Solar, wind, tide - now there is a new renewable energy which may bring light to the developing world. It is a simply-designed light powered by gravity with no running costs and no hidden extras. The ultimate goal of the gravity light is to wean the developing world off kerosene lights that can cause fires, are expensive and have been linked by researchers to millions of early deaths due to exposure to household air pollution. After a few more design tweaks, the product will be ready to ship out for trial, and if feedback is good, production will begin in earnest. Al Jazeera's Jessica Baldwin reports from London.