In today’s green world, hydroelectric dams are often unwelcome. Though their power is renewable and, on the face of it, carbon-free, there are lots of bad things about them, too. Blocking a river with a dam also blocks the movement of fish upstream to spawn and the movement of silt downstream to fertilise fields. The vegetation overwhelmed by the rising waters decays to form methane-a far worse greenhouse gas than carbon dioxide. The capital cost is huge. And, not least, people are often displaced to make way for the new lake. The question, therefore, is whether there is a way to get the advantages without suffering the disadvantages. And the answer is that there may be. The purpose of a dam is twofold. To house the turbines that create the electricity and to provide a sufficient head of water pressure to drive them efficiently. If it were possible to develop a turbine that did not need such a water-head to operate, and that could sit in the riverbed, then a dam would be unnecessary. Such turbines could also be put in places that could not be dammed-the bottom of the sea, for example. And that is just what is […]