Lightweight "triple-zero" house produces more energy than it uses

Dec 5, 2009 - Carina Storrs - Scientific American

NEW YORK—Overlooking the city of Stuttgart in southern Germany, a four-story modern glass house stands like a beacon of environmental sustainability. Built in 2000, it was the first in a series of buildings that are "triple-zero," a concept developed by German architect and engineer Werner Sobek, which signifies that the building is energy self-sufficient (zero energy consumed), produces zero emissions, and is made entirely of recyclable materials (zero waste).

Since the construction of the first triple-zero home, Werner Sobek's firm of engineers and architects, based in Stuttgart, has designed and built five more in Germany, with a seventh planned in France. The energy used by these buildings, including the four-story tower where Sobek resides, comes from solar cells and geothermal heating.

The most recent addition to the triple-zero series raises the bar for energy efficiency: It produces more energy than it uses, Sobek said. The one-story glass home, which seems to float in front of a backdrop of pine trees, "is a tiny power plant [which] feeds electricity into the public grid," he said during a lecture on his work on December 2. The lecture took place at The Cooper Union for the Advancement of Science and Art.

Sobek thinks that planners, builders and policy-makers must think about how to reduce the environmental impact of buildings at the same time that they try to reduce the footprint of the automobile and other industries. The building industry is responsible for 35 percent of the world's energy consumption and carbon emissions, and 50 percent of the waste produced in North America and Europe, Sobek said. His engineers and architects are working to reduce the energy required to maintain houses, office buildings, airports and bridges, as well the energy that goes into constructing and disassembling these structures.

Following the 1979 oil crisis, German engineers started to build "Passivhauses," or passive houses. These buildings retain comfortable interior temperatures without the use of active heating and cooling systems. Instead, passive houses receive warmth from sunlight through its south facing windows and underground air ducts in the winter, while the airtight seals prevent warm air from entering in the summer. But with the scant number of windows and 300 millimeters of thermal insulation, "you live like you are in a Styrofoam box," Sobek said.

Sobek strives for just the opposite effect of the Passivhaus, using thinner walls and bigger windows or, in the case of triple zero houses, all-glass walls. "I invented the so-called 'Aktivhaus' [or active house]—buildings which open your soul, which open your mind, which open your heart," said Sobek, who is also head of the Institute for Lightweight Structures and Conceptual Design (ILEK) at the University of Stuttgart and is the Mies van der Rohe professor at the Illinois Institute of Technology. These glass walls still provide insulation, however, because they are triple-glazed, meaning they have three layers of glass with air space in between the layers.

To achieve the lightweight quality of the triple-zero houses, Sobek and his architects and engineers minimize the use of steel and concrete, both of which are energy-intensive to manufacture. The carbon emissions caused by the production of cement, which is an ingredient in concrete, are greater than emissions from the aircraft industry, Sobek said. At his firm, engineers developed a technique to inject bubbles into concrete, halving the amount of concrete used, Sobek explained. The triple-zero houses also use less steel. Sobek's home, for instance, has a thin steel frame, which includes the cables in between the glass panels.

Perhaps what truly sets the triple-zero homes apart from zero-energy buildings is how their materials are put together. Glue is avoided wherever possible because, when it comes time to take down the building, glue makes it too difficult to disassemble the materials for recycling. "[In] passive buildings, between 15 and 20 materials…are all glued together. This is nothing but a toxic waste," Sobek said. "Nobody will ever sort those materials so the result is that you put them in a waste dump and hope that the next generation will not find them."

Instead, Sobek decided that the recycling requirement for buildings should be no less than what German law decrees for the cars manufactured in its country: Ninety percent of parts in German cars must be recyclable. Borrowing the automobile's assembly strategy, Sobek's engineers and architects put their triple-zero houses together with stainless steel bolts, which can simply be unscrewed using wrenches. These buildings are as safe as conventional buildings, Sobek said.

While Sobek's triple-zero houses are so far only in Germany, zero-energy buildings, which use only renewable energy—have begun cropping up in parts of the U.S., U.K. and China. Zero-energy homes in a neighborhood called BedZED, in the London Borough of Sutton, to the south of London, are made of glass and other recyclable materials and have solar photovoltaic cells that provide electricity. In Boulder, Colo., developers are planning to build 12 new zero-energy homes, which will be one of the first "eco-communities" in the country.

Image of the interior of a triple-zero building courtesy of flickr.com