Kinetic flow systems

All current schemes to extract power from rivers and lakes are essentially gravitational systems. They rely on water descending under gravity to provide power and not at all on the kinetic energy available from flowing water. Hydro-electric dams, pumped hydraulic storage and overshot water wheels all work on this principle. Overshot wheels are the type where the water flows onto top of the wheel from a weir or flume. The wheel is effectively a set of buckets and is driven by the weight of water in the buckets.

There are, however lesser known systems for using the kinetic energy in flowing water.

Undershot water wheels

These are a fairly ancient design that has fallen out of favour. The water flows under these wheels, which have paddles rather than buckets round the wheel rim. They are driven by the force of the flow passing under the wheel. They require a fairly fast flow to produce usable amounts of power, but their output is directly proportional to the width of the wheel. Double its width and its output is doubled. However, they only work well when the river level is between fairly narrow limits.

Hydraulic rams

These use the kinetic energy of a flowing stream to pump water. There's almost no limit to the height to which water can be raised by a ram, but there is a trade-off: the product of the mass of water raised and the distance it is lifted is a constant. These pumps only produce small flows but will run for long periods with little or no maintenance, so they are best suited to keeping tanks or animal drinking troughs topped up from the stream that drives them. They are stopped by back pressure and are self starting, so are easily controlled by a ball cock in the receiving vessel. Clemson University in South Carolina, USA, publishes a guide to making a hydraulic ram and a guide to likely performance. A typical ram will need a flow of eight times the volume of water raised by the pump but its energy efficiency approximates a useful 60%.

Transverse Horizontal Axis Water Turbine (THAWT)

This turbine design is being developed by Kepler Energy, a spin off from the Oxford University's engineering department, is designed to generate electricity from tidal flows or river currents and can generate 50% more power than a propeller type turbine on the same site because the profile it presents to the flow is rectangular rather than circular: thus the ratio of swept area to flow area is larger. In addition the structure (the blades form a stressed truss) is lighter and the turbine needs no reconfiguring when a tidal flow reverses.

Vortex flow round cylinders

A new development in 2008, known as VIVACE (Vortex Induced Vibrations Aquatic Clean Energy) is based on the discovery that a roughened cylinder placed in flowing water and free to move perpendicularly to the flow in a sliding mount will oscillate in the mounting plane. Springs can be used to drive a generator and extract useable amounts of energy from a flow of only 0.5 to 1.0 meters per second. An experimental rig using a cylinder 100 mm in diameter and 910 mm long produced 10 watts. Indications are that larger systems with outputs between tens of kilowatts and megawatts will generate electricity for around $US 0.05 per kWh. It is being developed by Vortex Hydro Energy. This system should work equally well in ocean currents and tidal flows.