ENERGY STAR: Thinking about Energy Modeling
At the recent U.S. Green Building Council’s Greenbuild Conference in Phoenix, I attended a session that dealt with energy modeling and how well energy is analyzed during the design process.
What a lively discussion it was! One panelist commented, “Energy modeling makes architects do dumb things.” Someone else exclaimed, “Energy modeling is based on perfect assumptions, and modeling as we know it deals with mechanical systems quite well but not the people and occupancy schedules in buildings.”
There seemed to be general agreement that energy modeling was designed to do trade-offs of equipment efficiencies for code compliance, not be a predictor of absolute or relative energy efficiency. Another recurring point was architects seldom have clearly defined energy-performance goals to use during the design process.
What Are We Measuring?
The nation’s buildings use $200 billion worth of electricity and natural gas each year. And the energy consumed by U.S. commercial and industrial buildings is responsible for nearly 50 percent of our national greenhouse-gas emissions. As our industry moves forward in designing, constructing and operating greener buildings, we must deliver on the promise that these buildings will indeed save energy, benefit the owner’s bottom line and reduce carbon-dioxide emissions.
What exactly should designers be measuring and how can we set and achieve the goal of saving energy and shrinking the carbon footprint of the built environment?
Let’s start with how energy performance is assessed during the design process. Energy modeling for code compliance, documented in study after study, indicates modeling to code is not a good predictor of performance. Meeting code requires evaluating equipment and system efficiencies but does not account for the entire energy use in buildings. That involves people. This may be a little unnerving for modelers who want to have complete control over their energy-load assumptions, but the reality is people occupy buildings and engage in multiple activities that affect energy use.
Process and plug loads are considered “unregulated” loads in most energy codes but need to be factored into the design team’s equation. All those little energy-sucking gadgets people plug in at their desks consume energy! Real-world scenarios require our attention if we are ever going to design buildings with the potential to save energy as modeled or predicted.
From now on, the energy metrics for design should account for all the variables required to put a building into service. These variables include computers, process loads for cooking, and equipment and system loads for keeping people comfortable. Whole-building energy analysis includes all systems, schedules, and plug and process loads; it defines the parameters for a comprehensive energy model.
The second part of the equation is having something to compare the modeled assumption against—a baseline of actual performance, not just a reference specified by the energy code where all variables are tightly controlled. Such a baseline can be derived from similar buildings in similar climate zones with similar operating characteristics. Energy consumption from a group of similar elementary schools is a better indicator of performance for a similar K-12 project than trade-offs for equipment and systems mandated for code compliance.
In a study at the University of California, Merced, engineers developed what was termed “benchmark-based performance targets” for a series of new buildings. They measured the energy use of existing buildings over time, evaluated building materials and systems, observed occupancy patterns and used the information to develop targets for future building projects. The methodology used in the UC Merced study was a thoughtful means to designing buildings with the intent to achieve energy-performance goals once they are operating. The objective was not to merely meet code nor achieve accolades; the UC engineers were seeking to design buildings to an efficiency level beyond what was typical or average for their campus facilities.
From Designing to Operating the Building
In 2004, the U.S. Environmental Protection Agency launched Designed to Earn the ENERGY STAR, a companion program to EPA’s successful existing buildings program. The name implies the goal of the initiative. A design project that meets the criteria established by EPA achieves this distinction. EPA’s energy-performance rating, which was initially developed to compare an existing building to a group of its peers, was retooled to help architects establish targets for design projects based on actual energy-consumption data (the baseline mentioned earlier).
Then as the design is developed and energy modeling is performed, the total estimated energy use can be plugged into EPA’s tool, and the design receives an estimated rating from 1 to 100. This rating provides a relative performance score of how well the design compares to a group of its peers. The rating is predicated on whole-building energy use.
For architects and building owners committed to helping our nation “green” future buildings, the easy-to-use, online EPA energy-performance rating system provides a relative performance goal, rating for design projects and existing buildings, and consistent metrics for the life-cycle of the building.
These metrics provide the design team with realistic energy intent to pass on to the building owner who, in turn, can measure the energy consumption of the operating building using the same efficiency rating. The metrics associated with the rating are typical of those found on utility bills and familiar to all involved in designing and operating commercial buildings. The rating system developed by EPA’s ENERGY STAR program provides a feedback loop between architects and owners that can close the performance gap between building design and operations. And it helps address the issues posed by the participants at the energy modeling session during Greenbuild!
Karen P. Butler manages Commercial Building Design for the U.S. EPA’s ENERGY STAR Program.