As more research is done on the internal data center environment, new and different cooling strategies are being applied to help reduce the operating cost of the cooling systems. Some of the research that ASHRAE has done (and is doing) helps the HVAC designer by providing important data points on how servers and other IT equipment react to different levels of temperature, moisture (humidity) and airborne contaminants.
A recent blog posting on the subject of free cooling (http://bit.ly/nVKpsU) got me thinking that I should write a brief primer on economizers and about the energy consumed by cooling systems in an economizer mode. Water economizer, the most time-tested economizer for data centers, doesn't really pose any problems to the internal environment because it uses water from the cooling towers, usually run through a heat exchanger, directly into the air-handing or CRAH units. The cooling towers, chilled water pumps and chilled water pumps continue to run - the chiller is shut down or running in a very reduced capacity. Depending in the climate, this type of economizer can save 14% on annual cooling costs in a climate like Denver, but only about 2% in Houston.
Turning to air economization, there are two primary types: direct and indirect. Direct, as the name implies, mixes the outdoor air stream with the indoor air. In an area that has very low outdoor particulates (from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray) and gaseous contaminants (sulfur dioxide, carbon monoxide and volatile organic compounds), direct air economization is very effective and can reduce energy consumption significantly. In Prague for example, using a direct outdoor air economizer will reduce cooling cost by 38%. Adding evaporative cooling reduces energy use by 45%. In Hong Kong the savings are approximately 25%.
Indirect economization uses the outdoor air to transfer heat from the data center but it does it by not mixing the airstreams. This is done using heat exchangers that come in a variety of configurations, including the option of adding evaporative cooling. These systems eliminate the problem of introducing contaminants into the data center; the downside (compared to a direct air economizer) is they have more components, have a lower efficiency and are typically larger in size. So if you want to use an indirect air economizer in Helsinki, you could expect to reduce your annual cooling costs by 45%. Not bad. But it still leaves 55%.
So "free" is more like "reduced". Different climates, systems and setpoints can make big difference in the ultimate quantity of energy that is used to cool a data center.