Ben Stewart, SVP of facility engineering at Terremark (, says:

Data centers strive to reduce energy consumption for two reasons.  First, there is the “green” component and all corporations have a responsibility to the environment and the community to be good stewards of energy.  Second, energy is often the highest cost for many data centers and energy reduction serves to reduce that cost.  Data centers that are highly energy efficient will have a lower overall cost of operation and will be more competitive than those data centers ignoring energy consumption.  Data centers that are not energy efficient will be determined obsolete and removed from service due to their associated high operational expenses.

The greatest opportunity to reduce data center energy consumption is in the cooling system and there are many methods employed to achieve those reductions.  Within the data center, means to segregate the cold air delivered to cool the equipment from the exhausted hot air is critical.  Energy is expended to create the cold air and it must be directed only to the equipment and not lost to mixing with the exhausted hot air.  Equipment aisles are aligned in hot aisle and cold aisle configurations with blanking panels in the cabinets preventing hot air from leaking into the cold aisle.  The cold aisle can be contained with doors and clear covers to prevent the hot air from getting into the cold aisle.  The hot aisle can be contained directing the hot exhaust directly back to the air handler preventing any mixing with the cold air.  
In each case, prohibiting cold air from mixing with the hot air, also referred to as bypass air, serves to ensure the air returned to the handlers is very hot.  Many data centers still control their air handler chill water valves on Return Air Temperature (RAT).  At these sites, the elimination of bypass air causes the RAT to rise and the air handler reacts by increasing the amount of chill water delivered to the coil to bring the RAT back to set point reducing cold aisle temperatures.  This represents an increase in energy consumed as more chill water is required.  Supply Air Temperature (SAT) control should be established prior to any attempt to reduce bypass air so that energy savings can be realized.

Another way to lower energy consumption is to reduce the shaft speed of the motors employed to cool the data center.  Motors are used to pump chill water and move the air.  Motor shaft speed and energy consumption vary as a cubic relationship.  This means that a 40% reduction in shaft speed can result in an 80% reduction in energy consumed by that motor.  Careful control of the amount of cold air delivered to the data center floor allows both chill water and air flow to be minimized.  Respective motor shaft speeds can be reduced resulting in significant energy reductions, so long as the motors are speed controllable.

Both the elimination of bypass air and motor shaft speed reductions can be achieved in any data center regardless of geographic location limiting energy consumption, reducing costs, and meeting corporate social responsibility goals.