Feature UPS & Standby Power Leading the field in modular power
As the drive for ever greener and more energy efficient data centre M&E infrastructure continues, power usage effectiveness (PUE) has emerged as the metric for measuring power usage efficiency improvement. Kenny Green, technical support manager for Uninterruptible Power Supplies (UPSL) explains
that time of cheap energy, users’ con- cerns centred on load protection rather than energy costs.
ment power. For many
P
UE compares the total amount of power used to that delivered to computing equipment: PUE = Total facility power/IT equip-
years designers have
focused on improving mechanical cool- ing system efficiency. As this has been achieved, sometimes drastically, atten- tion is returning to power system design.
UPS’ contribution to PUE Although a relatively recent innovation, the PUE reflects how designers have always needed to allow sufficient utility capacity for any planned ICT installa- tion. Unlike the PUE though, capacity planning must always allow for peak rather than annualised power demand. An early 1990s data centre of around 1MVA had a PUE of typically 3.5. It had an N+1 UPS of probably 3 x 500kVA and 88% efficiency. A com- pressor-based mechanical cooling system maintained tight temperature and humidity control, without the aid of free cooling coils, variable speed pumps or fans. Humidification, de- humidification and lighting energy consumption was relatively high as load power density was around 350- 500W/m2
UPS efficiency developments Moving from thyristor to innovative transistor semiconductors for inverters and then rectifiers, allowed UPS effi- ciency improvements by eliminating passive filters and transformers. Double conversion topology has also been enhanced with eco-mode opera- tion and modular design, enabling higher load factors. Efficiency has improved from 87% at full load for a 1990s transformer design to 96% in today’s transformerless double conver- sion implementations - with up to 99% being available if eco-mode oper- ation is allowed.
PUE and UPS impact and
The introduction of free cooling economisers
then ASHRAE’s
(American Society of Heating, Refrigeration & Air Conditioning Engineers - a technical committee that regularly issues and updates the limits for temperature, humidity and air- quality for ICT equipment) relaxation of temperature and humidity specifi- cations has drastically reduced cooling system power. Strict air management has been established and enhanced for partial load conditions by widespread use of variable drive pumps and fans. Cooling performance has improved by a factor of ten. To contribute to a target PUE below 1.2, the UPS must offer 0.05, and lighting and other con- sumers a further 0.05. UPS efficiency must exceed 95% at full load, and 94% at 30-40% load, unless right- sizing through modular topology can maintain the load above 70%.
Above: Kenny Green, technical support manager for Uninterruptible Power Supplies (UPSL)
Left and below: for many years designers have focused on improving
mechanical cooling system efficiency. As this has been achieved, sometimes drastically, attention is returning to power system design
Classification and impact on PUE Apart from the largest search engines and social media networks, data centre operations commonly feature partial loading. They can start with loads as low as 15% and take four to six years to exceed 65%, often never exceeding 80%. This creates a barrier to high efficiency. For single bus (tier I-III) ICT systems this can be miti- gated by scalable or modular topology that can be incremented to efficiently match changing loads. For example 5x500kW modules would support a 2MW load with 80% load factor and N+1 redundancy.
The UPS contribution to the PUE will be even higher for a dualbus (tier I-IV
installation), where the load
seldom exceeds 40% per bus and is often below 20%. However, the usual penalties of such light loading can be avoided using eco-mode operation.
Modular UPS technology For small and medium loads the advent of rack mounting UPS modules has made right sizing a UPS to its given load easier than ever - simply add or remove modules to change capacity. Module sizing and total rack capacity depends on the load steps anticipated and the ultimate load. For example, a 100kW ultimate load could be configured with 10kW modules, or 200kW modules could fulfil a 1MW load. In either case, the UPS should be loaded to 70-80%, allowing highest efficiency operation.
The future
ICT loads need uninterrupted power as much as ever, but UPS must deliver this with efficiencies above 95%, even at partial load to meet end user PUE expectations, future carbon reporting and possible legislation requirements. As energy costs rise, and eco-mode
from predominantly main- frame hardware. The result was an infrastructure with a seasonally con- stant utility load and a full load PUE which, if calculated, would have been around 2.5. However, full loading was seldom achieved as neither scalability nor variable speed drives were avail- able, so real PUE values were closer to 3.5. The UPS contributed only about 0.15 to the PUE, which was dominated by the mechanical cooling load. At
30
operation is proven in mature grids, future UPS efficiency of around 99% for over 90% of the year is likely. This, together with the most advanced cooling systems and LED motion controlled lighting will permit a PUE of typically below 1.10 throughout Europe.
Uninterruptible Power Supplies (UPSL)
www.upspower.co.uk T: 0800 171 2320
Enter 215 DECEMBER/JANUARY 2014 Electrical Engineering
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