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From time to time, when a building owner knows that an energy model has been used in the design of a building, he or she may ask, "How much will my utility bills be?" This is certainly a legitimate concern for all building owners, as they may need to know how to set up billing arrangements for their tenants or be interested in negotiating long-term contracts with utility providers based on future use. Unfortunately, energy models are not good at predicting the actual energy use in buildings with a high level of accuracy, especially models constructed during the design phase of new construction projects.
Even the ASHRAE Standard 90.1 User's Manual expresses concern that the results of energy models will be misconstrued. It states that the modeling guidelines presented in the standard are intended to provide a baseline for comparison of the estimated annual energy cost of the proposed building and the baseline building for the purposes of a rating. They are not intended to provide an accurate prediction of actual energy consumption or costs for the building as it is actually built. Although the energy analyst is expected to model energy use as closely as possible, there are many reasons why the actual building performance rating may differ from the predictions of the building performance rating method.
Those reasons include variations in occupancy, control, maintenance, weather, energy rates and the precision of the simulation program. When creating an energy model, these are all assumptions that must be agreed upon in order to get a result, but they can't be 100 percent accurate in predicting future energy consumption.
Even when the owner is consulted during the integrated design process to get an idea of the building's hours of operation, estimated occupancy counts and utility rates, it's hard to know exactly how the building will operate, so predictions could be way off.
For example, a newly constructed library attracted a lot of attention through its successful public relations campaign — which touted, among other things, LEED certification and energy-efficient design. The campaign was so successful that the library had significantly more patrons than anticipated and had to extend its hours of operation. As a result, its energy consumption during the first year was noticeably higher than predicted.
Even the best energy modeling tools make approximations and assumptions for how systems actually behave. For example, modeling programs typically assume that controls work perfectly and typically don't model operational inefficiencies. Theoretically, you could possibly de-rate your boiler's efficiency to try to estimate leaky steam traps, but do you really want to expect your building to operate in a less than ideal fashion? Instead, the as-designed energy model should be seen as a representation of the potential of the building's energy performance, and facility managers should strive to maintain the building's systems in peak condition in order to stay as close to the predicted performance as possible.
Bill Worthen, resource architect for sustainability at the American Institute of Architects, drives this point home: "Today's modeling tools are not intended to provide any higher degree of predictive certainty for actual utility bills than the miles-per-gallon ratings displayed on the window sticker of your last new car will predict real life mileage. Hopefully, that number influenced your selection of the car. But when you drive off the dealer's lot, most people don't drive their cars exactly the same way the mileage testing was designed."
An energy model is one of the best tools to make educated decisions in designing high performance buildings. Energy models are essential to achieving rebates, incentives, green building ratings and more. In the future, energy models will most likely be relied upon even more in the integrated design process to help meet increasingly stringent energy codes and net-zero energy targets in new construction.
Clark Denson is an energy engineer within the sustainable solutions group of SSRCx/Cotter, LLC, a strategic partnership between Cotter Consulting, Inc. and SSRCx. He has extensive experience in the design and modeling of energy efficient buildings and consults for project teams to help them meet their energy-saving goals. He is a Professional Engineer, Certified Energy Manager, ASHRAE Building Energy Modeling Professional, and a LEED Accredited Professional.
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