The Zero Effect  

Today you've exhaled, on average, 2.2 pounds of carbon dioxide (CO2), the greenhouse gas poster child for our current fascination with global warming. Just by breathing, you've entered a cycle of carbon emission and sequestration, an environmental chess game with consequences that are well documented, if little understood. Americans will emit roughly 44,000 pounds of CO2 per capita this year, an amount over three times the per capita average for the world. And if you're thinking of planting a forest to offset your last trip to Las Vegas, keep in mind the U.S. Department of Agriculture estimates a healthy tree can consume roughly 13 pounds of CO2 in a year.

Truth be told, human breathing contributes nothing to global warming. We each eat plenty of plants, which act as carbon sinks since they absorb carbon dioxide from the atmosphere, and thus by eating we remove a great deal of carbon dioxide from the environment on a daily basis. If anything, we can breath easier knowing respiration qualifies as a carbon-neutral activity. Such calculated explanations underscore the complications of accounting for carbon emissions across the broad spectrum of our lives-the food we eat, the places we go, the energy we consume, and increasingly, the buildings and cities we create.

Bert Gregory, FAIA, thinks about these kinds of things a lot. As president and C.E.O. of Seattle-based Mithun, an architecture and planning office with a sustainable design specialty, Gregory has literally gone back to nature. In his work on Mithun's Lloyd Crossing Sustainable Urban Design Plan and Catalyst Project, a 2004 master plan for a downtrodden section of downtown Portland, Oregon, Gregory and his team began its work by conceptually recreating the native ecosystem of the site's 54 acres to determine exactly how much CO2 would be absorbed and emitted annually if no human development had ever occurred. The redevelopment plan would then have to match, if not beat, this carbon footprint. "We couldn't analyze everything because the project resources weren't there, so we focused on high-level things," Gregory says, referring to the plan's key issues of energy and water.

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Lloyd Crossing, Portland, Oregon Mithun prepared a comprehensive urban-planning document for the revitalization of this downtrodden neighborhood into a carbon-neutral development with districtwide systems (right). Renderings: Courtesy Mithun

When setting up its predevelopment metrics, Mithun's team found that the yearly baseline of the native Northwest conifer forest landscape would have produced 32 million gallons of groundwater recharge out of 64 million gallons of total precipitation. Calculating that 8 million kilowatt hours (kwh) of solar energy contributed to photosynthesis per year, the team determined that 153 million kwh remained as reflected, absorbed, or re-radiated energy. For the site's carbon balance, the team found that annually the forest consumed 681 tons of CO2, released 495 tons of oxygen, and fixed 186 tons of carbon as biomass (such as new tree growth). The team now faced the question of how over the four decades of the plan's premise you develop an existing infrastructure into an economically viable district that would be a forest in function, if not appearance.

The plan, which won an AIA Honor Award in 2006 and is available on the firm's Web site (www.mithun.com), represents an attempt to define carbon-neutral design. While the term "zero carbon" gets bandied about, most designers mean "net" zero, where absorption of carbon is equal to or greater than emission, hence the neutrality argument. And when talk turns to carbon, more often than not the discussion concerns energy and its parent, fossil fuels. The Department of Energy (DOE) attributes 98 percent of America's carbon dioxide emissions to the combustion of fossil fuels. It's pretty straightforward to determine a building's energy use, to trace it to its source (probably a coal or natural gas-fired power plant), and to make a rough estimate of how much CO2 finds its way into the atmosphere every year when you turn on your lights or tweak your thermostat (see the EPA's quick calculator based on kilowatt-hours usage atwww.epa.gov/cleanrgy/powerprofiler.htm). Past utility bills or electricity-demand-consumption predictions by electrical engineers, environmental consultants, and utility companies-all of which are likely based on previously known consumption patterns-form the baseline for any discussion of a building's energy reduction or generation. But what do we mean when we set out to make a building "zero energy"?

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Main Station, Stuttgart, Germany Ingenhoven Architekten, of Dusseldorf, Germany, designed a zero-energy train station for the heart of Stuttgart. The building relies on passive ventilation through the train tunnels. The "light eye" scoops (right) transport daylight into the subterranean train platforms below (above) while also serving the additional duties of providing exhaust air louvers and smoke removal in case of fire. The eyes were digitally modeled to optimize their efficiency in directing light as well as funneling air out of the station. The project, now under construction, will be completed in 2013. Images: Courtesy Ingenhoven Architekten

Zero-sum games

The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) provided one of the more concise statements on zero-energy buildings in "Understanding Zero-Energy Buildings," which appeared in its September 2006 journal (www.ashrae.org). The authors, Paul Torcellini, an engineer at the National Renewable Energy Laboratory (NREL), and Drury Crawley, with the DOE's Office of Building Technologies, subclassify the concept for buildings in four schemes: net zero source energy, net zero site energy, net zero energy cost, and net zero energy emissions.

