Closed-Loop Cooling

Large computing facilities generate a lot of heat, and this heat has to go somewhere, otherwise the devices shut down. A widespread solution is liquid: water or a similar coolant absorbs heat much better than air. In closed-loop cooling, this liquid flows in an endless circuit through a sealed system of pipes. It picks up heat from the hot components, releases it elsewhere, and returns cooled down. Nothing is consumed in the process – the liquid stays in the system, just like the coolant in a car engine. The counterpart is called an open loop: there, water is deliberately allowed to evaporate, and evaporated water must be constantly replenished.

The dispute over data centers' water consumption

Data centers are buildings full of computers that run websites, streaming services, and AI services. Since the AI boom, they have been getting bigger and hotter, and their water consumption has become a political issue. Facilities with open evaporative cooling can consume millions of liters per day. In dry regions such as Arizona or Spain, this has led to protests from local residents.

A closed loop significantly defuses this problem. The liquid is filled in once and then runs for years afterward. Refilling is only necessary when small amounts evaporate or when liquid is drained during maintenance. Large operators are now openly advertising that new sites need practically no more water after the initial fill.

However, an important distinction must be made: saving water does not automatically mean saving electricity. The heat ultimately still has to be released into the environment, and that costs energy for pumps and fans. Anyone relying on evaporation in an open loop is exploiting a free physical effect and needs less electricity. A closed system, in other words, trades water for electricity.

From the hot chip to the outside air

The loop consists of four components. A flat metal plate with channels, the cold plate, sits on the hot chip. A pump drives the liquid through these channels. The now-warm liquid then flows to a heat exchanger. There it releases its heat to a second loop or directly to outside air, and flows back cooled to the pump.

Usually, two separate loops work in sequence. The inner loop touches the electronics and contains clean, treated water with additives against corrosion and algae. The outer loop carries the heat outside. The two liquids never mix; they only meet at the metal wall of the heat exchanger.

A common misconception is that such a system would be maintenance-free. The liquid ages, filters clog up, seals become porous. And a leak above a server containing chips worth tens of thousands of euros is expensive. That’s why loops in data centers are secured with sensors, emergency shutoffs, and drip trays.

From gaming PC to AI server hall

The technology you’re most likely to encounter is in your own computer. Pre-built water coolers for gaming PCs, often sold as AIOs, are exactly that: a sealed loop consisting of a pump, tubing, and a radiator with fans. The radiator in a car and a refrigerator also work on the same basic principle of a closed loop.

In the news, the term mainly comes up in connection with new AI data centers. Modern graphics processors for AI training give off so much heat that air alone is no longer enough. That’s why operators like Microsoft, Google, or Meta now explicitly announce new sites with closed-loop cooling. In permitting processes for new facilities, the question of water consumption is often decisive.

For investors and readers of business news, this is more than just technology. A whole market has sprung up around cold plates, pumps, and heat exchangers, with companies like Vertiv or Schneider Electric. When a report mentions orders for liquid cooling, it’s usually a sign of AI data center expansion in the background.