[NEWS] Energy Modeling for the Real World

07-Jun-2011 By Steven Straus, Glumac

The Mercy Corps Headquarters building in Portland, Oregon performs in line with its energy model and is LEED Platinum certified.

As design and construction teams work to achieve increasingly higher LEED ratings, they are pushing the envelope of energy efficiency. Unfortunately, many projects have fallen short of the team’s original predictions for energy savings.

The most significant concern for a finished building is the difference between the modeled and the actual energy consumption. Energy modelers are pressured by the design and construction teams to demonstrate significant energy savings to help earn a higher LEED rating for the finished project. However, in cases where owners see that first-year energy consumption costs are substantially higher than what the model predicted, they can claim they have been misled into making inappropriate investments that did not result in the efficiency they expected.

This is a simple mix-up that can be avoided very easily. Here are some tips for owners and design teams to minimize the potential for unhappy clients — or even litigation — for not delivering the energy goals that were promised:

•    Discuss model requirements. The U.S. Green Building Council (USGBC) and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) have specific requirements for modeling buildings to determine the energy efficiency of a proposed design. These guidelines are meant to provide a level playing field between different projects, even though some may have more specific requirements. For example, the ASHRAE 90.1 standard requires plug loads (e.g., computers, printers, etc.) be modeled based on an operating schedule that mimics a traditional workday, typically 7 a.m. to 6 p.m. for an office building. In reality, tenants who work late or leave their computers running all night create additional energy demands that fall outside of the schedule. Clients should be informed that if numerous electronics are left on after everyone has gone home, then they can expect higher energy consumption than predicted in the energy model. To avoid this, designers and energy analysts should provide at least four energy model results for the owner: a baseline and a building model for the project designed to meet USGBC or ASHRAE requirements, as well as separate models (baseline and actual) that use an operating schedule that designers and owner both agree upon. Perhaps the team will decide that some equipment will be left on at night, or that a small percentage of employees will sneak space heaters into their offices. Owners and designers should ensure everyone is on the same page as to what the hours of operation are. Surprisingly, this is not often done!

•    Deliver a similar-project database. Designers should provide the client with a historical database of operating costs of similar projects, from sources such as . the U.S. Environmental Protection Agency’s website of ENERGYSTAR buildings. By showing the client a comparison of code-conforming buildings and the actual data of energy efficient buildings, designers can begin to establish the groundwork for what the “real numbers” will be for the project.

•    Determine accurate plug loads. Plug loads vary by occupancy, owing to the number and type of computers within the workspace. While most office tenants have a plug load demand of 0.75-1.0 watts per square foot, one can see much higher loads of 2-3 watts per square foot, especially in the software engineering industry. In other cases, building owners are attempting to reduce real estate costs by increasing the occupant density by almost 50 percent, trying to fit far more people into a work space than they did just five years ago. Instead of using rules of thumb from experience with previous projects, designers should directly estimate the energy consumption per workstation, multiplied by the number of workstations in the space.

•    Don’t be overly optimistic. Energy specialists may be quick to believe manufacturers’ claims of energy savings or efficiency of a particular product, based on a best-case scenario with occupancy schedules and densities that differ greatly from a particular situation. One way to deal with this is to measure the actual energy demands or efficiencies from previous installations, to generate realistic assumptions for analysis.

•    Document the assumptions. It’s very important for energy modelers to document the assumptions used to create baseline energy models, and review them with the owner and design team to make sure the assumptions are reasonable. When differences exist between the modeling requirements of various agencies, these discrepancies should be brought to the team’s attention so that everyone can agree on estimates of energy use.

Since Glumac began commissioning projects, we’ve seen a trend where actual operating costs are often 50 percent to 100 percent higher than those predicted by the energy specialist. The baseline energy models should project similar increased operating costs, which would accelerate the payback for the energy-saving measures actually adopted.

Glumac’s recommended best practice is to track projects for several years after completion. If a building’s actual energy consumption doesn’t match the modeled results, designers and owners should identify the sources of these differences.

More often than not, a change in occupancy hours or building use is the cause. For example, Glumac recently evaluated a new college building where the actual energy usage was much higher than we initially estimated. We discovered that although our baseline model was created for 12 hours of operation, the university was operating the building 24 hours, owing to its increased popularity with the students.

This article was modified from its original publication in Sustainable Business Oregon.

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