Do flywheels really offer a valid, 'green' alternative to batteries in data centre energy storage?

Data centres are totally reliant on a consistent supply of clean power and any deviation from this creates severe repercussions. Therefore, Uninterruptible Power Supply (UPS) systems, such as those supplied by Chloride, are installed as the default backup source for protecting against this type of scenario, and the majority of them have traditionally relied upon batteries to provide this.

Typically, when a power interruption occurs, the UPS system draws power from a bank of batteries to provide ‘ridethrough’ power to keep servers online until the diesel generator can start up and begin powering the facility. However, as data centres struggle for space, the smaller footprint of flywheel-based UPS systems, which take up considerably less space than battery banks, are presenting a very viable and attractive alternative. Coupled with their longer life cycle, lower cost of maintenance and their environmentally friendly credentials as a result of not using hazardous and toxic chemicals, flywheels provide some obvious advantages over conventional battery systems.

The origins and use of flywheel technology for mechanical energy storage began several hundred years ago and was developed throughout the industrial Revolution. During the 1960s and 1970s, NASA sponsored programs proposed energy storage flywheels as possible primary sources for space missions. However, it wasn’t until the 1980s when the necessary components, such as magnetic bearing systems and high power density
motor-generators came into existence, that the technology was able to develop into the many current forms of energy storage that flywheels can now provide.

A flywheel energy storage system is basically a ‘mechanical battery’ that stores energy kinetically, in the form of a high speed rotating mass. When a utility outage occurs, the energy stored by the rotating mass is converted to electrical energy through the flywheel’s integrated motor generator and then passes through a bidirectional IGBT power converter. The system is designed so that the stored energy within the flywheel is sufficient to provide the required DC back-up power for a UPS system until the utility supply returns, or until the standby diesel generator comes online, and akes over the supply of power to the UPS system. As an example a single flywheel unit would support a 150 kVA UPS running at a typical 75% load for 30 seconds. Since 98% of all power outages last less than 10 seconds and most modern diesel generators can start and take load within 15 seconds, this makes flywheel bridging without batteries, perfectly feasible. Once either the utility is restored or the generator provides power to the input of the UPS, the flywheel system is recharged by taking current from the DC bus of the UPS until it is back up to full speed and its normal fully charged mode. Thus a flywheel can be regarded as a direct replacement or alternative to batteries. The additional inherent ability to recharge quickly makes the system more resilient than batteries due to the fact that, after a full discharge, flywheels can be fully recharged within as little as two minutes, whilst batteries always take many hours to achieve 100% capacity. If the standby generator set requires a longer time before accepting the critical load, flywheels can be paralleled for extended run time or power. Unlike lead acid batteries, there are no limitations on the numbers of flywheels that can be paralleled. A flywheel’s energy is created by means of a rotating mass, which is related to the inertia of the rotor (the weight of the mass) and the square value of its rotational speed (Revolutions Per Minute) or velocity, which is derived by the following formula:

E=kMw2
k -Depends on the shape
of the rotating mass
M - Mass of the flywheel
w - Angular velocity

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