So you’ve hunted down and exterminated all of your zombie and comatose servers. You’ve added blanking panels and sealed floor and cabinet openings to optimize airflow. Now what can we do to save even more energy in the data center?
Fans are an obvious consumer of energy in our mechanical cooling systems in the data center. One of the primary steps we take in this regard in the Data Center Energy Practitioner (DCEP) courses that I teach is to replace fixed speed fan motors with variable speed fan motors. The reason for this is that if we reduce fan speed (and airflow) we can get a lot of efficiency savings because of a relationship called the “Fan Cube Law.”
Fan Cube Law
The Fan Cube Law tells us that fan motor power is roughly proportional to the speed of the fan raised to the power of three. Because of this cubic relationship, we can effect significant fan motor energy savings with relatively slight reduction in fan speed. This is why the variable speed drives are so advantageous.
Here is the equation:
New Fan Motor Power = (Old Power) * (New Speed/ Old Speed) ^ 3
We can also treat airflow the same as speed.
New Fan Motor Power = (Old Power) * (New Airflow / Old Airflow) ^ 3
Big Impact to Data Center Energy Savings
So let’s do an example,
Assume we have an air handler with 50,000 CFM airflow, using 10 kW of fan motor power. If we reduce the airflow to 25,000 CFM, we have a new fan motor power of
10 * (25,000/50,000) ^ 3 = 1.25 kW
So we reduced our fan motor power by 88% with a 50% reduction in airflow! This is significant and it’s easy to see that if we convert from fixed speed to variable speed drives for all the fans in our data center how impactful to energy efficiency this would be.