Cooling plant optimization also includes several key elements that, when combined, can produce significant savings in energy use over the life of the facility.
The first element is supply temperature control. This is based on decoupling temperature and humidity and maintaining a constant discharge air temperature. Eliminating humidity control from the air conditioning units (ACUs) on the data center floor allows for the humidifiers and reheat coils to be removed from the ACUs. The result is a lower first cost and lower maintenance and operating cost for the system.
High delta T cooling is the use of the ceiling plenum as a means of returning the hot air from the hot aisle to the ACUs. It also involves connecting the top return on the ACUs with a duct to the return plenum, effectively returning the warm air from the back of the cabinet directly to the ACUs. Using a constant 60F supply temperature and 30F air temperature rise through the electronics equipment translates into a 90F return temperature to the ACUs. Extensive research with all major manufacturers of computer room ACUs has revealed a near linear relationship between coil delta T and cooling capacity. The result is a data center with bypass air reduced to about 10 percent versus 60 percent using conventional technology.
Because some outside air is required to ventilate the raised floor environment, an effective solution is central ventilation and humidity control which utilizes an ACU in a 2N configuration using 50 percent outside air and 50 percent return air to humidify and dehumidify the data center. The amount of moisture needing to be added or removed from the space is not a function of the total air circulated in the data center, but purely a factor of the condition of the outside air used for ventilation and the moisture migration through the building skin. This can be calculated, and the humidifier and dehumidification coil sized to match the required load.
An efficient option is ultrasonic humidification, which uses high frequency sound waves to evaporate water and operates on one-tenth the energy of a conventional electric system that boils water. Ultrasonic systems use de-ionized water. This requires the installation of a packaged de-ionized water system that some view as a complication not worth the hassle. However, modern packaged de-ionized water systems are highly reliable and provide a boon in maintenance savings. The savings come from having two dedicated ACUs with ultrasonic humidification versus humidifiers and reheat coils in half of the ACUs on the data center floor.
Elevating the chilled water temperatures is another element that will help optimize a data center's cooling plant. Because the data center can be cooled effectively with 60F supply air, it is possible to supply 50F water in lieu of standard chilled water temperatures of 44F. This increases the efficiency of the chillers, and significantly extends the annual hour of free cooling with a water-side economizer cycle.
In a typical N+1 ACU configuration, for every four ACUs there will be a fifth unit. With all five ACUs operating all the time, there is always 20 percent more "bypass air" than required. For cooling plant optimization, have all the ACUs operate at 80 percent air flow via the use of VFDs. In the event of an ACU failure or required maintenance, the other four ACUs would be ramped up to 100 percent airflow. The energy savings are huge. Using fan laws, when airflow is reduced by a certain percentage, the energy required is reduced by the cube of the air flow. Therefore at 80 percent airflow, the horsepower required is roughly 50 percent.
Rajan Battish is a principal charged with leading RTKL's mission critical design and engineering capabilities. R. Stephen Spinazzola, PE, LEED AP, is the vice president in charge of the Applied Technology Group at RTKL.
Data Centers Fight Energy Waste
Optimized Cooling Plants Help Data Centers Save Energy
