Start by measuring facility and IT energy, water use, and useful computing output. Then reduce avoidable IT load, improve airflow and controls, and assess cooling and water-treatment options against the site’s climate, equipment limits, water conditions, and uptime requirements. The right sequence can lower energy and water demand without treating any single technology or efficiency metric as a universal solution.
Measure energy, water, and useful output first
Establish a baseline over a consistent period before changing equipment or controls. Record facility electricity, IT equipment electricity, cooling energy where available, and water use. Track workload or useful-compute output as well, so a reduction in energy is not mistaken for progress if the facility is also delivering less service.
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Use annual PUE for infrastructure efficiency
Power usage effectiveness (PUE) is annual total facility energy divided by annual IT equipment energy. Comparing annual figures helps account for seasonal free cooling and changes in IT load. PUE describes infrastructure efficiency; by itself, it does not show how much useful computing the facility delivered, how much water it used, whether it reused heat, or the carbon impact of its energy.
Pair PUE with other measures
DOE’s Federal Energy Management Program (FEMP) recommends using PUE alongside water usage effectiveness (WUE), energy reuse effectiveness (ERE), carbon usage effectiveness (CUE), and an IT utilization or productivity measure such as transactions per watt or Gflops. These measures help assess different outcomes; agree on definitions and measurement boundaries before comparing sites.
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FEMP’s 2024 energy-efficient data-center design guide reports an average PUE of 1.6 and says several super-efficient data centers have achieved below 1.1. The same guide cites Uptime Institute’s 2022 average of 1.55 for large data centers in 2022. These are figures for different populations and should not be treated as directly interchangeable targets.
Reduce IT energy and heat at the source
Every watt used by IT equipment becomes heat that the facility must manage. Reducing unnecessary IT demand can therefore lower both direct electricity use and the cooling capacity required to remove that heat.
- Review server utilization and consolidate workloads where service, latency, resilience, and security requirements allow.
- Retire equipment that is no longer needed, rather than continuing to power and cool idle capacity.
- When replacing equipment, consider the efficiency of processors, fans, power supplies, and network devices.
- Review storage and power-supply consolidation, and use virtualization where it fits operational requirements.
FEMP’s 2024 guide identifies IT efficiency as an upstream opportunity with secondary benefits for cooling and electrical systems. It also notes that efficient IT equipment can account for a large share of energy use in a facility with efficient cooling. Efficient algorithms may offer another avenue, but they are beyond the guide’s hardware scope.
Improve airflow before adding cooling capacity
Cooling systems work harder when cold supply air bypasses equipment or mixes with hot exhaust before reaching the cooling system. Organize racks so cool air reaches equipment inlets and hot exhaust returns to cooling equipment with as little mixing as possible.
Separate supply air and exhaust
Hot-aisle/cold-aisle layouts position equipment to face cold supply air and direct exhaust toward a hot aisle. Aisle partitions or rack barriers can help maintain separation. Seal bypass paths and check for openings that allow supply air to miss equipment or hot air to recirculate. Blanking panels can close unused rack openings as one part of an engineered airflow plan, but FEMP’s cited guidance does not quantify savings from panels specifically.
Tune airflow and environmental controls
Check fan and pump speeds and review control sequences rather than assuming that more airflow or colder water is always better. Where equipment thermal guidance and operating limits allow, a higher inlet-temperature set point or less restrictive humidity control may reduce chiller demand and create more opportunities for economizing. Permitted conditions depend on equipment class, altitude, and other factors; do not apply a single set point across all facilities without checking the applicable equipment guidance.
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- PCI & HIPPA and EIA/ECA-310-E compliant
FEMP’s water-efficiency guidance, citing its 2024 design guide, says aisle separation can enable higher chilled-water temperatures and reduced airflow, resulting in 20% less chiller energy. That is an attributed potential result, not a guaranteed saving for every site.
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Economizers use favorable ambient conditions to reduce mechanical cooling. Their availability and net effect depend on the local climate, humidity, air quality, water conditions, control settings, and the existing cooling design.
Air-side economizing
An air-side economizer uses cool outdoor air rather than relying entirely on mechanical cooling. Assess contamination controls and safeguards before increasing outside-air use; outdoor conditions that are unsuitable for equipment can undermine reliability.
