02 February 2023
Liam Blackshaw, product manager for LV Drives, ABB
Data centre operators are continuously pushing for greater energy efficiency and lower emissions. Recently, rising energy costs have added even more pressure to make facilities as efficient as possible.
Operators traditionally measure energy efficiency using power usage effectiveness (PUE). A PUE of zero means that no useful work is being done and all the electricity is going to waste, while a PUE of one (unity) means that 100% of the electricity is going toward work.
PUE has become a central key performance indicator (KPI) for assessing a data centre’s efficiency because it is straightforward and easy to track over time. Unfortunately, PUE doesn’t tell the whole story. It only measures active power and fails to account for other sources of inefficiency, such as harmonic losses and reactive current losses on the cooling system.
Harmonic losses – electrical pollution on the network
PUE does not account for harmonics – disruptions to the electric supply’s ideal waveform. These disruptions, sometimes described as electrical ‘noise’ or ‘pollution,’ can significantly reduce the system’s energy efficiency. An ideal AC waveform is sinusoidal, but it can be distorted by power draw from equipment such as electric motors, battery chargers, uninterruptible power supplies (UPS), and more.
To deal with harmonics, operators often overprovision electrical equipment such as transformers. This increases the facility’s capital expenditures (CAPEX). The lower efficiency that results from harmonics increases the facility’s operating expenditures (OPEX). Harmonics can also damage equipment, increasing its failure rate and shortening its lifespan. In severe instances, electrical utilities may fine or even cut off facilities that introduce harmonics into the local grid.
Partial load – missing from the PUE calculation
The electric motors that power the cooling fans and pumps are among the most energy intensive pieces of equipment in a data centre. In fact, the cooling system typically ranks second only to IT load in terms of power consumption, using between 30-50% of the electricity supplied to the facility. While efficiency gains from upgrading IT equipment are often marginal, significant gains are possible when improving the cooling system.
Most cooling systems are designed for the worst-case scenario: maximum loading on the hottest day of the year. However, they rarely face these extreme conditions. For this reason, they almost always operate at partial load.
PUE struggles to account for partial load, especially since load and energy use are not in a linear relationship. The efficiency numbers from a piece of equipment’s catalogue entry do not always translate accurately to partial load. Further, calculating the efficiency of components in isolation means that operators are overlooking potential interactions that could improve or worsen efficiency.
Improving efficiency while addressing harmonics and partial load
The first step that operators can take to improve cooling system efficiency is adopting efficient electric motors. Many facilities use IE3 efficiency class motors – and some even use older IE2 models. By upgrading to more modern models, such as IE4 and IE5 electric motors, facilities can cut electrical losses significantly, greatly improving efficiency.
Each increase in IE number, such as upgrading from IE3 to IE4, represents a 20% decrease in electrical losses. Modern motors are also far more efficient at partial loads than older models.
Facilities should also pair motors with variable speed drives (VSDs). Drives adjust an electric motor’s speed to match the actual demands placed on it, rather than constantly running it at full speed. Installing a VSD can cut a motor’s energy use by up to 60%.
However, while efficient in one way, VSDs can introduce inefficiency in another: harmonics. Fortunately, active front end (AFE), or ‘ultra-low harmonic’ (ULH), drives are available to counteract harmonics at the source. These drives greatly reduce the total harmonic distortion (THDi) on the network, as well as improving energy efficiency.
Saving power, saving money
Facilities that upgrade to modern motors and AFE drives will benefit from significantly lower energy use – the largest OPEX for data centres. Although more advanced solutions have a higher upfront cost, they have a significantly lower total cost of ownership (TCO). While energy prices reach new highs, the payback period for these upgrades is also lower than ever – often a matter of months.
Eliminating harmonics ensures that other equipment on the network can reach its full operating lifespan without the risk of damage. When designing or upgrading other electrical components in a data centre, operators can also specify appropriately sized equipment rather than oversizing to accommodate harmonics.
Operators that look beyond PUE to consider the bigger picture of efficiency will be able to cut electrical costs, protect sensitive equipment from damage, and reduce emissions associated with the facility.
