Chris Cutler from Riello UPS examines how an uninterruptible power supply’s ECO operating mode works and explores whether the potential energy savings are worth any trade-off in reliability.
As energy costs continue to spiral, facilities managers across every industry are eager to reduce power consumption where possible and identify potential savings. UPS manufacturers play their part by developing ever more efficient models. Indeed, many modern UPS are capable of high efficiency (i.e. >97%) even in maximum protection online double conversion mode.
Most UPS systems now also incorporate ‘economy’ operating modes offering even greater efficiencies. But do the promised savings add up in practice?
Before we go on, we should first clarify what we mean when we talk about UPS efficiency. This term describes the ratio of power entering the UPS compared to the power that exits to supply your load.
Whenever electricity flows through a UPS’ components, a certain share dissipates as heat and sound. So, take a UPS with a 95% efficiency rating. That means 95% of the original input powers the load, with the other 5% ‘wasted’ running the actual UPS.
UPS efficiency has increased steadily in recent years, with ratings in excess of 95% now the norm rather than the exception. Most modern UPSs, especially transformer-free models, also have a flatter efficiency curve, which means they can achieve high efficiency even at loads of just 20-25%.
A typical industrial or data centre UPS will have a service life of up to 15 years. Now even a 1% or 2% improvement in operating efficiency over such a period can add up to significant energy savings. Not only is this positive financially, but there are also environmental benefits in terms of reduced power consumption and CO2 emissions.
ECO mode explained
ECO mode works in a similar way to an offline or standby UPS system. This means the bypass line (i.e. raw mains electricity supply) powers the load and the UPS’ inverter is switched off. Whenever there’s an issue with the mains, the critical load experiences a fractional break while the automatic bypass transfers it back to the inverter.
Operating in this way increases UPS efficiency to more than 99%, compared to the 93-97% efficiency of your typical unit running in online mode. That’s a difference of anywhere from 2-6%, which could equate to substantially lower electricity bills.
Another advantage of ECO mode is that there’s less wear and tear on some of the components inside the UPS’ inverter and rectifier.
Deciphering the drawbacks
Of course, there is one obvious downside to ECO mode – your IT load is exposed to raw mains utility power. If you’ve got a clean and stable mains supply, there’s relatively little risk. But if there’s any disruption to the mains, you’ve potentially got a serious problem on your hands.
If there’s an issue with the mains, your UPS goes through the following process:
- Detect the power problem
- Determine whether and how to respond
- Energise the inverter
- Open the static bypass switch
- Transfer the load onto the inverter output.
This process takes time. And even though it might only be 1-16 milliseconds, it’s still leaving your critical load exposed. While most modern servers can override even a 16ms loss of power, that’s not the case for other components or devices.
For example, just an 8ms loss to a transformer can cause it to surge and trip circuit breakers when the voltage goes back to normal.
As well as the loss of electrical protection, running in ECO mode runs the risk of other drawbacks too. Firstly, you might get damaging harmonics, as ECO mode disables the power factor correction you get as standard with an online UPS.
ECO mode also requires the inverter to restart in response to power events. Whether this happens once a month or once an hour, this change in power to the inverter causes a shock to the system. Such thermal transients are one of the main causes of failure in electronic power systems. And in ECO mode, these thermal transients occur at the exact time when you need your UPS to be at its most reliable.
Finally, there are issues with fault clearing. In online UPS mode, your unit will quickly detect a fault on the output, then switch to bypass to get the extra fault clearing that opens protective devices downstream of the UPS. But in ECO mode it’s tricky to distinguish between an output fault and a loss of input power.
The Active ECO alternative
Recent advances in firmware and electrical designs have resulted in another energy-saving mode called Active ECO. Sometimes referred to as ‘Advanced ECO’, it is similar to standard ECO mode as it still sees the mains supply power the load.
But the big difference is that the inverter remains on at all times, running in parallel with the input without actually carrying the load. As the inverter is always on, the UPS can take over the critical load far quicker than in standard ECO mode if there are any issues with the mains.
Active ECO also incorporates the power filtering that shields the critical load from any damaging harmonics.
Because the inverter circuit remains powered at all times, there is a 0.5-1% drop-off in efficiency running in Active ECO mode compared to standard ECO mode. But it’s still higher efficiency than online mode, so has come to be seen as some sort of happy medium between the two.
Looking to the future
Running in ECO mode can clearly deliver energy savings and reduce the overall total cost of ownership of the UPS, but for mission-critical sites, its use should be sparing. However, ECO mode might be appropriate out of hours when a facility’s critical loads are inactive.
Another option could be running the UPS in an N+X parallel redundant installation, where one unit operates in online mode as the primary UPS and the rest run in ECO mode until they are actively required to support the load.
The Green Grid, a not-for-profit data centre organisation, compares ECO mode to free cooling, where facilities take advantage of low outdoor temperatures to cool critical infrastructure; this isn’t perhaps a fair assessment.
That’s because ECO mode undoubtedly introduces a lower level of resilience both in terms of slower transfer time to backup power in case of a failure and generally inferior power quality to your connected loads. Such perceived risk is perhaps one reason why many operators of critical facilities remain reluctant to utilise ECO mode.
It’s likely that continued advances in UPS technology will reduce the energy savings associated with economy operating modes even further. Silicon carbide (SiC) semiconductors are one such example. These are far smaller and lighter than the silicon-based components used in UPS, while they can also run in much higher ambient temperatures.
Such qualities enable UPS with SiC to achieve efficiency of up to 99% in true online double conversion mode. This performance could potentially render ECO mode redundant as the UPS will deliver the same energy savings while also providing conditioned, reliable, and high-quality power.
