Cooling for Free (Almost)

Fresh air cooled data centres simply draw fresh air from outside (via filters) and pump it under the plenum floor rather than the refrigerated and recycled air that is used in 20th Century Data Centres (see picture above). This approach can reduce the energy required to cool a site by 80% plus as the only power draw is that required to drive the fans pumping fresh air. Hot air is immediately exhausted from the data centre halls via high level vents that lead directly to the outside atmosphere.

So why are all data centres not built this way? Fresh air cooled sites will not reliably maintain data centre hall temperatures between 20º Celsius and 24º Celsius. On cooler days where external temperatures are below 20º Celsius fan speeds can be reduced and hall temperature will rise to a set level. On days when external temperatures are greater than 20º Celsius fan speeds can be increased to reduce the delta between external temperature and internal temperatures. Nevertheless on these hotter days internal temperatures cannot be maintained with the ASRAE parameters without adding an additional (small) refrigeration plant into the air handling units. It is possible to dispense with this additional plant by specifying equipment that can operate reliably at higher temperatures.

In temperate climates like those in the UK (where, unfortunately, we rarely see days over 20º Celsius) this approach is highly efficient. Even in countries where quite high temperatures are seen in the summer months, benefits can be gained from using fresh air in winter and overnight. In colder climates quite low quantities of external air need to be moved thereby reducing the amount of power needed to provide cooling.

What are the drawbacks of using fresh air in our data centres? Well there are a few that we need to look at: The first is that as we draw in air from outside, contamination in that air will be drawn into our computer hall in quite large quantities. By filtering the air before allowing it into the building we can ensure that the larger particles are removed, however gaseous and microscopic particles (such as those emitted from diesel engines) can get through. Localised extreme conditions such as building fires can be traumatic.

Using fresh air has some benefits, one is that we need not be too concerned by humidity levels as we don’t use refrigeration, there are no cold surfaces for condensation to occur on. We can dispense with humidification a very great consumer of electrical power.

A less radical alternative to fresh air cooling is free cooling. Free cooling is used when the external ambient temperature is less than the temperature in the computer hall. In the the UK this occurs for approximately 40% of the year and yet is rarely taken advantage of. The cooler ambient air is used to cool a secondary medium, such as glycol or water. Free Cooling is achieved by circulation of the secondary medium, which either assists with the air conditioning’s prime circuit or when ambient temperatures are low enough, replaces it completely. Consequently during certain months of the year and often at night, cooling is provided without the need to run refrigeration plant. This significantly reduces power consumption, carbon emissions and running costs.