The following is a guest post from Vali Sorell, an associate partner in the critical facilities division at the Syska Hennessy Group. Sorell was a speaker at the ASHRAE Technical Committee (TC) 9.9 sessions in Chicago last week, and had some comments about our story on data center air-conditioning fans.
We all agree that reducing the fan speed saves energy, regardless of whether that fan is driven through a VFD (variable frequency drive) or if that fan is driven by an EC (electronically commutated) motor. The person from 365 Main stated that “it’s not worth the extra cost to have fans run at a lower speed for such a short time.” That misses the point – the fact is that it IS possible to save energy at all times, and that lower speed does not occur for a short time. That lower speed occurs forever! This is best explained by an example.
Assume a data center has 100 CRAC units, 80 of them are needed to meet the loads, and 20 of them are needed for redundancy. This amounts to 25% redundancy, which is very typical for most data centers. Let’s also assume that the load is constant from now till forever (meaning that the part load conditions are not considered, i.e. they are history).
Case 1: 80 units running at 100% speed consume 80/100 = 80% of the possible fan energy use.
Case 2: 100 units running at 80% speed consume 80/100 x 80/100 x 80/100 = 51% of the possible fan energy use.
Compared to normal operation, using all of the available redundant CRAC units at variable speed (regardless of whether that variable speed is achieved by EC motors or VFD’s) consumes 100%-(51%/80%) = 36% less fan energy than running the load-required complement of CRAC units at full speed. That’s not a small amount of energy!