Drives&Controls & BACK TO BASICS n SPONSORED BY Data centre cooling: PUE isn’t the whole story PUE is often used to measure the e ectiveness of data centre cooling systems. However, using PUE alone risks losing sight of the bigger picture when it comes to overall system e ciency, as ABB’s Carl Turbitt, ABB’s HVAC drives sales manager for the UK, explains. PUE (power usage e ectiveness) is the most widely used metric for calculating the energy e ciency of data centres. It measures the ratio of the energy used, compared to the energy delivered to the computing equipment. Anything that isn’t considered a computing device – for example, lighting or cooling – counts towards energy consumption. In theory, the closer PUE is to 1.0, the more e cient a data centre is. A perfect 1.0 ratio would imply that all of the power is being delivered to the IT equipment, with no losses. Anything above 1.4 for the latest data centres (or 1.5 for older centres) is considered to be less than ideal. While a PUE of 1.0 is theoretically possible, there are very few facilities around the world where it has actually been reached. In any case, using PUE as a performance metric for data centres is not without its aws. For instance, PUE only accounts for active power, ignoring harmonic losses and reactive current losses on the cooling system, so the formula does not account for a sizeable chunk of power losses. Harmonics, caused by switching devices such as variablespeed drives, can be a major issue for data centres, and are a blind spot in PUE calculations. They reduce energy e ciency, and can cause equipment to function erratically. This, in turn, reduces reliability. There are several options for mitigating data centre harmonics, varying in cost and complexity. A traditional solution to mitigating harmonics would be to add more harmonic lters, however these involve extra cost, space, cooling and installation. They also create a single potential point of failure – if a lter fails, the upstream system can trip, and so still needs to be oversized for the application. By contrast, active front-end (AFE) drives handle harmonic mitigation within the drive itself. They have an active supply and integrated line lter, which help to reduce harmonic content by up to 90%. While these drives typically have a higher upfront cost than conventional drives, they can save money within the context of wider systems by reducing the likelihood of downtime, and allowing smaller cables and transformers to be used. ABB will be exhibiting its AFE drives at the Data Centre World event in London from 6-7 March. To attend, register at https://www.datacentreworld.com 10.00am Tuesday 12th November - How smart maintenance can slash operating costs? 10.00am Tuesday th - How does efficiency and sustainability affect your bottom line? 10.00am Tuesday 11th June - Are robots a worthwhile investment for SMEs? 10.00am Tuesday 10th September - Machine safety: what are the latest regulations?
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