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Energy Model: Use To Compare, Not To Predict
OTHER PARTS OF THIS ARTICLEPt. 1: LEED Designs Don't Always Lead to Sustainable OperationsPt. 2: Controls, Commissioning Key to Realizing Green Building PotentialPt. 3: This PagePt. 4: The Curious Case of Northland Pines High School
But just how does one define that potential? Predicting potential energy use during design is a tricky proposition. Energy models are required for LEED certification, and critics of LEED point out that many LEED-certified buildings aren't as energy efficient as their energy models suggested that they would be. But that criticism is based on a misunderstanding about exactly what an energy model is supposed to reveal about the building.
"The energy model really informs the design team on making comparative decisions," says Steve Carlson, principal with CDH Energy. In other words, the purpose of an energy model is to compare the impact of different design options on energy performance — different window units, for example. "It shows the delta between choice A and choice B, but a lot of people think the energy model is supposed to predict reality," says Carlson.
As any experienced facility manager knows, as soon as the building is put into service, it's not unusual for many of the assumptions used in the energy model to fall by the wayside.
Just ask facility managers at Lincoln Heights Elementary in Spokane, Wash. The 56,000-square-foot school opened in the fall of 2006 and earned a LEED Gold certification in January 2008. The school was the subject of a Seattle Times article earlier this year alleging that the building, which paid $458,826 for green features (but received about $400,000 in rebates and saw a return on investment for the rest of 0.6 years), is underperforming because it uses more energy than expected. The criticism is unfair, say Greg Brown, director of capital projects, and Tim Wood, director of maintenance and operations, for Spokane Public Schools.
"(The writer) cherry-picked information that makes the school look bad," says Brown. The building was modeled to use 36.5 percent less energy than a standard ASHRAE 90.1 building. In the first year, it used 18 percent more than the model predicted. But after tuning the controls and setpoints, utility bills decreased by more than 6 percent, says Brown. Now, the school is the third best (out of 35 elementary schools) performer in the district on a total cost of energy per square foot basis ($0.82 per square foot compared with an average of $1.04).
The building still uses slightly more energy than the design model predicted, but the reason is not because it's a poorly performing building. The reason is that, because the building is the pride of the neighborhood, it's used for many more after-school activities than energy modelers had anticipated, and therefore uses more energy.
That seems to be indicative of the industry at large: "The largest divergences between energy models and actual performance that we've seen tend to be in buildings being used many more hours than anticipated in the modeling," says Mark Hanson, director of sustainable services for Hoffman, LLC.
Even so, Brown and Wood both confirm that the district is happy with the school. "Lincoln Heights is doing very well in terms of energy use," says Brown. "Those [energy-use] predictions are only as good as the assumptions they're based on."
Indeed, very few would dispute that the real measure of a building's efficiency is not how efficient it was on the drawing board, but how efficiently it operates over its life. "Both design and operations are important," says Hanson. "And falling down on either side will cause energy cost to go up." Operational efficiency, however, requires a never-finished process of tweaking and tuning, upgrading and downsizing. That's the only way a building's potential can be fully realized.
Room For Improvement
The poorer the energy performance when a building opens, the more opportunity there is to improve. Exhibit A: the Nathaniel R. Jones Federal Building and Courthouse located in Youngstown, Ohio. The building clocked in at the LEED Certified level in November 2002. The 50,000-square-foot building, which accrued only a single point in the Energy and Atmosphere category, had a staggeringly high energy use intensity of 102 kBtu/sq.ft./yr. in its first year. The reason, according to Deborah Vaughn, acting director, portfolio manager division, General Services Administration (GSA), is that six months before the building opened, the local utility cancelled plans to extend chilled water service to the site. So GSA had to construct its own chiller plant outside of the building. In the second year, after installing packaged air conditioning for the building's computer rooms to eliminate the need for cooling from the exterior chiller plant, the EUI dropped to 84. With some further tweaking and tuning, the building was down to 73.6 by the third year.
Vaughn says there have been periodic increases in consumption since then because of increased steam consumption. But after a recent lighting controls upgrade, which cut electric consumption by 7 percent, the building earned a 77 Energy Star score and stood at an EUI of 72.8. "We anticipate this number to continue to trend downward," says Vaughn.
— Greg Zimmerman
Energy Model: Use To Compare, Not To Predict