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Labyrinth to Store Energy in Basement for Later Use

May 29, 2009

Photo of two men standing in a construction area with concrete walls behind them.

NREL Construction Manager Carl Cox, and Shawn Bradfield with NREL Construction Safety, discuss an upcoming cement pour that will lay the floor of the labyrinth. The staggered walls behind them force air flowing through the basement of the RSF to cool and heat the concrete.
Credit: Heather Lammers

There's a labyrinth in the basement of NREL's newest building. The maze wasn't designed to hem in a mythical beast or to confuse workers, but it is a trap – one that will capture the heat of the day or the cool of the night, hold onto it and then slowly release the thermal energy to help warm or cool the building.

The Research Support Facilities now under construction on the U.S. Department of Energy's NREL campus will include two long wings, connected at the middle by a lobby and conference area. Each wing will rest on a low basement with concrete walls staggered to make the air take S-turns through the space, lingering awhile, losing its cool or — depending on the season — its heat.

"As the air goes through the maze, there's greater contact with the mass, thousands of tons of concrete," said NREL's Eric Telesmanich, project manager for the Research Support Facilities.

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. In winter, the labyrinth will store heat drawn from the computers in the new data center and outside air warmed by the sun beating down on a transpired air collector.

Waste Heat Won't Go Wasted

Photo of the RSF under construction shows building's stairwells, construction cranes and basement and foundations.

An aerial view of the Research Support Facilities under construction shows the south wing basement labyrinth's offset concrete walls. The staggered walls force air flowing through the basement to come into contact with more mass, cooling or heating the concrete.
Courtesy of Haselden Construction

As the waste heat from the computer center and the air warmed by the sun wends through the staggered concrete walls of the labyrinth, pulled by fans, the slabs absorb the warmth. The next day, the stored heat can knock off some of the morning's chill, when fresh air is pulled through the labyrinth before being heated and pumped into offices. The heat the computers shed also will be used to heat the building.

The transpired air collector — a metal sheet with tiny strategically placed holes to pull air through – takes advantage of what comes naturally on a cold Colorado day.

"The air might be pretty nippy," said Phil Macey, a senior associate at RNL, the design firm for the RSF. "But when you stand in the sun, there's lots of warmth."

NREL's Telesmanich said that the building's original design called for two walls — or a double skin — on the south side to preheat air. But that raised a lot of technical, cost and maintenance challenges. RNL went looking for a different solution, and came up with the transpired air collector, a concept NREL won an award for in 1994.

"So, a technology developed at NREL turned out to be the perfect solution for NREL's new building," Telesmanich said.

Pulling outside air through the labyrinth can warm it 5 to 10 degrees before it is further heated to warm the building. "That may not sound like much," Macey said, "but it is meaningful across the whole year."

Old Idea Brought Up To Date

The air flowing into the labyrinths starts two stories up and travels down ventilation shafts built into the RSF's stairwells. The labyrinth in the north wing of the office building will be for heating; the south wing labyrinth will help with both heating and cooling.

Storing heat or cool in a large dense mass isn't a new idea. Think of how refreshingly cool you feel walking into an adobe church or a gothic cathedral. Large masses absorb the heat of the day, slowly releasing it at night to keep the space warm. The reverse happens when the thermal mass has cooled and the outside temperatures begin to warm.

Drawing of the labyrinth shows stairwells and cutaway of basement with offset walls forming the labyrinths. Blue arrow shows the flow of cool air, red arrows show the flow of warm air.

Cool or warm air will be pulled into the south labyrinth (left) through ventilation shafts in the building's stairwells. And warm air, heated by the sun through a transpired air collector, will be pulled into the north labyrinth.
Courtesy of RNL

How can this effect be used to help save the energy it takes to heat or cool and office building? In the 1970s, drawing air across a bed of stones was considered. But, Macey said, because of the irregularity of stone sizes and shapes and the difficulty of controlling air flow through the rocks, the idea turned out to be impractical.

Poetry From a Challenge

His firm and engineering consultant Stantec revisited the idea because of Colorado's clay soils and climate. Here, builders need to remove the clay soil before building a foundation to prevent expansion and shifting. Often, a basement is the solution. But in buildings where a basement is not called for, typically the clay soil is taken away and replaced by better soil that then is compacted. This can be expensive.

And, the RSF doesn't need a basement. On the contrary, the building's design called for letting in as much light as practical.

The solution was to make a shallow basement that, for a small investment, could be used as a thermal mass to help reduce the energy used for heating and cooling the building.

It was an idea that "made poetry out of a challenge," Macey said.

Figuring out what to build was another challenge. Macey said that Stantec spent a few weeks writing a unique computer program to optimize the size and shape of the labyrinths and to calculate air flow. But that work can be used by others working on similar projects in the future, since the experience of the RSF will be shared to help design and construct other highly efficient buildings.

Energy Requirements Drive Design

Drawing of two wings of the building surround a courtyard and are joined by a lobby.

This computer rendering shows the Research Support Facilities main entrance facing west, along with the three-story south wing (left) and the four-story north wing (right).
Courtesy of RNL

The RSF will use about one half to one-third the amount of energy as other office buildings, and could be one of the most energy efficient office building in the world, Telesmanich said.

"The energy requirements drove the design of the building," he said. "That's a very different way of doing things."

Macey agrees. "We've learned how powerful climate is. Mother Nature gets a powerful vote. So, we let the energy of the environment drive the design and think of ways to finesse nature rather than challenge it."

Haselden Construction is building the 218,000 square-foot Research Support Facilities building, which is designed to be a model for sustainable, high-performance building design, and will provide DOE-owned work space for administrative staff who currently occupy leased space in the nearby Denver West Office Park. The RSF is planned achieve a LEED® (Leadership in Environmental and Energy Design) Platinum designation — the highest benchmark awarded by the U.S. Green Building Council. Construction on the RSF is expected to be completed in summer 2010.

Learn more about Sustainable NREL.

— George Douglas