Image caption Renewable power generation, such as wind turbines, can produce electricity when it is not in demand

Turning air into liquid may offer a solution to one of the great challenges in engineering - how to store energy.

The Institution of Mechanical Engineers says liquid air can compete with batteries and hydrogen to store excess energy generated from renewables.

IMechE says "wrong-time" electricity generated by wind farms at night can be used to chill air to a cryogenic state at a distant location.

When demand increases, the liquid air can be warmed to drive a turbine.

Engineers say the process to produce "right-time" electricity can achieve an efficiency of up to 70%.

IMechE is holding a conference today to discuss new ideas on how using "cryo-power" can benefit the low-carbon economy.

The technology was originally developed by Peter Dearman, a garage inventor in Hertfordshire, to power vehicles.

A new firm, Highview Power Storage, was created to transfer Mr Dearman's technology to a system that can store energy to be used on the power grid.

The process, part-funded by the government, has now been trialled for two years at the back of a power station in Slough, Berkshire.

More than hot air The results have attracted the admiration of IMechE officials.

Media playback is unsupported on your device Media caption Mr Dearman uses his garage as a laboratory

"I get half a dozen people a week trying to persuade me they have a brilliant invention," head of energy Tim Fox told BBC News.

"In this case, it is a very clever application that really does look like a potential solution to a really great challenge that faces us as we increase the amount of intermittent power from renewables."

Dr Fox urged the government to provide incentives in its forthcoming electricity legislation for firms to store energy on a commercial scale with this and other technologies.

IMechE says the simplicity and elegance of the Highview process is appealing, especially as it addresses not just the problem of storage but also the separate problem of waste industrial heat.

The process follows a number of stages:

"Wrong-time electricity" is used to take in air, remove the CO2 and water vapour, which would otherwise freeze solid the remaining air, mostly nitrogen, is chilled to -190C (-310F) and turns to liquid - this provides a compact storage medium that can later draw energy in the form of heat from the environment the liquid air is held in a giant vacuum flask until it is needed when demand for power rises, the liquid is warmed to ambient temperature. As it vapourises, the expanding gas drives a turbine to produce electricity - no combustion is involved

IMechE says this process is only 25% efficient but it is massively improved by co-siting the cryo-generator next to an industrial plant or power station producing low-grade heat that is currently vented and being released into the atmosphere.

The heat can be used to boost the thermal expansion of the liquid air.

More energy is saved by taking the waste cool air when the air has finished chilling, and passing it through three tanks containing gravel.

The gravel remains cool until it is needed to restart the air-chilling process.

Delivering durability

Highview believes that, produced at scale, their kits could be up to 70% efficient, and IMechE agrees this figure is realistic.

"Batteries can get 80% efficiency so this isn't as good in that respect," explains Dr Fox.

"But we do not have a battery industry in the UK and we do have plenty of respected engineers to produce a technology like this.

"What's more, it uses standard industrial components - which reduces commercial risk; it will last for decades and it can be fixed with a spanner."

In the future, it is expected that batteries currently used in electric cars may play a part in household energy storage.

But Richard Smith, head of energy strategy for National Grid, told BBC News that other sorts of storage would be increasingly important in coming decades and should be incentivised to commercial scale by government.

He said: "Storage is one of four tools we have to balance supply and demand, including thermal flexing (switching on and off gas-fired power stations); interconnections, and demand-side management. Ultimately it will be down to economics."

Mr Dearman, who also invented the MicroVent resuscitation device used in ambulances, told BBC News he was delighted at the success of his ideas.

He said he believed his liquid air engine would prevail against other storage technologies because it did not rely on potentially scarce materials for batteries. "I have been working on this off and on for close on 50 years," he told BBC News.

"I started when I was a teenager because I thought there wouldn't be enough raw materials in the world for everyone to have a car. There had to be a different way. Then somehow I came up with the idea of storing energy in cold.

"It's hard to put into words to see what's happening with my ideas today."

John Scott, from the Institution of Engineering and Technology (IET), added: "At present, pumped-hydro storage is the only practical bulk storage medium in the British grid.

"However, locations are very restricted," he told BBC News. "In the future, if new storage technologies can be deployed at a lower cost than alternatives, it would benefit the power system."

A spokesman for the Department of Energy and Climate Change (Decc) said it would shortly launch a scheme to incentivise innovation in energy storage. Other grants are available from Ofgem.

Follow Roger Harrabin on Twitter: @RogerHarrabin