As a consulting engineer during the 1970s, Greg Cunniff worked closely with another engineering professional, Dan Prill, at the request of Montana Bell. The project: a new 100,000-sq-ft facility to house telephone switching equipment.

"Our first energy crisis was upon us, and the telephone giant—with all that energized gear—wanted to find ways to reduce their energy consumption," Cunniff, now application engineering manager for Taco Inc., recalled.

Cunniff and Prill won the engineering contract based in part on their offbeat plan to build in "nighttime economizer cooling." Preparations would entail doubling the thickness of the facility’s standard 6-in. concrete slab.

"The foot-thick slab would allow us to embed 6-in.-round, nonmetallic sonotube ducts through the length of the floor slabs of the multistory building," Cunniff explained. "At night, we'd pull cool mountain air into the ducts, wicking away heat, creating a thermal-storage (cooling) system. During the day, warmer air within the structure was cooled by being blown through the ducts to air-handling units and discharged through diffusers at ceiling level."

The cooled air would cascade down into the large open spaces, measurably reducing the mechanical cooling load.

Cunniff was reminded of the Montana Bell project while attending ISH 2011 in Frankfurt, Germany. There, he saw an exhibit for a thermal-storage system coupled with radiant cooling for commercial facilities. Cold nighttime air is blown through a cooling tower. The resulting cooled water is moved through a plate-and-frame heat exchanger and circulated through cross-linked-polyethylene (PEX) tubing attached to the underside of floor slabs. During the day, warmer building air is blown through narrow plenums below the slabs. The air is cooled and discharged through perforated grilles in a radiant ceiling (Figure 1).

"The movement of cooler air formed a gentle convection within enclosed spaces, but not enough air volume for evaporative cooling," Cunniff said.

This approach has two advantages:

• Reduced energy consumption. The use of nighttime economizer thermal storage in building slabs can reduce daytime cooling energy consumption by up to 30 percent.
• Increased radiant-cooling or chilled-ceiling capacity. Chilled ceilings (Figure 2) have a fairly low cooling capacity of 20 to 30 Btuh per square foot. Passive chilled-beam systems, on the other hand, have a cooling capacity approaching 150 Btuh per square foot, while active chilled-beam systems have a cooling capacity approaching 400 Btuh per square foot.

Higher capacities are achieved by increasing the convective component of the chilled beam. This is done by increasing airflow over the chilled-beam coil, which increases heat transfer. In the case of passive chilled beams, the convection is natural (Figure 3), while in the case of active chilled beams, it is forced (Figure 4).

Blowing dedicated outdoor air over the underside of the floor slab can increase the capacity of this hybrid chilled ceiling by up to 30 percent.

Although building professionals in the United States have yet to embrace the concept of thermal-mass-linked radiant cooling, they are exploring the newest, coolest radiant technology: chilled-beam systems.

Chilled Beams
An alternative to conventional variable-air-volume (VAV) systems, chilled beams circulate chilled water through tubing embedded in a metal ceiling fixture to wick away heat. The business end of a chilled beam is made of copper tubing bonded to aluminum fins. The beam is housed in a sheet-metal enclosure typically placed at ceiling level.

There are three types of chilled beams: passive, active, and integrated/multiservice.

The difference between passive and active chilled beams, both of which are receiving significant attention in the United States, concerns airflow and the way fresh air is delivered to a space.

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