Free Cooling

In Data Centers 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:

Demands relating to safety, the autarky of the entire system and power management are also rising steadily. The remote maintenance of a computer center or of individual server locations is the current requirement of the industry. Practically all the cabinet parameters from temperature, moisture, smoke and vibration to door contacts and access check and mains-phase monitoring should be recorded and documented in a single unit.

	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 and simultaneous heat-recovery chillers for dehumidification.

Free Cooling

Supercooler as freecooling module placed on a roof

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.

Example:

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.

System I – direct expansion closed control units with aircooled condensers

	For small / medium thermal loads (max. 30 kW IT
	- load )
	Redudancy only with complete doubled systems
	Strong dehumidification cause of low
	evaporations temperature
	Humidification and heating neccessary with high
	operation costs
	Low invest, but high operating costs cause of energy
	and maintenance
	No emergancy-cooling-operation possible in case
	of blackout.

System II - DX – closed control units with free cooling exchanger and glycol cooler

	For medium IT-load & single rooms up to ca. 80 kW
	Bad for placing in the serverroom because of high effort in
	Maintenance and service
	Later extension (for increasing loads) not possible
	Same else system
	Relativ low invest, but also high operating costs because of
	stronger fans, circulatingpumps + high effort in maintenance

System III – central water chiller system with free cooling and watercooled closed control units

	Very flexible system for IT-loads from up 50 kW until
	2.000 kW (and bigger) with using of free cooling
	Very good controling of the performance with (n+1) water
	chiller and closed control units
	Emergancy running with buffertank and ups-buffered
	closed control units and circulation pumps
	Sensible operation with chilled water 12/18 °C & high
	coefficent by using screw- or scroll compressor
	Relativ high first-invest cost but minimum service- and
	maintenance cost

System IV – central water chiller system with free cooling

	Flexible cooling systems for IT loads from up 80 kW until
	2.000 kW with energyefficience free cooling
	Similar system configuration and operating costs like central
	water chiller system
	Less space for installation inside of the builing necessary
	High quality cooling technology, Placed weatherproofed
	outside