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No data center cooling method is best for both water and energy in every location. Evaporative cooling towers use water to reject heat; dry coolers can nearly eliminate on-site cooling water in suitable designs but need more space and can struggle in hot weather. Economizers reduce mechanical cooling when outdoor conditions allow, while direct liquid cooling moves heat efficiently away from dense computing equipment but does not determine how the facility ultimately rejects that heat. The right comparison is between complete systems, under local climate, water, power, and space constraints.
Why cooling water and cooling energy depend on the whole system
Data center cooling has two linked jobs: capture heat from IT equipment, then reject that heat outside. The first job may use room air, a liquid loop at the rack, or both. The second may rely on a cooling tower, a dry cooler, outdoor air, or a hybrid arrangement. A liquid-cooled server can therefore still be part of a water-using facility if its heat ultimately goes to an evaporative tower.
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In a common air-cooled arrangement, fans move room air through servers and cooling equipment, while chillers cool water and a cooling tower releases heat outdoors. Chillers, pumps, fans, and air movement all consume electricity. Tower evaporation carries heat away, and blowdown removes water in which dissolved minerals have concentrated. The U.S. Department of Energy (DOE) describes this baseline and the associated cooling-water practices in its federal data center cooling-water guidance.
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This distinction matters when a design is described simply as “air cooled,” “liquid cooled,” or “water free.” Those labels may refer to one part of the path rather than the whole facility. For a useful comparison, identify both the equipment-level heat capture and the final heat-rejection method.
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How the main cooling methods compare
| Method | Water implications | Energy implications | Main trade-offs |
|---|---|---|---|
| Air cooling with chillers and cooling towers | Tower evaporation consumes water; blowdown and treatment add to makeup-water needs. | Chillers, pumps, fans, and server airflow use power. | Performance depends on plant efficiency, airflow management, setpoints, and climate. |
| Air-side economizing | Can reduce tower water use when outdoor air replaces mechanical cooling. | Can reduce chiller operation during suitable weather. | Requires acceptable outdoor air quality and humidity, filtration, and controls suited to local conditions. |
| Water-side economizing | May reduce tower heat-rejection demand, but the tower may remain in service. | A heat exchanger can reduce or bypass chiller compressor work in mild conditions. | Needs suitable weather and integration with the plant; some applications depend on freshwater availability. |
| Direct liquid cooling, including cold plates | The equipment loop may be closed, but site water use depends on the downstream heat-rejection plant. | Liquid transports heat from high-density components and may reduce fan or chiller loads in a suitable design. | Usually requires a coolant distribution unit and facility-side loop; room-air cooling may still handle residual heat. |
| Immersion cooling | Like other liquid approaches, it does not by itself determine facility water use. | Captures heat in a liquid bath; total energy depends on the complete system. | It is a form of direct liquid cooling, not a universal guarantee of lower energy use or water-free operation. |
| Dry coolers | Closed-loop dry-cooler designs can use virtually zero water for cooling in the ASHRAE framework described below. | May avoid chillers when coolant temperatures and outdoor conditions are favorable. | Can require more footprint and lose effectiveness in hot weather; adiabatic assistance can use water. |
| Cooling-tower optimization and treatment | Better control of cycles of concentration can reduce blowdown and makeup demand; reclaimed or reverse-osmosis water may offset freshwater use. | Reverse osmosis adds energy use and operating requirements. | Water chemistry, maintenance, discharge rules, and local scarcity constrain the options. |
| Cold underground thermal energy storage | DOE describes storing cold water underground, but does not establish a general water-savings figure for the approach. | Can shift cooling demand to other times and reduce peak grid demand. | DOE describes a funded project exploring the approach, not a universal deployed solution. |
Evaporative towers: water is part of the heat-rejection process
A tower consumes water intentionally: some water evaporates to carry heat away, and blowdown is discharged to limit mineral buildup. Makeup demand depends on tower operation, treatment, and cycles of concentration—the number of times dissolved solids are concentrated before water is discharged. Improving tower management can lower water demand without changing the basic cooling architecture.
DOE’s 2019 guidance says increasing cycles of concentration from three to six can reduce cooling-tower makeup water by 20% and blowdown by 50%, citing the Federal Energy Management Program’s Cooling Tower Best Management Practice. These are the stated effects of that operational change, not a promise of the same savings at every site. The same DOE page reports that airflow and chilled-water practices enabling higher chilled-water temperatures and reduced airflow can lower chiller energy by 20%; that figure is not a whole-facility reduction.
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Economizers: use favorable weather, with the right safeguards
An air-side economizer brings outdoor air into the cooling system when conditions are suitable, reducing reliance on mechanical cooling. It can also reduce tower water use when outdoor air takes the place of cooling that would otherwise be provided mechanically. The design must filter the air and protect equipment from contaminants and humidity excursions.
