Mechanical cooling, depending on the efficiencies of the systems being used, can consume as high as 50% of the total power used in a data center. Good engineering practice, equipment efficiencies, and solid operational understanding can all benefit in a lower cost of ownership and operations.
Mechanical Economization or “Free Cooling”
The advent of “green” data center practices has ushered in a heightened interest in reducing mechanical systems energy use. As part of these efforts and in conjunction with data center design “best practices”, a means of mechanical economization or “free cooling” has become a design standard rather than a luxury.
Mechanical economization utilizes the ambient temperature of the local climate to provide an alternative means of heat rejection from standard mechanical systems. Two means of creating this ambient usage are through waterside or airside systems:
Waterside economization utilizes a liquid medium which is run through an outdoor series of coils to be cooled to a lower temperature. If the ambient cooling meets the necessary set point required for the supply water temperature, the chiller barrel never needs to run, thus greatly reducing the power required for the chiller. During periods where the ambient temperature are not at levels to provide 100% economization, partial “free cooling” can still be provided reducing the overall power needs of the chiller, while still providing some mechanical cooling to reduce the return water to a proper supply temperature.
Airside economization utilizes an air exchange through either a cross stream configuration (which mixes return air with outside air passing through a filter media to help create the supply air stream) or a heat wheel (also known as an enthalpy wheel which nearly eliminates outside air mixing, but typically requires a much larger footprint). This system essentially eliminates the water system medium requirements. These systems can outperform waterside economization in colder climates.
Mechanical Systems Controls and Monitoring
With mechanical systems improving their efficiencies through the equipment improvements and added systems designs noted above, controls and monitoring of these systems becomes more critical in order to maintain these efficient operations. CRAC unit manufacturers have added better controls for these units that allow for a more systematic approach for data center HVAC concerns. Units now communicate with one another throughout the data center and share individual operating conditions to assure a more singular response to the general room conditions.
Monitoring of these systems and trending data also benefit operations and maintenance personnel associated with the data center to better understand the effects of things like economization, maintenance, and other conditions which may affect the data center mechanical systems.
Motors and Drives
Because of reliability requirements in the data center, oftentimes mechanical systems are running at 50% or less their rated capacities during normal operation. This allows for failover scenarios to provide design loads even when a component in the system is not in operation. To further hamper efficient operation, most data center loads are typically below their maximum design capacities in order to plan for growth in the space.
In order to help alleviate the power consumption on equipment running at lower loads and help this equipment maintain better efficiency (as well as better life expectancy), the use of Variable Frequency Drives (VFDs) has provided a simple solution to allow better performance at lower loading while also reducing power consumption. A VFD is an electrical controlling device for motors varying the frequency to consume less power at
lower speeds when loads are not at their rated capacities. A motor can consume as low as 25% of the power required at 60% of the speed compared to 100% loading and speed. Additional benefits include reduced wear at start up and reduced over all motor wear by running it at lower rates than at its single speed maximum.
VFDs can be used in chillers, pumps, and cooling towers of a central system. VFDs can also be used on the air handler systems in the data center as well. VFDs can further provide more recordable information about power consumption for mechanical equipment. The recent improvements in the design technology of the new Variable Frequency Drives (VFDs) over the past few years have been substantial. The operation of HVAC equipment, especially the pumps and CRAC Unit Fans, at reduced speed can produce cost saving of almost 20 percent.