The approach "will help to preserve NYTimes.com's significant reach and advertising inventory, in that it will allow us to retain light users while aiming to convert heavier users to a digital subscription, " Times Co.

WSJ: New York Times Prepares Plan to Charge for Online Reading

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.

BBC: Petrol from air: Will it make a difference?

Aiming to create a green market for sustainable lighting, "A Liter of Light" has donated over 100 solar bottle bulbs to others across the Philippines to replicate the project.

CNN: Plastic bottles light up lives