A water-side economizer uses a heat exchanger to transfer heat without requiring the chiller compressor to do all the work. Depending on plant design and weather, it may reduce chiller load while leaving the cooling tower in use. Neither type has a fixed savings rate: the result depends on climate, operating hours, controls, and system configuration. ENERGY STAR reports a DOE estimate of 20% to 25% lower fan energy when airflow management is combined with containment; that estimate applies to fan energy, not guaranteed total facility savings. See ENERGY STAR’s airflow and HVAC guidance.
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Direct liquid cooling: efficient heat capture, not a complete cooling design
Direct liquid cooling brings coolant close to heat-producing components, commonly using cold plates, or immerses equipment in a heat-transfer liquid. DOE describes a coolant distribution unit (CDU) that transfers heat between equipment-side and facility-side loops. The facility loop still has to reject that heat, for example through a chilled-water system and cooling tower or through a dry cooler. Some designs also retain room-air cooling for heat that the liquid loop does not capture.
Higher rack power density is one reason to evaluate liquid cooling, but it does not select a single technology or dictate water use. DOE’s 2024 design guide cites 60 kW per compute rack in a 2013 example and more than 125 kW per rack in more recent examples; these illustrate high-performance-computing density, not universal thresholds for choosing liquid cooling. The guide describes the facility context in its Best Practices Guide for Energy-Efficient Data Center Design.
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Immersion and cold-plate systems should not be treated as interchangeable simply because both use liquid. Their equipment interfaces and operating requirements differ, while site water use and energy outcomes still depend on the complete design. ASHRAE’s AI Data Center Energy Performance Framework discusses liquid-cooling efficiency in the context of system-level performance rather than a technology label alone.
Dry cooling: a water-saving option with space and temperature costs
A dry cooler rejects heat through a closed loop and heat exchanger rather than relying on evaporation. In ASHRAE’s described framework, closed-loop dry coolers use virtually zero water for cooling; that does not mean the data center has no water use of any kind. The framework notes that dry cooling can require a larger footprint and becomes less effective at higher ambient temperatures. Hybrid or adiabatic assistance can improve hot-weather performance while using some water.
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ASHRAE also gives scenario-specific figures for high-temperature liquid cooling paired with dry coolers: a 300× water-efficiency improvement and approximately 10% lower total data center power in the framework’s discussion. Its modeled architecture comparison for a 50 MW IT load reports PUE values of approximately 1.40 for a traditional chilled-water plant and 1.10 for a GB200 dry-cooled architecture, with annual energy of approximately 613 GWh and 481 GWh respectively. These are framework scenario and modeled architecture figures, not universal measured results or a general ranking of all dry coolers. The assumptions and design context are presented in ASHRAE’s integrated design principles.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare water and energy figures fairly
Before comparing two facilities or proposals, check what each metric counts. PUE is total annual facility energy divided by annual IT equipment energy. It captures facility overhead relative to IT load, but a lower PUE alone does not establish lower water use. WUE expresses site-based water use relative to IT energy, commonly in liters per kilowatt-hour; the boundary and units should be stated. A lower on-site WUE does not, by itself, establish lower water impact beyond the site, including water associated indirectly with electricity generation. DOE defines these measures and discusses their use in its cooling-water guidance.
- Water boundary: distinguish water consumed or withdrawn on site for cooling from indirect water used to generate electricity.
- Heat-rejection path: identify whether heat goes to a tower, dry cooler, economizer, or hybrid system.
- Energy boundary: say whether the figure covers cooling equipment alone or the whole facility, and whether it includes chiller, fan, pump, server-fan, and treatment energy.
- Local conditions: account for climate, humidity, useful economizer hours, freshwater availability and quality, and hot-weather performance.
- IT load and residual heat: compare rack power density, heat captured at chips, and any remaining room-air cooling requirement.
- Build and operating constraints: include footprint, retrofit feasibility, piping, maintenance, controls, and water-discharge requirements.
A single percentage is meaningful only when its baseline, system boundary, operating conditions, and time period are known. There is no broadly applicable head-to-head performance figure that establishes one cooling method as the most water- and energy-efficient across all sites.
Choosing a design for a particular facility
- Establish the site constraints. Document seasonal temperatures and humidity, water availability and quality, space for heat-rejection equipment, and discharge requirements.
- Map the heat path. Identify how each IT load captures heat, whether through room air or a liquid loop, and where the facility ultimately rejects it.
- Match technology to the load. Evaluate direct liquid cooling where rack density and equipment design make it useful; assess economizers against actual local operating hours rather than annual averages alone.
- Compare complete-system performance. Request water and energy estimates with explicit boundaries, including residual air cooling, treatment energy, pumps, fans, chillers, and any hybrid water use.
- Include operations and resilience. Consider maintenance capacity, controls, water treatment, hot-weather operation, and how the system behaves when a component or water source is unavailable.
For federal data centers, DOE also describes cold underground thermal energy storage as a way to store cold water for later cooling demand and shift peak load. The agency’s Geothermal and Data Centers page presents this as a funded project exploring the approach; it does not establish a generally applicable water-savings result.
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