Because of their computer and HVAC equipment, data-processing centers are among the largest commercial users of electrical energy. While the energy consumed by data-processing equipment is fairly constant year-round, considerable energy savings are possible with an innovative approach to data-center HVAC design.

The entire electrical-kilowatt-power input to computer equipment is converted to heat and discharged into a conditioned space. Most small and medium-size data-processing centers utilize direct-expansion computer-room air conditioners — typically, split-air, water, or glycol-cooled. Large data centers predominantly utilize chilled-water-room units for several reasons, including:

• Increased efficiency of central chillers.

• Elimination of excessive refrigeration piping and charging.

• Simplified maintenance of the central refrigeration plant.

• Greatly reduced outside-equipment footprint.

Per American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards, typical data-center HVAC design conditions (68 to 77°F, 40- to 45-percent relative humidity) are required day and night, 365 days a year. Chilled-water temperature usually is required to be 45 to 50°F, depending on the computer-room-air-conditioning- (CRAC-) unit coil design for cooling and dehumidification. This continuous temperature- and humidity-control requirement also allows the possibility of substantial energy savings.

Two significant energy-saving possibilities for chilled-water systems are the use of free-cooling chillers (in central and northern states and Canada) and simultaneous heat-recovery chillers for dehumidification (in southern states).

FREE COOLING

The continuous cooling demand of data centers is largely unchanged during winter because winter building losses do not have a significant effect on total heat load. With the application of a free-cooling circuit to a central chiller, refrigeration compressors can be switched off for long periods of time during winter, spring, and fall. A water-cooled central chiller can be designed to switch automatically to free cooling as soon as a cooling tower's cooling water or glycol is at or below the return chilled-water temperature, while an air-cooled chiller with an integrated or separate free-cooling coil can switch to free cooling when the outside ambient temperature is lower than the return chilled-water temperature. Both of these energy-efficient designs for large data-center HVAC systems have similar advantages, including a direct reduction in energy costs and fewer compressor running hours. Further, the refrigeration compressors in an air-cooled chiller are never required to operate in very cold ambient temperatures, which eliminates any special low-temperature refrigeration controls and extends the equipment's life expectancy by eliminating the most difficult operating conditions for an air-cooled system.

Consider the savings possible in a New York City data center with 20 25-ton chilled-water CRAC units. This represents an installed cooling capacity of 500 tons. ASHRAE's historical weather data in New York City suggests a 99-percent winter design dry-bulb temperature of 11°F and an average of 3,750 hr of dry-bulb temperatures below 40°F per year. The chilled-water CRAC units typically require approximately 50°F chilled-water-supply and 60°F chilled-water-return temperatures. This means an ideal situation exists for winter free-cooling chiller savings.

Water-cooled chillers

A water-cooled chiller with an economizer is an energy-saving design solution. Winter energy savings from this system design are possible using cooling-tower water whenever it is below 60°F. An open cooling-tower winter economizer loop for chilled-water computer-room units would require an intermediate plate-and-frame heat exchanger to keep the data center's chilled water in a closed loop and free from external contamination. The tower must be winterized, via a variable-frequency drive or fan cycling, with heating and piping thermal insulation. Maintenance would include the chemical treatment of the makeup water, routine blowdown and cleaning, and fill replacement as required.

For redundancy in a critical data center, a duplicate parallel tower should be installed, along with a parallel plate exchanger and adequate filtration of tower water. These items, as well as the addition of piping for winter crossover operation, create a thermally efficient, but relatively expensive and complicated, piping system to provide required redundancy and winter energy savings.

If two 250-ton water-cooled chillers were installed in the New York City data center — each with compressor power of 188 kw, or 376 kw total (assuming two R-407C screw compressors per chiller) — the annual winter free-cooling savings would be:

• 376 kw × 0.25 of compressor savings (25-percent free cooling) × 2,734 hr × 8 cents per kilowatt-hour = $20,560.

• 376 kw × 0.5 of compressor savings (50-percent free cooling) × 1,697 hr × 8 cents per kilowatt-hour = $25,523.

• 376 kw × 1.0 of compressor savings (100-percent free cooling) × 742 hr × 8 cents per kilowatt-hour = $22,319.

The total savings would be about $68,402 more per year (conservatively rounded into 25-, 50-, and 100-percent free-cooling temperature bins) than they would with identical water-cooled chillers without free cooling.

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