Watermills in rain pipes
I moved to the city a couple of years ago, and have noticed how clean the roofs always are. No leafs get up there. Further, from every roof there runs a pipe all the way along the edges of the roof, and to the ground in order to make the rain that ends on the roof go to the ground without pouring down on the windows.
My idea is simply to put a little mill inside the pipes that go from the roof to the ground, the mill should run a simple generator in order to produce power. - Since the roofs are always clean in the city, the problem of dirt accumulating in the mill is not an issue.
It wouldn't be much of an extra cost to produce such pipes, since both the generator and the mill are easily made out of few raw-materials (plastic for the mill, and some copper and magnets for the generator).
There is quite a lot of square meters of roof in a city, and if these pipes were installed as a replacement of the old ones when the roof should be renewed anyway, it would propably accumlulate a neat amount of energy (although not nearly as much as a dam!).
Reward: A sample of the product?