With Energy Modeling, Virtual Models Lead to Real Sustainability
The greenest of the green come with more than a rating and a label, they come with energy modeling
Summary: Energy modeling consists of comparing how a proposed building design performs in terms of energy efficiency to a benchmark derived from real or virtual statistics on how energy efficient a comparable conventional building is. This performance measuring tool allows architects and engineers to track how every proposed change to their new design will affect their overall sustainability and energy consumption goals. EnergyStar®, Green Globes™, and LEED® all incorporate energy modeling into their ratings systems differently.
How do you ... take advantage of energy modeling and how it works with sustainability rating systems to design sustainable buildings?
If anything, the contemporary profusion of sustainable building rating systems have branded their ratings too well. Whether it’s the U.S. Green Building Council’s LEED, the Green Building Initiative’s Green Globes systems, or the Environmental Protection Agency’s EnergyStar—anyone can say that a Platinum rating is better than Silver, and that four Green Globes are better than one, but what does that really mean? How much better?
In terms of building performance, the answer to that question requires energy modeling—using collected data or computer models to predict how a proposed new design performs in terms of energy efficiency as compared to a standardized benchmark. And if you can answer this question, you can design a greener building. Energy modeling is a fundamental tool for delving beyond prescriptive sustainability building practices that can lead to little more than “greenwashing.” By benchmarking how a new design will reduce energy consumption over a more wasteful conventional building of the same type, architects gain knowledge and control over how their systems and design features fit into the project’s overall sustainability goals.
Energy modeling used to be the domain of engineers, but architects are finding that using it as early in the design process as possible allows for the most holistic integration of sustainability systems and best practices in their designs—and, of course, it helps to pick up that coveted rating label, too.
The best energy modeling methodologies contain two fundamental groups of data that are compared:
- An energy benchmark. This is what the proposed design will be compared to. It can consist of energy performance data collected from actual buildings, or from a fictitious conventional building that only exist as a computer model that is optimized to be as similar as possible to a proposed new design, sans sustainability features and practices.
- The proposed design. This consists of energy performance data from models of the new design. Comparing the data to a benchmark will tell the designer how much energy is being saved.
The Environmental Protection Agency’s (EPA) EnergyStar rating system incorporates this first set of benchmark data, but not the second. A government-sponsored system introduced in 1992, EnergyStar is designed to be easy to use, convenient, and economical so as to be an appropriate option for small firms and builders, although it can be applied to commercial, industrial, and retail properties as well as home appliances.
For commercial buildings, EnergyStar uses a simple, Web-based tool called Target Finder to rate the energy efficiency of a building
For commercial buildings, EnergyStar uses a simple, Web-based tool called Target Finder to rate the energy efficiency of a building. Target Finder has users input a building’s type, size, number of occupants, location, energy costs, and total energy use. It then generates a score that ranges from1 (least efficient) to 100 (most efficient). A score of 75 or greater earns the EnergyStar rating. This score is based on a Department of Energy study of hundreds of actual buildings across the nation called the Commercial Buildings Energy Consumption Survey (CBECS), and the EnergyStar rating essentially tells designers how energy efficient a building is, as compared to the applicable building types in the nation’s building stock as represented by CBECS. The idea here is that as EnergyStar is continually adopted, the building stock will become more efficient, and the bar for energy efficiency will be raised, according to David Bradley, a partner at Thermal Energy Systems Specialists (TESS), an energy modeling consulting firm in Madison, Wis. EnergyStar also allows architects and engineers to set goals for improved energy efficiency.
Without having the energy use and costs of a new design, the CBECS data in EnergyStar are primarily for establishing energy performance benchmarks. Users will have to go elsewhere (to modeling software or an outside consultant) for comprehensive modeling information about a new design.
“This tool was designed to take the things that you’re doing analysis on and see where it falls on the continuum,” says Karen Butler, EPA EnergyStar project manager for commercial building design.
She says EnergyStar’s strength is its ability to set efficiency goals quickly and easily that are grounded in the existing building stock. “Our metrics are big picture,” she says. “Right now, there are a billion things out there that let you get numbers, get energy, get better-than-code, but it doesn’t tell you in the grand scheme of things—compared to what?”
In 2004, the sustainability nonprofit Green Building Initiative (GBI) acquired the Green Globes system, which began as an environmental rating system developed in Canada. It assigns sustainability ratings along a 1,000 point checklist. One Green Globe is the lowest score, four Green Globes is the highest score. Categories the system evaluates are energy, indoor environment, emissions and effluents, resources, environmental management, and water. The energy category is weighted the heaviest, though some of these weightings will change soon. Green Globes users get their rating by filling out a Web-based list of 150 questions about their design, which Vicki Worden, the GBI’s vice president of commercial programs and business development, likens to a “Turbo Tax-like system.” This tool is designed to be easy to use and approachable, in the hopes of attracting architects and builders who are seeking an alternative to more unwieldy systems.
Green Globes users get their rating by filling out a Web-based list of 150 questions about their design
“We feel we can contribute the most by helping to bridge the gap between the early adopters of green buildings and those who are coming into the marketplace and want to apply green practices and green strategies to every building,” says Worden.
