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Free Cooling – Direct and Indirect

Free Cooling for Data Centers: this subject is on everyone's lips and is preoccupying specialists at conferences on Data Center infrastructure. There are now countless variations. But they all pursue the goal of lowering the Data Center's energy consumption and improving the PUE.

The term "Free Cooling" suggests that you don't have to pay for this type of cooling. That is a fallacy. Is anything free these days? Below I will describe the Free Cooling solutions in use today.

Free Cooling

Free Cooling means that the power consumption of the air conditioning system at the site is reduced to the necessary minimum by suitable means, without compromising on reliability and availability. The words "suitable means" and "at the site" open up a very broad range of possibilities.

Direct Free Cooling

To put it briefly, this could be described as follows: window open, blow cold air from outside through the Data Center, pick up the warm air, transport it back outside, voilà! And physically speaking, that's exactly what happens. Only the process of "moving the air" requires energy.

Unfortunately, in real life things are not that simple. Outdoor air is not always in a condition that the IT equipment is comfortable with. Sometimes it's hot and sometimes cold, sometimes it’s very humid and sometimes very dry. What's more, outdoor air is not always clean. The outdoor air is often full of particles which can be very hostile to modern IT equipment.

So, direct free cooling should only be used where the negative properties of the outdoor air can be controlled by financially acceptable measures.

Before the outdoor air is allowed access to the data center, it must be filtered – a process that can be expensive. If the outdoor air is too cold, a certain proportion of warm air from the data center must be mixed in with the cold air to supply controlled, tempered air to the IT equipment. The airflow concept and the control system in the data center must be designed to ensure this. If the outdoor air is too warm, additional mechanical cooling will be required. Therefore, in locations with very high average annual temperatures, direct free cooling may not be a feasible option.

Air humidity is yet another challenge. Although they are only machines, servers really don't like excessively dry or damp air. Measures to humidify or dehumidify the air can also be extremely expensive and complex.

Furthermore, in some places the law indirectly prohibits the use of Direct Free Cooling, as larger openings in the outer walls of buildings are not permitted for safety reasons.

So as we can see, the main advantages are put into perspective if expensive additional measures are necessary due to conditions at the site in question.

Indirect Free Cooling 

With Indirect Free Cooling, the window stays shut. The outside air, with all its bad characteristics, can't get into the Data Center and has to remain outside. All the potential disadvantages of Direct Free Cooling mentioned above do not arise with Indirect Free Cooling. Unfortunately however, Indirect Free Cooling is less efficient than Direct Free Cooling, as at least one heat transfer must always take place between the air in the Data Center and the outside air. At least one heat exchanger is required, and that means a loss of efficiency.

Indirect Free Cooling comes as a single or a two-stage system.

Single-stage Indirect Free Cooling systems

Single-stage Indirect Free Cooling systems transfer heat using an air/air heat exchanger. The air in the Data Center is circulated and conveyed through this air/air heat exchanger. The outside air is transported through the air/air heat exchanger on the outside. What's more, there is a mechanical cooling system, which is used when the outside air is too warm. Single-stage Indirect Free Cooling systems need a great deal of space, as the air/air heat exchangers are much larger relative to the cooling capacity and in comparison with heat exchangers in closed-circuit air conditioning units. Moreover, large holes in the building's outer wall are required, because large quantities of outside air must be fed to the air/air heat exchanger.

Two-stage Indirect Free Cooling systems

Two-stage Indirect Free Cooling systems have a considerably smaller footprint than their single-stage counterparts, but are somewhat less efficient as they function with two heat transfers. In the first heat exchanger, the heat from the air in the Data Center is transferred into a liquid. This is then conveyed to the outside in relatively thin pipes, where it is emitted into the outside air via a second heat exchanger. Again, we lose some efficiency through this second heat transfer. Here, too, the mechanical cooling unit is part of the system, and is only used to supplement the Free Cooling at high outside temperatures. If it is very hot, all cooling will take place mechanically.

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About the author

About the author

Benjamin Petschke was born in 1969 in Germany. After studying physics he joined STULZ in 1996 and worked since then in the R&D, Export and Marketing department on different positions. With 19+ years’ experience in the Data Center cooling industry, Mr Petschke is specialised in Data Center cooling design, energy saving and acoustic issues.

He works closely with the Joint Research Centre of the European Commission for the Code of Conduct on Data Centres on the Best Practice section and recently with the German DKE in development of the DIN EN 50600, Information technology – Data Centre facilities and infrastructures.

Mr Petschke authored White Papers on subjects like Best Practice for Data Centre Cooling and Indirect Free Cooling with Dynamic Control Logic.

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