Feature Article

Energy Modeling For Sustainability

Chief among factors determining the sustainable performance of a building are the amount of energy it consumes and how much of that energy is from renewable sources. Architects have long relied on engineers to perform the complex energy consumption estimations and select mechanical systems. This reliance is now transforming into collaboration as engineers are being invited into early design phases to help the team with decisions about orientation and massing. Three case studies show how the engineers’ work with energy simulation software informs those decisions. For many years, the U.S. Department of Energy (DOE) funded the development of the software now known as DOE-2. It analyzes the reaction of a building’s skin and geometry in response to internal loads and external climate conditions for each of the 8,760 hours in a year. These loads include solar heat gains; heat gains from occupants, electric lights, and equipment; and heat gained or lost through infiltration or by conduction through the walls, roof, and glazing. DOE-2 also models mechanical system performance and the local utility rate structure. The Department of Energy is now promoting the next-generation EnergyPlus. Like its predecessor DOE-2, EnergyPlus depends on third-party software to provide a user-friendly graphical interface.

Simulation software supplies a nuanced view of a building’s dynamic thermal performance that enables engineers to select and size equipment that is smarter, smaller, and often less expensive than the norm. For instance, cold but clear winter days may enjoy abundant solar radiation. Peak cooling loads may occur at a time of maximum daylight, which can replace some or all of the electric lighting. Economizers can use relatively cool outdoor air in lieu of some mechanical cooling. Looking at all these factors and their complex interactions is known as “whole-building simulation.” Optimizing building performance can often result in significant construction cost savings due to smaller systems and mechanical rooms.

In the design of Stopwaste.org, Rumsey Engineers used eQUEST energy modeling to simulate envelope insulation, glazing, electric lighting, and mechanical systems.

Photo: Rumsey Engineers

Chief among factors determining the sustainable performance of a building are the amount of energy it consumes and how much of that energy is from renewable sources. Architects have long relied on engineers to perform the complex energy consumption estimations and select mechanical systems. This reliance is now transforming into collaboration as engineers are being invited into early design phases to help the team with decisions about orientation and massing. Three case studies show how the engineers’ work with energy simulation software informs those decisions. For many years, the U.S. Department of Energy (DOE) funded the development of the software now known as DOE-2. It analyzes the reaction of a building’s skin and geometry in response to internal loads and external climate conditions for each of the 8,760 hours in a year. These loads include solar heat gains; heat gains from occupants, electric lights, and equipment; and heat gained or lost through infiltration or by conduction through the walls, roof, and glazing. DOE-2 also models mechanical system performance and the local utility rate structure. The Department of Energy is now promoting the next-generation EnergyPlus. Like its predecessor DOE-2, EnergyPlus depends on third-party software to provide a user-friendly graphical interface.

Simulation software supplies a nuanced view of a building’s dynamic thermal performance that enables engineers to select and size equipment that is smarter, smaller, and often less expensive than the norm. For instance, cold but clear winter days may enjoy abundant solar radiation. Peak cooling loads may occur at a time of maximum daylight, which can replace some or all of the electric lighting. Economizers can use relatively cool outdoor air in lieu of some mechanical cooling. Looking at all these factors and their complex interactions is known as “whole-building simulation.” Optimizing building performance can often result in significant construction cost savings due to smaller systems and mechanical rooms.

Because a complete simulation requires specifying thermal zones and mechanical systems, the venerable DOE-2 has proved daunting for architects to use, particularly early in design, when crucial decisions about form and orientation are made. To make these simulations more accessible, software developers have been creating “friendlier” user interfaces. Systems such as eQUEST (doe2.com/equest), also initiated by DOE, and EnergyPro, offered by the private sector, enable architects and engineers to take advantage of DOE-2’s analytic power during schematic design. They do so by offering intelligent defaults for as-yet-undecided design features. As the project develops, these defaults can be replaced by real design proposals, and the analysis becomes incrementally more precise. There is a price to pay, though, for simplicity. eQUEST, for instance, is limited in the complexity of geometry it can model. DOE-2 and EnergyPlus are not alone in their thoroughness and reliability. Trane Trace 700 and the British IES are comparable in calculating loads and sizing mechanical systems. IES goes further still to calculate carbon emissions and provide a computational fluid dynamic (CFD) analysis of airflow between thermal zones. Most energy modeling software supports energy-consumption comparisons among different scenarios to assist in design choices. However, no single software system does it all, and it’s common for design firms to draw on several to accomplish a complete simulation. This is particularly important for firms interested in still-out-of-mainstream technologies like photovoltaic cells, ground-source heat pumps, or natural ventilation in commercial buildings.

