How are data centers cooled?

Air cooling: hot aisle, cold aisle

The starting point is airflow management. DOE’s 2024 guide describes a hot aisle/cold aisle layout as one where racks and the air supply and return are arranged to keep hot exhaust air from mixing with the cool supply air drawn into the racks[3]. CRS notes servers tolerate higher temperatures but need lower humidity[5].

In small to midsize facilities, the cooling itself is often done by direct expansion units, usually called computer room air conditioners (CRACs), which LBNL says are common for their simplicity and cost-effectiveness[6]. Larger sites typically use water-cooled chillers, which LBNL says are widely used for high efficiency and capacity and release heat through cooling towers[6].

Free cooling: economizers and dry coolers

When the weather cooperates, a data center can skip much of the mechanical cooling. DOE says that on most nights and in mild winter conditions an air-side economizer, which brings in outdoor air, is the lowest-cost way to cool a data center, subject to an engineering evaluation of the local climate[3]. A waterside economizer instead uses a cooling tower’s evaporative capacity to make chilled water in mild conditions[3]. Dry coolers reject heat straight to the ambient air, allowing free cooling in favorable weather and conserving water compared with evaporative towers[6].

Liquid cooling and immersion

Direct liquid cooling means transferring waste heat to a fluid at or very near where the heat is generated, rather than to room air[3]. The physics is the reason it is spreading: liquid has roughly 1,000 times the cooling capacity of air, and rack densities of 60 kW or more can be handled with warm-water liquid cooling[2]. CRS says high-performance computing has required cooling closer to the chips, and that direct liquid cooling can address its higher power[5].

Immersion cooling goes further: EESI describes bathing servers, chips and other components in a non-conductive (dielectric) fluid that absorbs heat and passes it to a heat exchanger[7]. Closed-loop systems, meanwhile, circulate water within a sealed system without direct exposure to the environment, and CRS says they may generate more limited wastewater discharges during normal operation[8].

How dense are racks, really?

Headline AI figures are far above the current average. Uptime Institute’s 2025 global survey of operators found an average modal rack density of 7.5 kW, up from 6.8 kW in 2024, and more than 80% of respondents said their facility has no racks above 30 kW[9]. An industry executive quoted by IEEE Spectrum in October 2025 said average rack density was around 8 kW and is growing to 100 kW per rack for AI[10]. Both can be true: the fleet average moves slowly while new AI halls are built for much higher densities.

The water-versus-energy trade-off

Cooling choices trade water against electricity. CRS says evaporation-based cooling is generally more energy efficient than air-cooled chillers or other waterless systems, while air-cooled chillers use no water but comparatively more energy[8], and that water-cooled chiller systems typically have the highest (least efficient) water use, largely because of cooling towers[8]. LBNL notes evaporative systems consume substantial amounts of water even in configurations it calls highly energy-efficient[6]. EESI frames the choice by constraint: liquid cooling where water is limited, evaporative towers where the power grid is strained, since they need minimal power[7].

Companies are making that trade explicitly. Microsoft says its zero-water-for-cooling design will avoid more than 125 million liters of water per year per datacenter[11], and that replacing evaporative systems with mechanical cooling will raise its PUE, which it calls a nominal increase in annual energy use[11] (both company claims). For what individual projects report, see the water tracker.

Measuring cooling: PUE

Power Usage Effectiveness is total data center electricity divided by IT equipment electricity[6] — so a PUE of 1.6 means 60% more energy is used than the IT load alone. DOE’s 2024 guide says an average data center has a PUE of 1.6, while some recent super-efficient data centers are below 1.1[3]. Uptime’s 2025 survey put the weighted-average annual PUE at 1.54, the sixth consecutive year it has barely moved[9]; Google reports a fleet average of 1.09 for 2025[12]. DOE’s ARPA-E COOLERCHIPS program targets cutting total cooling energy below 5% of a typical data center’s IT load, anywhere in the U.S.[13].

Common questions

How are data centers cooled?

Either by moving air through ductwork or by moving water or another heat transfer fluid through pipes, according to the Congressional Research Service. Air-cooled halls use hot aisle/cold aisle layouts with CRAC units or chillers; denser AI halls increasingly use direct-to-chip liquid cooling or immersion.

How much energy does data center cooling use?

DOE has said cooling can account for up to 40% of a data center’s overall energy use, and a 2025 CRS report puts it at 38% to 40% of electricity consumption.

What is immersion cooling?

Servers and components are submerged in a non-conductive (dielectric) fluid that absorbs heat and transfers it to a heat exchanger, according to the Environmental and Energy Study Institute.

Do data centers have to use water to cool?

No. Air-cooled chillers and dry coolers use no water but more electricity, while evaporative cooling uses water and less energy. CRS describes this as the core trade-off, and some operators now build zero-water-for-cooling designs.

Sources

1. DOE Announces $40 Million for More Efficient Cooling for Data Centers
   U.S. Department of Energy · 2023-05-09
   Source excerpt

   “data center cooling can account for up to 40% of data center energy usage overall”

2. High-Performance Computing Data Center Warm-Water Liquid Cooling
   National Laboratory of the Rockies (formerly National Renewable Energy Laboratory) · 2025-12-04
   Source excerpt

   “Liquid has approximately 1,000 times the cooling capacity of air. … And rack power densities of 60 kilowatts (kW) per rack or more can be achieved using warm-water liquid cooling, which requires less floor space.”

3. Best Practices Guide for Energy-Efficient Data Center Design (Revised July 2024)
   U.S. Department of Energy, Federal Energy Management Program · 2024-07
   5 source excerpts

   “An average data center has a PUE of 1.6; however, several recent super-efficient data centers have been known to achieve a PUE below 1.1.”

