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Low PUE doesn’t necessarily mean low peak load

With data centre owners and managers focussed on getting the best PUE, the fact that PUE is not necessarily an indicator of peak power requirement is an important point that is sometimes overlooked. We all know that if we have a 100kW IT load and a PUE of 1.2 then the total power drawn by the data centre must be 120kW. That’s true – as an average over a full year. It doesn’t mean that it won’t be higher than that at times (and lower at other times).

When sizing the incoming mains supply, standby generators and switchgear it is the peak load that we have to design for.
In the past the difference between the average load and peak load was relatively small but two recent developments in data centre infrastructure design mean that this is no longer the case.

Free Cooling

Whereas a system without free cooling will draw much the same power in summer and winter, as soon as free cooling is introduced the difference can be huge. Take, for example, a direct air free cooling system which could draw as little as 3kW in winter to deliver 100kW of cooling. Every mission critical data centre will want the facility to isolate the data centre from outside air, either to prevent the ingress of contaminants (for example if there were a fire somewhere in the locality) or to contain the fire suppression gas if there were an incident inside the data centre itself. To facilitate this, a fresh air cooling system should have the ability to go to full re-circulation and any reputable supplier will offer this. However, once a direct air system goes to re-circulation no free cooling is available and the full cooling load has to be handled by a mechanical cooling backup. At best this is likely to have a COP of 3:1 and so will draw around 33kW.

With an indirect free cooling system, on the other hand, the annualised PUE may not be quite as good as with a direct system (although the latest developments in heat exchanger technology are changing that). However when looking at peak power an indirect system has the advantage, because with an indirect system the data centre is permanently isolated from outside. Mechanical cooling therefore will only ever be needed in extreme conditions and can be arranged so as just to “top up” the free cooling as ambient conditions require. In this case the peak mechanical cooling load might only be 30kW so using the same COP of 3:1 this system will draw just 10 kW

Energy Saving UPS

Most new data centres are now adopting UPS’s with an energy saving mode which bypasses the double-conversion circuit and give an efficiency of around 98%. So our 100kW IT load draws 102kW. However if after a power failure or if there are fluctuations in the mains supply the UPS will revert to double-conversion mode at which it will have an efficiency of perhaps 94%. So power drawn now is 106kW.

Combination Effect and Power Factor (PF)

The combination of these two together is that the cooling now has to cope with 106kW of load so with a direct air system 35kW power is required and total power drawn is 141kW.

This gets even worse when you convert kW to kVA, which you have to do to arrive at the required power supply. Large fans such as those used in cooling data centres tend to have poor power factors so the extra cooling load has a disproportionate effect.

Overall you can expect your mains supply and generator to be at least 30% bigger, and therefore more expensive, with a direct system as opposed to an indirect one. If you don’t have the space to install an air side system and have to have a chilled water system, expect the peak power requirement to be 50% greater than with an indirect air system.

So when comparing data centre designs look at PUE, of course, but also remember to look at peak load.