In 2003, the Dallas City Council mandated that all new facilities owned and operated by the city achieve Leadership in Energy and Environmental Design (LEED) Silver certification as a fundamental program requirement. Located on a 17-acre brownfield redevelopment site, the Northwest Service Center is Dallas’ new maintenance and operations facility that services the city’s fleet of over 1,200 service vehicles. The new center is also designed to meet city-mandated green requirements.
The facility’s designers had to develop an HVAC system that provides safe and adequate ventilation for a vehicle maintenance and operation facility while also meeting LEED Silver guidelines established largely for office buildings.
The solution was found in a careful balance of seasonal heating and cooling loads, and in a ventilation system designed around McQuay Vision™ air-handling units, energy recovery wheels, and variable air volume (VAV) control.
ASSIGNING POINTSThe LEED Green Building Rating System® is a voluntary standard. It was established by the U.S. Green Building Council in 1999 and is widely recognized as a third-party verification system and guideline for measuring what constitutes a green building.
A LEED-certified building means it has achieved at least a minimum standard as judged in six categories: sustainable sites; water efficiency; energy and atmosphere; materials and resources; indoor environmental quality; and innovation and design process. Points are awarded in each category, depending on how the facility meets each category’s requirements. A building must receive a minimum of 26 points for LEED certification; up to 69 points are possible. There are four levels of LEED certification: Certified (26-32 points); Silver (33-38 points); Gold (39-51 points); and Platinum (52-69 points).
The energy and atmosphere category, one-third of potential LEED points, directly addresses the HVAC system and its effect on the environment. This includes the amount of energy the HVAC system consumes, the environmental implications of generating that energy, and the ozone depletion potential of the refrigerant used in the equipment.
Once a building has met the three prerequisites - fundamental building systems commissioning, minimum energy performance, and fundamental refrigerant management - the LEED-New Construction (NC) provides up to 10 points for the percentage of energy performance increase beyond the minimum required in the prerequisite.
INDUSTRIAL CHALLENGESThe Northwest Service Center has approximately 16,000 square feet of office space for the streets and sanitation departments, the site, however, is primarily industrial space: 42,000 square feet for fleet vehicle maintenance; 10,000 square feet for vehicle fueling and washing; and 25,000 square feet for covered storage.
LEED-NC certification is structured on gains in energy cost savings compared to a typical office building as defined by ASHRAE 90.1 standards. At present, LEED certification programs do not take into consideration the unique requirements of industrial applications. For example, while air conditioning is the largest energy consumer in a typical commercial office building, the Northwest Service Center has a relatively small cooling requirement: only the 16,000 square foot offices are cooled.
“Industrial buildings typically don’t have a huge air conditioning load like an office building, so it’s harder to figure out ways to get efficiencies when you don’t have those big loads to drop down from,” said Charlie Aldredge, vice president and project manager, Huitt-Zollars Engineering Inc., Fort Worth, and a member of the Northwest Service Center project design team.
In the center, the largest source of energy consumption comes from a requirement for 100 percent outdoor makeup air exchangers in the vehicle service area in order to ensure adequate and safe air exchange for occupants. This is where the engineers had to gain the efficiencies necessary to meet the LEED requirement. Also, since the area is not air conditioned, those efficiencies had to be gained from winter heating loads rather than summer cooling.
RECOVERED ENERGYTo meet the 100 percent outdoor air exchange requirement in the vehicle service area, three large McQuay Vision air-handling units were installed. These pull in outside air at a total exchange rate of approximately 40,000 cfm (two units at 15,000 cfm and one unit at 10,000 cfm). In comparison, a typical office building pulls in 10-20 percent outside air.
To achieve the required energy efficiencies in the service area, Sergey Aleksanyan, P.E., at Huitt-Zollars and the mechanical engineer of record, specified energy recovery wheels for each Vision unit. These capture energy from the heated exhaust airstream and transfer it to the cool outside air being pulled into the space. Up to 75 percent of the energy from the exhaust air stream can be recovered.
“Our solution for meeting the energy requirement was to gain winter efficiencies to offset the lack of a cooling load,” said Aleksanyan. “The exhaust air energy is used for preheating the makeup air. The rotary wheels recover energy from the exhaust air and return it to the supply air.” In addition to achieving LEED points for energy efficiency, this solution earned the center an additional LEED point for innovation because energy recovery wheels are not considered standard design.
