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Hi Neil, We seem to be joining a conversation part way through here. 500kW is 1/2 a MW - that's a chunky bit of kit. Belgian Technical Cooperation and UNDP did a lot of work on this for the GoR about 6 years ago - mapping resources amongst other things. Sam Dargan would probably be a good contact: Great Lakes Energy Info@energyforafrica.com Tel. +250 (0)78.865.6025 Sam is more into solar PV systems but he is well connected in the renewables area in Rwanda. Say Hello from me if you contact him please.
I imagine you'll need to know what sort of system you plan to use before you can get a better idea on life. With a bunch of enthusiastic locals and missionaries I installed a pico-hydro system in DRC in 1993. The Pelton wheel has been replaced once - about 4 years ago and was ready for it! The biggest issues were with step up and step down transformers and with the Induction Generator Controller and the ballast load. Your system would need to be more complicated (a 500kW ballast is not feasible) and require hydraulic governing (I imagine). I don't know how long other impellers/turbines last - a lot will depend on the sand and gravels content in the water.
I wouldn't be concerned about decommissioning costs. Spoken like a heathen I know but if it's not nailed down it will walk to the scrap man anyway. Anything concrete can stay I would think - ready to be incorporated come the next revision.
If it was me, I'd be looking at building in enough capacity to supply a steady amount of power to a local business that wanted to run an energy hungry business. Send me a pm if this has got legs on it and you want some help.