It’s important to note that no simulation is a perfect representation of reality, and any prediction relies on many assumptions about conditions that will affect the operations of the building, such as weather, occupancy rates, and facility management practices. Results from these programs are useful for evaluating design options, but their ability to predict actual energy use is limited and should be tempered by good sense, professional experience, and actual performance data.

Toward Better Modelers

Another reason analytical systems have been traditionally difficult to use is that their interfaces for describing architectural form are rudimentary and must be supplemented by numerical data describing the thermal characteristics of each element. This input can take days or weeks depending on the size of the building, its complexity, and the number of options to be tested. CAD systems, especially those capable of building information modeling (BIM), have been facilitating this task by linking the architects’ 3D model to the simulation programs.

According to the engineers cited here, that software development is advancing rapidly, especially for preliminary studies, but no system has achieved full CAD simulation integration yet. Revit is developing integration with IES , and ArchiCAD links to EnergyPlus, Ecotect, and others. The Bentley Systems Building suite of multidisciplinary applications ties into EnergyPlus, Trane Trace 700, IES , and, most recently, Hevacomp Several CAD systems have the capability of exporting data in the gbXML format, which can then be subjected to analysis at the Green Building Studio Web site. There, the available tools are DOE-2, eQUEST, EnergyPlus, and Trane Trace 700. Exporting data from CAD to analysis is only half the story, though. Ideally, the results of an analysis would tie directly back to the design. A completely integrated, all-in-one design/analysis modeler is still a thing of the future. Autodesk has recently announced plans to acquire Green Building Studio and Carmel Software, which performs HVAC calculations. Acquisitions like these promise improvements in the CAD/analysis interface.

Energy modeling differs from architectural modeling in that building performance, not appearance, is of critical concern. An architect might sketch a window to portray a certain visual character and access to views, but whether that window opens and how it is shaded are more important to the analysis than the shape of the aperture. The energy analysis needs little information about structural materials except to the extent that they may affect the thermal mass of the building. So, for instance, an architectural choice between wood and concrete should be made early in design. Often more important than wall, floor, and roof materials is the amount of insulation. Energy modeling much concerned with color except insofar as that interior finishes affect internal reflectances and thus daylight performance, while exterior finishes can affect solar gain. On the other hand, some interior finishes should be considered early on; a concrete floor intended to contribute to thermal performance shouldn’t be covered up due to late-stage decisions about carpeting. Because there is so much difference between an architectural model and an energy model, the engineers cited here typically build their own models from scratch. This practice will surely evolve over time.

Nevertheless, according to CAD specialist Calvin Clark, of Oakland, California-based Rumsey Engineers, current practices are already helping achieve the goal of sustainability. He says Green Building Studio offers architects an accessible method for “tweaking variables and running alternatives” early in design. “For preliminary concepts,” he notes, “they can see, right off the bat, whether they’ll save energy, and they can ‘pre-optimize.’ This software is helpful in getting architects to do these things before they even come to us.”

Remodeling to LEED Platinum

Rumsey Engineers recently worked on a major renovation of a vintage 1926 two-story, 14,000-sf building in downtown Oakland. The new headquarters for Stopwaste.org, an organization that coordinates source reduction and recycling programs in the county, earned LEED Platinum certification. Komorous-Towey Architects of Oakland implemented a number of design strategies. They reworked the east-facing main facade and broke open the formerly solid north wall to more than double the amount of glazing. Rumsey energy modeler Kajal Chatterjee used eQUEST to simulate envelope insulation, glazing, electric lighting, and mechanical systems. She began by modeling the existing building as a baseline against which to gauge various design improvements.

