The Problem with Net-Zero Buildings (and the Case for Net-Zero Neighborhoods)
July 30, 2010
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Achieving a net-zero building with today’s technologies and occupant expectations is hard. There are a handful of projects out there proving that it is possible—for the right building in the right setting with the right team. But sometimes going after the goal of net-zero energy use in the building can have unwanted side effects. For example, a low-rise building on a low-density site will have a better chance of being net-zero with onsite renewables, but that type of development is often known as “sprawl.” The investment in dollars and resources to get to net-zero are significant and might be better spent on more cost-effective energy saving options, such as a more efficient building envelope or creating a district energy system that can serve an entire campus.
For all those reasons and more, some argue that while both have an important role to play, it’s more useful and important to work toward net-zero energy communities rather than net-zero buildings. Individual high-performing buildings don’t mean so much if the neighborhood as a whole is wasteful, while if an entire community is net-zero, that’s meaningful even if the individual buildings within it are not.
The Problem with Net-Zero Buildings
Net-zero energy is an ambitious goal for any building—one that can’t be achieved without scrupulous attention to every aspect of a building’s design, construction, and operation. Like the related goal of creating a carbon-neutral building, any net-zero building has to first achieve significant load reductions and system efficiencies, and then meet the remaining loads with onsite energy generation.
In some ways, net-zero is a tougher goal than carbon-neutral: By most definitions, a project could become carbon-neutral using biofuels from off-site—that’s not as widely recognized a solution for achieving net-zero. On the other hand, carbon calculations often account for the energy and carbon expended to create a building—its embodied carbon—which is not usually the case with net-zero energy.
Most buildings that generate their own energy do it with solar photovoltaics (PV). If we assume that a building has only its roof area available for mounting PV, then a single-story building is much more likely to achieve net-zero than a high-rise.
A 2007 report from the National Renewable Energy Lab (NREL) and the U.S. Department of Energy (DOE) analyzes the potential for buildings in the U.S. to achieve net-zero energy using energy technologies—both for energy efficiency and PV—that they expect will be widely available in 2025. The graph below shows how their predictions play out based on the number of stories. Getting to net-zero is extremely difficult for buildings of more than four stories. If the project includes energy-intensive data centers, labs, or other spaces, the challenge gets tougher.
Other impacts ignored—or worse
Even if cost is not an obstacle and the building has a low profile, getting to net-zero means that the solar panels can’t be shaded by trees or adjacent structures. That means that sprawling suburban homes are much more likely to achieve the net-zero goal than dense urban townhouses or apartments, and suburban office parks have a leg up on central business districts. This is a problem we’ve seen before, according to architect Muscoe Martin, AIA, of M2Architects: “In the ’80s a few solar architecture pioneers like Doug Kelbaugh and Peter Calthorpe noted that most of the highest–performing solar buildings were in rural or suburban locations, ignoring transportation and infrastructure energy use.” Martin notes, “Single-scale problem-solving leads to solutions that don’t always make sense.”
Transportation isn’t the only impact that is often (but not always) ignored when focusing on the building. The energy and environmental costs of supplying potable water and treating wastewater, providing energy and data infrastructure, and creating roads and transportation systems are all beyond the scope of most buildings, yet necessary to support their functions. The Living Building Challenge, created by the Cascadia Green Building Council and now managed by the International Living Future Institute, is the rare exception that accounts for these impacts by stipulating zero municipal water consumption and wastewater discharge, and requiring that all buildings be located on previously developed sites.
Version 2 of the Living Building Challenge, released in 2009, goes even further in this broader focus by requiring a focus on design for “car-free living.” It also anticipates the importance of reaching beyond the individual building with the concept of “scale jumping,” which allows project teams to link buildings together to achieve the requirements as a group.
Neighborhoods and communities, being larger than individual buildings, can support many technologies for low-impact heating, cooling, and electricity generation better and more cost-effectively. To some extent, this is simply a matter of scale: combined heat and power systems—especially those using biomass—are more efficient in larger sizes. Perhaps more important, they can support dedicated operations and maintenance staff to keep them working properly. The same is true for community-sized boilers, chillers, and many other high-tech solutions.
Many colleges and universities already have centralized energy infrastructure, as well as an entire community under common ownership, making them natural candidates for becoming net-zero. “University campuses are going to be the early adopters in this game,” says Eric Ridenour of SERA Architects in Portland, Oregon, citing long-term ownership of their buildings and access to patient capital (as opposed to investment funds seeking a quick return).
One example of a community-scale cooling system that would not have been feasible for an individual building is Cornell University’s deep-water cooling system in Ithaca, New York, using water from Cayuga Lake to replace three existing chiller plants. Reaching two miles out and 250 feet into the lake for cold water, this system cost about $60 million, some of which would have been spent anyway replacing chillers and CFC refrigerants. It saves about 25 million kWh annually, reducing Cornell’s total electricity bill by 10 percent.
