'Free' data center cooling: Methods and challenges

While air- and water-side free cooling aren't options in every climate, data center managers can improve data center energy efficiency by simply using outside temperatures.

Free cooling isn’t really “free” unless you just open some windows on a nice spring day and let the air blow through. However, when you dissipate the heat from a building or data center using lower temperature outside air instead of mechanical refrigeration, you can save so much in electrical costs that we label it “free cooling.”

Using the air-side method for free data center cooling

There are two forms of free cooling, commonly referred to as air-side and water-side. Water-side (or more generically, liquid-side) is still the more common form, and uses circulating water (or a water/glycol mix in freezing climates), to carry the heat to outdoor cooling towers. Large fans move the air through the water as it cascades down the cooling towers so that heat can be dissipated into the air, even if the outside air temperature is relatively high -- but this requires energy. When the air temperature is low, little to no fan power is required, so the energy used is mainly for the pumps that circulate water through the pipes.

Air-side free cooling just moves air without the liquid intermediary. There’s no electricity needed to pump liquid through pipes, so it should be the ultimate “green” energy saver. But is it really? Like so many things, it depends!

Many people still think the air-side method requires little more than opening a hole in the wall or extending a duct to the roof to bring in cool air at night or in the winter. Air-side cooling is becoming more popular, but it’s not really that simple. When transferring heat, water is many times more efficient than air per unit volume. Therefore, it takes a much greater volume of air to cool a data center than it does water. Nearly 40,000 cubic feet per minute (cfm) of air is needed to cool a 250 kW data center. For a 1 megawatt (MW) data center it’s more than 150,000 cfm. So you’d need a really big hole or a very large duct. The above numbers were figured at a flow rate of 2,500 lineal feet per minute (FPM), which is 28 mph. That’s quite a breeze! 40,000 cfm at that velocity requires at least 16 square feet of duct, which might be 2 x 8 feet. A better velocity would be 1,500 FPM, which, for 150,000 cfm, would require 100 square feet or something like a 4’ x 25’ hole or duct.

In large data centers, it’s usually not practical to move and filter so much air directly through ducts from outside. That much air, through such a big hole, can bring in a lot of contaminants, so filters just as big are needed to trap particulates. It takes large fans to pull that much air through heavy filters, which add a lot of air resistance when new, and even more as they fill up with dirt. That’s energy use. Then there’s filter replacement cost as well.

Addressing air-side cooling challenges

It is also important to control humidity. Cooling coils can be used to condense moisture out of the air if humidity is too high, or heating coils can be used if temperature and humidity are too low and moisture needs to be added. That’s energy again. Desiccant dehumidifiers (moisture absorbing chemicals) use much less energy than cooling coils, but still create some resistance to airflow that requires fan energy to overcome. These factors can quickly offset energy cost savings, so nothing is really “free.” And we haven’t even addressed gaseous contaminants that can ruin hardware over time and can be difficult to extract from the air.

In a sufficiently cool, clean climate, air-side free cooling may be a logical choice. In a “mid-climate” (including some places you might not expect, such as Hawaii) it could still be a consideration. In a hot climate, particularly one with high humidity or with blowing dust or pollutants, it’s unlikely to be worthwhile. Even with the expanded ASHRAE Thermal Envelope, which was, in part, developed to enable more hours of free cooling in more parts of the country, there are limits. Cost/benefit must always be considered when implementing any type of technology.

The “bottom line” is that, no matter what method is used dissipating heat is a process of transferring energy from one medium to another. With mechanical refrigeration, we use the unwanted heat to change a liquid refrigerant to a gas, and then use the mechanical system to change it back to a liquid so it can return through the conversion process and absorb heat all over again. Depending on the exact system used, the refrigerant may be circulated to a condenser that transfers heat to the outside air, or it may exchange the heat to water that circulates to outside cooling towers. Both methods take energy, as described above. Water-side free cooling is essentially a matter of adding valves that bypass the mechanical refrigeration system when it’s cool enough outside, and just circulate the liquid to carry the heat away. The control systems can get a little tricky, but in principle, it’s really rather simple.

With air-side cooling, we can avoid the contamination and humidity problems by using air-to-air heat exchangers. Their purpose is to transfer heat directly from the air inside the room to cooler air outside the room, with minimal loss of inside air or infiltration of new outside air. Of course, because air is so much less efficient than water for transferring heat, the devices that accomplish this must be quite large. One such unit that has now been well proven is the Kyoto Wheel.

Alternative data center cooling devices and methods

The Kyoto Wheel is a large, slowly rotating aluminum honeycomb -- a modern version of the decades-old “heat wheel” technology -- that is specifically tailored to the data center. It rotates at only 3 to 10 rpm and requires minimal energy to turn. Special fans have been added to more efficiently evacuate the air from the honeycomb, and they use energy too, but this wheel is still said to use only 8% - 25% of the power of equivalent mechanical refrigeration. There is also less than 1% air exchange between outside and inside, and the surfaces are coated to resist dirt buildup and corrosion that would reduce heat transfer efficiency. But it is large -- about 20 feet in diameter for a unit capable of 600-850 kW of heat removal -- and it is contained in a housing that takes up about 1,000 square feet of space on the roof or next to the building. However, the wheel can provide 100% free cooling in Dallas for 47% of the year, and in San Jose, Calif., 81% of the year at maximum wheel capacity. Those are pretty good numbers.

Another approach is evaporative cooling. This runs the hot return air from the data center through a metal tube, which is kept wet by a water spray, that cools the tube by evaporation. The cool tube, in turn, cools the air that moves through it. This system works best in drier climates where evaporation is maximized, but it still takes fan power to move the air. However, since the evaporative process can actually cool the air to below outside temperature, this type of system can be used more days of the year than the Kyoto Wheel, ideally saving more refrigeration energy than it consumes in fan power. A sophisticated example of this approach is the relatively new APC EcoBreeze indirect evaporative cooler.

The biggest drawback to data center cooling using the air-side method is that there are very few climates where free cooling can be expected to cool a data center 100% of the time. The newest ASHRAE Thermal Guideline may change that, but not with most of today’s computing hardware.  While a lot of electricity can be saved whenever free cooling is in use, mechanical air conditioning is still necessary for when it is not effective. With water-side cooling, the only real additions to the cooling plant are valves and controls. With air-side cooling, you still have the capital cost of the mechanical refrigeration system, plus the cost of the air-side cooling equipment. A thorough return on investment (ROI) analysis must be run to determine whether the investment in any free cooling solution is really worth it, and that’s even more necessary when considering “air-side” cooling. Regardless of the monetary ROI, free cooling will certainly be worth it environmentally, and the newer devices now available make it more realistic than ever to cool with outside air.

About the author:
Robert McFarlane is a principal in charge of data center design for the international consulting firm Shen Milsom Wilke. McFarlane has spent more than 30 years in communications consulting, has experience in every segment of the data center industry and was a pioneer in developing the field of building cable design. McFarlane also teaches the data center facilities course in the Marist College Institute for Data Center Professionals program, is a data center power and cooling expert, is widely published and speaks at many industry seminars.

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