EUI Energy Use Intensity
US United States
DOE Department of Energy
NREL National Renewable Energy Lab
RSF Research Support Facility
CO Colorado, alright you probably knew that
Most of these acronyms probably make quick sense, a government office building in Colorado. But EUI, Energy Use Intensity is a newer term and worth getting familiar with; several states have enacted a requirement to provide the EUI to buyers and leasers of commercial real estate. It is essentially the ratio of annual energy consumption per square foot. NREL issued the design-build contract for their new research facility (RSF 1) with targets of a LEED® Platinum rating and an EUI of 25. Contracts targeting a LEED rating aren’t new, particularly in public sector buildings, but EUI targets are.
A performance rating or EUI of 25 kBtu/s.f. is an ambitious goal, the national median for office space is closer to 90. Ultimately, NREL revised their goal to 35 based on planned use changes, such as the addition of a data center and increased occupancy.
NREL also specified a budget in range with typical office construction, and the final construction was about $258 per square foot ($288 for full project cost including furnishings). The 222,000 sf building opened in 2010 and houses approximately 800 staff.
An integrated design strategy was critical in meeting such an ambitious energy goal because the structure itself would have to play a significant role in heating, cooling, ventilation, and lighting. Concrete met structural and envelop requirements as well as functioning in critical elements for the control of the space temperature and the maintenance of thermal comfort. Specific applications were the use of insulated precast panels, radiant floors and a concrete labyrinth in the basement.
According to Stantec’s David Okada, Mechanical Engineer and Project Manager for RSF I and II, “After studying the performance of the remote mass labyrinth we found that the exposed concrete floor was the important element for thermal storage”.
An addition, RSF II, is underway and has provided an opportunity for the design/build team to apply lessons learned from RSF 1 and the actual performance data.
Building Shell Integral to HVAC and Lighting
NREL specifically contracted the project with an integrated design approach so that the design, structural and mechanical engineering would work together. Several strategies emerged from this collaboration including wings with a narrow (60’) deep floor plate to allow daylight to reach the full depth and a mass heat exchanger in the crawlspace. A slab for a large structure requires excavating and importing soil with better compaction. The designers chose to take the need for an excavated foundation further and use it in tandem with a solar heat collector which NREL had helped developed previously with the company Conserval. In the basement, a winding corridor of concrete stores and transfers heat to and from the air. Cooler air and warmed intake air move slowly through, exchanging their heat with the surface mass, and the ventilation system can tap different areas to provide ventilation or make up air with the least additional heating or cooling required. Transpired air collectors warm the air using perforated sheet metal with a dark coating that allow air in, and heat it with solar radiation before a fan draws it into the fresh air intake. The system increases the air temperature up to forty degrees depending on airflow and available sunlight.
In addition to an outer skin of thermal collectors, the shell consists of precast concrete panels with insulation encased by the interior and exterior finish surfaces. General contractor Haselden Construction placed panels on-site to create near finished walls. The mass and insulation provide a consistent thermal and air barrier, buffering the internal conditions from external changes.
The exposed interior mass is a critical part of the natural ventilation strategy and substantially reduces the need for cooling the building. Continuous insulation in the concrete sandwich panels provide the additional benefit of substantially reducing the thermal bridging which is weak spots in the building insulation that result in substantial heat loss.
Water delivers radiant heating and cooling through 42 miles of piping embedded in concrete floor panels. Reclaimed steel gas piping supports the structure. The final component of the shell designed to support passive energy management is triple glazed operable windows. The windows allow occupants to control natural ventilation. Under floor ventilation is available when outside air is too hot or cold. The windows were also carefully designed to allow daylight to reach throughout the space, reducing lighting energy intensity, yet with placement and shading to reduce glare and undesired heat gain and loss.
In practice the team has found that the concrete labyrinth works so well that not as much surface area is needed. The solar thermal skin is also working very well, but hasn’t been needed as much as anticipated because the heat recovery from the data center is so effective at warming outside air for ventilation. These observations are informing the design of the addition.
The Department of Energy does a quadrennial survey of energy use in commercial buildings. An EUI can be compared to that of other buildings of a similar type of use to benchmark energy performance. To simplify the analysis that would be required to factor in differences in building use, number of people, equipment, schedule of use and climate, Energy Star Portfolio Manager offers an index rating. After entering in building parameters and a history of energy data, Energy Star calculates the Site and Source EUI, but also offers an indexed score, on a scale of 1 – 100 – this time higher is better. A rating of 75 or greater qualifies for Energy Star Certification. Rating isn’t offered for buildings where the data isn’t adequate for statistical validity. With a couple years of utility bills, a count of people and computers, square footage, etc. anyone can create an account and evaluate their building. Visit energystar.gov for more info. The NREL Research Support Facility is too new to rank, since the score relies on actual operating energy rather than projected or modeled performance.
Model for Others
One of the real benefits of this project is its explicit intent to learn and teach by doing. NREL has the entire specification package available on line and it will no doubt be one of the most closely monitored buildings. What is particularly exciting is that the fundamentals that contribute to the buildings’ performance are readily available, concrete; cast in place foundation and precast walls, radiant heating. Not every project has the luxury of a large open space to site a building and arrange the footprint in the most optimal way, but the basic components and the way they work together offers designers approaches that can apply in any setting. http://www.nrel.gov/sustainable_nrel/rsf.html