On standby

A constant source of power can be maintained by utility providers with back-up power units that use compressed air. Siobhan Wagner reports.


Back-up power from compressed air is now a cheap and simple option, claims Energetix, a new energy specialist based in Chester.

The first commercial installation of its UK designed and manufactured power unit was for South African electricity utility Eskom Holdings, one of the 10 largest electricity producers in the world.

The Pnu Power TC1 compressed-air uninterruptible power supply device will be deployed to a live substation and provide more than 10 hours of standby power for relay-switching.

The key component of the device is its scroll, which has the same design as a car air-conditioning scroll except that it runs in the opposite direction.

The overall unit incorporates cylinders of compressed air stored at 300 bar. When the power trips, a signal to the control system opens the valves and the air is controlled through a regulator. The regulator acts as a power device so the more power that is needed, the wider the regulator opens. The air then goes into the scroll to drive the generator.

‘The fastest we’ve managed to get one to full power is 100 milliseconds,’ said Adrian Hutchings, Energetix Group chief executive.

The amount of power output depends on the size of the scroll. The company originally started using scrolls from cars and, later, buses.

Its latest project will attempt to use the same type of scroll that is used in the Airbus A380. ‘We’re looking to get 50kW of power out of that,’ he said.

But the ambition is to produce even more power. ‘Before the end of the year we want to have a 200kW device,’ Hutchings said. ‘Now if you have a 200kW device, what’s to stop you going up to a few megawatts?’

Energetix has been interested in using compressed air as an uninterruptible power supply for years, but its challenge was to find a prime mover for the device. The research and development team was looking for one that would be simple, have good expansion capability, was cheap and could be mass produced.

‘We knew that piston engines don’t work, and there are turbines, but they take up to five seconds to get up to speed,’ Hutchings said. That is when the team thought to try out a scroll.

The company has secured all the patents for using a scroll with this kind of a device and, as a consequence, Hutchings said other companies looking to make similar units must settle for more complicated solutions. He cited one company, Active Power, which has developed a compressed air back-up power unit that uses a turbine as a prime mover.

Graham Evans, national sales manager at Active Power UPS System, confirmed that these were the components in his company’s device, but noted that its back-up power units have made numerous commercial installations throughout the US since early 2006. ‘It’s good to see there is someone else out there offering this kind of energy solution,’ he said.

While compressed air back-up power devices have been championed for being both safe and environmentally friendly, there are still drawbacks. For one thing, the efficiency is poor compared with regular battery back-up power devices. ‘It’s not a high round trip efficiency device, but you don’t have things such as flow charge,’ Hutchings said.

He added that in terms of energy use, batteries always have to be topped up. ‘Not with our device,’ he said.

Hutchings stressed that Pnu Power is only ready to be used as back-up power. It could not be used, for instance, to store energy from wind turbines twice a day.

‘However that is an interesting challenge for us and an area we are currently looking at,’ he said.

One problem with using compressed air for power is there is a limited amount of the air that can be stored and used. An alternative would be to use liquefied gas such as liquid nitrogen. ‘Liquid nitrogen, which is effectively liquid air, is the same density as compressed air at 700 bar,’ Hutchings said.

Energetix has carried out initial tests with liquid nitrogen and Hutchings said the device operates just as efficiently with it. ‘I wouldn’t be surprised if next year we had a commercial liquid nitrogen operating device out there,’ he said.

The next big challenge for the company has less to do with improving the design or performance of its technology than with finding new commercial applications for it.

Hutchings suggested its device would be ready-made for use in data centres. One advantage of a compressed air device for this application is its two-fold ability to provide power and air-conditioning.

‘As air expands, it gets colder, so it delivers not just back-up power but back-up cooling as well,’ Hutchings said. ‘At a data centre, 25 per cent of the power used is in air conditioning. Our system and the amount of power it needs to provide back-up to a data centre is only 75 per cent of the power that would be needed from batteries.’

Energetix is not the only company to offer this sort of back-up power and cooling technology. Evans from Active Power pointed to his company’s CoolAir uninterruptible power supply unit, which is based on the company’s thermal and compressed air energy storage technology.

Hutchings argued that Energetix technology is simpler, so less expensive. However, he believes any alternative to batteries is likely to be a cheaper solution.

He cited a study from Telecom Italia that demonstrated batteries are unpredictable, expensive to maintain and require a high replacement rate, which is costly. ‘Also, at the end of their life cycle they must be treated as hazardous material,’ he said.

With Pnu Power’s first commercial application at Eskom’s substation, Energetix hopes it can prove its device is more reliable and less expensive than alternative back-up power technologies.

Hutchings said he is confident. ‘We see the device as a technology platform,’ he said. ‘One of the hardest things for us now is to choose which market we should take it to next.’