   “A basic hot aisle/cold aisle configuration is created when the equipment racks and the cooling system's air supply and return are designed to prevent mixing of the hot rack exhaust air and the cool supply air drawn into the racks.”

   “Most nights and during mild winter conditions, the lowest cost option to cool data centers is an air-side economizer; however, a proper engineering evaluation of the local climate conditions must be completed to evaluate whether this is the case for a specific data center.”

   “Free cooling can be provided via a waterside economizer, which uses the evaporative cooling capacity of a cooling tower to produce chilled water to cool the data center during mild outdoor conditions.”

   “Direct liquid cooling refers to a number of different cooling approaches that all share the same characteristic of transferring waste heat to a fluid at or very near the point the heat is generated, rather than transferring it to room air and then conditioning the room air.”

4. High-Performance Computing Data Center Cooling System Energy Efficiency
   National Renewable Energy Laboratory (nrel.gov) · 2025-04-10
   Source excerpt

   “IT equipment, such as compute clusters and data storage systems, produces heat as a byproduct.”

5. Data Centers and Their Energy Consumption: Frequently Asked Questions (R48646)
   Congressional Research Service · 2025-08-26
   3 source excerpts

   “computer servers tolerant of higher temperatures but requiring lower humidity. … There are two types of centralized cooling resources: (1) those moving air through large ductwork to deliver chilled air and remove warm air; or (2) those moving water or other heat transfer fluid through a piped cooling loop that exchanges heat with the environment.”

   “The cooling systems could account for another 38% to 40% of electricity consumption in a data center.”

   “High-performance computing (HPC) equipment has necessitated cooling methods that are thermodynamically closer to the chips and intercept the energy before it has substantially raised the room air temperature. These direct liquid cooling technologies can address the higher power of HPC.”

6. 2024 United States Data Center Energy Usage Report
   Lawrence Berkeley National Laboratory · 2024-12-20
   5 source excerpts

   “Direct expansion systems, typically called computer room air conditioners (CRACs), are commonly employed in small to midsize data centers because of their simplicity and cost-effectiveness.”

   “Water-cooled chillers are widely used in data centers, owing to their high efficiency and capacity to manage substantial cooling requirements. These systems use water-cooled condensers to extract heat from the system, subsequently releasing it into the environment via cooling towers.”

   “Dry coolers reject heat to the ambient air and enable "free" cooling during favorable weather conditions, with or without adiabatic assistance. … This system conserves water compared to evaporative cooling towers.”

   “While highly energy-efficient, this system consumes substantial amounts of water, like all evaporative cooling systems.”

   “Power Usage Effectiveness (PUE) is defined as the total electricity demand of the data center divided by the electricity demand of the IT equipment.”

7. Data Centers and Water Consumption
   Environmental and Energy Study Institute · 2025-06-25
   2 source excerpts

   “Immersion cooling in data centers involves bathing servers, chips, and other components in a specialized dielectric (or non-conductive) fluid. … The non-conductive liquid absorbs the heat from the chips and transfers it to a heat exchanger”

   “In areas with limited water availability, server liquid cooling is the best choice, as it requires minimal water consumption. Conversely, in areas with a strained power grid, an evaporative air cooling tower is a suitable building design, as it requires minimal power usage.”

8. Data Centers and Water: Frequently Asked Questions (R49057)
   Congressional Research Service (via EveryCRSReport.com) · 2026-07-31
   3 source excerpts

   “Closed-loop cooling systems, which circulate water within a sealed system without direct exposure to the environment, may generate more limited wastewater discharges during normal operational phases.”

   “Trade-offs in water consumed and power usage exist between evaporation-based cooling systems and air-cooled systems. Evaporation-based cooling systems are generally more energy efficient than air-cooled chillers or other waterless systems. While air-cooled chillers use no water, they use comparatively more energy.”

   “Water-cooled chiller systems typically have the highest WUE (i.e., are less efficient at using water), which is largely attributed to the substantial cooling tower water usage.”

9. Uptime Institute Global Data Center Survey 2025
   Uptime Institute · 2025-07
   2 source excerpts

   “Average modal density excluding these outliers comes to 7.5 kW in our 2025 survey, up from 6.8 kW in 2024. … More than 80% of the operators responding to our survey say their facility has no racks above 30 kW — about the same share as last year.”

   “In 2025, respondents' annual PUE had a weighted average of 1.54 (see Figure 2), marking the sixth consecutive year that this headline figure has virtually stood still.”

10. Next-Gen AI Needs Liquid Cooling
    IEEE Spectrum · 2025-10-13
    Source excerpt

    “The average power density in a rack was around 8 kW … For AI, that's growing to 100 kW per rack. That's an order of magnitude.”

11. Sustainable by design: Next-generation datacenters consume zero water for cooling
    Microsoft · 2024-12-09
    2 source excerpts

    “this design will avoid the need for more than 125 million liters of water per year per datacenter.”

    “Replacement of evaporative systems with mechanical cooling will increase our power usage effectiveness (PUE). … The result is a nominal increase in our annual energy usage compared to our evaporative datacenter designs across the global fleet.”

12. Power usage effectiveness – Google Data Centers
    Google · 2026
    Source excerpt

    “In 2025, the average annual power usage effectiveness for our global fleet of data centers was 1.09.”

13. COOLERCHIPS
    U.S. Department of Energy, ARPA-E
    Source excerpt

    “The target for COOLERCHIPS is to reduce total cooling energy expenditure to less than 5% of a typical data center's IT load at any time and any U.S. location for a high-density compute system.”

Each source was opened and checked against the excerpt shown. Company statements are identified as such. Figures are as published; where sources differ, each is shown.