A novel way of storing electric power by liquefying air is using induction motors as generators.

The CryoEnergy system, being developed by London-based Highview Power Storage, is based on a cryogenic process. It uses cheap off-peak electricity to power an air liquefaction plant, storing liquefied air in an insulated tank at –196°C. When the energy is needed, the liquefied air or nitrogen is released from the tank, pumped to a high pressure and allowed to warm and expand to its gaseous state, increasing in volume about 700 times. This high-pressure air drives an expansion turbine, which rotates an ABB motor acting as a generator.

A pilot plant being hosted by SSE (Scottish and Southern Energy), and part-funded by a £1m-plus grant from the UK’s Department of Energy & Climate Change, uses a 500kW motor (shown above) as the generator. It can supply electricity to several hundred homes.

“With the drive for a low-carbon economy, there is increasing emphasis on wind and solar sources,” explains Highview’s CEO, Gareth Brett. “However, by their nature, these forms of generation are intermittent and there is thus a need for a viable energy storage method to act as a buffer to store electricity when it is being produced but perhaps not needed, and release it to the grid at peak times.

“The application required the use of an induction motor as a generator,” he continues, “so a simple, robust machine was required with no special starting or cooling requirements. Our prime mover is very sensitive to torque transients as well as speed excursions. ABB were able to provide transient torque plots under fault conditions, as well as the torque-speed relationship at a part of the curve which would not normally be looked at for a motor application.”

Highview plans to scale up the system to the 3–5MW range, and will probably change to using a synchronous generator.