Net zero source energy compares the building's energy consumption and production to that of the utility source. Since utilities rely on multiple generation plants and transmission systems, this concept generally proves too difficult to quantify and is therefore rarely used in architecture. The net zero site energy concept measures energy consumption within the boundaries of the building's site, ignoring whether the utility source is coal or wind. This applies more generally to what architects try to achieve, since even so-called "off-the-grid" buildings loaded with photovoltaics (PV) and wind turbines still typically connect to utility transmission lines for backup power. Skidmore, Owings & Merrill's design for Pearl River Tower [record, December 2006, page 172], in Guangzhou, China, exemplifies this approach. The third element, net zero energy cost, relies on volatile energy rates-a notoriously difficult metric-to reach a balance between the energy generated on-site and sold to the utility versus the energy supplied by the utility. Finally, the concept of net zero energy emissions only measures the emissions produced by the generation of power to meet the building's total energy needs, which can also prove difficult with a utility company dependent on multiple sources at any given time.

Architects approach this topic in multiple ways, but the ASHRAE article stresses the need to define the project's goal from the beginning to guide the design team in its decisions. The Dusseldorf, Germany−based firm Ingenhoven Architekten established a net zero site energy constraint on its competition-winning scheme for the Stuttgart Main Station, an expansion and upgrade to the city's train station. The design of the building, for completion in 2013, essentially amounts to a roof to cover below-grade train tracks, a public park, and some interior circulation space. Christoph Ingenhoven, the firm's principal, describes the design as a 21st-century response to a 19th-century problem. "It's very difficult to get a zero-energy building," he says, noting that although his own house is nearly zero energy, he still purchases some power from a renewably sourced utility company.

With the program of the train station, Ingenhoven's firm focused on passive strategies-a popular tactic of energy reduction first, followed by an investigation of on-site generation to make up the difference. The design includes 28 so-called "light eyes," which serve multiple purposes: daylighting the underground station, relieving exhaust air, and passively removing smoke in the case of an emergency. (Ingenhoven observes that architectural systems serving more than one purpose are a hallmark of sustainable design.) Computational fluid dynamic modeling, as well as conventional wind tunnel tests, proved that a natural ventilation scheme would work since the station's tracks slope nearly 16 feet across its length and trains push air in and out. And since Germany has a relatively temperate climate, expected interior temperatures range from 46 degrees Fahrenheit in the winter to 78 degrees in the summer-not uncomfortable temperatures for passengers seasonably dressed. PVs and renewable power purchase agreements make up the station's only electricity needs for elevators and lighting.

Public, private, power

It's conventional wisdom that public institutions have been in the forefront of adopting most design and technology supporting sustainable goals. City train stations, built to last decades, must meet higher operational standards and can afford to take a longer view than the classic developer model building that requires a payback in a handful of years. At least, developers have conditioned architects to expect this.

Zero-energy house, Chicago Zoka Zola, AIA, has designed a site-sensitive single-family home for an inner-city lot that takes advantage of passive heating and cooling strategies. The house, which goes into construction this spring, will eventually support a rooftop PV system. Renderings: Courtesy Zoka Zola

The Los Angeles Community College District (LACCD) has adopted an ambitious plan to transform its nine campuses with a net zero site energy policy. With an existing building base of 5 million square feet and plans to add 3 million more, the LACCD worried it wouldn't have money for increased energy costs for new building. Larry Eisenberg, executive director of LACCD's facilities planning and development department, says the district decided to just eliminate its $9 million annual energy bill. "A comprehensive strategy is key," Eisenberg says.

For the nine campuses to reach zero energy, the LACCD has implemented a three-part plan: improve, reduce, and generate. First, the district intends to convert each of its campuses to a central plant model, an improvement that allows systems to operate more efficiently with fewer maintenance costs. Second, the district has enlisted a third-party contractor to perform an efficiency study of its campuses and to install energy-saving technology, such as occupancy sensors, low-E glass, and better insulation. The district pays for this service out of the money the technology saves on its annual energy bill. These two preliminary strategies ensure the success of the third: the generation of power through a 9-megawatt (mw) photovoltaic installation (or 1 mw per campus).

What's more, the district will subcontract out the system cost and installation to a contractor/supplier (requests for proposals are due back this spring). That third party supplier could then reap federal and state tax credits to eliminate an immediate 20 percent of the PV installation cost. Further, in tax parlance, rapid depreciation of the PV system will pay for another 20 to 25 percent. Green tags-renewable energy credits-can sell on the open market to generate another 5 to 10 percent of the cost. Adding it up, Eisenberg estimates the 9 mw system will cost between 10 to 20 cents on the dollar. With these incentives, Eisenberg projects 1 mw of PVs costs between $1 million and $2 million, which translates to a one to two year payback for a campus with a $1 million annual energy bill. Eventually, the LACCD will buy the system back from the contractor. To skeptics who point to the erratic performance of PVs, which need good sunlight exposure, Eisenberg says the district is exploring a few promising options in energy storage technology, such as hydrogen-powered fuel cells.

