Located in midtown Atlanta on the site of the old Atlantic Steel Mill, Atlantic Station is a 140-acre environmentally conscious, live-work-play village. Among the many attributes contributing to this development’s green status is the integration of the region’s most environmentally friendly central cooling system.

Maxon Holdings, LLC, the developer, owner, and operator of the central cooling system at Atlantic Station, partnered with Johnson Controls, which provided YORK® chillers to create the new cooling system. “The entire development will be about 13 million sq ft,” according to president of Maxon, Marc O’Connor. “We’ll have about 10 to 12 million sq ft on our central energy plant and maybe two million sq ft will be residential high-rise condos, hotels, office buildings, retail buildings, conference center hotels, and the like.”

Miles And Miles

The Atlantic Station development consists of three phases. Upon completion of the final phase, a 25,000-ton central cooling system will circulate approximately 40,000 gpm of chilled water throughout a network of buildings. Several miles of 36-in. and 24-in. chilled water piping installed by Maxon connect the buildings to the plant. Because the plant operates 24/7 and is the entire building network’s sole mechanical cooling source, the system’s operational uptime is paramount.

To ensure Phase I of the chilled water plant met the critical needs, Maxon designed and built a refrigeration plant consisting of three 2,500-ton dual-compressor YORK MaxE™ centrifugal chillers. “The next phase will probably start in ’08,” said O’Connor. This phase may increase the central cooling system by as much as 9,000 tons, depending upon how fast the development, as a whole, grows. Phase III will most likely commence in 2012.

LEED® Points And Savings

From an economic perspective, the motivation to balance physical plant size, refrigeration capacity, operational efficiency, and capital cost led Maxon to install three YORK Model YD chillers. The chillers provide excellent unloading capability, minimized floor space usage, and partial redundancy. They also use HFC-134a refrigerant, and because each building in the development uses chilled water from the central plant, each building captures the assigned LEED® points.

Because the YD chiller uses a single control panel, the dual-compressor chiller operates as a single chiller with optimized unloading capability. When system load and/or entering condenser water temperatures (ECWT) decrease, the control logic allows for optimized sequencing and unloading of the dual compressors, reducing energy consumption.

Although a district cooling plant in Atlanta will tend to have a significant base-load, the compressor’s energy consumption is also influenced by cooling load and ECWT. In fact, even in Atlanta (where the typical design ECWT is 85°F), tower water temperatures below 80° are available more than 88% of the time. With reduced ECWT, the chiller can deliver the design cooling capacity, while consuming a reduced amount of energy.

To realize the full energy savings potential available from using colder tower water, the YORK MaxE chiller is capable of accepting an ECWT as low as 55°. While producing 2,500 tons of cooling and receiving 55° ECWT, the chiller’s efficiency is .47 kW/ton. Operationally, the central plant incorporates a waterside economizer, thus allowing mechanical refrigeration to shut down when outside-air conditions support economizer operation. Typically, transitioning between economizer operation and mechanical cooling operation happens when tower water temperatures are below 55°. 

There For All To See

Maxon also integrated the YORK color graphic display control panels into the plant’s EMS by directly linking the more than 60 available control points to the Johnson Controls Metasys® automation system. The finished product includes plasma display monitors mounted throughout the plant, allowing operators and visitors to monitor the system’s intricate mechanical operation from multiple vantage points.

“It’s laid out so education is a big focus,” said O’Connor. “We have plasma TVs inside the plant just to gather around to look and educate them [the residents] on what is working and where water is going and why it does what it does. We actually got LEED points for our innovation in education for developing a plant that everyone at the development gets to enjoy.”

The project employs one of the most environmentally friendly central cooling systems in the Southeast, and serves as an excellent model for other aspiring green developments. “It doesn’t look like any district energy plant you’ve seen,” said O’Connor.

“It’s a pretty sexy looking plant with glass windows and neon lights, and we light up the chillers and cooling towers [at night], and we have neon signs and plasma TVs. It’s a pretty rock and roll plant.” ES