Data centers are among the most energy-intensive types of facilities, and they are growing dramatically in terms of size and energy intensity.1 As a result, there has been increasing interest on the part of stakeholders—ranging from data-center managers to policymakers—over the last few years to improve data-center energy efficiency. Several industry and government organizations have developed tools, guidelines, and training programs to help work toward this goal.

There are many opportunities to reduce energy use in data centers, and benchmarking studies reveal a wide range of efficiency practices. Data-center operators may not be aware of how efficient their facilities are relative to those of their peers, even those with similar service levels. Benchmarking is an effective way to compare one facility to another and track the performance of a given facility over time.

Toward that end, this article will present key metrics that facility managers can use to assess, track, and manage the efficiency of the infrastructure systems in data centers and thereby identify potential efficiency actions. Most of the benchmarking data presented in this article are drawn from the data-center benchmarking database at Lawrence Berkeley National Laboratory (LBNL). The database was developed from studies commissioned by the California Energy Commission, Pacific Gas and Electric Co., the U.S. Department of Energy, and the New York State Energy Research and Development Authority.

DATA-CENTER INFRASTRUCTURE EFFICIENCY
Data center infrastructure efficiency (DCIE) is the ratio of information-technology- (IT-) equipment energy use to total data-center energy use. DCIE can be calculated for annual site energy, annual source energy, or electrical power. For example:

DCIEsite = IT site energy use ÷ total site energy use
DCIEsource = IT source energy use ÷ total source energy use
DCIEelecpower = IT electrical power ÷ total electrical power

Data-center energy use is the sum of all of the energy used by a data center, including campus chilled water and steam, if present. An online data-center energy profiler, available at http://dcpro.ppc.com, can be used to assess DCIE for site and source energy.2

DCIE provides an overall measure of the infrastructure efficiency (i.e., lower values relative to the peer group suggest higher potential to improve the efficiency of the infrastructure systems, such as HVAC, power distribution, and lights, and vice versa). It is not a measure of IT efficiency. Therefore, a data center that has a high DCIE still may have major opportunities to reduce overall energy use through IT efficiency measures, such as virtualization. DCIE is influenced by climate and tier level. Therefore, the potential to improve it may be limited by those factors. Some data-center professionals prefer to use the inverse of DCIE, known as power-utilization effectiveness, but both metrics serve the same purpose.

The benchmarking data in the LBNL database show a DCIEelecpower range from just over 0.3 to 0.75 (Figure 1). Some data centers are capable of achieving 0.9 or higher.3

TEMPERATURE AND HUMIDITY RANGES
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) guidelines provide a range of allowable and recommended supply temperatures and humidity at the inlet to IT equipment.4

The recommended temperature range is between 64˚F and 80˚F, while the allowable is 59˚F to 90˚F (Figure 2). A low supply-air temperature and a small temperature differential between supply and return typically indicate the opportunity to improve air management, raise supply-air temperature, and thereby reduce energy use. Air management can be improved through better isolation between cold and hot aisles using blanking panels and strip curtains, the optimization of supply-diffuser and return-grille configuration, better cable management, blocked gaps in floor tiles, etc.

The ASHRAE-recommended humidity ranges from a dew point of 42˚F to a dew point of 59˚F, with 60-percent relative humidity (Figure 3). The allowable relative-humidity range is between 20 and 80 percent with a maximum dew point of 63˚F. A small, tightly controlled relative-humidity range suggests opportunities to reduce energy use, especially if there is active humidification and dehumidification. Centralized active control of humidification units reduces conflicting operations between individual units, improving energy efficiency and capacity.

Because temperature and humidity affect the reliability and life of IT equipment, any changes to air management and temperature and humidity settings should be evaluated with metrics, such as Rack Cooling Index, which can be used to assess the thermal health of IT equipment.5 Many data centers operate well without active humidity control. While humidity control is important for physical media, such as tape storage, it generally is not critical for data-center equipment. Studies by LBNL and the Electrostatic Discharge Association suggest that humidity may not need to be controlled as tightly.

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