Monday 5 March 2012

Air Battery UPS

An interesting technology at Data Centre World was the Air Battery. It is a system designed to provide 30 seconds of uninterrupted power to a Data Centre electrical power UPS system. The technology uses stored compressed air to drive fast starting in-cabinet expanding scroll rotors. Ultra capacitors are used to provide energy during the brief start up time of the scroll rotors. The  endurance of the Air Battery can be extended by use of additional stored compressed air tanks.

The advertised power ranges are from 3KW right through to 3MVA. The technology has been deployed in National Grid environments, so it is no lightweight technology struggling to find a market. The units depend on having a fast start generator to take up the power load as the air supply depletes. To facilitate fast generator start up, heat from the air battery can be used to pre-heat the diesel motor. Such diesel generators need to be kept at 35 degrees Celsius to aid fast starting. When the air battery is triggered and producing power there is a by-product of cold exhaust air which can be used to contribute to the cooling of a data centre while the air conditioning await the power from the standby generator..

The manufacturers claim lower operating costs, lower maintenance and greater reliability over a stored energy rotor system because there are no moving parts until the scroll rotors are activated. (excepting recharge compression of the air storage tanks.  Bearing friction, air friction and even the centrifugal effect of the earth's rotation can affect the storage efficiency of a rotor system.

Similarly this system is claimed to have a a lower ongoing operational and maintenance cost when compared with a lead acid battery power stage system. Batteries need a trickle charge to maintain power over long periods. In the case of gel acid lead batteries there is a cost a relatively short life. If you keep the gel acid battery cool it should give a five year lifetime. Wet acid batteries can survive 10 - 15 years depending on their construction and storage conditions. If the ambient temperature in the battery room is allowed to rise over 20 degree Celsius (about 70 Fahrenheit) the battery life will be shortened. So there is also an implied air conditioning cost unless free cooling can be used.Wet acid batteries generally need a specially constructed secure room with good ventilation, temperature conditioning and an acid proof tanked floor.

We managed one trading floor data centre implementation where the lead acid batteries had to be located away from the UPS power unit by reason of the weakness of the floor in the Data Centre Room. We had to install a very thick copper cable to take the low voltage power from the batteries to avoid too much voltage drop before it reached the UPS unit. Going from the basement to the fourth floor meant we were working at the absolute low voltage power cable length recommended by the manufacturers of the UPS. If we'd been able to use a compressed air battery we'd have been able to use much longer compressed air pipes.

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