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Briefly, so my analysis may be too rushed here:
Flow figures used will be conservative because floating balls on the surface run at a lower speed than the body of the water so you are on the safe side with your assumptions.
Rule of thumb: litres/second x head (m) x 10 /2 = Power (W) The /2 aspect is for efficiency at 50%. 780 x 80 x 5 = 62400 x 5 = 312,000 W Take 20% of the flow so 312,000/5 = 60,000W = 60kW roughly.
Watch out for seasonal variation but using only 20% may guard you from this?
Other issues: Ask if you should only take 20% - if the fall is very steep then will you be causing any ecological impact ? But if taking a bigger factor there may be a need to seriously examine environmental impact depending on seasonal variations. Impact of other works? Land ownership - compensation - security - potential for disputes? Political economic risk factors: power structures - who loses, who gains?
At 80 m I would use a Pelton wheel form Gilbert Gilkes and Gordon up in the Lake District (UK). Probably a 7inch and use a 4 pole machine (1500rpm) directly mounted.
You should be able to find a nice old cast squirrel cage 3 phase motor of around 10kW and use this with an Induction generator Controller from Nigel Smith who runs Sustainable Control (http://www.sustainablecontrol.com/) who makes IGCs and knows all about setting up the electronic control side of these systems.
Such an arrangement is very reliable PROVIDED lightning strikes are catered for.
I installed a micro-hydro system like this in the DRC in 1993. It has been mechanically overhauled once since then but the biggest nuisance is from lightning strikes. Don't despair if the 60kW figure is correct compared to the 110kW - the system I installed only produces around 2kW but has been a real life-line for the staff at the hospital where it is installed. Sounds tiny but having cheap power for lighting, and refrigeration for over 20 years, 24 hours a day - that's worthwhile . You can do a huge amount with 60kW.
Penstock depends on length but 156 l/s through a 12" ID pipe looks like 2.2m/s which is getting towards the limiting speeds to avoid wear according to http://www.engineeringtoolbox.com/flow-velocity-water-pipes-d_385.html For a proper analysis you will need to know the pipe material so as to get a roughness factor.
You may also want to know about how to limit load consumption at intakes so as to avoid people plugging cookers and heaters in ......
If you want any more info feel free to PM me but Adam Harvey and particularly Andy Brown authors of the Micro-Hydro Design manual: http://www.amazon.co.uk/Micro-hydro-Design-Manual-Small-scale-Schemes/dp/1853391034
both ex-ITDG would be best placed to help.
Exciting stuff - always wish I could do more of this!