There is no single best data center cooling method. Air cooling moves heat through the data hall in air; evaporative cooling uses water evaporation to cool air or reject heat; liquid cooling carries heat away from IT equipment in a fluid loop. The right choice depends on rack density, local climate, water availability, energy goals, retrofit limits, resilience needs, and lifecycle cost. These methods can also work together: liquid-cooled servers still need a facility system to reject heat, and often need room-air cooling for residual heat.
How do air, evaporative, and liquid cooling differ?
The terms describe different parts of a heat-removal system, not always mutually exclusive facility designs. Air and liquid describe how heat leaves IT equipment. Evaporation describes a way to cool air or reject heat from a system.
- Air cooling: IT heat enters room air, which fans and cooling equipment move to heat-rejection equipment.
- Evaporative cooling: Water evaporation lowers air temperature or helps dissipate heat, with water consumption as a trade-off.
- Liquid cooling: A circulating fluid carries heat from IT components to a heat exchanger or coolant distribution unit (CDU), then to facility heat rejection.
For an overview of these system boundaries, see DOE FEMP’s data-center cooling and water-efficiency guidance.
How air cooling works
In a conventional air-cooled data hall, servers release heat into the room. Computer-room air-conditioning equipment draws in that heated air, transfers the heat to a chilled-water system or another heat-rejection arrangement, and supplies cooled air back to the IT space. Fans and server airflow do much of the work of moving heat to the room cooling equipment.
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Airflow management and economizers
Separating server exhaust from cool intake air helps limit mixing and supports efficient airflow. DOE FEMP’s Best Practices Guide for Energy-Efficient Data Center Design reports 20% less chiller energy for the cited hot/cold-aisle and airflow practices; treat that as a guide-specific reported result, not a guaranteed saving for every facility. DOE FEMP
When outdoor conditions are suitable, an economizer can reduce or avoid mechanical refrigeration. Direct air economizers bring outdoor air into the data hall; indirect air economizers transfer heat through a heat exchanger without mixing outdoor and indoor air; indirect fluid economizers use an intermediate fluid loop. “Free cooling” still uses fan or pump energy, and direct outdoor-air systems need controls for air quality and humidity. Whether an economizer is useful depends on local weather and the IT operating envelope. ASHRAE Handbook Chapter 20
How evaporative cooling works—and how much water it uses
Evaporative equipment uses water’s phase change to absorb heat. Its performance depends on ambient wet-bulb conditions, system design, operating mode, and the facility’s water constraints. It is not accurate to assume that every evaporative system always saves energy or water.
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Evaporative air cooling
Direct evaporative cooling passes air over wetted pads or through a spray. As water evaporates, the air’s dry-bulb temperature falls and its moisture content rises; its temperature approaches the ambient wet-bulb temperature. Indirect evaporative equipment transfers cooling through a heat exchanger, so the delivered air is not directly humidified. ASHRAE Handbook Chapter 41
Evaporation at heat rejection
Cooling towers use evaporation to release heat. They consume water through evaporation, and also require blowdown to control dissolved minerals. Wet heat rejection is typically more energy efficient than dry heat rejection, while dry operation saves water and can help during drought contingencies. Hybrid equipment can switch between wet and dry operation as conditions change. DOE FEMP; ASHRAE Handbook Chapter 20
How liquid cooling works
Direct liquid cooling transfers heat from IT equipment into a recirculating fluid loop rather than first transferring all of it to room air. In DOE FEMP’s schematic, a closed loop carries heat from racks to a CDU, where it transfers to another loop for rejection. A CDU or heat exchanger connects IT-side fluid distribution with the facility system; the exact arrangement varies by design. DOE FEMP
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Liquid cooling is often considered for dense IT loads, but it does not remove the need for facility heat rejection. Depending on design, that heat may go to chillers, cooling towers, dry coolers, or a combination. Room air can also remain necessary for residual equipment heat. Liquid loops need appropriate redundancy and coordination between IT and facility operations. ASHRAE Handbook Chapter 20; ASHRAE’s liquid-cooling white paper
What the SuperMUC-NG case does—and does not—show
ASHRAE’s 2021 white paper describes direct warm-water cooling at 40°C–45°C at SuperMUC-NG, the Leibniz Supercomputing Centre, and reports 30% energy savings in that facility’s configuration. The case combines lower server-fan power, reduced cooling power, energy-aware scheduling, and less mechanical refrigeration; it is not a controlled universal comparison of liquid and air cooling. ASHRAE liquid-cooling white paper, 2021
