Building energy research in Germany finds inspiration from polar bears.
By Mark Zeh
Fabric materials offer quite a few advantages in the construction of roofs, such as efficiency of material use, ease of transport and good light transmissivity, but a big disadvantage is their poor thermal insulation performance. Over the last several years, many advances have been made in this area, such as the Tensotherm® material from Birdair and work on improvements in “Gray Energy” use, but there is still a lot of work to be done.
In 2010, Wagner Tragwerke, an engineering firm in Stuttgart, Germany, and the Institute for Textile and Process Engineering of Denkendorf, Germany, together with four other partners, began a project to develop thermal energy-efficient fabric materials inspired by nature. The project, called “bear skin house,” was funded by the Ministry of the Environment, Nature Conservation and Transport, of the German state of Baden-Württemberg and the European Regional Development Fund.
The design team for this project based their work on the thermal energy management principles of the polar bear’s hide. Polar bears are wonderfully adapted to function in their extremely cold climates. In fact, research reveals that a polar bear is almost invisible in an infrared photograph, only losing heat through the area around its eyes and muzzle.
The polar bear’s secret
It’s a little-known fact that the skin of the polar bear is black, and its multi-layered coat is actually transparent—only appearing to be white because of its high-refractory properties. The hairs are also hollow, providing maximum insulation. The coat consists of a dense under layer of fur and an outer layer of guard hairs.
Polar bears are able to harvest solar energy with their black hide, while the light-scattering properties of its hairs reflect heat back to the skin. Apparently, the fur serves to almost completely block UV wavelengths. Solar gain on their skin is partially managed by making the fur stand up at various angles—it lies flat at night, so that a maximum amount of radiated heat is reflected back to the skin. Additionally, polar bears have quite a large layer of fat under their skin and store thermal energy collected in their large bodies, which feature relatively low surface area-to-volume ratios.
Translating the principle into fabric
Two problems faced the “bear skin house” team in applying this principle from nature: the first was how to translate the thermal energy collection principle to a mechanism involving fabric materials. The second was how to store the thermal energy and manage its re-distribution.
Key to resolving this question was managing reflection and transmission of the various waveforms of light. ETFE film was chosen as a key layer in the resulting sandwich of membrane layers. The basis for this is its high reflectivity to infrared light (heat).
Incoming light, including near-infrared and all visible light colors, are admitted through the transparent layer. The inner black layer absorbs the thermal energy. Forced air convection carries heat away from the surface of the black layers. This energy goes into the storage media.
Some percentage of the thermal energy stored in the black layer is re-radiated as heat (middle infrared) from the black layer. This is re-reflected back through the moving air stream, back to the black layer, by the inner layer of ETFE film. Thus thermal energy is trapped and captured for transport to the storage media.
Note that there are “lofting” layers of transparent polyester fibers in this sandwich of materials. The upper layer serves to create an insulating layer of non-moving air (a sandwich of ETFE membranes separated by polyester fibers), the lower layer serves to separate the transparent heat insulation sandwich from the black membrane, allowing forced air to move over the black membrane layer as a thermal-energy collection mechanism.
Creating a thermal mass
Harvesting and storing thermal energy turns out to be one of the biggest challenges in the design of this system. The polar bear has insulating layers of fat and a large body mass to serve this purpose. In contrast, a house is generally filled mostly with air and offers few opportunities to construct a thermal mass, which can be regulated.
“When considering how to store heat collected from the membranes, we considered two mature technologies,” explained Dr.-Ing. Thomas Stegmaier, head of technical textiles at the Institute of Textile Technology and Process Engineering. “Large masses of zeolite or silica gel, storing energy on the basis of absorption, are safe, proven ways to store and return thermal energy. Additionally, the process of regulating them is well-proven.”
“There are several tons of silica gel held inside individual containers,” says Dr. Rosemarie Wagner, of Wagner Tragwerke. “Each container sits on its own scale and the dry mass of silica in each container is known. We are able to calculate how much thermal energy is stored in each cell by weighing how much moisture has been released by each mass of silica; the lighter the cells are the more heat is stored.”
“The thermal air system operates as a closed system in two circuits,” says Stegmaier. “The first, containing partially fresh air, runs under the floor of the house, providing the comfort of a heated floor. The second, the solar thermal circuit, is about 70 percent efficient in transferring sunlight into heat energy.”
The test house is planned to remain in its present location in front of the Institute of Textile Technology and Process Engineering for three years to aid long-term data-gathering, proof-of-concept, and technology transfer.