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Best to Take a Systems View on Free Data Center Cooling

Free cooling for your data center. It sounds great, doesn’t it?

You are likely familiar with the term, but perhaps want to know more about its limitations, and wise use of the solutions involved.

In its most basic variation—free cooling could consist of opening a window and letting cool air in. While this may work at home for keeping oneself cool, using this method in a data center is unwise for your equipment and future employment prospects.   This is because of the harmful effects that sharp swings in humidity and temperature would have on data center equipment. So let’s take a look at what practical free cooling actually means for data centers.

Surprisingly, a data center cooling solution is not designed to provide cooling, but rather to remove heat. Heat is the “waste product” of running your data center.

Removing the heat in the data center involves the deployment of dedicated equipment known as CRAH or CRAC (computer room air handler/conditioner) units. Removing the heat trapped by a CRAH/C involves another piece of equipment, either a chiller or a condenser. The purpose of these two units is to release the heat generated by the servers into the outside environment. This heat removal is where free-cooling comes into play.

If the ambient conditions are right, fresh outside air can be used to remove the heat trapped in the chilled water loop or the refrigerant loop (if using a condenser).

If the ambient conditions are not quite right, only part of the heat will be removed. The remainder would need to be removed with the aid of the mechanical cooling system (a chiller or a condenser, with the latter also being known as a “dry cooler” when the condenser is fitted with free-cooling coil).

What is a ‘right’ condition?  Places with low ambient temperatures and low relative humidity provide the best opportunity to leverage free cooling. So if you live near the tropics and have high ambient temperatures and high relative humidity much of the year, free cooling gear will probably not provide you with the performance you would expect or desire. The good news, however, is that many regions of the world can leverage free cooling effectively most of the year.  You don’t need to live inIcelandto take advantage of the substantial time and money savings that free data center cooling provides.

In fact, low ambient relative humidity (RH) plays a critical factor even more than the actual temperature: in general, the lower the RH, the better.

If you have the right conditions, you will also need the right solution. Getting a free-cooling chiller won’t necessarily guarantee performance and savings. Like any other systems it all comes down to how you run it.

Let’s consider a non-IT example. If you drive a Toyota hybrid Prius at 90 miles per hour on the highway, you won’t match the listed highway fuel economy. But this does not mean the Prius is inefficient—you are just using the right system in the wrong way.

The same concept applies to free cooling gear. The control logic that runs the unit needs to be smart enough to understand ‘when’ free-cooling can be implemented, in ‘which’ proportion and for ‘how’ long. If the return chilled water temperature is lower than the ambient conditions, switching on the free-cooling coil will only increase the water temperature and not lower it as desired.

Remember, the main purpose of a free cooling solution is not to save money at all cost, but to support your data center operation in the most efficient way, guaranteeing business continuity 24/7.

Another trend in free cooling is growing popularity of indirect evaporative cooling solutions, or units specifically design to exploit low ambient temperature to cool down the air in the data center.

The benefits of free cooling clearly exist, even though it takes a bit of study to understand the variations. Take a look at Schneider Electric white paper 132, “Economizer Modes of Data Center Operations,” for more detailed information. Also check out this post by Damien Wells on cost/benefit considerations. With a bit of study and a “systems view” of how free cooling is best applied, free cooling is a great opportunity for many data centers.

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