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Breakthroughs alter the face of power delivery

Posted: 10 Feb 2009     Print Version  Bookmark and Share

Keywords:data centre  UPS  power supply 

A lot of attention has been given to cars and heavy industry, but the pristine IT data centre also hides a monstrous consumer of energy that is rapidly increasing its carbon footprint. Using energy equivalent to that of 25,000 to 35,000 homes, these centres are causing alarm to accounting departments and green activists worldwide.

Figure: The power chain in a typical U.S. data center has two primary obstacles to efficiency—the number of power conversions in the chain and the distribution loss.
(Click for a larger image.)

At the heart of the problem is inefficient power delivery, specifically power conversion, memory leakage, cooling and distribution losses. In recognition of this, the world's top engineers have scrutinised everything from power distribution to server and point-of-load power supplies to minimise power waste. Their work has led to architectural breakthroughs that look to fundamentally change the face of power delivery for decades to come.

Commenting on the problem of efficiency, Stephen Oliver, vice president of marketing and sales at Vicor (Andover, MA), said "For every watt of power used usefully you have to put 2.3W into the building just to get there." Oliver was essentially referring to a metric called power usage effectiveness, or PUE. PUE was defined by an organisation of IT professionals called The Green Grid that was formed to promote energy efficiency in data centres. PUE is the ratio of total power used by a data centre divided by the IT equipment power.

The aggregate inefficiency comes as a result of all of the power conversions in a data centre and the efficiency of components such as processors, memory and disc drives. For example, a server power supply might offer 90 per cent efficiency. That 10 per cent of energy that is not used effectively adds to the PUE. Moreover that 10 per cent of wasted energy is dissipated as heat. The power required to cool the equipment is also captured in PUE.

Dallas Thornton, division director at the San Diego Supercomputer Centre (SDSC) located at the University of California San Diego (UCSD), believes data centres can hit a PUE goal in the 1.3 to 1.4 range. Thornton is currently working on an addition to the SDSC. He states, "Typical data centres today are in the 1.8 range. Inefficient data centres can be over 2."

Attacking efficiency in the power system
To improve efficiency, the industry needs more-efficient power supplies and improved power-systems management. The industry must also consider more efficient power-distribution systems. And certainly the efficiency of the cooling scheme must be optimised. With the exception of cooling systems, all of the other efficiency initiatives rely on engineers adapting and optimising power supply and server designs.

The figure depicts a simplified power chain in a typical U.S. data centre. An ac input feeds an uninterrupted power supply (UPS) typically at 480V phase-to-phase, 277V phase-to-neutral levels. The UPS outputs the same voltage level to a PDU (power distribution unit) that includes circuit breakers and a transformer that steps the voltage down to 208V. The server #LINKKEYWORD1# (power supply unit)converts the 208V input to 12Vdc.

The power system shown has two primary obstacles to efficiency. First, there are a number of power conversions in the chain. Bill Tschudi, programme manager at Lawrence Berkeley National Laboratory (LBNL), states, "Every time you do a conversion from ac/dc or vice versa, there is a loss. That loss becomes heat and that heat has to be taken out." In fact, the typical double-conversion UPS design first converts ac to dc to charge the battery, and then back to ac to feed the PDU. The PDU transformer adds to the loss as does the PSU that does another ac/dc conversion.

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