Climate, Cooling and Data Center Site Selection

A data center is a machine for turning electricity into computing and heat. Climate decides how easily that heat can be released. In cool weather, outside air or water can carry heat away with little energy. In hot weather, the facility needs compressors, evaporation or both, which use more power or more water.

That has four practical effects in site selection:

• Energy use over the year, measured through PUE (Power Usage Effectiveness, a metric from The Green Grid).
• Peak electrical demand on the hottest days, which sets how much power the site has to request.
• Water use, if the design relies on evaporation.
• Equipment size and cost, since cooling plants are sized for extreme conditions, not average ones.

ASHRAE Technical Committee 9.9 publishes the thermal guidelines most data centers design around. Its recommended range for air entering IT equipment is 18–27°C (64.4–80.6°F), with humidity limits expressed mainly through dew point. Wider allowable ranges apply to equipment classes A1 through A4, and many operators run warmer than they once did to save cooling energy.

For liquid-cooled equipment, ASHRAE defines separate classes based on the facility water temperature supplied to the system. Higher supply temperatures let the facility reject heat to outside air for more of the year, which reduces the influence of climate.

The higher the operating temperature a design allows, the more hours a given climate can provide free cooling. Climate and design assumptions have to be evaluated together.

An economizer uses outside conditions to cool the facility without running compressors. Air-side economizers bring filtered outside air in directly or pass heat through a heat exchanger. Water-side economizers use cooling towers or dry coolers to chill water when the weather allows.

Which weather measure matters depends on the system. Dry systems depend on dry-bulb temperature, the ordinary air temperature. Evaporative systems depend on wet-bulb temperature, which reflects humidity and sets how much cooling evaporation can provide. A hot, dry climate can be good for evaporative cooling and poor for dry cooling; a mild, humid climate can be the reverse.

To compare sites, engineers run hourly weather data for a typical year through the proposed cooling design and count the hours each mode can run. That analysis is more reliable than comparing average temperatures.

These are general patterns. Local factors such as elevation, proximity to water and microclimate matter, so evaluate each site with its own weather data.

Annual averages drive energy cost, but extremes drive capacity. Cooling plants and the electrical service are sized for the hottest expected conditions. ASHRAE publishes climate design data for weather stations, including dry-bulb and wet-bulb values exceeded 0.4%, 1% and 2% of the year, as well as extreme values with longer return periods. Data center designers often choose conservative values, then add margin.

This affects power directly. On the hottest day, cooling energy peaks at the same time the IT load may be at its highest, so the site’s peak demand rises. A hotter site may need a larger utility request for the same IT capacity. Air-cooled chillers, dry coolers and generators also lose capacity at high temperatures, and at higher elevations thinner air reduces the output of air-cooled equipment and engines.

Many designers also allow margin for warmer extremes over the life of the facility, since a campus is built to operate for decades.

High-density AI racks produce more heat than air can practically remove, so they increasingly use direct-to-chip or immersion liquid cooling. Liquid systems can often run at warm supply temperatures, which means dry coolers can reject the heat for many more hours of the year without evaporation or compressors. In effect, liquid cooling widens the range of climates that work well.

Climate still matters. Peak days still set the size of the heat rejection plant, and many facilities mix air-cooled and liquid-cooled equipment. Our guide to AI data center site requirements covers how density changes the rest of the site program, and data center water requirements covers the water side of the tradeoff.

Climate is more than temperature. Several related factors affect design and risk:

• Severe weather: hurricanes, tornadoes, high winds, hail, snow loads and ice storms, which affect structural design and the reliability of power and fiber.
• Air quality: dust, salt air near coasts, agricultural particulates and wildfire smoke, which can limit air-side economizers and raise filtration needs.
• Freezing: cold climates need freeze protection for water systems and attention to generator and fuel performance.
• Flooding: heavy rainfall drives stormwater design and floodplain risk.

Our guide to natural hazard risk covers how to screen for these.

In most site searches, climate ranks below power, land, entitlement and fiber. A site with power available in a reasonable timeframe in a hot climate usually beats a cool site with no power path. Climate becomes a tiebreaker between otherwise similar sites, or a factor in the design budget and operating costs.