck

Abstract

Solar thermal collectors used at present consist of rigid and heavy materials, which are the reasons for their immobility. Based on the solar function of polar bear fur and skin, new collector systems are in development, which are flexible and mobile. The developed transparent heat insulation material consists of a spacer textile based on translucent polymer fibres coated with transparent silicone rubber. For incident light of the visible spectrum the system is translucent, but impermeable for ultraviolet radiation. Owing to its structure it shows a reduced heat loss by convection. Heat loss by the emission of long-wave radiation can be prevented by a suitable low-emission coating. Suitable treatment of the silicone surface protects it against soiling. In combination with further insulation materials and flow systems, complete flexible solar collector systems are in development.

Keywords:

1. Introduction

(a) Potentials of textile technology in bionic developments

Fibre-based materials and technologies offer a great potential for successful bionic developments, because there are different similarities to living nature.

  1. Starting from the small and smallest construction units in nature bigger systems are built up. On the contrary, our technology tends to start with large volumes of raw materials, which are gradually processed into smaller functional units and subsequently assembled. The growth in nature can result in very complex systems with functionality and efficiency that far exceed any of our technical products, especially in terms of consumption of materials and energy. Textile processing technologies offer remarkable analogies to natural growth processes.

  2. Starting from small units of single fibres, down to nanometre dimensions, larger elements can be ‘composed’ in staged processes. This method, in principle, functions without producing much waste and requires relatively small amounts of energy.

  3. Hairy structures in plants and animals are responsible for special functions and mechanisms. Hairs can be found on the upper and undersides of insects, between the parts of an insect exoskeleton, in the feathers of birds, in the coats of animals and in spiders' webs. We know a lot about the functions of the hairs, but we are far from knowing everything.

  4. The many different methods of fibre processing, fibre orientation and finishing are absolutely suitable for meaningfully transferring biological functions to technical products.

  5. Fibre reinforcement is an essential tool in nature's constructions for strong and lightweight materials in many different forms. The closer these materials are analysed, the more astonishing the findings are. Fibres at the nanoscale, gradual transitions, high-tensile materials and functional cross sections can be found in nature's constructions. Composites occur in soft and hard forms in bones, stalks, leaves and surfaces, and are composed of organic and inorganic materials. Of great interest for the developments in future in advanced technologies are the hierarchical structures in natural composites.

(b) Fibres for successful bionic activities

Over the last decade at ITV Denkendorf, Germany, intensive bionic research and development have been carried out (Stegmaier et al. 2006, 2008a). This basic and applied scientific work covers not only surface-related functions (such as self-cleaning in the case of the lotus effect (Stegmaier et al. 2008b), not wetting in water in the case of the water spider and harvesting fog as in the dessert beetle) and structural functions (oil absorbing such as the functions of oil bees (Scherrieble et al. 2008), adaptive filtration such as in sponges (Linke et al. 2005) and fibre-reinforced composites such as helmets (Milwich et al. 2006)) but also energy-related developments (energy-independent fluid transfer functions as in trees (Stegmaier et al. 2008a; www.kompetenznetz-biomimetik.de) and solar thermal materials (Stegmaier et al. 2007)), which will be described in the following, and breathable membranes such as the stomata in leaves.

In most of these developments, engineers are working close together with biologists of universities. In this way, the necessary synergies can be generated.

2. Materials for harvesting solar radiation energy for heating

In view of the fact that fossil energy sources will become more and more rare, the development of new systems for the use of solar energy is an essential task of our time (Ladener & Späte 1999), as the potential for sustainability is given. The transport of energy from the Sun to the Earth is quite large: for instance, the middle annual insolation power for Munich shows a maximal value of 1.150 kW h m−2, while in the Sahara it amounts maximally to 2.200 kW h m−2 (Ladener & Späte 1999). One has to consider that the solar radiation on the Earth's surface contains approximately 3 per cent ultraviolet (UV), 46 per cent visible and 51 per cent infrared (IR) radiation. In nature, sunlight is the number one in energy supply. Only very few plant organisms can use thermal or chemical sources. A great number of plants use sunlight. The technical use of sunlight is a great challenge to humans, since the fossil resources are decreasing dramatically. It is a global task to use this natural energy for heating of houses and liquids.

