NREL also had the opportunity to test some in-house innovations on the project. The scheme includes a system called a "transpired solar collector" integrated into the south facades of the building. The collector essentially creates a vertical cavity on the outside of the south-facing walls. Using dark perforated metal to absorb heat from the sun, a fan then draws heated air down into a massive storage area in the crawlspace. The mechanical crawlspace (actually a result of poor soil conditions) provided an opportunity for a "thermal labyrinth," which serves to store thermal energy. In the summer the building and labyrinth are flushed with cool night air to supply cooling during the day. "NREL developed the transpired solar collector for warehouses," says RNL Director of Sustain- ability Tom Hootman, AIA. "We liked the look of the perforated metal panels and the opportunity for capturing heat." The team also used the labyrinth for storing heat from the data center, he adds.

Climate and building type are critical to the viability of high-performance build­ings. A warehouse building in southern California's moderate climate is relatively easy to push toward net-zero. The job is made easier because the building usually has one level with a lot of roof surface area for photovoltaics (PVs), temperature flexibility, and very low energy demands per square foot. A multistory medical center in hot, humid Houston has huge internal loads and fewer opportunities for renewables, making it an impractical candidate for net-zero balance.

Because of low roof areas and small footprints, high-rise buildings present a particularly difficult scenario for net-zero performance. The challenge has been taken on by the SERA Architects and GBD Architects team, who are currently working on the feasibility of the 13-story Oregon Sustainability Center (OSC) in downtown Portland, Oregon. All stakeholders are committed not only to zero-energy but to meet the Living Building Challenge. "Our task evolved into determining what is the largest net-zero building possible in this climate, on this particular site," SERA's Clark Brockman, AIA, says of reducing the building's energy loads. "The first step was to develop a design that would be about 63 percent more energy efficient than a comparable code compliant building." 

The scheme maximizes roof area with a sweeping cantilevered top that captures both energy and water. The PV-covered roof funnels water into a cistern that supplies the building's water needs. Since horizontal surfaces are limited on a high-rise, it also in corporates PVs in the double-duty sunshades that wrap the south facade and building-integrated amorphous thin-film PVs that cover the south-facing spandrel panels.