More cost-effective energy generation
In a report published in 2009, Srinivas Katipamula and others at Pacific Northwest National Lab (PNNL) compared five different scenarios for achieving net-zero energy use for a mixed-use community of about 16,000 people. They assumed that all buildings in the community would be, on average, 70% more efficient than typical U.S. buildings. They then compared the cost of each of the following options for both Chicago and Phoenix:
• PV on each building;
• PV in an adjacent solar farm;
• concentrating solar thermal electricity;
• wind turbines on leased land;
• and wind turbines on purchased land.
They were surprised to find that, on a cost-per-kWh basis, wind on leased land was the winner even in Phoenix, which has abundant sunlight and a relatively poor wind resource. PV on individual buildings was the second-to-last choice in both cities, beating only solar thermal electricity in Chicago, and wind on purchased land in Phoenix (see graph).
Communities also have a mix of occupancies and uses, which can support more efficient use of infrastructure and cascading uses of energy. Offices use most of their energy by day and can go dark at night, while for residences it’s just the opposite. That means that a single heating or cooling plant serving both can be not much bigger than a plant serving just one of them. It is also sometimes possible to share energy—using waste heat from data centers, for example, to heat water for use in apartments. Looking into the benefits of this diversification for neighborhoods in Portland, Oregon and Seattle, the architecture firm Mithun found the ideal mix of residential to commercial uses to be 75% to 25%, according to president Bert Gregory, FAIA.
Mithun and engineering firm Arup are now designing new mixed-use facilities for a state office campus in Baltimore, Maryland, and exploring potential synergies. One discovery they made is that they can get many of the benefits of a central heating and cooling plant with equipment distributed in the basements of several buildings by linking the equipment together into a “virtual central plant.” That solution gives them the benefit of modular equipment, so each device can come online as needed and operate at optimum capacity. More important, it solved a phasing problem by removing the need to invest in a large central plant before all the demand was in place.
Criterion Planners of Portland, Oregon, has estimated that a full 25% of the building energy savings in a prototype LEED for Neighborhood Development community can come from district systems, including both plant efficiency and peak load diversity. Another 10%, in their model, comes from the increased density.
A more inclusive scope
A net-zero-energy community is not simply a collection of buildings that, taken together, achieve the goal of net-zero-energy. A community includes loads and energy uses that are not often included in the equation for individual buildings, such as wastewater treatment and other community infrastructure, not to mention designing to support low-impact transportation options. These represent opportunities for environmental gain that extend well beyond energy efficiency, including water conservation, rainwater infiltration to reduce runoff, the social benefit of reduced car dependence, and even urban agriculture for a local food economy.
All of these infrastructure and planning-scale issues directly or indirectly involve energy use: wastewater and stormwater treatment have energy costs for the city; transportation and transporting food both require fuel for vehicles. They are also tied into quality-of-life issues that are best addressed at the neighborhood scale—by creating spaces that make dense, urban living appealing, for example.
The Irvine, California, consulting firm CTG Energetics has created a sophisticated community-impacts calculator that aggregates data from a range of sources, including energy models of typical buildings in the community, transportation models, and water-related energy use, to quantify the impacts of community design choices. By addressing infrastructure in addition to individual buildings, and resources beyond energy, it gives project teams a framework for considering tradeoffs and avoiding the pitfall of too narrow a goal.
For an individual building project, site constraints and preexisting street grids may make it impossible to orient the building for optimal daylighting and passive solar heating or cooling. “As much as 50% of the heating and cooling energy can be saved by going from the worst orientation to the best orientation, and street layout has a tremendous impact on the orientation of buildings,” said Norbert Lechner, architect and professor emeritus at Auburn University.
When designing at the community scale, those considerations can be addressed in ways that help make the individual buildings more efficient at minimal cost. The best example of this is Village Homes in Davis, California, where all 200 houses are on east-west streets even though the parcel of land is mostly north-south, according to Lechner. “Not only does every house save energy by receiving mostly winter sun and little summer sun but the cooling systems are also smaller and less expensive,” he said. Village Homes was built in the 1970s with a focus on solar design of individual homes and innovative stormwater management, but it doesn’t appeal to transportation-minded planners today, who see it as yet another example of a car-dependent, suburban neighborhood.
Challenges at the Community Scale
Despite all the advantages, there are many reasons why community-scale approaches are not more common.
Large, developer-led, mixed-use projects such as Dockside Green in Victoria, British Columbia, or London’s Beddington Zero-Energy Development (BedZED) consolidate the ownership and management into one entity, so that they can coordinate design, construction, and management of an integrated project. There is even greater potential in revitalizing existing urban neighborhoods, but doing that introduces many new challenges.
Mithun has explored several projects that could become urban ecodistricts, linking together many different buildings and uses, owned by separate entities, into a coordinated network. In the process, they’ve discovered a range of challenges. Who owns the infrastructure that’s needed to share utilities? How do you create governance structures that encourage private investment while coordinating it in a way that optimizes community-scale design? How do you get permission to connect buildings across public streets? And what do you do about utility regulations that make it difficult to create small, neighborhood-scale utility companies? Without a single owner to be accountable and responsible for these systems, all kinds of legal and commercial problems emerge.
