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How green is your data centre?

Rising energy price trends, more stringent carbon taxation, and a need to present green credentials are all motivating data centre operators to cut energy consumption wherever possible.

In this article, Alex Emms, operations director at Uninterruptible Power Supplies Ltd., a Kohler company, shows how choosing the right UPS topologies, and operating them efficiently, contributes to meeting these objectives.

According to a Guardian article , our growing appetite for digital services means the data centres that power them are responsible for about 2% of greenhouse gas emissions; a similar share to aviation. While an individual’s everyday browsing has a minimal impact, it’s a different story with technology giants like Facebook and Google, and their billions of users worldwide. Google’s carbon footprint for 2013 was 1,766,014 tonnes CO2 equivalent, mostly generated by their data centres.

While most organisations are far smaller than these giants, the percentage of their energy bill attributed to their data centre has nevertheless grown with their increasing dependency on IT. This has major cost implications, both through direct energy costs and the impact of the Government’s Climate Change Levy – an energy tax payable by non-domestic users, intended to provide an incentive to increase energy efficiency and reduce carbon emissions. On top of this, shareholders, customers, employees and the media increasingly seek evidence of green credentials as part of their evaluation of any company they associate with.

As UPSs usually handle all of a data centre’s ICT power, any improvement in UPS efficiency will contribute significantly to reducing its energy demand. Fortunately, UPS efficiencies have been improving; 10 years ago, UPS efficiency levels were typically around 94%, but, today, 96% is possible, with 97% expected soon.

So, let’s look at the factors that contribute to UPS efficiency, and how they can be invoked to maximum effect.

Transformerless topology and modular UPSs
The most important single game changer of recent years was the appearance of transformerless topology, as this has brought both direct and indirect benefits. Firstly, going transformerless immediately improves efficiency by up to 5% across most of the load spectrum, as shown in Figure 1. This not only yields a direct reduction in energy use, but also saves on energy needed for cooling.


Fig.1: UPS AC to AC efficiency curves

Additionally, the conversion electronics used by transformerless UPSs present an input power factor that’s much closer to unity, and more load-independent, than previously. This improvement reduces the magnitude of the input currents, minimises associated cabling and switchgear sizing, and in some circumstances reduces electricity running costs. Also, input current total harmonic distortion (THDi) of less than 3.5% virtually eliminates harmonic pollution of the mains supply. This saves unnecessary oversizing of gen-sets, cabling and circuit breakers, avoids extra input transformer heating and extends the overall lifetime of all input components.

Yet transformerless topology also brings indirect energy-saving benefits; the associated space and weight savings are so significant that implementing a UPS as a rack-mounting module rather than a large, floor-standing system becomes viable. This means that UPSs can be built as modular solutions that can be incrementally sized to closely match their critical load. Unnecessary UPS capacity – and its power and cooling demand – is eliminated. This remains true even as the load grows, as the UPS can be scaled to match changing load requirements.

Eco mode: improved efficiency, but higher risk
The PowerWAVE 9500DPA UPS, a racking solution with 500 kVA capacity, by using this design approach, achieves 96% efficiency in normal operating mode. However, this UPS, like many others from various manufacturers, also offers an option for further energy efficiency; it can be operated in Eco mode, to achieve efficiencies of 99% or better.


Fig.2: UPS Off-line or Eco mode

Eco mode in dual conversion online UPS systems is similar to the default operating mode in offline types, as shown in Figure 2. During normal operation, the critical load is powered from the bypass line; it is therefore subjected to the raw mains supply and all the transients, brownouts and other disturbances it may bring. Although some spike suppression and RF filtering may be provided, protection is not as effective as having the UPSs power conversion components in line between the raw mains and the critical load; no correction is applied to frequency, voltage or waveform variations.

Another issue is that, if the mains supply does fail, there will be a supply break during transfer to and from the inverter, although this is brief and typically of 2 – 10 ms duration. Overall, for most data centres, cleanliness and continuity of supply take the highest priority, so the Eco mode is used with caution if at all.
Improving efficiency at low loads

There is a further method available for saving energy. It addresses the fact that transformerless UPSs’ efficiency drops significantly at loads below 25%. Known as Xtra VFI, this smart technique exploits the modular topology of UPSs like the PowerWAVE 9500DPA. When enabled, it automatically adjusts the number of active modules to meet changing load requirements. By being continuously and dynamically resized in this way, the UPS’s capacity can be kept more fully-loaded, and therefore optimally efficient in operation.

Modules that are not needed are switched to standby but remain in a state of readiness, primed to start up and transfer to active mode if the load increases. This mode of operation is particularly effective in mitigating the transformerless UPS’s above-mentioned loss of efficiency when loads drop below 25% of the UPS’s full capacity. Another benefit is that Xtra VFI involves rotating the modules between active and standby, therefore extending the service life of the UPS.

Fig.3 below shows how the Xtra VFI operating mode can enhance efficiency when running at a low load level for a PowerWAVE 9500DPA modular UPS system, with 10 UPS modules (100 kW) and total available capacity of 1 MW.


Fig.3: Improving UPS energy efficiency at low loads

For example, a typical system could be supporting an 800-kW maximum load with N+2 redundancy. It could be implemented in two fully-populated 500 kW UPS frames together containing ten 100 kW modules. If the load demand drops to 200 kW, the Xtra VFI control would reduce the number of active modules to four, to match the reduced load requirement while maintaining N+2 redundancy. Each module would be 50% loaded and operating within its optimal efficiency/load range. The other six modules would be passive.

The active modules would be working in double-conversion mode; the load is shared equally between all of them. The passive modules would be on standby, with their inverter switched off, ready to transfer to active double conversion operation when the load increases.

Conclusion
According to energy price comparison service UKPower.co.uk, energy prices look set to increase steadily for the foreseeable future, influenced by increases in demand, transportation costs, wholesale prices, and government legislation . Carbon emission taxes, and the need to present green credentials, will also remain as issues.

With this pressure to reduce energy consumption and costs, choosing the right UPS topology and deploying it for optimum practical efficiency have now become essential issues for senior management.

For more information visit www.upspower.co.uk, email sales@upspower.co.uk or call 0800 731 3269.

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About Sarah OBeirne

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