Spreadsheet Sustainability

Visionaries may talk up instant output and 3-D digital models for complex projects, but the bulk of analysis begins and ends with by-the-numbers spreadsheets.
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From GreenSource
Russell Fortmeyer

"All water and waste water had to be processed on-site, so we used the model to capture all of the related interactions from reducing the scale of the waste stream of the water," says Meacham. "It not only reduces the scale and site impact of the waste treatment, but also the energy used to treat that water." Since all site energy was generated from imported oil transported on barges, there were both direct and indirect correlations between water and fuel consumption. "If we reduced the amount of water consumed on-site, we found that would reduce energy consumption by a factor of three, once we accounted for all of the processing and purification that went into providing the water," he says.

These kinds of synergies occur in sustainable design practices routinely, but they can take weeks or months of analysis to fully appreciate when calculated separately. Typically, site-water-balance calculations, transport analyses, and energy modeling are discrete exercises, often performed by different consultants. A combined multi-criteria model may not speed this up excessively, but it makes connections more apparent and flexible, enabling designers to consider multiple solutions to this balancing act without bogging down decisions. "It's a highly quantitative tool, but there are many kinds of qualitative factors that fall on top of it," says Meacham, adding that aesthetic considerations or market studies may overrule a life cycle assessment that concluded a specific system was best.

Meacham thinks flexibility explains why commercial software developers have yet to fully address this market, although he acknowledges there are products that assist in larger planning projects that take a more top-down approach rather than the bottom-up approach espoused by CTG. For Meacham, the benefits of spreadsheets not only include flexibility, where new databases and systems can be integrated into an existing model on the fly, but also data reliability, since equations can be readily tracked for accuracy.

Shifting Between Programs

But spreadsheets should not be confused with more complicated environmental simulation engines, such as the Department of Energy's stalwart, EnergyPlus, for calculating a building's annual energy consumption. CTG uses these programs, feeding them into their spreadsheet models. There are many such software engines available, depending on a project's focus. Engineers at Buro Happold's Los Angeles office combined four software packages as part of their multi-criteria analysis of the natural ventilation strategy for the Transbay Terminal project in San Francisco. Designed by Pelli Clarke Pelli Architects, the project features a 5.4-acre rooftop garden over 600,000 square feet of bus and rail platforms and is in construction until 2017.

To argue for extending skylights in the Transbay Terminal, Buro Happold used radiance daylight modeling (1, 2).

Images: Top, courtesy Transbay Joint Powers Authority; 1, 2 courtesy Buro Happold


The goal for Alper Erten and his mechanical engineering colleagues at Buro Happold, was to convince the client and architect that an energy-efficient natural ventilation strategy would maintain acceptable comfort conditions for the project's Grand Hall. On the ground floor of the terminal, it features a glazed exterior wall and a large central skylight that penetrates the roof garden. Naturally ventilating this space, as opposed to providing a dedicated mechanical cooling and heating system, could significantly decrease the project's annual energy demand, but it relies on considering a complex interplay of environmental and programmatic concerns.

Happold’s engineers began by importing the architect's three-dimensional geometry model, which existed in a Rhino format, into the Autodesk Ecotect environmental software, which they then exported into Radiance, the industry's de facto daylighting engine. With Radiance, they could analyze the configuration of skylights and advise on appropriate glazing types for daylighting. They also developed a thermal model using Integrated Environmental Solutions Virtual Environment (IESVE), a proprietary software package in wide use in engineering. IESVE has a Radiance interface that allowed them to consider the thermal advantages of increased daylighting through the use of daylight-dimming systems. "Software like Ecotect is great if you have a simple, boxy office," says Orla Williams, one of Erten’s colleagues at Happold. "But with the complicated geometry of Transbay, we needed to be more accurate and have more confidence in our results." While software like Rhino has allowed architecture to become more complex and blobby, environmental modeling software remains geometrically challenged. Ecotect works well for simple projects or in early phases of design where its capability includes solar, daylighting, and overshadowing analysis. But a natural ventilation strategy depends on more than knowing internal and external heat gains. "We then used IESVE's MacroFlo model to simulate the buoyancy flows between inner zones by measuring temperature and pressure," Erten says. "We can also calculate the flows in external vents to understand differences between openings." They then exported the resultant surface temperatures from IESVE into a software called FloVENT, which is a computational fluid dynamic (CFD) analysis package.

Where MacroFlo is a bulk air flow analysis that demonstrates how air moves between spaces to show that natural ventilation is possible, FloVENT's CFD capability provides a more nuanced and rigorous analysis of the airflow rates, supply and extract locations, and how air interacts with other components in the space. CFD models can indicate air velocities and highlight dead spots where the age of air may exceed a target value, which can lead to uncomfortable conditions for occupants. Finally, once they generated hourly performance data in IESVE for the building's mechanical system, which included a large central plant with chillers, they could then export this data to another software called TRNSYS, short for Transient Energy System Simulation Tool. Overseen by a consortium of universities, led by the University of Wisconsin in Madison, TRNSYS is a component-based energy-modeling software that is particularly suited for simulating alternative equipment in more complicated central plant systems, but it's not that adept at modeling complex architectural geometries. Simulink/Matlab is another component-based proprietary software package used by engineers to model complex building-controls systems, which makes sense given the software's origins as a tool for automotive designers.

For Transbay, the engineers used TRNSYS to analyze the project's proposed geothermal heat pump system, using the IESVE data already embedded with the simulations of the architecture and mechanical system. "With the TRNSYS model, we were interested in comparing energy use and savings, as well as water use and savings in comparison to a cooling tower-based system," says Erten. "To balance heating and cooling, we also needed to look at ground temperatures over 10 years." With a geothermal heat pump, incorrect sizing could cause ground temperatures to reach unviable levels, leading to an inefficient mechanical system and the need for a costly retrofit. Buro Happold's engineers used the TRNSYS model to test parameters like borehole depths and separation sizes, too, finding that their results were approximate to the final design provided by the specialist geothermal consultant. Williams says such findings give them more confidence that their simulation processes are accurate for early design advice.


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Originally published in GreenSource
Originally published in May 2011