Why do datacenters continuously use more water instead of recycling the same water in a closed loop system?
In: 72
AH! a question in an area of my expertise. There are at least three variants of water-consuming datacenter cooling.
1. The datacenter captures the hot air coming out of servers. Radiators with cool water circulating through them are used to cool this hot air. The water is heated while the air is cooled. That water is then pumped outside and poured over big fiber boards while a fan blows outside air past them. This evaporates some of the water, and the remainder is cooled by the evaporation. The remaining cooled water is pumped back inside the datacenter and the process repeats.
2. The datacenter uses industrial air conditioning units called CRACs, which have radiators outside the datacenter. The air conditioner compresses a gas, heating it up, and then pumps it through the radiator while a fan blows outside air across the radiators. Once the compressed gas is cooled, it is brought back into the datacenter and allowed to expand, which makes it cold. Air inside the datacenter is blown over a different radiator filled with this cold gas, cooling the datacenter air. On very hot days, the CRAC sprays the outside radiators with water, which evaporates on the radiator and cools it more than the air alone would.
3. The datacenter brings in outside air directly to keep the inside air cool. On hot days, the datacenter’s systems will spray a mist of water into the air on its way in. That mist evaporates quickly and cools the outside air further before it reaches the datacenter floor.
It all leads back to heat exchange and thermodynamics.
First, not a datacenter guy – so I’m not overly familiar with the details of their cooling water setups. But here’s my best attempt…
Something like a datacenter generates a lot of heat. Water is a great (and generally cheap) way of exchanging heat. Closed loops can effectively do this, but even with refrigerants heat doesn’t just go away. You need a way to cool that closed loop as well. Evaporation is a tried and true way to cool off something. Like other posts have eluded to, that’s how our bodies works. We sweat, the sweat evaporates, and the temp of our body decreases.
Cooling towers use this same principle. They spray water in an environment that has airflow (generally through the work of a fan, but hyperbolic cooling towers use some even “cooler” science to create the same effect, if you’re ever interested in researching those). By creating airflow and an increased surface area of the water you’re increasing the evaporation rate of the water.
However, the water leaves behind a lot in the process of evaporating. All those solids in the water stay in the the open water loop. Also, things like alkalinity and microbiological growth begin to change. This combo can lead to scale, biofilm, and corrosion which are detrimental to heat exchange. The solution to this is dilution. You send the concentrated water down the drain and make up water with the water source of choice. It’s generally most cost effective to use whatever water source the plant uses as a whole. You can soften the water, use reverse osmosis, deionize or distill the water to decrease water usage, but often times that costs significantly more than using a raw water source and then you’re still concerned about how corrosive the water is to the the metallurgy of the system. And something like reverse osmosis still dumps all the concentrated water down the drain, so you aren’t getting a huge payback on water usage.
All that being said, the main cause of water usage is evaporation. Which is again the most reliable way to cool water that then cools the closed loop. If there was a solution to the fundamental laws of thermodynamics where heat could disappear without the loss of something else (in this case water), then that problem solver would be generationally wealthy. But as it stands today you have to give up something in order to get rid of heat. So far, the best answer we’ve come up with is water. Now, that evaporated water does go back into the hydrologic cycle, but it’s still a drain on water sources like Lake Mead and other non renewable fresh water sources, so it’s far from perfect.
I’ve seen several instances of server rooms chilled with heat pumps that just use tap water dumped to drain. It’s incredibly wasteful and just due to taking shortcuts. It’s not the norm though.
DC engineer here.
There are two “basic” DC designs. One that uses little water, and one that continuously consumes water.
Let’s start with the basics. When you remove heat from the air, you tend to strip out humidity. If you’ve ever seen a home furnace with an AC, it has a pipe that typically runs outside or to a drain. This is to catch the condensation due to removing heat.
But why remove humidity? Static electricity. Dryer climates can promote static which is killer for computers.
The little water users typically only consume water to humidity the air for the servers, and also for the people occupying the building. The cooling is typically done by RTUs (roof top units) or CRACs (computer room air chillers). These units in this configuration use DX (direct expansion) to cool. This is the same cooling style as a refrigerator. These take hot air and run it over radiators. The extracted heat is expelled into the atmosphere.
For the continuous users, there are typically two water systems. The first is a closed loop (water or glycol) which feeds CRAHs (computer room air handler). This coolant is chilled by very large water chillers which are also DX. These chillers develop an immense amount of heat which is removed via a second water loop. This loop is “open” which means it touches air.
Household AC units, and RTUs vent the excess heat by using air. Think of the fan on the home units. Water chillers use water to extract the heat. Hot water from the chillers will go to cooling towers. These towers spray water while a large fan pulls air over the water. Doing this makes some of the water evaporate dropping the temperature of the water. Think of this as a giant swamp cooler. The cooler water is then cycled back into the water chillers.
Now because some of the water evaporates, you need to replace it; hence why this style of DC continuously uses water.
I hope that explains it. Feel free to message me if you want to dive deeper!
Latest Answers