Water-side economizing
A water-side economizer can use a heat exchanger and cooling tower to cool the chilled-water loop while bypassing or reducing chiller operation. The benefit depends on when outdoor conditions make this possible and on the energy and water required by the system. Evaluate the whole cooling system, not just chiller runtime.
Reduce cooling-tower water losses carefully
Cooling towers use water through evaporation and blowdown. Blowdown removes water containing concentrated dissolved minerals; makeup water replaces losses. Cycles of concentration describe the relationship between makeup water and blowdown. Raising the cycles can reduce water demand, but the practical limit depends on water chemistry, treatment, fouling, corrosion, and equipment requirements.
FEMP’s cooling-tower guidance, published in 2019 and citing its Cooling Tower Best Management Practice, reports that increasing cycles of concentration from three to six reduces cooling-tower makeup water by 20% and blowdown by 50%. Treat these as results attributed to that guidance, not as a promise for a particular tower or water supply.
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Assess treatment and reuse options
FEMP also identifies side-stream filtration and reverse-osmosis treatment of cooling-tower blowdown for reuse as options. They can add maintenance, energy use, cost, and—in the case of reverse osmosis—reject water. Compare them with local water scarcity, discharge rules, treatment needs, and total lifecycle costs before selecting an upgrade.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose cooling architecture for the site and workload
Liquid cooling, dry coolers, hybrid heat rejection, and heat reuse are design choices, not automatic upgrades. Rack density, climate, water availability, equipment thermal limits, reliability needs, air quality, controls, and lifecycle cost all affect the result. FEMP emphasizes that there is no single most-efficient data-center design.
Direct liquid cooling
Direct liquid cooling transfers heat from IT equipment into a liquid loop, which can move heat more effectively than room air for suitable equipment. Designs vary: some retain chillers and cooling towers, while others combine liquid cooling with dry or hybrid heat rejection. FEMP notes potential benefits for PUE and WUE, along with additional control loops and the need for a detailed operations and maintenance plan.
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DOE/FEMP reports that a National Laboratory of the Rockies facility achieved PUE 1.06 and WUE 0.7. These are reported results at that facility, not typical performance values or a prediction for another site.
Heat reuse and dry heat rejection
Heat reuse can make sense when a nearby user needs heat at compatible temperatures and times, and the infrastructure exists to deliver it. Dry coolers can reject heat without the evaporative water use associated with cooling towers, where the design and climate permit. DOE/FEMP/NREL’s stated priority sequence is to reduce energy use first, reuse waste heat where useful, reject as much remaining heat to dry coolers as possible to save water, and maximize renewable energy.
Compare options on more than a single efficiency score
Before approving a project, compare its effect on annual electricity, cooling energy, water use and source, useful compute, reliability, and total cost over the system’s life. Account for maintenance and control complexity as well as equipment purchase and installation.
| Measure or design choice | Potential benefit | Key site-specific checks |
|---|---|---|
| IT consolidation and efficient equipment | Reduces IT electricity and the heat that must be removed. | Workload utilization, service and latency needs, resilience, security, and replacement lifecycle. |
| Airflow separation and control tuning | Can reduce wasted airflow and cooling demand. | Rack layout, bypass paths, equipment thermal guidance, and operating limits. |
| Air- or water-side economizing | Can reduce mechanical cooling when ambient conditions are favorable. | Climate, humidity, air quality, water conditions, safeguards, and hours available. |
| Cooling-tower water optimization | Can reduce makeup water and blowdown. | Water chemistry, treatment, corrosion, fouling, discharge rules, and equipment limits. |
| Liquid cooling, dry heat rejection, or heat reuse | May suit high-density loads, reduce water use, or make waste heat useful. | Workload and rack requirements, climate, nearby heat demand, controls, maintenance, and lifecycle cost. |
FEMP Energy Program Manager Kendall Kam said in a DOE/FEMP interview published December 11, 2024: “Data centers are expensive, so whether you’re in the process of building or renovating one, if you can plan for energy- and water-use efficiencies in advance, you can get the most bang for your buck.” Planning matters because the strongest choice depends on site conditions and operational requirements, not on one headline metric or technology.
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