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Courtesy Integral Group
The east wing (right) has radiant cooling instead of variable air volume (VAV) systems.

PROJECTS:

SDB1 Office Building

Integral Group
Hyderabad, India

Energy Showdown: Side-by-side installations of mechanical systems show savings from radiant cooling.

By Nadav Malin
July 2012
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With 130,000 employees at 10 locations in India, technology giant Infosys manages a whopping 28 million square feet of floorspace and uses about one million kilowatt-hours per workday. Until 2008 its buildings were designed to impress visitors but not necessarily to save energy. That changed, however, when Rohan Parikh was hired as head of green initiatives and charged with making the company's SDB1 office building on its Hyderabad campus the most energy-efficient building in the world.

TEAM & SOURCES

Architects, interiors and structural design Sundaram Architects

Daylighting and energy consultants EDS (Environmental Design Services)

HVAC principal consultant Integral Group

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After extensive research, and with the support of Peter Rumsey, managing partner of the consulting engineers Integral Group in San Francisco, Parikh recommended radiant cooling in the slabs instead of standard variable air volume (VAV) systems for the 240,000-square-foot building. Radiant cooling has been used for centuries to enhance comfort in open-air palaces throughout India, but its use in sealed office buildings is relatively rare, and unheard of in India.

Not surprisingly, engineers at Infosys's local firm, ARCOV, were skeptical that a radiant system could work in Hyderabad's climate, which Rumsey describes as feeling like "Houston half the year and Phoenix the other half." ARCOV's team argued that radiant slabs wouldn't be able to maintain comfort and that the cool surfaces of the slabs would attract condensation in the humid months.

The debate resulted in a Solomonic decision from the head of facilities at Infosys: Split the building in half, using conventional high-efficiency VAV systems for one wing and radiant slabs for the other. After getting over their initial disappointment at not being able to control the entire project, Rumsey and Parikh realized they had been handed a great opportunity: "A gargantuan HVAC show-down," says Rumsey. Infosys would then apply lessons from this side-by-side test to future offices.

Before designing the HVAC systems, Rumsey and his team insulated the building envelope and shaded the windows, with a goal of avoiding any direct sunlight on the glass. The installed lighting load is only 0.45 watts per square foot, and with the daylight controls it's operating at a level equivalent to 0.17 watts per square foot.

Challenged to match the efficiency of the radiant systems, the ARCOV engineers didn't hold back on the VAV side, using "all the bells and whistles," according to Rumsey, including variable-speed fans, chillers, pumps, and cooling towers, and avoiding reheat.

Making the comparison between the two systems even starker, there was no first-cost premium for the radiant cooling approach. The construction cost of the two systems was nearly identical. That pricing makes sense to Rumsey, who has been frustrated by contractors in the U.S. who quote $10–$15 per square foot to install radiant tubing in a slab, even though the tubing only costs 20 cents per square foot. Large companies with strong purchasing power and big open slabs typically pay $3 per square foot, Rumsey claims.

In addition to radiant tubing in the slabs, which cools the space both from above and from below, the more efficient system uses dedicated outdoor air system (DOAS) ventilation, providing cooled outdoor air to all spaces. This air is dehumidified to slightly below required levels in order to account for moisture added by occupants.

Now that SDB1 has been occupied for nearly a year, the results are unequivocal. The VAV (air system) wing uses less than half the energy for cooling and ventilation than Infosys's other buildings on the Hyderabad campus, but the radiant wing beats that by another 33 percent, making it three times more efficient in terms of HVAC energy in spite of being more densely occupied. (Researchers from the Braunschweig University of Technology in Germany independently verified the results.)

The radiant system has two primary advantages in terms of energy efficiency. First, it takes a lot less energy to move heat by pumping water than by blowing air. Second, the water is chilled to 55 degrees Fahrenheit as opposed to 45 degrees Fahrenheit for the air system, so the radiant system avoids the need to chill those last 10 degrees—the most energy-intensive part of the cooling load. The outdoor-air dehumidification coils do require colder water, however.

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