Why does PUE only describe a part of data center energy efficiency?

Time of issue:2021-02-25


Power usage efficiency (PUE) is one of the most important key performance indicators for data center energy efficiency. It can show the energy efficiency of data centers.


PUE is a ratio defined as the power used by the data center divided by the power used by its IT equipment. Specifically, it shows how much electricity is used by the actual IT equipment compared to the electricity used by all services in the data center (including cooling, lighting, grid equipment, etc.).


By adopting best practices, the average annual PUE may reach 1.1 or even lower.


It is useful to understand PUE, but be careful when interpreting what it actually displays. This is because PUE is only the ratio of active power measured in watts (W), and the power supplied to the data center includes active power and reactive power.


Reactive power does not do any actual work, but needs to supply power to inductive or capacitive loads to maintain the voltage stability in the network.


Typical inductive loads in data centers include motors running cooling applications, and computer server power supply units are good examples of capacitive loads.


If the reactive power is not managed immediately at the electrical load, it may cause huge losses to the entire grid.


It is also important to remember that non-linear loads such as variable speed drives (VSD), LED lighting, UPS and servers with switching power supplies also consume reactive power.


The special way they absorb current can cause current distortion. In addition to the active (fundamental) current, there is also a reactive current component called harmonics.


Harmonic is a kind of electrical pollution in the network, which will increase energy loss, reduce the reliability of the grid, and shorten the service life of connected equipment.


In order to estimate the reactive power present in the network, a value called power factor (PF) is used, which shows the relationship between the active power at work and the total power supplied to the circuit. The closer the power factor is to 1, the smaller the reactive power in the network, and the higher the efficiency and reliability of the network.


Utility companies often punish consumers for lower power factor, because this requires the utility company to provide higher power generation and distribution capacity.


When taking measures to improve PUE, such as installing a VSD for cooling applications, it is important to check the impact on the data center power supply network.


During the cooling process, the drive can save 20% to 60% energy on average. But their downside may be increased energy losses in the grid-and PUE will not reflect this.


The standard VSD that uses capacitors in the design can usually well compensate the reactive power of the inductive load it controls. The driver uses a capacitor to provide reactive current to the motor and protect the power supply facility from the source of reactive current itself.


However, more complex drivers with active front ends (AFE) and DC capacitors, such as ABB's ultra-low harmonic (ULH) drivers, can further improve network efficiency by compensating other network inductance or capacitive loads.


The situation with harmonics is different. Harmonic performance largely depends on the design of the drive.


The influence of harmonics is measured by the percentage of total harmonic distortion (THD), which is the relationship between all current or voltage harmonics and the fundamental current or voltage. In the absence of voltage or current harmonics, THD is 0%.


The THDi of a typical 6-pulse driver with built-in impedance is about 40%. Compared with a system without harmonics, this will result in an 8% increase in line current and a 16% increase in energy loss.


Instead of using an additional filter to solve the harmonic problem, why not use a frequency converter that will not cause harmonics? Active front-end drivers will produce abnormally low harmonic content even under partial load, thereby reducing the risk of grid failure and improving efficiency.


Although it is important to keep PUE near 1, it is also important to pay attention to the VSD technology used to control cooling applications to reach this level. This is because the choice of VSD will not only affect the efficiency of the cooling process, but also the efficiency of the grid-which is not reflected in the PUE.


In the final analysis, the efficiency of all systems (including cooling and power networks) determines the true energy efficiency of the data center.