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Dual-Wheel Energy Recovery, Chilled Beams Drive Savings for Transit Operations Facility

Sept. 1, 2012
Exceeds ASHRAE 90.1 efficiency by 30-plus percent

The state of North Carolina mandates that all government buildings over 20,000 sq ft exceed ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, requirements by at least 30 percent. The Transit Operations Facility in Raleigh, N.C., completed in spring 2011, exceeds them by 40 percent.1

Funded in part with $11.1 million from the American Recovery and Reinvestment Act, the facility consists of a 27,400-sq-ft, two-story operations administration building; a 56,000-sq-ft, single-story bus-maintenance facility; a 7,500-sq-ft bus-wash building; and a two-lane fueling depot. The facility accommodates 125 buses, with the capacity for expansion to 200.

To satisfy the state's efficiency mandate, Mike Talbot, president of mechanical-systems designer Talbot & Associates, advocated an energy-recovery-and-chilled-beam strategy proven successful in many large government and institutional facilities, including several in the southeast.

The Strategy
Thirteen Pinnacle primary ventilation units and 80 Flexicool active chilled beams from SEMCO LLC serve the administrative, operations, and maintenance wings of the facility. The Pinnacle units deliver 100-percent outdoor air to the administrative and operations spaces continuously and provide all of the preconditioned air supplying the chilled beams.

Throughout the year, three dual-heat-wheel Pinnacle units work in concert with integral heating and cooling coils to deliver preconditioned air to the chilled beams at suitable wet-bulb temperature. The units provide all necessary dehumidification during cooling/latent-load periods, as well as humidification during winter, recovering energy from building exhaust to the extent possible. The heating and cooling coils inside of the energy-recovery units modulate as needed to further dehumidify or heat supply air.

Additional space heating and cooling is carried out via the chilled beams. During cooling season, supply air must be dehumidified before entering the chilled beams to avoid condensation. The decoupling of this aspect of the cooling system via the Pinnacle units is what makes the configuration work so well.

The chilled beams provide sensible heating or cooling as needed, while the Pinnacle units provide all necessary ventilation and latent conditioning. Because of the excellent induction rates of the chilled beams, the only ducted airflow needed is what is required to meet outdoor-air requirements. According to Talbot, compared with a typical variable-air-volume system, this significantly reduces the amount of system fan energy that is consumed. The energy performance also is superior to that of the traditional European approach to chilled beams, which utilizes larger quantities of higher-dew-point air and not just ventilation air. Furthermore, chilled beams utilize a higher chilled-water temperature—approximately 57°F—to cool spaces, which lightens the load on the chilled-water system.

Enhanced Comfort for Drivers and Mechanics
Bus drivers have a comfortable indoor environment awaiting them when they return from their routes. With the time between routes sometimes two hours or more, drivers have a place to rest, eat, and even exercise, saving time and fuel they otherwise might spend driving home. Meanwhile, mechanics in the maintenance facility get year-round comfort from radiant heat and a displaced air-delivery system that keeps conditioned air concentrated where it is needed most, at floor level. Traditional heating and cooling would not have been practical, given the large doors, which frequently are open. Single-wheel SEMCO energy-recovery ventilators (ERVs) help to keep the space comfortable, especially compared with other garages, most of which are unconditioned or are heated only during winter.

Talbot estimates a 10-year payback or better for both the chilled beams and the ERVs. However, this payback likely is to be reduced, given the strong likelihood the facility will add a third shift to its bus-driver operations.

"This is definitely the most cost-effective way to achieve 30 percent better than ASHRAE 90.1 and still meet your outdoor-air requirement," Talbot said. "You have to have a dual-wheel dehumidification system to use in conjunction with the chilled beams. It is the only way to provide neutral dehumidified air to the space so you don't run the risk of overcooling."

A Leg Up on LEED
From geothermal heat pumps to an employee-maintained organic vegetable garden, the transit facility is a virtual showplace for environmental stewardship. Leadership in Energy and Environmental Design (LEED) Platinum certification is anticipated, with the application of the chilled beams and ERV units providing the lion’s share of energy-reduction points, as well as points associated with water savings and increased ventilation.

"We really feel this is one of the most cost-effective strategies a facility of this scale can have for energy reduction and overall comfort," Talbot said.

1) Compared with the ANSI/ASHRAE/IESNA Standard 90.1 baseline system, savings are projected to be 40 percent. Compared with the old facility, the savings per square foot are approximately 65 percent.

Information and photograph courtesy of SEMCO LLC.

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