Why do data centres use so much water, and where does all that water go?

Data centres and water have almost become synonymous. Whenever data centres are mentioned, water comes up. There is now intense discussion about it on social media. But really, what do data centres do with all that water?

Many people imagine that, like the coolant in a car engine, water flows through rows of computers here too. But that is not how it works. The servers never get wet; if they did, that would be the end of the story! The water in a data centre mostly stays inside pipes, and most of it escapes as vapour through the roof.

In a country like the United States, data centres now consume more than 4 percent of all electricity. And almost all of that electricity eventually turns into heat. A computer chip stores no energy and converts none of it into motion. It switches on and off billions of times a second, and each time, the silicon inside gets a little hotter.

So that the servers do not get cooked by the heat, it has to be taken out of the building. The work happens in two steps. First, the heat from the chips is absorbed with the help of the air or liquid inside the server hall. This is a closed process, in which the same water or air keeps circulating and nothing is lost.

Then comes the second step. This is where the water arithmetic really begins. The most popular way of getting heat out of a building is the cooling tower. The tower works by evaporating water.

Why does water carry away so much heat when it evaporates? On a hot day, a little running makes our body sweat. That sweat is not made to wet the body, but to evaporate. And as it evaporates, the sweat draws a great deal of heat from our body. When water is heated, its molecules only move a little faster. But when water evaporates, its molecules have to separate from one another and fly off into the air. That takes a lot of energy. The energy needed to turn a liquid into a gas without raising its temperature is called 'latent heat' in the language of science.

The arithmetic is striking. Raising the temperature of one kilogram of water by 10 degrees Celsius takes about 42 kilojoules of energy. But evaporating that same amount of water at a comfortable temperature (about 30 degrees Celsius) takes about 2,430 kilojoules! In other words, evaporating water absorbs about 58 times more heat than just heating it.

In a data centre, all of the electricity per kilowatt-hour (3,600 kilojoules) turns into heat. Removing that heat requires evaporating about one and a half litres of water in the cooling tower. If the heat were removed only by heating water, without evaporating it, water consumption would rise about 60 times!

A data centre cooling tower is not like the giant chimneys of a power plant. They are usually shaped like rectangular boxes, with large fans fitted on top. They sit on the building's roof or in an open space beside it. The work inside is quite interesting. Hot water coming from the servers is sprayed over a structure like a plastic honeycomb. At the same time, the fans above draw outside air through that water. A small amount of water evaporates and mixes with the air, and the rest of the water becomes completely cool. That cool water goes back to fetch heat from the servers. And the vapour escapes through the roof like smoke.

The tower loses water in mainly three ways — evaporation, blowdown and drift. The tiny droplets of water carried away by the fan's air are drift. The real cost is evaporation, which is what the tower is built for. And blowdown hides a little! When water evaporates, the minerals inside it stay behind in the tower. Much like salt left at the bottom of a pot after boiling for a long time. If these are allowed to build up, the minerals will jam the pipes. So the tower occasionally drains away this salty water and draws in fresh clean water.

This is where two different words for water accounting come from, which many people confuse. One is withdrawal, the other is consumption. The amount of water a data centre draws in is withdrawal. And consumption means the part of the water that evaporates. With good quality water, about 80 percent of the water drawn in evaporates, and the rest goes down the drain as blowdown. In reality, this arithmetic differs from place to place. Some large cloud companies use only about 1 litre of water per kilowatt-hour, because for much of the year they use cold outside air. In desert areas like Arizona, on hot days it can reach 9 litres, because the hot and dry air makes the tower work much harder.

When the weather is good, many data centres cool their servers with outside air alone. But there is a small obstacle here, known in science as the 'wet-bulb temperature'. A cooling tower can cool water down to this wet-bulb temperature. But running only radiators and fans without water can at best cool to the same temperature as the outside air. That is why even ordinary 'free air' data centres switch on the water evaporation system once the outside temperature crosses 29 degrees Celsius.

Water use in data centres is measured by 'water usage effectiveness'. It means how many litres of water are evaporated for each kilowatt-hour of electricity. Across the whole United States, data centres use on average slightly more than a third of a litre of water per kilowatt-hour. Calculations show that in 2023, data centres in the United States evaporated a total of 66 billion litres (6,600 crore litres) of water, almost three times as much as a decade ago! As the business of the big cloud companies expands, this water consumption is rising rapidly.

Now to the real debate. The water evaporated at a data centre is actually a very small part of the total cost. Electricity has to come from somewhere, and most power plants are giant water kettles! They boil water into steam to spin turbines, and use more water in their own cooling towers to cool that steam. Adding this indirect calculation, the total water cost behind data centres in the United States comes to about 800 billion litres! In other words, power plants evaporate about 12 times more water into the sky than the data centres themselves consume.

The answer to whether a chatbot or AI wastes water is hidden here too. A study on the older model GPT-3 found that it consumes as much water as a half-litre bottle for 10 to 50 medium-sized answers. But the surprising part is the split. Of that half litre, only about 2 millilitres is consumed at the data centre itself, while about 15 millilitres of water evaporates at the power plant that supplies electricity to that data centre!

Can data centres run without using water? Data centres use so much water because they generate enormous heat. And under the laws of physics, the cheapest way to remove that heat is the evaporation of water. When water evaporates, it carries away 58 times more heat than just getting hot. Of course, cooling is also possible with machines like air conditioners. They work in any weather and use no water. But they need a lot of electricity, and generating that electricity consumes water again!