EARNING SILVER?To achieve additional efficiencies, the office area is conditioned by two McQuay Vision air-handling units with variable air volume (VAV) controls, energy recovery wheels, and hot water heating. The VAV controls provide energy efficiency by matching airflow volume to load requirements; the motor runs at a lower amp draw under part load, thereby using energy only as needed to meet conditions. The energy recovery wheels lower heating and cooling costs by pre-treating the outside air using building exhaust air. A high-efficiency McQuay Model AGZ air-cooled, scroll compressor chiller (130 tons) provides chilled water for cooling. The fact that the chiller operates using an HCFC-free refrigerant (Râ€'407C) gained the project an additional LEED point.
“Energy model data shows that energy costs for the total project will be 67 percent of those for a typical LEED-NC building,” said Rachael Green, LEED director for Mitchell Enterprises LD, the building project’s general contractor. “That’s a 33 percent savings, and enough to get four LEED points for optimizing energy. The McQuay high-efficiency chiller, VAV control, and innovative energy wheels - along with other factors such as high-efficiency boilers, domestic hot water demand reduction, good insulation and windows, and lighting efficiencies, - all contributed to the total energy savings.”
These plans and solutions put the Northwest Service Center on track to exceed the Silver requirement with an expected 44 LEED points, enough to achieve LEED Gold certification later this year.
“Silver, I thought, would be very tough to get when we first started because of the largely industrial nature of these buildings,” said Bobby Williams, chief estimator for RMF Contractors who installed the air conditioning and plumbing systems. “If we get a LEED Gold rating, then you know everyone has done exceptional work.”
“From roof color to glass selection - it’s not just equipment - it all has to work in harmony to get the maximum energy reduction,” said Williams. “We could put the most efficient equipment in the world in the building, but if they put a black roof on a building in Texas, or use uninsulated glass or inefficient light fixtures, you’re not going to see the benefit of that efficient equipment.”
MORE THAN HVACOther LEED accomplishments on this project included a 68 percent wastewater generation reduction, 56 percent recycled content in materials, 99 percent construction and demolition materials recycled, and over 90 percent locally-produced materials. IAQ requirements were achieved through installation of IAQ monitoring systems and control of indoor chemical and pollutant sources, including low VOC materials such as mastics, insulation, and sealants.
“Designing the facility to LEED requirements was a tough challenge,” said Jack Ireland, director of equipment and building services, city of Dallas, commenting on the rigorous process changes required to meet LEED certification.
“But in the long run the city will see the savings in energy consumption, in water consumption, and hopefully in less employee illness and absenteeism from better IAQ - not only for the shop workers, but for the office workers as well.”
For more information, visit www.mcquay.com.
Sidebar: How Energy Recovery Wheels WorkEnergy recovery wheels reduce energy costs by transferring heat and moisture from one airstream to another. In the winter, they capture heat and humidity in the exhaust air stream and transfer it to the incoming fresh air. This saves much of the cost of heating and humidifying the incoming cold, dry air. Energy recovery wheels can recover 75 percent of the energy from an exhaust airstream and can cut winter humidification energy costs by up to 60 percent. (See Figure 1.)
In the summer, energy recovery wheels work in reverse: They capture heat and humidity in the incoming fresh airstream and transfer it to the exhaust airstream. This saves much of the cost of cooling and dehumidifying the incoming airstream. Energy recovery wheels can cool outdoor air to about 67°F, significantly reducing the load on conventional cooling equipment. (See Figure 2.)
Heat and moisture are captured in the energy wheel’s corrugated desiccant material. The wheel rotates between the incoming and exhaust airstreams where variances in pressure and temperature cause heat and moisture to move from one airstream to the other. Only heat and moisture transfer through the energy recovery wheel. Air is purged from the wheel before entering the other airstream. (See Figure 3.)
For example, the Vision air handlers in the Northwest Service Center vehicle maintenance area exhaust 100 percent of the air in the facility. In the winter, their energy recovery wheels capture heat in the exhaust airstream before it leaves the building and transfers it to the incoming airstream. Contaminants carried in the airstream are exhausted outside.
Publication Date: 06/25/2007