Fans use a lot of energy
Computer room air conditioning (CRAC) unit fans account for 5% to 10% of a data center's total energy use.1 (Typically, only cooling compressors use more energy in a data center.) CRAC units with standard fans are unable to vary fan speed to match the data center heat load, which fluctuates as the services provided evolve and equipment is replaced. Because data center environments constantly change, variable-speed fan drives (or VSDs for short) should be used wherever possible. Retrofits for many CRAC and CRAH (computer room air handling) units are available. Many energy-efficiency measures—hot aisle/cold aisle, containment, increasing server inlet temperature, air side and water side economizers—impact cooling loads and depend on VSDs to fully realize their energy-saving potential.
Small Reductions in Fan Speed = Large Savings
A fan’s power draw varies with the cube of its operating speed. This relationship means that:
- Reducing the fan speed by one-half will reduce energy consumed by one-eighth!
- Two CRAC units running at half speed use one-fourth the fan energy of one CRAC unit running at full speed!
This same exponential relationship between speed and power consumption applies to variable speed pumps installed in chilled water systems.
Electronically commutated (EC) fans are inherently more efficient than traditional centrifugal fans, and are 100% speed controllable without needing to install added components like VFD’s. This video explains EC plug fans and their advantages in air handling systems.
Savings and Costs
Installing variable speed drives is one of the most common data center energy efficiency retrofits:
- Four ENERGY STAR certified data centers — operated by BNY Mellon, Kaiser Permanente, RagingWire, and Target — installed variable speed drives to save hundreds of thousands of dollars annually. They saw paybacks ranging from 0.54 to 1.7 years.
- EBay completed a case study (PDF, 578 KB) with EPA ENERGY STAR that documented a 1.6 year payback (with a utility incentive) for variable speed drive retrofits at their Phoenix data center. (2.6 years without the utility incentive).
- The Green Grid completed a comprehensive analysis (PDF, 2 MB) detailing the return on investment (ROI) and power usage effectiveness (PUE) for associated with several data center efficiency upgrades. Payback for VSDs was 16 months.
- The Green Grid’s Chiller System Optimization: Project Impacts on Data Center Sustainability (PDF, 1 MB) examines the ROI of variable speed drives installed on 6 chiller pumps.
Tips and Considerations
- Not all manufacturers offer retrofit kits for their older CRAC and CRAH units. Your only option may be to ensure that your next CRAC or CRAH comes with variable speed fans.
- With VSD fan retrofits, reduced fan speeds can lead to condensation and freezing on the AC coils , and should be closely monitored. Newer CRACs and CRAHs with variable-speed fans use digital compressors that slow down with fan speed.