The next evolutionary step on the path to carbon neutrality is a functioning, super-energy-efficient, 220,000-square-foot office structure that is the largest net-zero energy building in the United States.
Opened in 2010, the Research Support Facility (RSF) in Golden, Colorado, a $64 million complex built by the Department of Energy’s National Renewable Energy Laboratory (NREL), showcases what is technologically possible and commercially viable.
A new term that seeks to define a new and more sustainable way of designing and constructing commercial spaces is thermally activated building system, or TABS. Like the Earth itself, a TABS structure uses its mass to absorb or emit heat through its conditioned surfaces to regulate then interior environment. Two key features typically characterize TABS structures:
- high-performance enclosures
- a reliance on concrete slabs embedded with hydronic tubing, usually made of crosslinked polyethylene tubing (PEX), for low-temperature radiant heating (emitting energy) and high-temperature radiant cooling (absorbing energy).
Facilitating compliance with ANSI/ASHRAE Standard 55-2010: Thermal Environmental Conditions for Human Occupancy, the radiant slabs help to cut energy use by more than 50 percent over ASHRAE Standard 90.1(2004): Energy Standard for Buildings Except Low-Rise Residential Buildings.
NREL Senior Engineer Paul Torcellini said, “It isn’t enough to be energy-efficient when commercially viable technology exists to make buildings energy-neutral.”
Among the innovations that made the facility possible was a new method for installing radiant heating and cooling systems: the Uponor Radiant Rollout™ Mat, which enabled mechanical contractor Trautman & Shreve to dramatically slash labor time and costs. These savings, in turn, helped NREL meet its budgetary goals and tight construction schedule.
Housing 800 staff members in an open work environment, RSF boosted NREL’s campus square footage in Golden by 60 percent, but increased campus energy use by only six percent. Achieving this outcome wasn’t accidental; thorough planning with the following mission-critical goals helped to guide the design process:
- Design and build a safe work environment.
- Achieve a LEED Platinum rating.
- Aim for Energy Star “Plus” in terms of energy usage.
“Our goal was to maximize the passive performance of this facility,” says David Okada of Stantec in San Francisco. “Then we focused on making the engineered systems as efficient as possible. Thermal and energy modelling provided the information the design-build team needed to keep the design true to the project’s aggressive goals.”
Energy Efficient Ventilation
Principles of thermal mass heating and cooling not only applied to the core TABS radiant system, but also included separate design innovations to improve the pre-heating and pre-cooling of the ventilation system. A unique and innovative ventilation heat exchanger, the labyrinth, was constructed out of the buildings lower level with concrete airflow diverters/partitions. This maze captures the heat of the day or the cool of the night, holds onto the thermal energy and then slowly releasing it to help warm or cool the ventilation supply air.
The RSF includes two long wings, connected at the middle by a lobby and a conference area. Each wing rests on a low basement with concrete walls staggered to make the air take S-turns through the space, lingers awhile, and then loses its cooling or its heating, depending on the season. The air has greater exposure to the concrete thermal mass and this prolonged exposure ensure more effective heat transfer between the two.
That way, the labyrinth acts as a thermal battery, storing the chill of the night air to reduce the building’s cooling load in summer by pre-cooling the ventilation air. During winter, the labyrinth stores heat drawn from two sources: 1) computers in the facility’s new data center; and 2) outside air warmed by the sun beating down on a transpired air collector.
Transpired air collector systems essentially consist of a dark-colored, perforated sheet metal façade installed on the building’s south-facing wall. A fan draws ventilation air into the building through the perforated absorber plate and across the plenum (the air space between the absorber and the south wall). Solar energy absorbed by the dark absorber and transferred to the air flowing through it can preheat the intake air by as much as 40°F.
Unique Radiant Installation
The twin performance criteria of energy efficiency and cost control complicated the job, which installed the radiant heating and cooling in the slabs. Project manager Tony Barela needed an ultra-efficient way of keeping the project on schedule.
The team devised a pre-fab plan for the radiant zones. After mapping out all the zones, Trautman & Shreve purchased PEX tubing in standard 1,000- and 500-foot rolls. Then, using three-foot plastic rails (with loops in 6″ to 10″ spacing’s to hold the pipe together in an even width), they prefabricated the radiant mats.
The mats were a precursor to the Radiant Rollout™ Mat, a product then under development and eventually introduced in 2010. The mat is a custom-designed, prefabricated, pre-pressurized network of PEX-a tubing connected with ProPEX engineered plastic fittings. These mats can install approximately 85 percent faster than conventional radiant tubing methods.
Once the RSF decks were ready, a crane was used to lift the large bundles of tubing. A crew unrolled the tubing, tied it down and quickly made the necessary connections. The entire tubing-installation task took only two days, saving a total of 28 days on the construction schedule.
A Way Forward
NREL’s Paul Torcellini believes that the PEX-based, radiant heating and cooling slabs are one of the keys to the energy performance at RSF.
“Logic will prevail,” says Torcellini. “Water is a much better conductor of energy than air, and employing hydronic systems as a pathway for energy will be one of the strongest tools in rewriting our energy profile.”
Installation tools like the Radiant Rollout Mat dramatically speed the installation of radiant systems. In doing so, such innovations help lower the up-front system costs for building owners, making sustainability even more economically viable and contributing to a very different and brighter energy future for commercial buildings.