Some really effective systems are already developed: thermal solar collectors are able to change solar radiation power into usable heat (Stine & Geyer 2007). An absorber is used to transfer the heat into a carrier medium (air, water and glycol water mixtures; Wagner 1999). For the absorber, good heat conducting materials such as metals or some kinds of plastics are used. The absorbers should have a high absorption rate and convert solar radiation as completely as possible into heat.

For the cover of such solar collectors, but also for translucent thermal insulation (TTI) of buildings (Löffler et al. 2007), materials have to be used with preferably high translucence and simultaneously high thermal insulation characteristics. The principle of such materials is simple: the sun shines through a transparent front sheet and warms up with its radiation (mainly visible and UV rays) a dark absorber sheet lying behind. The absorber releases the heat to the brickwork and thus into the building. This principle does not function in the reverse direction, because an air cushion lets through the radiation, but blocks the convection and conduction heat losses. As façade elements in winter months these materials can generate heat for the rooms by absorbing sunlight and at the same time reduce the heat loss of the building. In summer months, a sun protection in the front (sunblind) protects against excessive heating of the building.

For TTI, glasses and other materials with excellent optical characteristics and/or fine capillaries are applied (Hausner et al. 1996).

But up to now the available materials for TTI are plate-shaped, inflexible, rigid and additionally heavy and fragile due to the panes of glass. Therefore, the collectors available are suited only for local use.

For absorbers, flexible materials are well known but they are unsuited for TTI.

3. Development of a translucent thermal insulation

(a) Polar bear and its fur: an archetype for solar thermal functions

A living example for such a flexible solar material is the fur and skin of the polar bear, which has to survive in the arctic cold at −50°C (Nachtigall 1998; figure 1). It is the biggest living predator on the Earth. To survive, these huge animals have an extensive fat layer of up to 10 cm, which helps to protect against the cold. The bulky fur is white in colour and helps the bear to camouflage itself in the snow-covered and ice environment. Looking through an IR camera, which makes heat radiation apparent to our eyes, the polar bear is nearly invisible. UV cameras have to be used to find the bear amidst the white environment.

ITV scientists looked in detail at the fur and hairs. The polar bear hairs are nearly transparent and show a hollow structure with foam in the core (figure 2). The captured air in these water-repelling hollow hairs as well as between the hairs leads to them being highly insulating.

Figure 2

Hollow fibre in polar bear fur.

A further speciality is the black skin of the polar bear (figure 3). Scientists assume that the white hairs reflect sunlight along their length until it is transferred into heat in the black skin (Nachtigall 1998; Blüchel & Malik 2006). So the black skin catches the sun's radiation. But the matter of light transfer in the jacket of the hollow fibre is discussed controversially in different scientific papers (Bohren & Sardie 1981; Koon 1998). It is certain that only little of the generated heat is transferred through the dense fur to the outside (figure 4). That is the reason why it is impossible to view the polar bear with an IR camera.

Figure 3

Yellowish fur and black skin of the polar bear.

Figure 4

Solar thermal functions of polar bear fur.

So, the fur is not only camouflage but also a light trap, and at the same time an ideal insulation material. In combination with the thick fat layer (up to 10 cm) warmth is retained well and ensures the survival of the polar bear.

(b) The process of abstraction by physical understanding

At ITV Denkendorf, the principles of solar technology of the polar bear were analysed. Especially, the physical functions of transferring solar radiation from the outside to the absorber, the thermal insulation of the system as well as the low heat radiation emission were of great interest for technical development. Figure 4 demonstrates the solar thermal functions of polar bear fur, which were the initial point for the development of a technical product.

Figure 4 shows that the sun's radiation is transferred through the air-holding sheet (yellowish fur) to the black skin, which has the function of an absorber. Owing to the fat layer as well as the fur with the heat insulation property, the heat is not able to be lost by convection. Furthermore, the IR (heat) radiation from the body is reflected by the skin and hairs in order to avoid heat loss.

