In 2008, Raleigh, North Carolina, dug its tar heels into sustainability. With the establishment of an Office of Sustainability, local government officials introduced a requirement that all new public facilities totaling more than 10,000 square feet perform to LEED-Silver standards. The city also secured a 23-acre site just east of downtown to develop a new Transit Operations Facility for its growing bus fleet. Transit ridership has doubled in the last decade, and the replacement facility would approximately quadruple the capacity of the four-service-bay building that had been serving Raleigh since the mid-1970s.
Courtesy Williard Ferm Architects
The city hired local studio Williard Ferm Architects to design the project through preparation of bridging documents, which it undertook with consultants Maintenance Design Group and Talbot Associates Consulting Engineers. On behalf of the city, Williard Ferm also oversaw the subsequent design-build team’s completion of construction and LEED documentation. The new complex measures 110,000 square feet and includes administration space, as well as bus maintenance, fueling, and wash facilities, and it is expected to earn LEED-Platinum certification.
The rating is a first for projects of this type, and the sustainability strategies to achieve it include radiant floors heated from geothermal wells, reflective exterior surfaces, electric illumination tied to daylight and occupancy sensors, and rainwater collection. According to Williard Ferm architect Paul Stewart, “One of the greatest challenges was to economically cool the maintenance areas to an acceptable level.” Indeed, regular use of large garage doors threatens these areas with outdoor air penetration. To achieve a consistent temperature of 80 degrees, the design team decided to employ chilled beams, in which water coursing through suspended heat exchangers chills the surrounding air. Although the system has been available for decades, it is more regularly employed in Europe, where it has proven to require less energy for ventilation.
One reason chilled beams have not enjoyed widespread adoption stateside is the persistent belief that they fail in humid environments. Yet reports of interior rain are exaggerated. Talbot & Associates had used the system throughout the Southeast prior to the Raleigh commission, about which Stewart says, “When the engineer focuses on humidity control first, outdoor air humidity is no longer a concern.” For the Transit Operations Facility, the design team conceived an efficient dedicated outside air system in which 13 heat-recovery wheels operate with 80 chilled beams in administrative, maintenance, and operations areas.
Working in tandem, outdoor air enters the enthalpy system, where it cools and dries before reaching the beams. Standard exhaust fans and intake louvers enhance this cycle, too, though they kick in only when outdoor air conditions are acceptable.
Regarding penetration of humid air in the maintenance areas, Stewart says, “one benefit of utilizing a dedicated outdoor air system to serve the beams is that the beams are positively pressurized by dehumidified air. So if there is a larger source of humidity surrounding the beam, the supply air will actually keep the beam from condensing.” To be sure, there are still conditions that chilled beams cannot surmount, according to engineer Michael Talbot. “I would say the only places we would not use chilled beams are around entrances and other areas that have potentially high-humidity environments,” he says, and his designation includes indoor pools as well as the repair garages one would find abutting the maintenance areas in Raleigh.
Because the interior flirts with moisture in this project, high technology assists the chilled beams’ old-fashioned convection. Dew point sensors are mounted throughout the Raleigh Transit Operations Facility: If the water is a lower temperature than the surrounding dew point, flow to the beams is interrupted. In a similar vein, carbon dioxide sensors trip a ventilation flush when localized emissions are too high.