Economizers, evaporative cooling and water use

Chillers are needed only when the outdoor environment is too warm to absorb the heat directly. For much of the year in most of North America, outdoor air or water cooled by evaporation is colder than the 18 to 27 °C air that servers need, or the 20 to 40 °C water that liquid-cooled equipment can accept. Economizers exploit that, cooling with outdoor conditions instead of compressors, and they are the main reason modern facilities achieve PUEs of 1.1 to 1.2. Evaporation is the most powerful way to extend economizer hours, but it consumes water, which in many regions is scarcer, and more politically sensitive, than electricity.

This lesson covers air-side and water-side economizers, evaporative cooling, how climate determines economizer hours, how much water evaporative systems use and how it is measured, and the trade-off between water and energy.

Air-side economizers

A direct air-side economizer brings filtered outdoor air into the data hall and exhausts the hot return air, mixing in return air when the outdoor air is too cold. When outdoor air is cooler than the required supply temperature, no mechanical cooling is needed at all. Direct systems are simple and very efficient, and several large operators use them in suitable climates, but they expose IT equipment to outdoor contaminants (dust, salt, corrosive gases, wildfire smoke) and humidity swings, so they need good filtration (commonly MERV 13 or better), humidity control, monitoring, and a way to close the dampers and switch to recirculation and mechanical cooling during smoke or pollution events.

An indirect air-side economizer keeps outdoor and indoor air separate, transferring heat through an air-to-air heat exchanger, such as a plate heat exchanger, heat pipe or rotating heat wheel. Indoor air never mixes with outdoor air, which avoids contamination at the cost of a temperature approach across the exchanger (a few kelvin). Many indirect units add evaporative cooling of the outdoor airstream (indirect evaporative cooling), which lets them cool below the outdoor dry-bulb temperature without adding moisture to the hall.

Water-side economizers

In a chilled-water plant, a water-side economizer is a plate heat exchanger between the condenser water loop and the chilled-water loop. When the cooling towers (or dry coolers) can produce water cold enough, it cools the chilled water directly and the chillers turn off. Operation has three modes:

• Full economizer: tower water alone, through the heat exchanger, meets the chilled-water supply temperature. Chillers are off.
• Partial (integrated) economizer: the heat exchanger pre-cools the return chilled water and the chillers finish the job, with a reduced lift.
• Mechanical cooling: outdoor conditions are too warm for the heat exchanger to help; chillers carry the full load.

The temperatures at which the modes change follow from the approaches of each component. With a tower approach \(a_t\) and a heat exchanger approach \(a_{hx}\), full economizing is possible when the wet-bulb temperature is at or below \(T_{\text{chws}} - a_{hx} - a_t\), and partial economizing when it is below \(T_{\text{chwr}} - a_{hx} - a_t\), where \(T_{\text{chws}}\) and \(T_{\text{chwr}}\) are the chilled-water supply and return temperatures.

Air-cooled chillers with integral free-cooling coils work the same way using dry-bulb temperature: when outdoor air is cool enough, glycol from the loop passes through the free-cooling coils and the compressors stop. With liquid-cooled IT equipment accepting facility water at 30 °C or more (Lesson 9), dry coolers can provide all the cooling, without chillers or water, in most climates for all or nearly all hours.

Evaporative cooling

Evaporating water absorbs about 2400 kJ per kilogram at typical conditions, so a little water removes a lot of heat. Direct evaporative cooling passes air through wetted media: the air is cooled toward its wet-bulb temperature and gains moisture. The effectiveness of the media (typically 80 to 90%) gives the leaving temperature:

Indirect evaporative cooling cools a secondary airstream or water that then cools the supply air through a heat exchanger, so no moisture is added to the hall. Cooling towers are evaporative heat rejection for water loops.

Climate and economizer hours

The value of an economizer depends on how many hours per year the outdoor wet-bulb (for evaporative and tower systems) or dry-bulb (for dry systems) temperature is below the changeover points. Designers use hourly weather data for the site, often typical meteorological year files, sorted into temperature bins, and simulate the plant's energy hour by hour. Both the typical year and the extremes matter: the plant must still meet the load on the hottest, most humid design day, often taken as the conditions exceeded only 0.4% of hours (about 35 hours a year) or, for critical facilities, the extreme recorded values.

US climates vary widely: in the Pacific Northwest and the northern states, outdoor conditions allow economizing for most of the year, while hot, humid regions such as the Gulf Coast and the Southeast have long periods with high wet-bulb temperatures, and hot, dry regions such as Arizona have high dry-bulb but often low wet-bulb temperatures that favor evaporative cooling but put pressure on scarce water. Northern Virginia, the largest data center market, has humid summers but cool conditions for much of the year.

Water use

Evaporative systems consume water in three ways. Evaporation is the water that actually removes heat: about 1.5 liters per kWh of heat rejected by evaporation (3600 kJ divided by 2400 kJ/kg). Blowdown is water deliberately drained to stop dissolved minerals from concentrating as water evaporates. Drift is the small fraction carried out of a tower as droplets, typically under 0.005% of circulating flow with modern eliminators. Blowdown is set by the cycles of concentration \(C\), the ratio of dissolved solids in the tower water to those in the make-up water:

where \(E\) is evaporation, \(B\) blowdown, \(D\) drift and \(M\) make-up. Raising the cycles from 3 to 6 cuts blowdown from half the evaporation to a fifth, which is why water treatment matters for water use, not just equipment protection.

Water usage effectiveness

The Green Grid's water usage effectiveness (WUE) metric, standardized as ISO/IEC 30134-9, is the annual site water use divided by the IT energy:

Facilities with water-cooled chillers and towers commonly report WUE between about 1 and 2 L/kWh; those using evaporative cooling only on the hottest days report a few tenths; and those with air-cooled chillers, dry coolers or closed-loop liquid cooling can approach zero. A source-based version also counts the water consumed in generating the electricity, which for thermal power plants can be of the same order as on-site water use, and near zero for wind and solar.

The water-energy trade-off

Evaporative cooling saves energy: a water-cooled plant uses less electricity than an air-cooled one on hot days, and evaporative pre-cooling lets dry coolers meet loads they otherwise could not. Removing water from the design therefore usually raises PUE somewhat. Whether that is the right choice depends on the local water situation (drought risk, competing uses, groundwater depletion), the carbon intensity and water intensity of the grid, and community expectations. Strategies that reduce the trade-off include warmer facility water (so dry coolers suffice for more hours), hybrid systems that evaporate only on the hottest days, use of non-potable sources such as treated municipal wastewater or harvested rainwater, and heat reuse, which replaces heat rejection altogether by sending warm water to district heating networks, greenhouses or industrial users.

In the US, water rights and withdrawal permits are governed by the states, under prior-appropriation systems in much of the West and riparian systems in much of the East, and data centers usually buy water from municipal utilities whose supply agreements may cap consumption or impose drought restrictions. Discharge of blowdown to sewers must meet the local pretreatment limits under the Clean Water Act. Several water-stressed communities have scrutinized or restricted water-intensive data center cooling.

Lesson 2 explains the psychrometrics behind economizer and evaporative performance, and Lesson 12 shows how climate data turn these modes into annual energy and water use.