The original building was poorly insulated, so the designers considered how much insulation to add to the roof and walls. By incrementally testing various amounts of insulation, Chatterjee was able to demonstrate an optimum level. The software indicated a clear point of diminishing returns, where the energy-benefit curve ceased to climb steeply. The amount of rigid insulation varied due to roof slope but averaged 14 inches.

Because of efficient fixtures, photosensors that turn off unneeded lights, and increased daylighting, Chatterjee was able to document a drop in the lighting power density (LPD, or watts per square foot) from 1.11, on par with a minimally code-compliant building, down to 0.86. While the electricity consumed went down, the quality of light went up.

Chatterjee used Lawrence Berkeley National Laboratory’s program Window to obtain data for the overall glazing system. “Manufacturers usually just provide a U-value and solar heat-gain coefficient (SHGC) for glass,” Chatterjee explains. “We run that together with the frame type to get the overall U-value. Output from Window went into eQUEST.”

Then, based on solar-radiation and cloud-cover information from historical weather data for Oakland, eQUEST calculated the amount of daylight the building would receive and lowered the predicted lighting energy use accordingly. Chatterjee notes “There’s a lot of interaction between daylighting and electrical lighting, and it affects the loads and the mechanical system. It’s all connected.”

The choice of mechanical system was influenced by the super insulated shell. They chose a packaged rooftop variable air-volume (VAV) unit with a gas furnace and no reheat. “No reheat is unusual in California,” Chatterjee points out. “We put more insulation in the walls, so the perimeter areas wouldn’t need the reheat. That’s where we think the energy conservation lies.” The mechanical system was also less expensive than a conventional one would have been, and it supports natural ventilation when conditions permit. Calculations predict the building will outperform Title 24–2005, California’s already stringent energy code, by 40 percent.

Clean Mountain Air

A second case study involves a classroom building for Sierra College in Truckee, California, elevation 5,800 feet, characterized by snowy but sunny winters and cool, low-humidity summers. Sacramento architects Lionakis Beaumont Design Group worked with Vail, Colorado-based Beaudin Ganze Consulting Engineers (BGCE) to take full advantage of climate factors to reduce fossil-fuel consumption. They oriented the building to the south to maximize solar access for heat, light, and snow melt. In keeping with a ski-lodge vernacular, they used stone, wood, and exposed structure but stopped short of roof overhangs because of the structural load and hazards snow would cause. Now in construction, the building is expected to achieve LEED Silver.

BGCE senior associate Wesley Ploof, PE, reports on the firm’s energy modeling effort. He explains they chose EnergyPro as their primary software because its output directly documents Title 24 compliance. “For most of our work outside California, we prefer to use Trane Trace. It’s very effective all-around software—it documents LEED compliance in terms of comparisons to a minimally ASHRAE-compliant building.”

Ploof’s colleagues modeled a variety of wall insulation levels and glazing types before establishing an optimum combination that fit the budget. When they realized they wouldn’t be able to rely on overhangs for solar shading, they gave extra emphasis to high-quality, spectrally selective glass to keep unwanted solar heat gain down. They recommended smart controls to regulate temperature and lighting, room by room, hour by hour, according to daylight availability and number of occupants. Because the modeling looked at hourly interactions of external and internal thermal conditions, BGCE was able to recommend a mechanical system without refrigerants. The system instead relies on evaporative cooling, economizer cycles, and natural ventilation. As a result of all this moving air, the energy required for fans is actually higher here than in a comparable conventional building, while the overall mechanical system is far less energy consumptive.

These reduced operating costs will pay for the extra design effort over time because, unlike many commercial building owners, the institutional owner is also the tenant and will directly benefit from future savings.

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.

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.”

 

Published April 1, 2008