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Data centers are being challenged to increase energy efficiency without damaging reliability. One major area for improvement? Power.
Uninterruptible power supplies (UPSes) have made great progress. Insulated gate bipolar transistors (IGBT) bring electrical efficiencies above 90%, and transformerless designs add another 2% improvement. Load matching, phase balance and higher voltage to IT hardware also yield small gains.
It's great to save $25,000 in annual electrical costs with a high-efficiency UPS, but ROI is still a concern for many data center operators. For a 500-kW UPS, a 2% to 3% efficiency improvement puts the ROI at two or three decades. New UPS designs promise higher efficiency, lowering power demand and making the major UPS upgrade more appealing.
Economy-mode UPS systems -- also called energy-saving systems, econo-mode or eco-mode -- power IT equipment from the incoming utility until that source fails or voltage goes outside of a threshold. Then, the batteries take over. Sound scary? Not necessarily.
"If operating in eco-mode does not compromise the requirement to power the critical load when the input power to the UPS is out of tolerance, and we can demonstrate the immediate transition from eco mode to battery/inverter, it will meet our expectations for UPS performance," said Ed Rafter, VP of technology for The Uptime Institute, a body that ranks data center resilience and redundancy. He added that there are "other details to consider."
Eco UPS options
There are three generally recognized UPS technologies: double-conversion, voltage-independent (VI) line-interactive and standby. Voltage and frequency independent double-conversion UPS systems are considered the most robust UPS, but at best efficiencies hit 96%.
Most eco-mode UPSes are sophisticated adaptations of line-interactive or standby designs. Some eco UPS systems are configured as double-conversion VI, with the bypass switch normally closed.
VI line-interactive UPSes have a controlled output voltage, but the same output frequency as the input. The result is about 98% energy efficiency.
With standby UPS, the inverter doesn't run until incoming power fails. Eco UPS systems on voltage and frequency dependent (VFD), or standby UPS, designs keep the IGBTs ready so the inverter can produce power within 2 ms after activation. These designs enable efficiencies as high as 99%.
Risks and concerns
Users have two concerns with eco UPS operation: utility power anomalies getting through to IT equipment, and stable power starting quickly enough to keep servers running through an outage. UPS designers have taken significant steps to quell fears.
Anomalies should not be a concern when running in eco-mode, especially with a developed, mature power infrastructure. Eco UPSes have some filtering and should use surge protection devices on the bypass circuit to avert serious power line spikes and transients. UPSes based on VI topology provide more protection from power issues than VFD systems. Locations with unreliable power are poor candidates for eco UPS systems. Everywhere else, the exposure should be minimal with well-designed systems, and the potential savings could be significant.
Transformers lose about 2% efficiency, so eco UPS models are transformerless. Without a transformer, electrical designs must include protection against fault currents: short circuits that cause current (amperage) to instantly upsurge.
Recovery time on a UPS system includes many variables. IT power supply capacitors store power that rides through short interruptions (8.3 ms average). Sensor circuits must detect the problem with incoming power and open the bypass switch -- and start the inverter in a VFD system -- before full power goes to the IT equipment. If the UPS restores stable power quickly, the IT equipment should continue normal operation -- but there is no guarantee. All electrical systems take time to stabilize when suddenly hit with a full load, adding a millisecond or more to the total recovery time.
Old capacitors may not fully charge, shortening ride-through time. An eco UPS switches to battery when voltage exceeds limits, which can be +10% to -15%, so a VFD UPS delivers reduced voltage until reaching the limit. Even new capacitors may not fully charge at reduced voltage, such as during brownouts, also shortening ride-through time.
In a rare combination of circumstances, eco UPS operation could conceivably cause some servers to crash. That's the tradeoff for gaining maximum energy efficiency.
"Experience indicates that operating in this [eco] configuration does present some risk," said Uptime's Rafter. "From a purest perspective, it has to. Nothing is free, not even eco-mode."
Utility power restoration
Battery stress is a concern with any UPS, as use and recharging shortens the battery's service life. VFD designs don't stress batteries until switchover occurs, but VI systems could, because the inverter is always engaged. Good VI designs take supplemental power from the rectifier without discharging batteries until power actually fails.
Not sure how UPS systems work? The author defines three types of UPS designs: voltage and frequency independent, voltage independent, and voltage and frequency dependent, in accordance with IEC standard 62040.
When input power is restored, whether utility or generator, it is usually unstable for a few seconds. Utilities may even go on and off several times. An eco UPS must decide when to switch back to bypass, so there is usually built-in delay and monitoring logic to avoid returning to line power too soon. Unless you're almost out of battery, this should be no different than a double-conversion UPS.
Longer battery run time won't help. Without a generator to run cooling systems, both batteries and UPS will overheat.
Actual operation efficiencies
The real efficiency of an eco UPS is statistical, accounting UPS design, utility power reliability and data center operations. So manufacturers can't actually provide a number. A system based on VFD topology could be 99% efficient in eco- mode. One based on VI architecture will be slightly less efficient due to the small power draw of the inverter. One UPS manufacturer claims 98.3% efficiency with their VI design -- only 0.7% additional loss.
The real determinant is actual time running in economy mode. One installation reports logging 1,050 hours in double-conversion mode out of 21,000 hours total -- 95% eco operation. If eco-mode is 99% efficient, and double-conversion is 95% efficient, the actual efficiency is 98.79%.
A financial industry data center runs double-conversion during its critical daytime hours and eco-mode at night. This setup reduces savings, but maximizes reliability during its most critical hours.
Expect to see an eco option on every new UPS. Determine when and how to use it for your data center -- how much it saves will only be known from actual experience over time.
About the author:
Robert McFarlane is a principal in charge of data center design at Shen Milsom and Wilke LLC, with more than 35 years of experience. An expert in data center power and cooling, he helped pioneer building cable design and is a corresponding member of ASHRAE TC9.9. McFarlane also teaches at Marist College's Institute for Data Center Professionals.
To further reduce energy use in the data center, replace outdated mechanical cooling systems with designs based on close-coupled cooling, variable speed fans, electronically commutated motors and atmospheric free cooling.