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Air vs. evaporative vs. liquid cooling: practical comparison
| Decision factor | Air cooling | Evaporative approaches | Liquid cooling |
|---|---|---|---|
| Heat path | IT heat enters room air; fans and room cooling equipment move it to heat rejection. | Evaporation cools air or rejects system heat; it may be direct, indirect, or used in a cooling tower. | IT heat enters a circulating fluid loop; a CDU or heat exchanger transfers it to facility heat rejection. |
| Climate dependence | Economizer opportunities depend on outdoor conditions and the IT operating envelope. | Wet-bulb conditions affect performance; climate and water availability matter. | Warm-water operation may reduce chiller dependence, but final heat rejection still depends on design and ambient conditions. |
| Water implications | Air-side economizing can avoid cooling-tower water during those hours, depending on the rest of the system. | Evaporation consumes water; tower blowdown adds to make-up demand. | A closed IT coolant loop does not prove the facility uses no water; downstream heat rejection may use dry, wet, or hybrid equipment. |
| Density and integration | Requires planned airflow and separation of hot exhaust from cool intake; capacity depends on site design. | Can support air cooling with evaporative stages; design depends on humidity, water, and climate. | Often considered for dense IT; requires fluid distribution, CDU or heat-exchanger integration, maintenance, and redundancy. |
| What to measure | Whole-facility energy, IT energy, and direct water use, with clear boundaries. | Both water and energy outcomes, rather than energy efficiency alone. | Facility and IT energy boundaries, cooling auxiliaries, water use, and thermal conformance. |
This is a qualitative comparison, not a performance guarantee. DOE FEMP; ASHRAE Handbook Chapter 20; ASHRAE Handbook Chapter 41
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Which data-center cooling method is most efficient?
There is no supported universal ranking. Efficiency depends on the complete plant, local weather, IT load and operating temperatures, redundancy, part-load behavior, and the energy and water boundaries used in measurement. Evaporation can improve cooling performance while consuming water; dry heat rejection saves water but may use more energy. Liquid cooling can reduce some cooling loads, but its result depends on the facility system that ultimately rejects the heat.
Use PUE and WUE with their boundaries
- PUE is annual total facility energy divided by annual IT equipment energy. DOE FEMP says highly efficient facilities can approach the theoretical minimum of 1.0; that is a limit, not a typical result.
- WUE is annual site water use in liters divided by IT equipment annual energy use in kWh. State the site-water boundary when reporting it.
PUE alone is not a fair way to rank unrelated facilities. ASHRAE Handbook Chapter 20 says it was not intended for cross-facility efficiency comparisons because climate zone, redundancy, and other conditions affect the number. Compare facilities only with consistent boundaries and context. DOE FEMP; ASHRAE Handbook Chapter 20
Account for operating temperature and heat reuse
ASHRAE’s AI Data Center Energy Performance Framework lists W17, W27, W32, W40, W45, and W+ classes. Each class embeds its upper temperature limit, and all share a lower limit of 2°C (35.6°F). The applicable class and equipment requirements matter when evaluating whether a cooling design can operate at a warmer temperature. ASHRAE AI Data Center Energy Performance Framework
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Also assess heat-reuse opportunities where outlet temperatures and nearby demand make reuse practical. Evaluate expected part-load operation, since plant load changes over time and part-load efficiency can affect the outcome. ASHRAE Handbook Chapter 20; ASHRAE liquid-cooling white paper
How to choose a cooling approach for a specific site
- Define the IT requirement. Document current and expected load, rack density, equipment temperature limits, growth plans, and resilience requirements.
- Map the existing facility. Identify cooling equipment, available space, distribution paths, retrofit constraints, and any existing liquid loops or heat-rejection assets.
- Model local conditions. Evaluate weather and likely economizer hours, outdoor-air quality and humidity, water source and water stress, and local energy and water tariffs.
- Compare complete operating modes. Include wet, dry, and hybrid heat rejection where relevant; model expected part-load behavior rather than only peak conditions.
- Set consistent measurement boundaries. Compare energy, water, PUE, and WUE on the same basis, including cooling auxiliaries and heat-rejection use.
- Check lifecycle and resilience needs. Include capital and operating costs, maintenance, redundancy, drought contingencies, and the operational coordination required between IT and facilities.
- Assess heat reuse. Consider it only where the heat’s temperature and a nearby, dependable demand make reuse practical.
Specific costs and optimal designs cannot be determined without local climate, utility rates, water conditions, load profile, equipment, and resilience requirements.
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