STULZ worldwide

AER – A new efficiency indicator for airflow in Data Centers

AER - Fan power consumption

Recently, a value has repeatedly popped up in air conditioning system specifications, defining the maximum permitted power consumption for the fans at a certain airflow.

In the past, the primary consumer of energy in an air conditioning system was the compressor. Today, most Data Centers use air conditioning systems with Free Cooling. Now mechanical or compressor cooling is only required if it is very warm outside, and the Free Cooling is insufficient for transporting heat out of the Data Center. Due to these changes in air conditioning systems, fan power consumption is now moving into the spotlight, as air needs to be conveyed through the Data Center even in Free Cooling mode. Therefore, these days the fans in the air conditioning units are frequently the primary energy consumer.

To demonstrate how efficiently air is conveyed through a Data Center, we look at the ratio of fan power consumption to airflow. And so that this child has a name, we have called it AER, which stands for Airflow Efficiency Ratio.

The AER describes the ratio of fan power consumption to the airflow of an air conditioning unit at a given external static pressure. The unit used for the AER value is W / (m³/h). In order to obtain numerical values that are easy to handle, we did not choose kilowatts - the usual means of measuring fan power consumption - as the unit, but watts instead. The smaller the AER value, the better. The less power consumption required to achieve a certain airflow, the better.


Here are two examples:

  1. A precision air conditioning unit achieves an airflow of 30,000 m³/h at a static pressure of 20 Pa in the raised floor. The fans have a power consumption of 3.3 kW, or 3,300 watts. This translates as AER = 3,300 / 30,000 = 0.11 W / (m³/h).
  2. For a typical air handler, which conveys air at a rate of 80,000 m³/h at an external static pressure of 50 Pa in the ducts to the Data Center, a power consumption of 28.0 kW results in AER = 28,000 / 80,000 = 0.35 W / (m³/h). The AER can be used to compare different air conditioning units, identical units with differing airflows (e.g. with and without active standby units), or different air conditioning systems in comparable conditions.

It is safe to assume that in future the AER will become a common item among the technical data in air conditioning unit brochures.

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 Center Cooling and Indirect Free Cooling with Dynamic Control Logic.