Hurdles for Free Cooling

It is indeed a lot easier for Facebook, Google and Microsoft to operate data centers with "free cooling". After all, the servers inside those data centers are basically "expendable"; there is no need to make sure that an individual server does not fail. The applications running on top of those servers can handle an occasional server failure easily. That is in sharp contrast with a data center that hosts servers of hundreds of different customers, where the availability of a small server cluster is of the utmost importance and regulated by an SLA (Service Level Agreement). The internet giants also have full control over both facilities and IT equipment.

There are other concerns and humidity is one of the most important ones. Too much humidity and your equipment is threatened by condensation. Conversely, if the data center air is too dry, electrostatic discharge can wreak havoc.

Still, the humidity of the outside air is not a problem for free cooling as many data centers can be outfitted with a water-side economizer. Cold water replaces the refrigerant, pumps and a closed circuit replace the compressor. The hot return water passes through the outdoor pipes of the heat exchangers. If the outdoor air is cold enough, the water-side system can cool the water back to the desired temperature.

Google's data center in Belgium uses water-side cooling so well that it
does not need any additional cooling. (source: google)

Most of the "free cooling" systems are "assisting cooling systems". In many situations they do not perform well enough to guarantee the typical 20-25°C (68-77 °F) inlet temperature the whole year around that CRACs can offer.

All you need is ... a mild climate

But do we really need to guarantee a rather low 20-25°C inlet temperature for our IT equipment all year round? It is a very important question as the temperature in large parts of the worlds can be cooled with free cooling if the server inlet temperature does not need to be so low.

The Green Grid, a non-profit organization, uses data from the Weatherbank to calculate the amount of time that a data center can use air-side "free cooling" to keep the inlet temperature below 35°C. To make this more visual, they publish the data in a colorful way. Dark blue means that air-side economizers can be efficient for 8500 hours per year, which is basically year round. Here is the map of North-America:

About 75% of North-America can use free cooling if the maximum inlet temperature is raised to 35°C (95 °F). In Europe, the situation is even better:

Although I have my doubts about the accuracy of the map (the south of Spain and Greece see a lot more hot days than the south of Ireland), it looks like 99% of Europe can make use of free cooling. So how do our current servers cope with an inlet temperature up to 35 °C ?

Free Cooling for the Data Center? Servers and High Inlet Temperatures
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  • iTzSnypah - Tuesday, February 11, 2014 - link

    I wonder why nobody has tried geothermal liquid cooling. You could do it 2 ways. Either with a geothermal heat pump set up or cut out the middle man and just use the earth like you would a radiator in a liquid cooling loop. The only problem would be how many wells you would have to drill to cool up to 100MW (I'm thinking 20+ at a depth of at least 50ft). Reply
  • ShieTar - Tuesday, February 11, 2014 - link

    Its kind of easier to just use a nearby river than dig for and pump up ground water. That's what power stations and big chemical factories do. For everybody else, air-cooling is just easier and less expensive. Reply
  • iTzSnypah - Tuesday, February 11, 2014 - link

    You wouldn't be drilling for water. You drill a well so you can put pipe in it, fill it back up and then pump water through the pipes using the earth's constant temp (~20c) to cool your liquid which is warmer (>~30c). Reply
  • looncraz - Tuesday, February 11, 2014 - link

    I experimented with this (mathematically) and found that heat soak is a serious, variable, concern. If the new moisture is coming from the surface, this is not as much of an issue, but if it isn't, you could have a problem in short order. Then there are the corrosion and maintenance issues...

    The net result is that it is cheaper and easier to just install a few ten thousand gallon coolant holding tanks and keep them cool (but above ambient) and to cool the air in the server room(s). These tanks can be put inside a hill or in the ground for extra installation and a surface radiator system could allow using cold outside air to save energy.
    Reply
  • superflex - Wednesday, February 12, 2014 - link

    You obviously dont know have a clue about drilling costs.
    For a 2,000 s.f. home, a geothermal driller needs between 200-300 lineal feet of well bore to cool the house. In unconsolidated material, drilling costs per foot range from $15-$30/foot, depending on the rig. For drilling in rock, up the cost to $45/foot.
    For something that uses 80,000x more power than a typical home, what do you think the drilling costs would be?
    Go back to heating up Hot Pockets.
    Reply
  • chadwilson - Wednesday, February 19, 2014 - link

    That last statement was totally unnecessary. Your perfectly valid point was tarnished by your awful attitude. Reply
  • nathanddrews - Tuesday, February 11, 2014 - link

    Small scale, but really cool. Use PV to power your pumps...

    http://www.overclockers.com/forums/showthread.php?...
    Reply
  • Sivar - Tuesday, February 11, 2014 - link

    Geothermal heat pumps are only moderately more efficient than standard air conditioning and require an enormous amount of area. 20 holes at a depth of 50ft would handle the cooling requirements for a large residential home, but wouldn't even approach the requirements for a data center.
    One related possibility is to drill to a nearby aquifer and draw cool water, run it through a heat exchanger, then exhaust warm water into the same aquifer. Unfortunately, water overuse has been drained aquifers such that even the pumping costs would be substantial, and the aquifers will eventually be drained to the point that vacuum-based pumps can no longer draw water.
    Reply
  • rkcth - Tuesday, February 11, 2014 - link

    They are a lot more efficient at heating, but only mildly more efficient at cooling. They also are really storing heat in the ground in the summer and taking it back in the winter, so if you only store heat you can actually have a problem long-term. Your essentially using the ground as a long-term heat storage device since the ground is between 50-60 degrees depending on your area of the country, but use of the geothermal changes that temperature. An air source makes much more sense since you share the air with everyone else and it essentially just blows away. Reply
  • biohazard918 - Tuesday, February 11, 2014 - link

    Wells don't use vacuum based pumps most aquifers are much to deep for that instead you stick the pump in the bottom of the well and push the water to the service. Reply

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