A microhydropower system needs a turbine, pump, or waterwheel to transform the energy of flowing water into rotational energy, which is then converted into electricity.
Turbines are commonly used to power microhydropower systems. The moving water strikes the turbine blades, much like a waterwheel, to spin a shaft. But turbines are more compact in relation to their energy output than waterwheels. They also have fewer gears and require less material for construction.
There are two general types of turbines: impulse and reaction.
Impulse turbines, which have the least complex design, are most commonly used for high-head microhydro systems. They rely on the velocity of water to move the turbine wheel, which is called the runner. The most common types of impulse turbines include the Pelton wheel and the Turgo wheel.
The Pelton wheel uses the concept of jet force to create energy. Water is funneled into a pressurized pipeline with a narrow nozzle at one end. The water sprays out of the nozzle in a jet, striking the double-cupped buckets attached to the wheel. The impact of the jet spray on the curved buckets creates a force that rotates the wheel at high efficiency rates of 70%-90%. Pelton wheel turbines are available in various sizes and operate best under low-flow and high-head conditions.
|Pelton wheels, like this one, can be purchased with one
or more nozzles. Multi-nozzle systems allow more water
to impact the runner, which can increase wheel output.
The Turgo impulse wheel is an upgraded version of the Pelton. It uses the same jet spray concept, but the Turgo jet, which is half the size of the Pelton, is angled so that the spray hits three buckets at once. As a result, the Turgo wheel moves twice as fast. It is also less bulky, needs few or no gears, and has a good reputation for trouble-free operation. The Turgo can operate under low-flow conditions but requires a medium or high head.
Another turbine option is called the Jack Rabbit (sometimes referred to as the Aquair UW Submersible Hydro Generator). The Jack Rabbit is a drop-in-the-creek turbine that can generate power from a stream with as little as 13 inches of water and no head. Output from the Jack Rabbit is a maximum of 100 watts, so daily output averages 1.5–2.4 kilowatt-hours, depending on the site.
Reaction turbines, which are highly efficient, depend on pressure rather than velocity to produce energy. All blades of the reaction turbine maintain constant contact with the water. These turbines are also often used in large-scale hydropower sites.
Because of their complexity and cost, reaction turbines aren't usually used for microhydropower projects. But one exception is the propeller turbine, which comes in many designs and works much like a boat's propeller.
Propeller turbines have three to six usually fixed blades set at different angles aligned on the runner. The bulb, tubular, and Kaplan tubular are variations of the propeller turbine. The Kaplan turbine, which is a highly adaptable propeller system, can be used for microhydro sites.
Conventional pumps can be used as substitutes for hydraulic turbines. When the action of a pump is reversed, it operates like a turbine. Because pumps are mass produced, they can be found more readily than turbines. Pumps are also less expensive. For adequate pump performance, however, a microhydropower site must have fairly constant head and flow. Pumps are also less efficient and more prone to damage.
The waterwheel is the oldest hydropower system component. Waterwheels are still available, but they aren't very practical for generating electricity because of their slow speed and bulky structure.