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Are Energy Models Really Accurate?
For all of its many pluses, building energy modeling also has drawbacks. Modeling is expensive, time-consuming, and complicated.
It’s also not infallible. “What you predict may not always be what you get,” says Skelton. “A model is only as good as the information you put into it.”
Unfortunately, energy modeling is not taught in most architectural or engineering schools, where building design professionals get their education, according to Muehleisen, “Energy modeling requires a good background in building science in order to make sure a model is an accurate representation of the design intent,” he says.
The lack of educational training, leads to fewer people and companies that can offer building energy modeling, according to Skelton.
ASHRAE offers a building energy modeling engine test certification through Standard 140. The International Building Performance Simulation Association (IBPSA) – USA, offers a modeler map to help identify qualified modeling service providers. IBPSA-USA also provides training for professionals. “Together with ASHRAE, we have been presenting a full-day training session on the fundamentals of building energy modeling for many years,” explains Erik Kolderup, president, board of directors, for IBPSA-USA. Kolderup says the organization is working to get more training videos online.
If an investment in modeling is going to be made, it’s essential that the results be used. “The expense should be justified based on the value provided,” adds Franconi. “The findings won’t result in savings unless the design team or facility manager acts on the recommendations.”
Skelton also believes building energy modeling still suffers from doubts about its accuracy. “People don’t always trust energy modeling,” he says.
Experts advise facility managers to use energy models with their eyes open. “Modeling should be used for decision making, and we modelers should understand the assumptions, uncertainty and risk associated with the output we report from our modeling efforts,” says Knight.
“Modeling can have many objectives,” explains Eldridge. “The expectation for the deliverable should be clear in evaluating projects — total building annual energy use or annual energy cost and so on. Building operations also have a significant impact that may be unknown, or not provided to the modeler, during the design stage.”
While energy modeling has gained ground for some design applications, other efforts have not been successful. “The value of using it to predict energy cost for an owner to use for budgeting a building’s financial performance has been a big failure,” says Knight.
Over the years, experts have learned to clearly define modeling’s limitations to clients, make appropriate building system trade offs, and gain insights into operational issues.
“Energy modeling can be an important tool for project teams, particularly for deep retrofit projects at existing buildings” says Eldridge. “Clear definition of client expectations for the deliverables allows the modeling results to meet the project’s needs. Clarity in deliverables also prevents people from using the results for analysis that the modeling wasn’t actually intended to address, causing problems for the project.”
Franconi has learned to make trade offs between different building system components. For example, improved envelope and lighting can reduce the size of or even eliminate some mechanical systems, which Franconi has found “results in overall capital costs savings in addition to energy savings.
“In addition, insights into operational issues can be gained by comparing predicted energy use to actual energy use,” says Franconi. “This can help facility managers track down and address operational issues.”
“Less is more,” observes Skelton. “The more complicated you make the model, the more inaccurate your results can be. As Albert Einstein said, ‘Make things as simple as possible, but no simpler.’”
In the end, Knight points out that “energy modeling is only one, in some cases small, contribution to a successful project outcome.” Producing a facility that offers its occupants a safe, healthy, and secure environment requires more than what today’s energy models can predict and achieve, maintains Knight. What is required to achieve those facility goals is “an integrated, multidisciplinary, life-cycle approach to the design, construction and operation of buildings,” says Knight. “That requires continuous monitoring and active management at each phase.”
“We will never model that perfectly — even with artificial intelligence,” Knight insists. But he is hopeful that future energy modeling will do so “with ever increasing precision and accuracy.”
Rita Tatum, a contributing editor for Building Operating Management, has more than 30 years of experience covering facility design and technology.
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Are Energy Models Really Accurate?