Banki turbine. Image credit; European Communities, Layman's Guidbook (on how to develop a small hydro site)

The Banki turbine (also known as the Michell, Crossflow or Ossberger turbine) is a hydropower system similar in appearance to an over-shot water wheel. Unlike the water wheel, however, it uses a nozzle and blades instead of buckets. The "middle" of the Banki turbine is left open and the blades that would normally form the walls of the buckets are angled. It is, in effect, a "leaky" water wheel.

Water flowing into the top of the turbine not only spins the wheel by its weight, but as it flows past the blades and into the middle of the turbine, its direction is changed. This extracts additional power via Newton's Third Law. A venturi enhances this effect by funneling the water at a high speed onto the blades.

The Banki turbine uses concepts from both impulse and reaction turbine design. This allows it to perform well in a wide range of heads. The system is somewhat similar to the Pelton wheel in concept, but requires less engineering in the wheel itself.

The Banki turbine has lower efficiency than other turbine designs but enjoys a niche market for low cost and home made installations. A reasonably efficient turbine runner can be constructed from pipe sections that are slit to form the blades (70% - 80% efficiency at 10 feet of head). The venturi and associated plumbing may be fabricated with sheet metal tools. It is somewhat more complex than in a over-shot wheel, but by no means difficult. A home made Banki turbine that has same power as a large Diesel engine is within the means of most small and amateur workshops.