Decarbonated living

Large-scale projects garner attention and offset the most CO2 emissions in a single gesture, but zero-energy single-family homes represent a growing market in nearly every corner of the country. A February 2006 report for the NREL prepared by the National Association of Home Builders (available athttp://www.toolbase.org/pdf/casestudies/zehpotentialimpact.pdf) found that the concept of zero-energy homes would be part of the mainstream residential building market by 2012, and by 2050 could result in reducing by 17 percent the electricity demand for the entire U.S. single-family-home sector. Many of the homes referred to in the study, however, appear business-as-usual, incorporating PVs with higher-performance building materials in a conventionally designed tract home on a cookie-cutter subdivision site. As any sustainable-building consultant will tell you, 90 percent of your opportunities for designing a zero-energy home begin with site orientation.

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Zero-carbon beach resort, Nungwi, Zanzibar Richard Hywel Evans Architecture and Design have planned to use photovoltaics and wind turbines to power a beach resort for ecoconscious tourists (left). The resort will offset the carbon emissions associated with guest travel. Rendering: Courtesy Richard Hywel Evans Architects

Zoka Zola, AIA, who practices in Chicago, says unless an architect can decide where to place windows and how to take advantage of natural ventilation on a site, reaching a zero-energy goal through passive strategies becomes difficult, if not impossible. "The discussion should be how to make the building as efficient as possible through its general configuration," Zola says. With her design for a zero-energy house in Chicago, she included south-facing windows, specified 25-percent-fly-ash concrete to provide thermal mass, and devised a layout with courtyard gardens to combat heat island effects. She also helped the client pare his space needs, avoiding the desire to build out the maximum allowable square footage for the inner-city site. Zola planned infrastructure for the eventual installation of PVs on the roof, but advised her client to wait until PV efficiency reached a higher level to bring the house to full zero-energy status.

May contain hazardous materials

Embedded energy or carbon, which takes into account the energy used to manufacture a construction material or product, constitutes an altogether trickier component of the zero-sum game. Zola did not consider the embodied energy in the house, simply because the material information and tools to quantify such things are sketchy at best. For the Lloyd Crossing plan, Mithun did not consider the occupants or the furnishings of a building and only encouraged the use of construction materials with low embodied energy levels. Mithun's Gregory says any master plan undertaken today should consider occupant and operational factors for individual buildings. "This is an emerging issue," he says, noting that most clients interested today are college campuses and other tightly focused building constituencies.

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Hull Architecture Centre, Hull, England Níall McLaughlin Architects created a zero-energy temporary structure to raise design awareness in Hull. Wind turbines and photovoltaics mounted in a plaza supply the building's energy and feed back into the city's grid. Photography: © Nick Kane

Ingenhoven began the design process for the Stuttgart station with simple models to test structural solutions. "We wanted to minimize the amount of concrete used, so we needed to find a purely pressure-loaded structure," Ingenhoven says. With Frei Otto and Buro Happold as consultants, as well as a team of German university researchers, Ingenhoven used experience to shape the structure with the models prior to building a digital model that could optimize it. The work paid off in a slim, 14-inch, reinforced-concrete slab across the tracks (compared to Toyo Ito's 7.8-inch slab roof for the Kakamigahara Crematorium, on page 166). Saving concrete cuts down on cement plant production, a notorious source of CO2 emissions.

Although accounting for embodied energy in our buildings represents a challenge (see the related story RECORD, March 2007, at the top of page 170), raising awareness of architecture's effects on carbon emissions has reached a fever pitch. The U.S. Green Building Council announced in November that it would require all buildings going for commercial certification to achieve a 50 percent CO2 reduction over current levels [RECORD, January 2007, page 127] through stricter enforcement of the energy and optimization points in the LEED rating system. As exhaustively noted in the February 12 issue ofEngineering News-Record, there is no shortage of CO2 emissions news-such as the January call from an unlikely industry coalition, including Alcoa, General Electric, and Dupont, for instituting national-emissions limits or the release of the Intergovernmental Panel on Climate Change's Climate Change 2007 report-helping architects wade through the competing claims, various options, and unwieldy concepts remains a daunting challenge. Mithun's Gregory stresses the combined wisdom of multidisciplinary teams as the short-term solution. On the Lloyd Crossing project, the Mithun team included architects, energy engineers, civil engineers, economists, landscape architects, and even a branding company to help the team communicate its ideas to the community. While meaningful change will take time, Gregory notes, "There aren't enough carbon offsets for the entire world."