Most large, mixed-use projects are created in phases rather than all at once. That makes it hard to invest early in efficient, centralized infrastructure such as HVAC systems, transportation networks, and ecological wastewater solutions. But if those systems are not in place early on, then individual buildings and units have to build separate systems to meet those needs, creating obstacles to doing centralized systems later. Financing those investments before there is a critical mass of occupants to use them is a key challenge for many large projects.
The more sophisticated systems that are possible in a community-scale project require dedicated management, so there has to be an organizational infrastructure in place to support the physical infrastructure. This problem gets even worse for communities that are pursuing the goal of net-zero energy, because they tend to seek out the most advanced technologies available rather than the tried and true. At BedZED, the combined heat and power system using biomass never worked as advertised and was replaced by gas boilers after a couple of difficult years for residents, who endured winters with limited heating. The onsite wastewater treatment system also failed repeatedly.
Overreaching for unreliable technologies is one problem, but any large, mixed-use project lends itself to complicated systems and potentially unwieldy design and construction, warns Mithun’s Sandy Mendler, AIA. “Added size sometimes creates complexity that pushes against the efficiencies of scale, and there is an extra coordination effort when a large team is involved.” That complexity gets even worse when there are multiple owners and regulatory agencies involved.
Letting buildings off the hook
Clark Brockman, AIA, of SERA, acknowledges that net-zero individual buildings may not be the right goal: “I don’t believe that we’re ever going to get to a time when all the buildings are net-zero,” he says. But he does believe that every designer should have the experience of creating a net-zero building, because until they’ve done that they won’t understand what it means to design to absolute, physical limits. “It will change the way you work,” Brockman says.
Any model for net-zero communities assumes that the buildings themselves, while not necessarily net-zero, are extremely energy efficient. A risk of focusing on the community is that the efficiency of individual buildings will get short shrift. This fear is corroborated by conversations Mendler has heard among the team working at the University of California’s new green campus in Merced, suggesting that they needn’t push the efficiency of individual building envelopes as much because the central plant is so efficient. This dynamic can occur any time a centralized solution is perceived to “solve the problem” for the project.
Every net-zero-energy project depends on the full participation of its occupants to get that way. “In the case of individual buildings, the onus is on the owner or occupants to balance consumption with generation,” notes University of Florida’s Charles Kibert, Ph.D. “I don’t see this as achievable at larger scale because we are back to the ‘commons’ again—there is no vested interest for all the individual recipients of energy to limit their consumption,” he adds.
The project team for the Oregon Sustainability Center—a 200,000 ft2 (18,600 m2) multi-tenant project pursuing Living Building certification in Portland, discovered just how critical the occupants would be when they broke out the energy loads in their net-zero-energy model and found that occupant loads accounted for a full 50% of the energy use. “That changed the whole game,” says Brockman. “Now we have an entire section of our energy model that is devoted to occupant behavior,” he adds, noting that in addition to predicting usage schedules and thermostat set points more precisely, they also have to codify those assumptions for the tenant guidelines.
Brockman isn’t daunted by the need to expand that level of occupant engagement to the community scale, however. He envisions a “fractal dashboard” model, with outputs at various scales displaying actual performance in real time at different levels of detail, from the community all the way down to the individual occupant. Net-zero communities won’t be achieved without effectively driving down energy use in each individual building—it’s just that not every build ing will get to net-zero.
What’s the Right Scale?
“The right scale is the scale that you have before you,” says Muscoe Martin, arguing that it’s better to do what you can within the scope you’re given than to do nothing at all. At the same time, he notes, it’s useful to think in terms of “nested scales” and consider the impact of your project on the products and systems within it, and on the systems in which it fits. Every project should help make its neighborhood more pedestrian-friendly, for example, even if it can’t redesign the entire streetscape.
Net-zero is a compelling goal, both for individual buildings and for communities. Like any generic goal, however, it has to be applied wisely. “There is no question that you have to focus on every individual building because you want to drive down the loads as much as possible,” says Malcolm Lewis, president of CTG Energetics. “The trap is thinking that you have to make each building completely self-sufficient, because in doing that you sacrifice opportunities.” Some buildings won’t get there—so how do you set equally compelling targets that those buildings can achieve? Other buildings might get there, but with unfortunate side effects, such as increasing car-dependence. In those cases it’s important to expand the scope of the challenge to include transportation impacts and other community level impacts, and optimize the design based on a wider view of the goal.
It’s not all about technology and design, however. No matter how cleverly they are built, we won’t have net-zero buildings or communities unless we change the way that we live and work in them. That’s a challenge that we’re all facing together.
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- Describe several unintended consequences of focusing on net-zero-energy buildings.
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- Describe several strategies used in the net-zero-energy projects.
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