What is everyone planning for their datacenters? We have plenty of cooling from the neighboring physical plant, but can currently only supply 5kW per rack.
That's really rather wimpy to be honest.
Rear door coolers are good for in the region of 30kW per rack. I think we are pulling on average 32kW on ours when they have compute nodes in. However it is just brutal opening the rear door when everything is screaming. Good for a laugh for unsuspecting vendor service engineers. With the door closed you would not have a scooby how hot is about to become
For the vast majority of people this is the way forward. It is basically just a large car radiator in the back door of the rack. I know when I was a small child car radiators where not terribly reliable, but I can't think of when I have known someone outside of a crash having a leaky car radiator or even hoses. Being in a datacentre is a way more benign enviroment. We have been using them for a nearly a decade now. They also mean you don't need to bother with the hot/cold isle stuff and your PUE jumps dramatically over traditional free air cooling.
The current limit is I think around 130kW per rack with on die water cooling. But that's really specialist stuff. You have to vendor supplied heat exchangers to maintain the water quality in your rack row. It effectively has to be ultrapure and yeah that's a real term for water quality.
However I recall proposals for much
higher limits for exascale that my back of the envelope calculations suggest would turn it all into a molten puddle of metal in a minute or so should the cooling ever fail. If I recall correctly it was around one megawatt per rack. Yep I nearly fell off my chair when I heard that too. Personally I don't think HVAC is anywhere near reliable enough for that, because you have one maybe two seconds to react before everything in the rack is scrap on a cooling failure so not a chance in hell of a orderly shutdown, you are going to have to throw breakers on the basis of incoming cooling water temperature being out of spec and or not flowing.
As for free cooling, the best proposal I have seen is to drill down and install a heat exchanger with the water in the abandoned flooded coal mines below the data centre. Year round free cooling from that.