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Free-air cooling – using natural cooling in the data centre

In many circumstances, the most cost-effective means of maintaining an optimum temperature within the data centre is by using natural cooling, or free-air cooling.

In the past, when the accepted temperature for a data centre was in the range 19 degrees Celsius to 21 degrees Celsius, natural cooling

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only applied to certain geographical environments and only at certain times of the year. Consequently, natural cooling wasn’t used much. But, with the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) now accepting temperatures of up to 27 degrees C (and with many IT equipment vendors accepting higher temperatures of up to 34 degrees C) as being within working limits, free-air cooling is worth considering again.

Natural cooling, however, requires more than just opening a few extra windows in the data centre.  The air’s humidity is also an important factor and sometimes far more important than the temperature.  A high moisture level in the air can lead to rapid deterioration of some metals, whilst air that is too dry can lead to issues with static electricity, as well as the growth of dendrites, little whiskers of metal on printed circuit boards (PCBs) that can cause paths to short out in high-density systems.

So, even if air is at a temperature suitable for cooling, it still has to be treated to ensure the right humidity as well as filtered to catch particulates that could also cause problems in the data centre.

But you can still save on energy requirements for chiller units. You can also save on energy by operating large, slowly running fans rather than the high-pressure, fast spinning fans that are needed to push air through the smaller-bore ducting used in standard computer room air conditioning (CRAC) systems.

The most basic free-air system draws in air through filters to remove particulates. This system measures the moisture content and remedies it through drying or by adding small amounts of additional moisture as required. The main use of free-air cooling tends to be around the use of “economisers”, using liquid or air-based systems to provide a relatively closed system. 

A water-side economiser uses an evaporative tower or a dry cooler to deliver the cooling needed for a heat rejection loop. An air-side economiser uses ducting, filters and dampers alongside sensors and controls to allow filtered, treated cold air into the data centre as required. 

Keeping a watch on outside temp

Such systems can lower energy requirements by 30% to 60% over standard CRAC units, but they can be highly dependent on external temperatures for their overall efficiency. When external temperatures are too warm, other cooling will still be required, which makes the financial returns on free-air cooling a little more complex.

Other approaches to natural cooling are heat wheels that exchange the heat from the data centre with cooler outside air via large metal surface areas; and the superior thermal mass of metal to improve overall heat exchange. But even these systems bring a considerable amount of external air into the data centre, and the air still requires filtering and moisture controlling. 

However, the Kyoto wheel or “KyotoCell” (an energy recovery heat exchanger) attempts to get around this issue by using an enclosed system that allows just tiny amounts of external air to enter the data centre (less than 4% per wheel revolution). This minimises the need for filtering and moisture control, making the overall system far more energy efficient.

KyotoCooling International BV, the company that sells Kyoto wheels, uses four sections to achieve this energy efficiency. The heat exchange wheel, made from wound corrugated metal, rotates through each section. Internal hot data centre air enters in one section, transfers its heat to the wheel and then exits as cool air to be used back in the data centre through the next section. Cold external air enters in the third section, strips the heat from the hot wheel and exits to the outside air through the fourth section.

Again, external temperatures are an issue, but because of the small surface areas involved in the heat exchange mechanism, less cooling is required compared to using direct cooling via CRAC units or other free-air cooling systems.

Capturing heat through evaporation

Evaporative cooling is another natural cooling approach. As a liquid evaporates, it absorbs heat from its surroundings.  Therefore, air circulating past a liquid that can be evaporated at a relatively low temperature will be chilled down as that liquid evaporates. Think of it in terms of a school physics lesson where air blown through a straw into a beaker of chloroform placed on a thin puddle of water causes the water to freeze. 

For very warm climates, a contained system with a liquid that evaporates at a low temperature can use solar energy to cause evaporation and to cool the secondary liquid or air. The evaporated liquid can then be condensed to fall back again and maintain the constant cooling during the day.  At night, provided the air temperature falls far enough, standard free-air cooling can then take over. Even if the night air is not cool enough, standard CRAC units can be used as necessary, and benefits still exist as the units are only used for 30% to 40% of the time.

I would recommend free-air cooling to all data centre managers building a new facility or retrofitting an existing facility. The potential savings of using natural cooling in the data centre are appreciable, and with energy prices remaining volatile, any approach that gives you some control over cooling systems is worthwhile.

Clive Longbottom is a service director at UK analyst Quocirca Ltd. and a contributor to SearchVirtualDataCentre.co.uk.

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This was first published in September 2011

 

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