(c) Technological developments

Bionic transfer was first examined with an artificial fur made of a pile fabric with light-conductive fibres and a black-coated back side (figure 5). In principle, the pile fabric worked from the point of view of solar activity but the cleaning behaviour under technical environment was not acceptable. Contrary to the polar bear, which cleans its fur inter alia with its tongue, in technical application it was not possible to ensure the necessary properties of the material.

Figure 5

Pile fabric with light-conductive fibres.

With this first step—a kind of biomimicking—the desired technical performance could not be achieved in the generated materials. The close-to-reality copy of the biological archetype does not fulfil the criteria for the technical environment. So, we had to go on a necessary creative process of new ideas to fulfil the limitations of the production processes and the demands of consumer products.

In this way, a new idea was created using modern textile technologies. With the combination of a flexible spacer textile with a smooth foil on the top and the bottom the ITV scientists obtained the principle for a technological product—a coated spacer textile (figure 6; Stegmaier et al. 2005). It consists of

  1. light stable polymer fibres, which form the insulating spacer,

    Figure 6

    Construction of a spacer textile composite for solar thermal energy harvesting.

  2. highly light- and temperature-resistant coating, the colour of which can be changed from transparent to black,

  3. a transparent coating letting visible sunlight pass through, but absorbing the dangerous UV light, which protects the polymer fibres underneath,

  4. a special top coat that works as in the case of the lotus effect and keeps the surface clean, and

  5. a black pigmented coating on the back side, which is used as an absorber to transfer sunlight into heat.

The developed solar textile is characterized by the following properties:

  1. high translucent and/or black pigmented silicone coating,

  2. open textile structure for a high light transfer,

  3. translucence for incident light of the visible spectrum and impermeability for short-wave UV radiation,

  4. strongly reduced heat loss by convection,

  5. heat loss reduction of long-wave (thermal) radiation by a suitable coating, and

  6. dirt resistance by a special coating.

This kind of TTI is firstly completely flexible and has special advantages for solar technology: high transparency, high mechanical stability (break proof, tear proof and elastic), high thermal stability, high flexibility for arched forms and high chemical durability due to the choice of materials.

Table 1 shows the technical data of a double-side coated translucent spacer textile (figure 7) and those of commercially available TTI hollow chamber panels and structures, which are inserted into double panes of glass. It is clear that the flexible textile TTI has advantages in respect of

  1. low weight,

    View this table:
    Table 1

    Technical data of different translucent heat insulation materials.

    Figure 7

    Translucent coated spacer textiles.

  2. high light transmission, and

  3. low thermal transition coefficient (U-value);

and in addition in respect of

  1. high mechanical stability (unbreakable, tear proof, elastic),

  2. high thermal stability (approx. up to 110–160°C),

  3. flexibility, i.e. arched structures are feasible,

  4. deep drawability within certain limits, and

  5. chemical resistance.

4. Potential application areas

Both described applications (TTI and absorber) can be generated independently by adapting the pigments in the coating layer. In the combination, special and flexible solar thermal collectors are possible (figure 8).

Figure 8

Solar thermal collector made of textile composites (layers from top to bottom: transparent thermal insulation; air or water floated absorber; thermal insulation).

The international builder of solar collector systems Solarenergie Stefanakis is using this material as a cover of its hemispherical shaped collector systems (figure 9; Stegmaier & Stefanakis 2007). These are able to use sunlight in an optimal way by different daytime or time in the seasons. To reduce heat losses, especially in the night, and as transparent heat insulation owing to the encapsulated air layer and reduced convection, the absorber has been insulated by a cupola of synthetics until now. But the high heat losses need to be overcome. As an improvement, the spacer textile with only one side coated in a transparent way is used as cover. Now, high insulation values are combined with low weight and due to the high elasticity a new dimension of break proofing is achieved.

Figure 9

Half-spherical collector with flexible textile transparent thermal insulation.

With a modified structure these developed materials are also of interest in the construction industry as front elements and for roofs too. New ways of design are possible.

Footnotes

  • One contribution of 9 to a Theme Issue ‘Biomimetics II: fabrication and applications’.

References

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