No data center cooling method is best for both water and energy in every location. Evaporative cooling towers reject heat using water; dry coolers can bring on-site cooling-water use close to zero but need space and can lose effectiveness in hot weather. Economizers reduce mechanical cooling when outdoor conditions cooperate, while liquid cooling can move heat away from dense computing equipment without deciding how the facility ultimately rejects that heat.
The useful comparison is therefore the whole system: how it captures heat at the servers, how it rejects heat outdoors, and what local climate, water supply, rack density, and available space allow.
Compare the whole cooling path, not just the server loop
A data center cooling system has two linked jobs: capture heat from IT equipment and carry it out of the building. A cold-plate loop or immersion bath addresses the first job; the facility still needs a way to reject the heat, such as a cooling tower or dry cooler. That downstream choice is a major determinant of on-site water use. The U.S. Department of Energy (DOE) describes data center designs in which liquid cooling equipment transfers heat through a coolant distribution unit (CDU) to facility-side systems, including chilled-water loops and cooling towers. DOE, Best Practices Guide for Energy-Efficient Data Center Design (2024)
| Method or system element | Water implications | Energy implications and limits |
|---|---|---|
| Air cooling with mechanical chillers and cooling towers | Tower evaporation and blowdown require makeup water. | Chillers, pumps, fans, and air movement use electricity; results vary with climate, setpoints, plant efficiency, and airflow management. |
| Air-side economizer | Can reduce tower water use when outdoor air replaces mechanical cooling. | Can cut chiller operating hours; air quality, humidity, and climate constrain when it can run. |
| Water-side economizer | May reduce tower demand but does not necessarily eliminate tower use. | A heat exchanger can reduce or bypass chiller compressor work in suitable conditions; integration and local conditions matter. |
| Direct liquid cooling, including cold plates | A closed IT loop does not guarantee low site water use; the heat-rejection plant determines much of the outcome. | Can move heat efficiently and reduce fan or chiller loads in suitable designs; some room-air cooling may remain. |
| Immersion cooling | As with other liquid approaches, downstream heat rejection determines site water use. | Captures heat in a liquid bath; whole-system performance depends on the complete design. |
| Closed-loop dry cooler | ASHRAE describes virtually zero cooling water in the framework’s dry-cooler design. | Can avoid chillers with suitable coolant and ambient temperatures; may need more footprint and perform less effectively in hot weather. |
These are architectural tendencies, not a universal ranking. DOE and ENERGY STAR guidance discusses operational improvements as well as equipment choices, while ASHRAE’s quantitative comparisons are tied to specific scenarios rather than all facilities. DOE, Cooling Water Efficiency Opportunities for Federal Data Centers; ENERGY STAR, Optimize Airflow and HVAC; ASHRAE, Integrated Design Principles
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Evaporative towers: effective heat rejection with a water demand
Cooling towers reject heat in part by evaporating water. As water evaporates, dissolved minerals remain behind, so tower systems discharge some concentrated water as blowdown and replace it with makeup water. Makeup demand depends on operation, water treatment, and cycles of concentration—the number of times dissolved solids are concentrated in tower water before blowdown.
Water management can reduce demand without changing the heat-rejection method. DOE’s Federal Energy Management Program reports that increasing cycles of concentration from three to six can reduce cooling-tower makeup water by 20% and blowdown by 50%. Those figures describe that specific operating change, not a guaranteed saving for every tower. DOE also discusses reverse-osmosis (RO) reuse as a way to offset freshwater needs; RO adds energy use and operating requirements, so the water benefit should be weighed against treatment energy, water quality, maintenance, and discharge constraints. DOE, Cooling Water Efficiency Opportunities for Federal Data Centers
Filtration and maintenance help a tower operate as designed, but do not by themselves establish lower water demand. Site water quality, treatment practices, and scarcity all affect whether a tower is a practical choice.
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Economizers: use outdoor conditions to reduce mechanical cooling
Air-side economizing
An air-side economizer brings suitable outdoor air into the data center, reducing the hours when chillers must mechanically cool the building. The approach depends on the local climate and the outdoor air’s temperature, humidity, and cleanliness. Filters and controls must protect equipment from contaminants and humidity excursions; air-side economizing is not a simple matter of opening a vent whenever outside air feels cool.
Water-side economizing
A water-side economizer uses a heat exchanger to transfer heat when outdoor conditions make it possible to reduce or bypass chiller compressor operation. It can lower chiller load during mild periods, but the system may still rely on a cooling tower. Integration, outdoor conditions, and—in some applications—fresh-water availability affect its usefulness. DOE, Cooling Water Efficiency Opportunities for Federal Data Centers
Operational improvements matter alongside economizer hardware. DOE cites a potential 20% reduction in chiller energy from airflow and chilled-water practices that allow higher chilled-water temperatures and reduced airflow; this is not a whole-facility energy guarantee. ENERGY STAR reports a DOE estimate of 20% to 25% lower fan energy when airflow management is combined with containment, also not a guaranteed facility-wide saving. DOE, Cooling Water Efficiency Opportunities for Federal Data Centers; ENERGY STAR, Optimize Airflow and HVAC
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Direct liquid cooling and immersion: capture heat close to the hardware
Direct liquid cooling is an umbrella for approaches that use liquid to remove heat at or near computing components. Cold plates transfer heat from selected components to coolant; immersion places hardware in a liquid bath. Neither label tells you whether the facility will use water for its final heat-rejection step, nor does either one alone establish a whole-system energy saving.
