• Updated with a whitepaper download link that's open to the public

The Green Grid has published a whitepaper outlining in detail the most important points data center managers need to consider when evaluating the possibility of running their Uninterruptible Power Supply (UPS) systems in Eco Mode.

The Eco Mode is part of The Green Grid’s Data Center Maturity Model and one of the model’s energy-savings recommendations.

The paper’s main editors are from Eaton and GE, and its list of contributors also includes people from Emerson Network Power and Dell. Eaton, GE and Emerson are all UPS manufacturers.

The Eco Mode in UPS products provides slightly higher energy efficiency than the voltage/frequency independent mode (VFI), or double-conversion.

In a best-in-class data center, the Eco Mode can provide 99% efficiency, compared to 96% in VFI mode. The difference is greater in typical modern data centers that are not best in class: 98% efficiency in Eco Mode and 94% in VFI.

The difference in efficiency is yet greater in legacy data centers, where an Eco Mode UPS can operate with 98% efficiency, while VFI mode offers 92%.

When a UPS is operating in Eco Mode, it is at its most energy efficient, but the mode may also require performance trade-offs, according to the paper’s authors.

Key considerations in Eco Mode performance should include transfer time, design ride-through in IT power supplies, availability of surge protection in Eco Mode, quality of internal site power, static transfer switch compatibility and reliability and quality of utility power.

According to The Green Grid, transfer-time is “one of the most salient performance characteristics for Eco Mode.” This is the amount of time during which the UPS will not output any power.

In UPS systems with static switches the transfer time is shortest, whereas mechanical transfer devices are much slower. When examining the possibility of using the Eco Mode, data center managers should examine transfer mechanisms between two power sources both ways: from UPS to alternative source, and from alternative source back to the UPS inverter, the paper’s authors recommend.

Another important consideration is design of the IT power supply units. They can store some energy in capacitors and provide a small amount of ride-through time.

The energy stored by the power supply unit needs to be taken into account because an Eco Mode UPS will supply transient inrush load current to recover energy the power supply lost after an outage. Because of line impedance between the UPS and the power supply, this current surge may drop power supply input voltage.

More details on the Eco Mode considerations listed above in The Green Grid’s whitepaper on the org's website.