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In a well-designed data centre cooling system, the chiller can use a large share of the cooling electricity and still do only a small part of the work. In one calculation for a typical Danish year, free cooling with a dry cooler delivered 97.7 % of the cooling. The chiller used 42 % of the cooling electricity to deliver the last 2.3 %. That is not a fault. It is what happens when the chiller only handles the warmest hours.

The numbers come from a LinkedIn post that started a useful discussion among engineers. This article explains the calculation and the physics behind it, and answers two questions from the comments.

What did the calculation show?

At Next Generation Cooling, we calculated a full year of cooling at 22 °C supply and 32 °C return, running around the clock. We used hourly temperatures for a typical year at Tirstrup (Aarhus Airport) from the Danish Meteorological Institute, DMI. Free cooling through the dry cooler delivered 97.7 % of the cooling, and the chiller covered the remaining 2.3 %. The calculated pPUE for the whole cooling system was 1.0099.

This is a calculation, not a measurement. The free cooling share always depends on supply temperature, climate and design. In another optimised case, it was 92 to 96 %.

Why can a dry cooler handle most hours of the year?

A dry cooler uses outdoor air and fans to cool the water, with no compressor. It can bring the water down to around 4 °C above the outdoor temperature. With a 22 °C supply, it can carry the full load whenever it is below about 18 °C outside.

According to DMI, Denmark’s normal annual mean temperature is 8.7 °C, based on the 1991 to 2020 climate normal. That puts most hours of the year well below 18 °C.

On the warmest hours, the dry cooler cannot bring the water all the way down. Then the chiller takes over, but only for the last few degrees.

Why does the chiller use 42 % of the power?

A chiller uses a compressor to lift heat to a higher temperature so it can be released outside. That takes energy. A chiller can use up to 10 times as much energy per kW of cooling as free cooling.

So even a small share of the work shows up as a large share of the electricity. In the calculation, 2.3 % of the cooling became 42 % of the cooling power. Because the total is so low, the whole system still ends at a calculated pPUE of 1.0099.

Warm water helps the chiller too. The warmer the water it receives, the smaller the temperature lift to outdoor air, and the more efficiently it runs. The rule is simple: let free cooling take the heat first, and leave only the remainder to the chiller.

What happens if the system runs on colder water?

Turn the design around and the picture changes. In a system running on 12 °C supply water, the dry cooler can only carry the full load when it is below about 8 °C outside. In a Danish climate, that is a much smaller share of the year. The chiller becomes the workhorse instead of the backup. We explain the wider trade-off in The Value of Hot Water.

For the operator, this is the economic point. Higher supply and return temperatures mean more hours of free cooling and fewer hours of compressor cooling. The chiller becomes insurance against the warmest hours, not the item that sets the electricity bill. Read more on our energy efficiency page.

Does a warm year change the result?

Yes. The calculation uses a typical year and constant load. In 2025, Denmark’s mean temperature was 9.7 °C, 1.0 °C above the 1991 to 2020 normal, according to DMI. In a year like that, the chiller’s share rises. In a warmer climate than Denmark’s, the calculation looks different again. The principle stays the same: the higher the supply temperature, the more of the year free cooling can cover.

FAQ

Is free cooling the same as a dry cooler?

Not quite. Free cooling is the operating mode: cooling without a compressor. A dry cooler is one way to achieve it. A chiller can also work together with a dry cooler, so the two are not always separate machines.

How do you know how much energy the chiller actually uses?

Measure it. Separate sub-meters on the chiller, the dry cooler fans and the pumps show where the cooling electricity goes. Without them, a high chiller share is easy to miss, and so is a well-performing system.

What supply temperature gives the most free cooling?

The higher, the better, within what the IT equipment and cooling units allow. In our designs, supply is typically 18 to 22 °C. At 22 °C, free cooling can carry the full load below about 18 °C outside.

Sources

• DMI, Danmarks klima 2025 (DMI Report 26-04): 2025 mean temperature 9.7 °C, 1.0 °C above the 1991 to 2020 normal
• DMI, hourly temperature data for Tirstrup (Aarhus Airport), typical year, via DMI’s open data API
• Next Generation Cooling, cooling calculation for Tirstrup at 22/32 °C, whitepaper section 6.2

This article builds on a LinkedIn post by Ole Kristensen, CTO: read the original post and the discussion.