But last night I had an idea. It is still in its earliest stages, and I really don't know if it would work but I thought i'd run it past you folks to see if you had any thoughts.
I was struck by the relative similarity between a pelton turbine and a centrifugal pump. So I thought, there's a lot of these lying around (certainly more than there are pelton wheels), and if you ran them in reverse, you should be able to turn the shaft. You might even be able to use the original motor as the generator. So, like with a pelton wheel you would run a pipe from upstream which would have a high pressure by the time it reached the pump, the water jets in, spinning the pump blades and turning the motor or shaft.
So i don't have great electrical knowledge, but the system I am making my bike generator with is fairly simple. We use a 24v 200w electric scooter motor in reverse. My idea is to attempt to connect one of these up to an easily available centrifugal pump and see what it can do. This is the type of pump that sprung to mind.

It is a pump designed to be powered by an electric drill, which usefully means it has a protruding shaft, and it can be bought for £3 off ebay, Now obviously its not going to be the sturdyist device, and its not built to last, but I thought I might as well try it to test the principle. I think the difficult part will be linking the axels and getting them lined up perfectly. Also, it is likely that there will be issues with speed and torque and the pump may not even be able to turn the motor....
Any feedback on my idea? Is it doomed to failure?
Pete