Energy Modeling For Sustainability

Evolving software for modeling energy dynamics makes it easier for architects and engineers to design green buildings
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From GreenSource
B.J. Novitski

Sculpting Air and Light

A third case study demonstrates the interrelatedness of architectural factors that affect energy consumption. Washington, D.C.-based Bowie Gridley Architects were asked by the Howard Hughes Medical Institute to add 115,000 sf to the historic Hayes Manor, in Chevy Chase Maryland, to create a headquarters for the institution. From the beginning, they committed to collaborating with engineers from the Syska Hennessy Group to ensure the addition would be built sustainably. The project is now in construction and expected to earn LEED Gold.

The engineers began by conducting an intense energy audit of the existing building, installing meters and studying occupant behavior. They simulated the envelope of the proposed addition and ran a series of studies of lighting, daylighting, and the interconnection of new and existing chiller plants to take advantage of the latest utility rate structures.

They used Ecotect for preliminary climate analysis and basic massing studies, then continued the simulation with eQUEST. From site and weather data, they determined daylight availability and used Radiance to visualize the distribution of light in a typical perimeter office. Syska's Shreshth Nagpal, based in New York, explores ways to minimize discomfort glare without interrupting views and overall illumination from daylight. He simulated several daylighting and glare-control options, while optimizing internal surface reflectance values to achieve the desired visual comfort conditions. With the option selected, daylight enters through the upper glazing, reflects from the top surface of a lightshelf, and is distributed throughout the room, while views are maintained through the lower glazing. An external sunshade blocks much of the direct sunlight coming through the view window, but in some cases the external shade is ineffective, such as in the late afternoon in a west-oriented office. "But even if you pull the blinds against glare, the daylight levels from above are still effective," Nagpal says.

 

Engineers working on an addition to a facility at the Howard Hughes Medical Institute (top) began with a minimally ASHRAE-compliant building (pie chart, middle) and examined various energy-conservation options (graph, bottom).

Images courtesy Syska Hennessy Group

 

The simulation grew more interesting when Nagpal tested the lightshelf and room geometry in a computational fluid dynamics (CFD) analysis in IES . For various reasons, the preferred option of under-floor air distribution was not feasible, and there was concern that the lightshelf would impede airflow from the ceiling diffusers. Indeed, the first CFD visualization showed a pocket of cold air over the lightshelf and excessive heat buildup in areas subject to internal gains: The view window, occupant, and desktop computer. "Our concern was to effectively improve air distribution while not compromising on the lightshelf or daylight quality," he explains. "With the solar angles in mind, we sized a slot along the perimeter for a six-inch gap between the window and the lightshelf." They chose special jet-air diffusers to create a laminar airflow along the internal surfaces. Airflow through the slot reduces the cold-air pocket above the shelf, improving distribution. It would have been more challenging, if not impossible, to come to this seemingly simple result without the feedback provided by the light and airflow simulations.

Is all this extra design effort worth it? David Callan, PE, Syska's national director of sustainable design and high-performance building technology, concludes on an encouraging note: "It doesn't necessarily cost more," he asserts. "Sometimes it costs a lot less. If you take the long view on building performance, you see that most of the money you spend over the lifespan of a building isn't in up-front costs. About 85 percent of your costs are in people. So if you can make a design choice that allows the people inside your building to be delighted and energized by their environment, more comfortable and therefore more productive, you're going to see a big payoff."

 

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Originally published in GreenSource
Originally published in April 2008

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