Image caption Can "petrol from air" help fuel future energy needs?

An idea has hit the news on Friday to produce petrol from air and water - removing CO2 from the atmosphere, combining it with hydrogen split from water vapour and turning it into a fuel that can go straight back into the petrol tank.

It's like combustion in reverse, and in essence it is what powers plants: CO2 and water in, energy-rich sugar molecules out.

But in matters of energy, nothing comes for free.

Just as plants need sunlight to pull off the trick, Air Fuel Synthesis, the firm profiled in the UK's Independent newspaper, need to use good old-fashioned electric energy to pull off theirs.

As with any novel fuel production or energy storage method, it is the numbers that matter: efficiency is king.

The degree to which this technique can fulfil its promise to lower CO2 and provide a sustainable fuel source depends crucially on the balance of energy it requires and the energy it stores.

First things first - squashing CO2 back into a molecule packed with energy is not a new idea.

For example, work done at Princeton University in the US and published in 1994 to make the fuel additive methanol from CO2 has more recently been refined and spun into a company called Liquid Light that is aiming to do the same thing.

In Iceland, Carbon Recycling International opened a plant at the end of 2011 drawing waste CO2 from a power station, with capacity to produce five million litres of methanol per year.

Air Fuel Synthesis build on these methods by turning the methanol into something more like petrol, using processes well entrenched already in the petroleum industry.

The firm, so far, has made five litres of their fuel in a two-year demonstration experiment in which they have invested £1m.

Peter Harrison, the firm's chief executive, told BBC News that the demonstration did not focus on efficiency, but rather a proof of principle.

"All we're trying to demonstrate is that here in the UK we can make petrol from air," he said.

"[These processes] are all capable of working at industrial scale, and we've brought it down to container scale. There' a lot of work to do to develop the supply chains and to reduce the costs.

"We've got a design now for a one-tonne-a-day unit, and we expect to be in production by 2015."

'Minority player'

Their first market is "greening the motorsport industry" by offering their pure fuels to racing teams.

But in the bigger picture, the petrol-from-air idea joins a legion of others that are trying to crack the far bigger problem of storing energy produced by renewable sources.

Like the recently reported "liquid air" approach, Air Fuel Synthesis wants to use renewable energy which may come when there is no consumer demand for it.

Mark Carpenter, research fellow at Cranfield University's Chemical Safety, Fuels and Environment Group, said that hydrocarbon fuels were "a very good way of storing energy" but that linking the process to renewable energy was the only way to make it tenable.

"If they can get enough renewable, low-cost electricity, that's a big determinant," he told BBC News. "It could be a very clean and green way of producing hydrocarbon fuels."

"But it'll be a minority player in terms of trying to meet global fuels demand - very, very small.

"It's certainly interesting that someone's considered this as a method, it's just whether you can make the chemistry work and get a reasonable degree of efficiency."

As with many other energy production and storage methods, only time and market forces will determine how far the petrol-from-air idea can go.