Article in EE Times last week on the inefficiencies of power delivery in datacenters these days, particularly supercomputing data centers. The problem? Conversions and cooling and distribution loss.
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 organization of IT professionals called The Green Grid that was formed to promote energy efficiency in data centers. PUE is the ratio of total power used by a data center divided by the IT equipment power.
…Dallas Thornton, division director at the San Diego Supercomputer Center (SDSC) located at the University of California San Diego (UCSD), believes data centers 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 centers today are in the 1.8 range. Inefficient data centers can be over 2.”
But what’s a mother to do about all of this?
The power system shown has two primary obstacles to efficiency. First, there are a number of power conversions in the chain. Bill Tschudi, program 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.
The second obstacle is distribution loss. The PDU outputs a relatively low 208-V level. And while the PDU looks simple in the diagram, in reality it may feed many racks of servers. A relatively low voltage means the system needs relatively high current. High current is bad because distribution loss is driven by the square of the current. To maximize efficiency, Vicor’s Oliver states “Keep the voltage as high as you can as far as you can.”
More in the article.