Why PUE tells only part of the data center energy efficiency story
Power usage effectiveness (PUE) is one of the most significant key performance indicators to show how efficiently a data center uses energy.
PUE is a ratio defined as the power used by a data center divided by the power used by its IT equipment. Specifically, it shows how much power is used by the actual IT equipment as compared with the power used by all the data center's services, which also includes cooling, lighting, power network equipment and so on.
By adopting best practices, it is possible to reach an average annual PUE of 1.1 and even lower.
It is useful to know the PUE, but care should be taken when interpreting what it actually shows. This is because the PUE is only a ratio of active power measured in Watts (W), whereas power supplied to a data center consists of both active and reactive power.
Reactive power doesn't do any actual work, but it needs to be supplied to inductive or capacitive loads to maintain the voltage stability in the network.
Typical inductive loads in a data center include motors running cooling applications, while computer server power supply units are good examples of capacitive loads.
If reactive power is not managed immediately at the load consuming it, it could cause massive losses across the whole network.
It is also important to remember that non-linear loads like variable speed drives (VSDs), LED lighting, UPS and servers with switched-mode power supply consume reactive power as well.
The specific way they draw current can cause its distortion. Besides active (fundamental) current there is a reactive current component present called harmonics.
Harmonics are a sort of electrical pollution in the network, causing increased energy losses, decreased power network reliability and reducing the lifetime of connected equipment.
To estimate how much reactive power is present in the network, a value called power factor (PF) is used – it shows the relationship between the active power that does work, and the total power supplied to the circuit. The closer the power factor is to 1, the less reactive power is present in the network and the more efficient and reliable the network is.
Utilities often penalize consumers for a low power factor since it requires utilities to provide increased power generation and distribution capacity.
When taking measures to improve PUE, like installing VSDs for cooling applications, it's crucial to check how the data center power network is affected.
Drives can save on average 20 to 60% energy in cooling processes. But their downside may be increased energy losses in the power network – and the PUE will not reflect this.
Standard VSDs that feature capacitors in their design are generally good at compensating for the reactive power of inductive loads they control. Drives use their capacitors to feed reactive current to the motors and protect the supply utility from being the source of the reactive current itself.
However, more sophisticated drives with an active front end (AFE) and DC capacitors, such as ABB's ultra-low harmonic (ULH) drives, can go a step further by also compensating for other network inductive or capacitive loads for even better network efficiency.
The situation with harmonics is different. Harmonic performance depends very much on the drive design.
The impact of harmonics is measured as a percentage value known as the total harmonic distortion (THD) which is the relationship between all the current or voltage harmonics and the fundamental current or voltage. Where no voltage or current harmonics exist the THD is 0%.
A typical 6-pulse drive with built-in impedance has a THDi of about 40%. This results in an 8% increase in line current and 16% higher energy losses compared to a system with no harmonics.
Rather than using additional filters to tackle harmonics, why not employ drives that do not cause harmonics in the first place? Active front end drives produce exceptionally low harmonic content even at partial loads, reducing risks of power network failure and increasing its efficiency.
While it is important to keep the PUE close to 1, it is also critical to pay attention to the VSD technology used in controlling the cooling applications to achieve that level. This is because the choice of VSDs affects not only cooling process efficiency but also the power network efficiency - which is not reflected in the PUE.
Ultimately, it is the efficiency of all systems including the cooling and power network, that determines the true energy efficiency of a data center.