Power usage effectiveness: not the whole story

By Liam Blackshaw, Product Manager for LV Drives, ABB
Liam Blackshaw, Product Manager for LV Drives at ABB, explains why PUE is not the ultimate efficiency metric, and offers operators an alternative

Rising energy costs and sustainability focused regulations have propelled energy efficiency to the top of the agenda for data centre operators. Historically, operators have measured efficiency using power usage effectiveness (PUE) – a number between zero and one (unity) which, in theory, represents how much of the electricity used goes toward useful work. PUE is undoubtedly a useful metric, but operators should be cautious of treating it as a holistic reflection of their facility’s efficiency. 

As focus on efficiency increases, it is essential that operators develop a comprehensive, system-level approach to understanding their facility’s energy use. This understanding must go beyond PUE to consider other important factors such as load and harmonics. 

What does PUE miss out?

PUE is the traditional key performance indicator (KPI) for a data centre’s efficiency. It is a simple, trackable metric, so it is understandable that it has become the industry standard. However, it doesn’t reflect several important realities of a data centre’s power use. PUE only measures active power and does not account for important factors like harmonic losses and reactive current losses on the cooling system. At best, this means that it is inaccurate, and at worst it can be misleading. 

Other high-level metrics, such as carbon usage effectiveness (CUE) and water usage effectiveness (WUE), have similar issues. For example, reducing the speed of electric motors in the cooling system would save energy as well as reducing emissions. However, because the other metrics stem from PUE, they are also unable to account for the full range of variables affecting real world efficiency. 

Partial load efficiency: a missing link 

To make data centres more efficient, we first need to understand where power is being spent. Typically, the largest consumer of power is the IT load, so operators often start here when looking to improve efficiency. However, there is relatively little margin for efficiency improvement on IT equipment without sacrificing performance. The next largest consumer of power, accounting for an average of 30-50% of total energy consumption, is cooling. Fortunately, there often is significant room for improvement here. 

Operating efficiently at partial load is a significant challenge. Most data centre cooling systems are designed for peak loads under the worst cooling conditions, such as on the hottest day of the year. However, they rarely operate under such stress, so they are significantly overprovisioned for their average workload. 

Working at partial load makes PUE more challenging to calculate and can produce misleading information. The efficiency numbers from a catalogue entry can’t always be accurately extrapolated. For this reason, it is essential to calculate efficiency at a system level rather than considering each component in isolation. 

Harmonics: electrical pollution 

An ideal AC electric supply takes the form of a sinusoidal waveform. However, various pieces of equipment – such as laptops, battery chargers, uninterruptible power supplies (UPS), and more – can disrupt or distort this waveform and introduce electrical “noise” or “pollution” in the form of harmonics. Unfortunately, PUE does not account for harmonics. 

Excessive harmonics have a detrimental effect on the system’s efficiency. Addressing them also requires increased capital expenditures (CAPEX) and operating expenditures (OPEX). Harmonics increase CAPEX by requiring operators to overprovision electrical equipment such as cables and transformers. They increase OPEX by increasing a facility’s power demand, and raising the failure rate of equipment and reducing its lifetime. Electrical utilities may also apply penalties or even cut off facilities that introduce harmonics to the local grid. 

Truly efficient solutions 

Once they have an understanding of these issues, data centre operators can take meaningful steps to improve efficiency, particularly on cooling systems. Many facilities still use older, less efficient IE2 or IE3 electric motors to power their cooling system. By switching to more modern models, such as IE4 or IE5, a facility can cut electrical losses on motors by up to 40%, contributing significantly to lower energy use. 

More efficient motors – such as synchronous reluctance motors (SynRM) – are often drop-in replacements, and, with the high price of electricity, the payback periods are now a matter of months. Importantly, modern motors are also significantly more efficient at lower speeds, meaning that they are an ideal way to address partial load inefficiency.

Facilities should also use variable speed drives (VSDs) to address partial load. Without a VSD, fans and pumps operate at full speed all the time, wasting power. A VSD adjusts a motor’s speed and torque to match the actual requirements of its task. This can decrease a motor’s energy use by 20-60%. 

However, VSDs can introduce harmonics. Since PUE does not account for harmonics, facilities may be reducing their real efficiency without realising it. Fortunately, data centres can address both issues simultaneously by specifying active front end (AFE), ultra-low harmonic (ULH) drives. These reduce electrical losses, increasing efficiency, and ensure that harmonics are not introduced to the network. 

By counteracting harmonics, operators can specify appropriately sized components, reducing their total cost of ownership (TCO). This also eliminates the risk that harmonics will damage or shorten the lifespan of equipment on the network. Some ULH drives also include a “voltage boost” feature to compensate for voltage fluctuations, protecting sensitive IT equipment. 

Efficient technology in action

A multinational, multi-sector Chinese technology company operates data centres in over 24 regions and 74 availability zones worldwide. These data centres are the backbone of the company’s retail, e-commerce, and other businesses, but many of them were not as efficient as they could be. 

Looking beyond PUE, the operator determined that harmonics were the issue. They specified ABB ACH580 drives and passive filters, which are designed for heating, ventilation, and air conditioning applications. As a result, total harmonic distortion (THDi) was reduced to under 10%, greatly improving efficiency and lowering TCO. 

Keep PUE in perspective 

Treating PUE as the one true representation of a facility’s efficiency hides the big picture. By considering a data centre at a system level and measuring real power use, operators can uncover and address the underlying issues. This is in every operator’s interest, since efficiency is the key to cutting both costs and emissions. 

Learn more at https://new.abb.com/data-centres


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