In a cold-plate design, the IT-side coolant loop can pass heat to a CDU, which then transfers it to a facility loop. DOE describes configurations that retain computer-room air handlers for residual room heat, as well as systems that carry heat through chilled water to a tower. Those arrangements mean a liquid-cooled rack may coexist with air cooling and evaporative heat rejection. DOE, Best Practices Guide for Energy-Efficient Data Center Design (2024); DOE, Cooling Water Efficiency Opportunities for Federal Data Centers
Higher rack power density is one reason to examine liquid cooling, but it does not automatically dictate cold plates versus immersion. DOE’s 2024 guide gives contextual high-performance-computing examples of 60 kW per rack in 2013 and more than 125 kW per rack in more recent examples. These examples illustrate rising density; they are not thresholds that apply to every data center. DOE, Best Practices Guide for Energy-Efficient Data Center Design (2024)
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Dry cooling: reduce site cooling water, with space and heat limits
A closed-loop dry cooler rejects heat without relying on evaporation, so it can sharply reduce on-site cooling-water demand. The trade-off is that dry heat rejection needs adequate heat-transfer area and favorable ambient conditions; ASHRAE notes the potential need for a larger footprint and reduced effectiveness in hot weather. A hybrid design may use adiabatic assistance during hot periods, trading occasional water use for improved performance.
ASHRAE’s AI Data Center Energy Performance Framework discusses high-temperature liquid cooling paired with dry coolers as a particular design scenario. In that context, it describes virtually zero cooling water, a 300× water-efficiency improvement, and approximately 10% lower total data center power. These figures belong to the framework’s scenario and should not be read as typical results for every dry cooler or liquid-cooled site. ASHRAE, Integrated Design Principles
The same framework models a traditional chilled-water plant against a GB200 dry-cooled architecture for a 50 MW IT load: approximately 1.40 versus 1.10 PUE, and approximately 613 GWh versus 481 GWh of annual energy. This is a modeled architecture comparison, not a measured result that can be generalized to other buildings, climates, or operating profiles. ASHRAE, Integrated Design Principles
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How to compare water and energy claims fairly
Before comparing options, identify the measurement boundary. The figures below address different questions, so one cannot substitute for the other.
- PUE (Power Usage Effectiveness): DOE defines it as total annual facility energy divided by annual IT equipment energy. It indicates facility energy overhead relative to IT energy, not water use. DOE, Cooling Water Efficiency Opportunities for Federal Data Centers
- WUE (Water Usage Effectiveness): DOE describes site-based water use relative to IT energy, expressed in liters per kilowatt-hour. State the metric’s boundary and units when reporting it. A site-based figure does not by itself capture indirect water use from electricity generation. DOE, Cooling Water Efficiency Opportunities for Federal Data Centers
- Cooling-system energy: Specify whether the figure includes chillers, fans, pumps, server fans, and water treatment—or only some of them. A chiller or fan saving is not automatically a whole-facility saving.
- Water use: Distinguish on-site cooling water from water used indirectly to generate electricity. Also identify whether the figure counts makeup water, consumption, or another boundary; do not compare unlike measures as if they were equivalent.
A lower PUE does not establish lower water use, and a lower on-site WUE does not establish lower total water impact. The sources do not provide a broadly applicable, head-to-head performance figure across all cooling methods, so a single universal water-and-energy ranking is not supported.
Choose a system around the site
- Map the heat path. Identify how heat leaves the IT equipment, whether it passes through a CDU or air-handling system, and what rejects it outdoors.
- Set the operating constraints. Assess outdoor temperature and humidity, air contamination risks, water availability and quality, discharge limits, rack power density, and space for equipment.
- Compare complete operating loads. Include chiller, pump, fan, treatment, and any remaining room-air cooling energy. Compare water and energy with matching boundaries and operating periods.
- Check retrofit and operations requirements. Account for piping, plant footprint, controls, maintenance, and whether the existing building can accommodate the proposed design.
- Evaluate seasonal and hybrid options. Economizers can be useful only during suitable conditions; hybrid heat rejection may use water during hot periods while avoiding it at other times. Model performance across the site’s actual conditions rather than relying on a technology label.
Cold underground thermal energy storage is another emerging design consideration: DOE describes storing cold water underground for later cooling demand, potentially shifting peak loads and reducing peak grid demand. DOE’s cited page describes a funded project exploring the approach, not a universal deployed solution or a quantified general water-saving result. DOE, Geothermal and Data Centers
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