The same CBECS benchmarking data that EnergyStar uses are built into the Green Globes online questionnaire through the use of Energy Star’s Target Finder program. These baseline energy performance data can then be compared to results from the Green Globes Life Cycle Assessment (LCA) Credit Calculator. It analyzes the proposed design’s energy performance characteristics over time, though it and the CBECS-derived benchmark data are not directly integrated. Also built into the online questionnaire, the LCA predicts the performance of hundreds of building assemblies and takes into account many other variables that affect a building’s performance: materials, embodied energy, solid waste, air and water pollution, and global warming potential.
Green Globes was praised by a 2006 University of Minnesota study for featuring a distinct rating criterion for life cycle performance, which LEED does not yet have, though the study did say that both rating systems need to address the issue of life cycle assessment more robustly. Eighteen states have recognized the Green Globes rating system in sustainable building legislation and regulation.
The U.S. Green Building Council (USGBC) launched LEED in 1998, and since then it’s become the most widely used sustainable building rating system. Like Green Globes, it’s formatted around a checklist of sustainable features and practices. There are 69 points available on the LEED (Leadership in Energy and Environmental Design) scale, which are divided into four certification levels: Standard, Silver, Gold, and Platinum. LEED evaluates six environmental performance categories: sustainable sites, water efficiency, materials and resources, indoor environmental quality, energy and atmosphere, and innovation and design process. Energy and atmosphere is weighted as the heaviest, and energy alone is worth 10 LEED points, though this number will likely increase even more when LEED revamps its criteria next year. It’s also where energy modeling is applied to the rating system.
LEED uses a different benchmark model than EnergyStar and Green Globes. It doesn’t use modeling standards of its own, and instead refers users to models used by various industries representing the appropriate building type
LEED uses a different benchmark model than EnergyStar and Green Globes. It doesn’t use modeling standards of its own, and instead refers users to models used by various industries representing the appropriate building type. LEED for New Construction uses the ASHRAE 90.1 model, though its housing guidelines are derived from more prescriptive guidelines, according to Bradley, the energy modeling consultant. Instead of using data from the existing building stock like EnergyStar and Green Globes to form a conventional building benchmark, ASHRAE 90.1 uses fictionally averaged computer modeled buildings that set minimum energy performance baselines. These computer-generated models span different building types, sizes, locations, and climate zones and reflect the same kinds of systems, materials, and building assemblies that will be used in the proposed design whenever possible.
To get the most accurate picture of how much energy their building will save, architects have to run their own energy performance models on their proposed designs, compare this to the ASHRAE models, then bring them both back to the USGBC so they can assign them the appropriate rating level. “It basically tells you how to set up your energy model so that you can determine the energy performance of your building based on the materials, the orientation, [and] the mechanical systems,” says Cory Enck, the USGBC’s manager of LEED certification.
In reality and practice
Neither the actual building nor the virtual building benchmarking method is perfect. Irregular data from extremely efficient or inefficient buildings can skew building stock-derived systems like CBECS. Comparing a proposed design to a building that will never be built and only exists in the absolute, most controlled of environments presents its own discrepancies. The only way to get completely definitive energy modeling comparisons of how much more sustainable one building is than another is to “build a conventional building, and then build your proposed building right next to it and compare their metering,” says Bradley. “Of course, nobody ever does that.”
But architects can measure the performance of a proposed design after it’s built. For example, Morphosis, the firm of Thom Mayne, FAIA, has been steadily ingesting performance and user operation data from its San Francisco Federal Building, completed in 2007. This kind of research will likely guide the next generation of energy modeling standards and sustainability rating systems, as it can give architects actionable knowledge about how their sustainability systems and practices work once they’ve moved beyond a computer screen.
Kubala Washatko Architects in Cedarburg, Wis., hired Bradley’s firm TESS to create energy models of their design for the Aldo Leopold Legacy Center. The Legacy Center won a 2008 AIA Committee on the Environment Top Ten Green Award and qualified for LEED Platinum and beyond. It’s completely carbon neutral. Located in Baraboo, Wis., the 13,000-square-foot complex is a research and conservation institution that furthers the work of naturalist Aldo Leopold, author of “The Land Ethic,” a mid-20th century clarion call for a collaborative and conservationist relationship with the natural world.
The buildings that make up the Aldo Leopold campus are made out of wood harvested directly from the site, cut with ample windows for daylighting. The roofs of the low-slung stone and wood buildings are covered in photovoltaic solar panels, and it’s heated and cooled using a geothermal radiant slab floor system. A similar geothermal air tube system delivers heated and cooled ventilation air that’s been circulated through the earth. Despite the tight integration of all these active sustainability strategies, project manager Joel Krueger says the soul of the building’s sustainability mandate is passive. “We made a building that, in a sense, referenced the way buildings were built in history before we had heating and cooling, so it worked well just because of the bones of the building.”
Kruger says TESS’s energy modeling services allowed his firm to do accurate cost-benefit comparisons of sustainability features and practices so that they could understand, for example, how much altering the thickness of a panel of insulation might save in heating bills. In a broader context, energy modeling allowed Kubala Washatko to alter their sustainability techniques and practices while still staying locked into their overall energy consumption goals. Finally, energy performance modeling allowed Kruger and his team to model, test, and ultimately adopt energy-saving systems they were otherwise unfamiliar with, like the air tube ventilation system. “I’ve never done that before, but the model was terrific in informing us that it was worth it.”