Is there a way to use all the light and heat and noise that we produce as by-products of other activities? Of course we should above all aim to reduce our energy use but there will always be a certain excess. Take light: there are many buildings that need to stay lit all night - hospitals, factories, car parks, hotels, prisons. Is there a way to capture this light and put it to use. Given that the world is soon to face a food shortage - 80% of arable land is already in use and the world population is set to increase to 9 billion by 2050 - would it be possible to incorporate greenhouses into well lit buildings? The light given off by artificial lights is weaker than direct sunlight but perhaps it could function as a top up light source for plants? In some of the cases mentioned above - hospitals and prisons for instance - plants would have a therapeutic benefit as well as providing nourishment for the people who live and work there. In others the plants would have to be transported elsewhere for processing and consumption but any activity that uses light can't be that far from a population that needs feeding so there would be limited transport costs involved. People like Dickson Despommier in the US and Plantagon in Sweden have developed prototypes for multistorey urban greenhouses that would occupy entire city blocks and produce food where it is needed - for the first time ever more than half the world's population lives in cities and that figure is set to grow - but until food gets so expensive that farming is more lucrative than real estate I can't see this idea getting off the ground (pardon the pun). As things stand the owner of a plot of land will make more money by building flats or offices on it than by building a greenhouse. But maybe if you could persuade a property developer to include some plant growing activity - perhaps through a system of government subsidies - in his or her plans we could at least see if the idea has legs.