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Comment by minimaltom | Hacker News Reader

For datacenters specifically I've never understood what specifically consumes the water. Arent the water-cooling loops closed, so the water just cycles around and around and around?

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At the datacenter side, it depends on the method of cooling. You can chill the air or the chips directly (or both), doesn't matter, you still need to cool, and that still needs water. The question is, where is the water being used?

- If they use either evaporative cooling or a liquid-cooled heat exchanger, that uses tons of water consistently. This requires less energy (it's mostly passive) so you use more water.

- If they use closed-loop water cooling and/or heat pumps/electric chillers, that uses much less water - at the DC. But it does require more energy to circulate the water, run fans, etc. If you are using more energy, where is the energy coming from? It's coming from power plants, which require... you guessed it... more water (e.g. thermoelectric, hydroelectric, geothermal, concentrated solar). They need water in order to generate the power, and lots of it. Coal, natural gas, nuclear, and concentrated solar, all use steam to generate energy. Nuclear also uses water to cool the reactor. And water is used extensively to extract coal, oil, and natural gas. Geothermal uses water in the ground.

You can't not use a ton of water in one fashion or another. It just depends what method, and on what end the water is used. And the crazy thing is, most new datacenters are being built in places with extremely little water. Guess how that's gonna work out as the planet gets hotter?

I don't know why I got downvoted to hell for stating facts every datacenter architect knows. HN be HN'in.

They evaporate the water which is what makes it cool so effeciently.

Evaporative cooling does not necessitate an open loop system

The system which runs coolant over the chips can be closed but the part which uses an evaporative system to cool that is still open loop and vents water into the air, no?

Nope, it doesn't have to be open loop!

Example of such a system being used specifically for datacenters: https://blog.vantage-dc.com/2026/04/22/cooling-without-the-d...

Evaporated water is condensed, and in the process transfers its heat into another place that removes it. Another simple example is a pot of boiling water with a lid on it.

The link you cited is not evaporative cooling and a pot of boiling water with a sealed lid on it is a pressure vessel which eventually explodes.

If you were to remove the heat at a sufficient rate by, say, turning the lid into a heat exchanger, you would have a stable system.

How is that different from not using evaporative cooling and just putting the heat exchanger on the burner?

Water is being used to get heat from one place to another. The idea is being able to separate the heat generation and the heat extraction

You are adding an extra step though. Use a closed loop coolant and remove the heat from that coolant with the heat exchanger. Why would evaporation in between be more efficient?

That's the problem, removing heat at a sufficient rate. Of course it can be done, but the most efficient way (in terms of cost) is just open loop evaporation.

I'm not a datacenter engineer, but I used to work in the ski industry. Snowmaking systems use vast quantities of compressed air. It works better if that air is cool. Blowing hot compressed air out of a snow cannon means the air temperature (wet bulb to be specific) needs to be colder to make snow.

Anyways, most air compression stations use water to cool the air, and then evaporative coolers to cool the water. The water is reused, but a ton (not sure of the percentage) is lost into the air. It's more or less a tower with a big fan on top, and water percolates down from the top, being cooled by the air as it goes. The water is then collected and pumped through the system again (but of course has to be always topped up to counteract what was lost to evaporation).

Anyways, long story short is it's most cost effective to just spray water into the air to cool water, as long as water is free/cheap.

Oh like one of those scenic cone towers like on a nuclear power plant?

Iiuc youre saying: its more cost-efficient to waste water using evaporative cooling so thats what we'll get, not that a closed loop with a heat exchanger is technically infeasible?

Closed loop is of course feasible, and closed loop with an evaporation step is also feasible, but open loop evaporative is the most efficient in terms of total energy usage (cost) and most cost effective in general, so that's what we mostly get right now.

Exactly. A car is a closed loop system. Coolant (which is water with some chemicals) cools the engine, then the hot coolant flows through a radiator which transfers that heat to the air, and then it just keeps cycling through. If there's no leaks then no coolant is lost.

The above method could be scaled up to data center levels, but is less efficient in terms of energy usage and cost. Cheaper/easier to just spray water into the air and get "free" cooling that way, as long as you have a source of free/cheap water to replenish what is lost to evaporation.

I'm not a nuclear engineer either, but I assume this is what is happening inside of the big iconic nuclear stacks. That's just cooling water being evaporated off to keep it cool.

I didn't say evaporative HAS to be open loop, but many are, which answers the question why they use up a bunch of water.

Yes they are for water cooling.

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