Waterwheels

Waterwheels, although not as efficient as turbines, are still a viable proposition for producing electriciy for domestic purposes. They are simple to control and aesthetically pleasing. Although they run realtively slowly and requrie a high ratio gearbox to drive a generator, for low powers - say below 5kW - and heads below 3m, they are worth consdiering.

Waterwheels are often overlooked for generating electricity but can be successfully employed at many low-head micro-hydro sites abd have a number of advantages over conventional approaches with turbines:

• Output reduction due to screen blockages is avoided since fine intake screens are not required.

• Part-flow performance of waterwheels can be very good without requiring complex control systems.

• Often minimal building work is required, particularly at former watermills if there is a vacant wheel pit.

• Waterwheels have obvious aesthetic benefits over turbines and provide an excellent attraction at sites where visitors are encouraged.

The principal challenge of the waterwheel is the low rational speed, which means that significant gearing up is required to match generator speeds. However, high power gear units are now much more widely available and have improved the economics of waterwheel power schemes up to 50kW.

Waterwheels are widely regarded as being rather inefficient compared with turbines. This is not necessarily the case as studies have shown that waterwheel efficiency can be in excess of 80% for overshot waterwheels and 75% for breastshot waterwheels [Muller 2004]. This in combination with hoghly respectable part-flow performance and lack of fine intake screening requirements can often result in very worthwhile overall energy capture.

[Source: Pico Energy Ltd and HydroWatt GmbH]

Types of Waterwheels

Vertical axis wheels are common in Nepal and North India, but for low heads horizontal axis wheel are used. There are three basic types:

Undershot - This is probably the oldest design. The paddles are flat and are simply dragged round by the flowing water. The undershot wheel is not the most efficient – at most 307 and very low output.




Breast Shot - The water hits the breast shot waterwheel much higher than on the undershot wheel and is more efficient.





Overshot- This type of waterwheel can achieve an efficiency of 60% with careful design. The disadvantage is that it must have a diameter almost equal to the head. This can lead to very large wheels (e.g. the Laxey Wheel in the Isle of Man is 125ft in diameter).

Variations on the overshot wheel worth considering, both of which are capable of efficiencies of over 60%, are the Poncelet wheel and the Pitchback wheel. Each of these has a large number of blades, usually curved to smooth the entry of water, and both rotate in the direction of the water flow at the base. The Poncelet is an undershot wheel where the water is introduced as a jet from a sliding gate at its base, and the Pitchback is a high breast shot wheel where the water is introduced well above the axel at about the 11 o’clock position. Many wheels of this type were built in the 19th century at textile mills in Northern England and Scotland.

Gearing Up

A number of ingenious solutions have been used by do-it-yourself hydro developers.

If the power is taken from the axle of the wheel, the torque on the gearbox is high. Solutions in the past have included the use of back axle and differential gearboxes from tractors and other motor vehicles to raise the speed enough to use a second, more conventional gearbox. An elegant though more expensive solution is to use a combined gearbox motor unit as the generator.

Taking the power from the rim of the wheel is possible for low powers. Ideally, the rim gear from an old wheel could be used. Running a car wheel (complete with tyre) on the rim and taking the power off from the car wheel axle has been used successfully but needs careful engineering.

Basic Formulae

The power produced by an overshot or breast shot wheel is the weight of water passing per second multiplied by the height between the upper and lower water level times the efficiency of the wheel, gearbox and generator.

This can be expressed approximately as:

Power in Watts = 4 x Q x H

The optimal rotational speed of a breast shot or overshot wheel is approximately:

(D = diameter of the wheel in meters)

For traditional undershot wheels, the formulae are:

Approximate power produced (assuming a water to wire efficiency of 20%)

100 x A x V³

A = Area of paddles in the water (square meters)

V = Velocity of the stream in meters per second

Safety

Although waterwheels are safe for the passage of fish, they do not pass people as kindly. Make sure that adequate fencing is provided to protect the curious.