Electronically Controlled Exterior for Green Buildings

A sensor-tuned facade lets fresh air in without throwing energy out

Photo: KFW Bankengruppe

Sensible System: The Westarkade tower integrates data from dozens of sensors to keep the tower from consuming power.

You don't need to seal yourself in to live green. That's the message from the engineers of the Westarkade, a colorful high-rise that officially opened in Frankfurt in July.

The 15-story glass tower glows with natural light and offers windows that open—a comfort that can wreak havoc with energy efficiency. But the Westarkade's first-of-a-kind "pressure ring" facade and sophisticated, sensor-rich control scheme promise to consume no more than 100 kilowatt-hours of energy per square meter per year. That would make it a world-class energy miser, using half as much energy as a conventional office building in Europe and as little as a third of the U.S. average.

"It's an aggressive goal, especially with this building that's all glass," says Mark Perepelitza, a facade expert with Zimmer Gunsul Frasca Architects, based in Portland, Ore.

The Westarkade's dynamic facade is a descendant of a 1990s German design that was developed to manage excess sunlight in glass towers. Horizontal venetian blinds installed outside of the tower's glass envelope reject unwanted summer heat, while a pane of glass installed over the blinds and ventilated at the top and bottom protects the blinds from high winds. The "double skin" design comes up short, however, when architects add windows that open, which is typically mandated under European labor codes. Winds create a pressure differential around the tower, sending drafts ripping across floors when workers exercise their right to natural ventilation. "Sheets are blowing from the tables and the doors are smashing and the heat is going out," says Tom Geister, a senior architect with Berlin-based Sauerbruch Hutton, the firm that designed the Westarkade.

Sauerbruch Hutton worked with Stuttgart-based firm Transsolar KlimaEngineering to solve the problem. The firms installed 180 vertical ventilation flaps in the outer skin to manage air flows, creating what they call a pressure-ring facade. The building's control system takes constant feedback from a rooftop weather station and from 40 sensors deployed throughout the building that measure temperature, pressure, and sunlight. The system continually opens and closes individual flaps to maintain a ring of consistent positive pressure around the structure, preventing strong winds from entering.

Photo: KFW Bankengruppe

Second Skin: The Westarkade's control system manipulates ventilation flaps on the building facade to maintain a ring of air pressure around the building.

That should keep windows from sabotaging the Westarkade's high-efficiency heating and cooling systems, which rely on such tricks as geothermal heat exchangers, heat captured from a basement data center, and heat recovered from vented air. Better still, in the fall and spring, air from the open windows should eliminate the need for mechanical ventilation altogether. Throughout the year, the control system will advise occupants via an LED panel in offices when open windows are a good idea, but it gives the occupant the last word.

Of course, that's all theory. "We will see if it works," said Axel Hinterthan with a nervous laugh, as he led journalists around the nearly completed Westarkade a few months ago. Hinterthan is director of project management for KfW Bankengruppe, which commissioned the Westarkade as an expansion of its Frankfurt headquarters complex. He admits that KfW is taking a chance on new technology to further green-building design, which is an important role for the state-owned development bank. Created decades ago to implement the Marshall Plan, KfW now administers, among other things, low-interest federal loans for energy-efficiency upgrades.

Transsolar engineer Björn Röhle says his company is confident that the pressure-ring facade is the most efficient means of building windows into a tower. He says that a team at the University of Karlsruhe, in Germany, will monitor the Westarkade's energy consumption for two years to test whether this approach works.

Oregon architect Perepelitza expects that it will take two years just to tune the Westarkade's control systems for optimum energy performance. He cautions that the double-skin design was overhyped and that some similar early projects "didn't live up to the promise." But he is nevertheless bullish about the pressure ring's potential, which he says was extensively modeled by Transsolar and builds on the earlier designs. He thinks it could point the way forward for glass towers as natural ventilation and energy efficiency grow in importance. "This building represents a new generation," Perepelitza says.

This article originally appeared in print as "Frankfurt Tower’s Power Trick".

About the Author

Peter Fairley is a contributing editor to IEEE Spectrum and the Energywise blog. In the June 2010 issue he wrote about how energy technology giant GE intends to compete in solar using cadmium telluride technology.

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