Data Center Cooling: 25 Questions and 7 Options Before Tendering

This white paper is for people who design, build or upgrade a data center and want a cooling system that matches the resources on their site: power, water and natural gas. We condensed project practice into a matrix of 7 standard cooling options and 25 questions that the team (chief engineer, IT director and finance) should answer before putting equipment out to tender. Each question narrows the choice, and some rule out half of the options at once. The main conclusion: for new AI/HPC clusters, a closed wet cooling tower without a chiller uses 10 times less electricity for cooling than a conventional packaged air-cooled chiller. For a 10 MW data center that means hundreds of millions of rubles a year. The PDF is in Russian; the key points are below in English.

What is inside the white paper?

• Why cooling is the largest OPEX item in a modern data center.

• 7 standard options, from a packaged air-cooled chiller to a closed cooling tower without a chiller: when to choose each one, strengths and limits.

• A selection algorithm based on the energy available on site: 5 scenarios, from "power only" to a new AI/HPC data center.

• 25 questions in 6 blocks: site basics, water and space, power and gas, operations, redundancy and risk, economics and decision-making.

• A comparison matrix of all 7 options for a 1,000 kW data center: CAPEX, OPEX, water, PUE, main limitation.

• A total cost of ownership (TCO) model for a 10 MW AI/HPC data center.

Key takeaways

Air has hit its ceiling. Twenty-five years ago a rack drew 2 to 5 kW; AI and HPC loads now need 200+ kW per rack. Even with aisle containment and optimised CRAH units, air can remove no more than 30 kW per rack; beyond that only liquid at the chip level works. By volume, water carries 3,500 times more heat than air.

Cooling takes 30 to 50% of a data center's electricity. For a 10 MW facility with a PUE of 1.4 that is about 2 MW of average power, roughly 17.5 million kWh and more than RUB 120 million a year at RUB 7 per kWh. The choice of external heat rejection determines 80 to 85% of that amount.

Screen by site resources first, then compare money. What the site has (power, water, gas) decides which of the seven options are even on the table. Without a water connection, only the packaged chiller and the free-cooling chiller remain. If the grid limit is no more than 1.5 times the IT load, a refrigerant chiller with a PUE of 1.4 will not fit: you need options with PUE of 1.2 or below, or a gas-fired absorption chiller.

Supply temperature decides whether you need a chiller. At 7 °C supply a refrigerant chiller is mandatory. At 10 to 18 °C, a chiller with free cooling. At 25 to 35 °C with liquid cooling, the chiller can be removed entirely.

A cooling tower instead of a refrigeration machine. A closed finned-coil cooling tower made by Cold Energy (NCT) costs less than a refrigerant chiller of the same capacity. At +35 °C outside it delivers +25 to 30 °C water, enough for a supply of about 30 °C to the chips in a direct-to-chip system. The fan sits at the bottom, so the unit physically cannot freeze. In our model for Astrakhan (dry bulb 39 °C, wet bulb 28 °C), the tower holds the required temperature without a chiller for 99%+ of the hours in a year.

10 MW: RUB 122 million versus RUB 18 million a year. The option with a refrigerant chiller uses about 2 MW of average power for cooling: 17.5 million kWh and RUB 122 million a year. A closed evaporative cooling tower without a chiller uses about 0.3 MW: 2.6 million kWh and RUB 18 million. That saves RUB 104 million a year, and the extra CAPEX, including a backup chiller for peak hours, pays back in 1.5 to 2.5 years.

The chiller stays, but as a backup. For an AI/HPC cluster in central Russia we recommend a closed cooling tower with a backup chiller sized at 10 to 20% of capacity. It covers 100 to 500 peak hours a year: heat waves and cooling tower maintenance stops.

Refrigerant in the plant room is a fire risk. In a fire, a refrigerant chiller can release 50 to 500 kg of refrigerant, with possible phosgene formation. Refrigerant-free options (absorption chiller, adiabatic dry cooler, cooling tower without a chiller) do not carry this risk.

Who is it for?

• Data center designers and chief engineers choosing a cooling scheme for a new build or an upgrade.

• IT directors planning AI/HPC racks of 50+ kW.

• Finance directors comparing options on a 10-year TCO.

• Owners with a tight grid power limit or no spare water on site.

FAQ

Which data center cooling option is cheapest to run?

For a new data center designed for liquid cooling, a closed cooling tower without a chiller. In the comparison matrix for 1,000 kW its OPEX is about RUB 1.5 to 2 million a year, against about RUB 14.5 million for a packaged chiller, with a PUE of 1.06 to 1.12. The one limitation: it only works with rack-level liquid cooling and does not suit a conventional white space with 7 °C supply.

When does a gas-fired absorption chiller make sense for a data center?

When natural gas is available at an industrial tariff of RUB 6 to 9 per m³, when there is cogeneration on site, or when the grid power limit is tight. The absorption chiller itself draws about 3 kW of electricity per 1 MW of cooling; with the cooling tower and pumps, about 28 kW. At an electricity tariff of RUB 10+ per kWh it pays even without waste heat. The downsides are CAPEX 2 to 3 times higher than a packaged chiller and a start-up time of about 30 minutes.

Why do people still install packaged chillers?

For a cheap start: about RUB 13 million for 1,000 kW, no water and no water treatment, only a power connection. It suits regional data centers up to 700 kW, test labs and backup halls. The price is the highest OPEX: with a COP of about 3, every kilowatt of cooling costs 0.33 kW of electricity.

Related articles

• Data center cooling: 5 options for 5,000 kW and their annual cost

• Liquid cooling for data centers without a chiller: water to the chip

• Absorption chiller for data centers: free up 1,437 kW of grid power

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