Gravity light

Last week an article in the weekly Guardian caught my eye. This time it wasn't the one on the science pages, which incidentally was about the Higg's Boson and dark matter, it was the one about the gravity light in the International development section. The idea is to enable people to replace kerosene lamps with a cleaner, cheaper alternative. The principle is simple, convert the potential energy stored in a bag of rocks into light. The bag of rocks is lifted about 2m and connected to a device containing gears and a generator. The trick is to allow the bag to fall very slowly without losing too much energy, somehow the internal gears facilitate this. Teaching physics for 30 years has made me rather sceptical so I wanted to see the figures. Can this really be enough energy to provide light to read by? Assuming the system is 100% efficient (no one is claiming that by the way) then the energy stored in 9kg of rocks lifted 2m is given by mgh = 9x10x2 = 180J. The claim made in the article is that this can produce light for 30minutes so the power would be energy/time=180/30x60 = 0.1W. Is this enough light to read by? Well for Christmas I got a small keyring flash light that has one LED. This provides enough light to search for my keys if I drop them on the floor of a dark garage, at least I could if it wasn't attached to my keyring. The power of a typical LED is 0.12W so the data matches. What a great idea and in Norway we have lots of rocks already ontop of hills so no need to lift them.