Why are NYC area data center owners paying expensive utility bills to cool their mission critical facilities in the dead of winterwhen temperatures barely reach 40^°F? Data centers must be cooled24hoursaday, 365 days ayear to prevent IT equipment from overheating and preserve uptime, but new technology can reduce the annual operating hours of cooling equipment. One of the most efficient ways to reduce energy consumption is to harness the winter temperatures by installing economizers. Economizers draw in outdoor air and reject heat directly to the environment without the use of mechanical refrigeration. This process is known as “free cooling,” because mechanical equipment can be shut off or operated at reduced loads.

Though effective, economizers cause concern to some data center owners, especially in urban environments. Along with cooler temperatures, outside air carries water vapor, and contaminants, which increase humidity control and filtration requirements. Economizers can also pose security, space availability, and operational and maintenance challenges. Recently, Willdan Energy Solutions wrote a white paper exploring the innovative approach of using water-side economizers with closed-circuit cooling towers, coupled with air cooled chillers, to alleviate many of the common concerns associated with economizers.

Closed-circuit cooling towers, sometimes called evaporative fluid coolers, essentially combine a heat exchanger with a cooling tower. In these systems, cooling energy from the outside air is transported indirectly through liquid rather than directly from the air like traditional air-side economizer systems. Airside economizers typically require large ductwork and intake and exhaust air openings while closed circuit cooling tower based waterside economizers only require space for fluid coolers and pumps. While it may seem inefficient to use an indirect heat transfer medium rather than a direct-air economizer system, water can move pound-for-pound nearly 42 times the amount of heat as compared to air.

We evaluated the benefits of this approach by analyzing a 10,000 sq ft, Tier III data center in NYC with a cooling load of 386 tons and an IT load of 1,200 kW. The closed-circuit cooling tower-based economizer saved 35% in energy and resulted in a 10% reduction in the mechanical PUE over a traditional data center with an air-cooled chiller plant with no economizers. Waterside economization with a closed-circuit cooling tower is useful whenever the combination of wetbulb temperature plus the closed-circuit cooling tower approach is lower than the temperature of chilled water returning from the load. The wetbulb temperature is always equal to, or lower than, the drybulb temperature of the air, which provides an advantage for evaporative cooling based economizers.

Waterside economizers with closed-circuit cooling towers, coupled with air-cooled chillers, offer significant operating energy savings compared to air-cooled chillers alone, while simplifying the control of humidity and contaminants in data center facilities. On existing data centers with air cooled chillers, waterside economizers can be implemented easily using closed-circuit cooling towers on a retrofit basis.