Data Center Flywheel 101

Tuesday, 14 October 2008 13:34

Written by Rakesh Dogra

Of course due to the large amounts of power required only a diesel generator can fulfill the power requirements of the data center if the power outage is sufficiently lengthy but then generators are mechanical devices which do need some time to start however small that time period may be.

Hence the main problem is not about having a power back up for the longer time period but the main issue is to handle those few seconds immediately succeeding power failure. Conventionally these tasks have been handled by battery operated UPS (Uninterrupted Power Supplies) Systems quite successfully but there are several disadvantages of using batteries. Batteries are not very environmental friendly, so to speak, since they contain toxic substances and their disposal is a problem as well. Moreover due to the acid they contain, they require very careful handling from the human and fire safety points of view as well. But of course the main subject of discussion in this article is not about defaming our good old friends (batteries) but to highlight one fast growing alternative to them, namely flywheels.

Most of us understand what is meant by a flywheel in the context of mechanical systems. It refers to a large rotating mass which stores energy in the form rotating energy which is a form of kinetic energy. Usually flywheels are used at one end of mechanical engines in order to smoothen out the effects of jerky motion due to cylinder firing etc. The same principle is used in case of flywheels used for temporary power backup in UPS systems. It consists of a mass of rotating matter which stores kinetic energy which can be used for short duration once the main power fails, before the diesel generator starts or the grid power is restored. Normally this time period is very short of the order of few seconds to one quarter of a minute but this is very critical to ensure continuous availability of power during this time span.

Though batteries have been in use for much longer period of time as compared flywheels which are relatively newer on the horizon, yet their popularity is certainly on the rise due to several advantages they have to offer over conventional battery backed up UPS Systems. These include but not limited to lower maintenance, lesser carbon footprints and so forth. But flywheels do cost more than conventional batteries and hence their upfront cost is certainly more than having batteries. Moreover there is a huge difference in the time for which a flywheel system can handle a power outage vis-à-vis batteries. Although it depends on the exact number and capacity of the installed system a rough example is that whereas a flywheel backup can take care of the power for roughly 15 seconds, the time span for batteries is much greater in magnitude ranging for upto 15 minutes.

But there is also a joke in data center circles that if you are not able to start your generator in 15 seconds, you cannot do it in 15 minutes either, hence the justification of using flywheels despite their short power back up. Jokes apart, sometimes flywheels are not used in isolation as independent or sole power source but used to complement the conventional battery back up system as they help to reduce the sudden load on batteries. This certainly increased the life of the batteries since many times there are sudden dips in voltage which the batteries take upon themselves and though normal operators may not notice them, the effect of the batteries is certainly detrimental, and this can be avoided by clubbing the flywheel arrangement along with the battery back up system.

There are several companies which specialize in manufacturing flywheels for huge installations such as data centers and some of the popular companies include Hitec, Pentadyne, Active-Power and so forth. There are several new data center projects coming up which utilize solely the flywheel systems for back up rather than batteries. One such project in Culpeper, Virginia that is South West of  the Washington DC metro area, where a 10, 000 KVA power back using flywheel UPS systems is being built. The builders claim that they will make the data center save huge amount of space since the lines of batteries need not be installed and flywheels occupy much lesser space. This example is not alone and there are several project either underway or in the plans to become a reality soon.

The setup of flywheels is such that they are connected to the AC or the DC bus of the UPS system depending on the make and model. For example Hitec is AC while Pentadyne solution is DC attached and targeted at battery replacement in (Emerson Network Power) Liebert UPS systems, ActivePower and Piller can both be AC attached without the need for AC to DC to AC conversion. These flywheels draw energy from them to keep the masses in motion so that they are filled with rotational kinetic energy. Once the power fails the same energy is transferred back to the UPS system which in turn delivers it to the data center for a short duration of time till the masses “die-out” due to loss of motion. This is a very similar concept to having batteries which are kept in the charging mode via the UPS and once the power fails they return the same charge to the UPS for the time period when main power isn’t available or the generator gets going.

Hence this discussion can be concluded by stating that though flywheels may cost higher at the installation stage, they certainly need to be taken seriously by the data center companies because of the numerous advantages they tend to offer as highlighted in this report. In my personal opinion their environmental friendliness of these data centers is one of the greatest virtues since we are living in an arena where environmental issues are taking prominence in all activities. It is surely hoped that flywheels will be adopted more and more in the times to come.

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