it can be possible to generate electricity from an old mill site, but if the site has very little vertical drop then the costs can be higher than for 'high-head' sites, so careful planning is needed to ensure it will be worthwhile and effective.
A large, slow-moving body of water gives a high torque (turning force), and a waterwheel can make use of this to operate machinery directly. However, the low rotational speed makes it difficult to use a waterwheel for electricity generation; it’s easier to make electricity with a fast flow of water that can be channelled to hit a turbine at high pressure. Waterwheels are also more expensive to construct than small turbines and need more maintenance.
Having said that, with some 8,000 mills or mill sites recorded in Britain, a small number may be suitable for generating electricity - so it could be worth looking into. A hydro turbine installed a few years ago at Gants Mill in Somerset generates up to 12kW of electricity and feeds into the local grid.
The most suitable type of waterwheel for conversion to electricity production is the overshot style, as it has the highest head. It often proves worthwhile to increase the head by raising the headrace and/or lowering the tailrace. Some types of waterwheel can operate at a very low fall of only a few metres – you’d then need large flows of water to get reasonable amounts of power out of them.
A recent development is the use of Archimedes Screw turbines on low head sites. The case studies on this webpage give an idea of the large flow rates required when the drop is only a few metres: www.mannpower-hydro.co.uk/casestudy.php
Generators operate most efficiently at high speeds. Motors or generators that run at very low rpm (revolutions per minute) are large and expensive - a 1000rpm motor is much bigger than a 1500rpm one. Therefore, it may be more practical to gear up to a faster turbine, or consider installing a micro-hydro turbine instead of a waterwheel.