23 Jul 2024


Data centre cooling: PUE isn’t the whole story

PUE is often used to measure the effectiveness of data centre cooling systems. However, using PUE alone risks losing sight of the bigger picture when it comes to overall system efficiency, as ABB’s Carl Turbitt, ABB’s HVAC drives sales manager for the UK, explains.

PUE (power usage effectiveness) is the most widely used metric for calculating the energy efficiency 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 efficient 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 flaws. 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 variable-speed drives, can be a major issue for data centres, and are a blind spot in PUE calculations. They reduce energy efficiency, 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 filters, however these involve extra cost, space, cooling and installation. They also create a single potential point of failure – if a filter 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 filter, 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.