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more exhaust fans must operate during economizer periodsand the greater airflow required to remove the increased heat load ismore sensitive to the increase in static pressure from duct use.Because of the high fan energy use, a significant improvement inefficiency is observed in the Economizer Plus scenario inwhich thereis a larger temperature difference between the supply and return air.Even though the supply air temperature ismaintained at the Baselinedesign setpoint, cooling energy use is nearly eliminated in themilderclimate regions for mid-tier data centers with the Economizer Plusdesign.The use of water-cooled chillers for the cooling system in enter-prise data centers provides for the most efficient calculated Baselinedesign PUE values and these results are nearly identical across allclimate regions. Outside wetbulb temperature, which varies lesswith outside air conditions than drybulb temperature, limits theefficiency ofwater-cooled systems. In Dallas and Richmond, Baselinedesign PUE values for the enterprise data center are even lower thanthe respective server room and localized data center PUE values forthe Economizer design. The high airflow required for enterprise datacenters causes significant fan energy demand that is furtherincreased by the addition ofmultiple exhaust fans in the Economizerscenario. Without increasing the difference in temperature set-points, this high fan energy demand partially offsets the largeenergy savings gained from reduced chiller operation duringincreased hours of economizer use. However, in the Economizer Plusdesign, the high efficiency of the water-cooled chiller-based coolingsystem means that the increased chiller activity that results fromonly raising the return air temperature (and maintaining the supplyair temperature at the Baseline setting) is small compared to the fan energy savings associated with lower required airflow rates. WithPUE values of 1.30e1.46, depending on climate, the energy efficiencyof the Economizer Plus scenario for enterprise data centers is the bestof all the cases evaluated in this study.In general, relaxing humidity restrictions in the Economizer andEconomizer Plus scenarios improved PUE values by allowing for anincrease in the hours of economizer operation. However, whenapplying the Baseline temperature settings with a DX coolingsystem, completely removing the humidity restrictions (1e100%RH) resulted in increased cooling energy demand in more humidclimates (Chicago, Richmond, Dallas). Cooling during high humidityperiods, when the outside temperature is below the return airdrybulb setpoint and upper dewpoint economizer lockout, canresult in significant latent cooling demand. The increase in coolingenergy associated with relaxed humidity restrictions did not occurafter shifting from DX cooling to chiller systems in the larger datacenters and did not occur in any of the cooling systems when thetemperature setpoints were raised in the Economizer Plus scenario.The PUE results fromFig.1 are applied to the IT energy estimatesin Table 1 to provide the US national data center energy use esti-mates presented in Fig. 2. The 2008 data center energy use estimateof 69 billion kWh would decline to 63 billion kWh when applyingthe Baseline PUE values calculated in this study rather than theuniform PUE of 2.0 as is commonly assumed to apply. The 10%difference in annual energy use may be largely attributable to theidealized airflow and control system characterization used in themodel, suggesting that improved operation of current non-econo-mizer data centers (e.g., better airflow management andcommissioned control sequences) might yield substantial energysavings. The Economizer scenario, inwhich economizers are used inall data centers except for server closets while maintaining theBaseline temperature and humidity restrictions, reduces the annualnational data center energy use estimate to 57 billion kWh.Enterprise data centers provide the greatest savings in the Econo-mizer scenario, owing to significant PUE improvements and thelarge IT energy use attributed to this data center space type. TheEconomizer Plus scenario yields an additional savings of 4 billionkWh/year (to 53 billion kWh/y nationwide) from operating theeconomizers under relaxed humidity and temperature conditionsin local, mid-tier, and enterprise data centers. Although enterprisedata centers represent the largest portion of energy demand,