Small Hydropower System Components

Small run-of-the-river hydropower systems consist of these basic components: 

  • Water conveyance—channel, pipeline, or pressurized pipeline (penstock) that delivers the water 
  • Turbine or waterwheel—transforms the energy of flowing water into rotational energy 
  • Alternator or —transforms the rotational energy into electricity 
  • Regulator—controls the generator 
  • Wiring—delivers the electricity. 

Many systems also use an inverter to convert the low-voltage direct current (DC) electricity produced by the system into 120 or 240 volts of alternating current (AC) electricity (alternatively you can buy household that run on DC electricity). Some systems also use batteries to store the electricity generated by the system, although because hydro resources tend to be more seasonal in nature than wind or solar resources, batteries may not always be practical for hydropower systems. If you do use batteries, they should be located as close to the turbine as possible, because it is difficult to transmit low-voltage power over long distances. 

Channels, storage, and filters 

Before water enters the turbine or waterwheel, it is first funneled through a series of components that control its flow and filter out debris. These components are the headrace, forebay, and water conveyance (channel, pipeline, or penstock). 

The headrace is a waterway running parallel to the water source. A headrace is sometimes necessary for hydropower systems when insufficient head is provided. They often are constructed of cement or masonry. The headrace leads to the fore-bay, which also is made of concrete or masonry. It functions as a settling pond for large debris which would otherwise flow into the system and damage the turbine. Water from the forebay is fed through the trashrack, a grill that removes additional debris. The filtered water then enters through the controlled gates of the spillway into the water conveyance, which funnels water directly to the turbine or waterwheel. These channels, pipelines, or penstocks can be constructed from plastic pipe, cement, steel and even wood. They often are held in place above-ground by support piers and anchors. 

Dams or diversion structures are rarely used in microhydro projects. They are an added expense and require professional assistance from a civil engineer. In addition, dams increase the potential for environmental and maintenance problems. 

Turbines and waterwheels 

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. 

Turbines are more commonly used today to power small hydropower 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 classes 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 rates of 70 to 90 percent. Pelton wheel turbines are available in various sizes and operate best under low-flow and high-head conditions. 

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's also less bulky, needs few or no gears, and has a good reputation for trouble-free operations. The Turgo can operate under low-flow conditions but requires a medium or high head. 

Pelton wheels, like this one, can be purchased with one or more nozzles. Multi- nozzle systems allow a greater amount of water to impact the runner, which can increase wheel output.
Pelton wheels can be purchased with one or more nozzles. Multi- nozzle systems allow a greater amount of water to impact the runner, which can increase wheel output.

Pelton wheels, like this one, can be purchased with one or more nozzles. Multi-nozzle systems allow a greater amount of water to impact the runner, which can increase wheel output. 

Another turbine option is called the Jack Rabbit (sometimes referred to as the Aquair UW Submersible Hydro Generator). The Jack Rabbit is the drop-in-thecreek turbine, mentioned earlier, 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 W, so daily output averages 1.5 to 2.4 kilowatt-hours, depending on your 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 often used in large-scale hydropower sites. Because of their complexity and high , they aren't usually used for microhydro projects. An exception is the propeller turbine, which comes in many different 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. 

Pumps as substitutes for turbines 

Conventional pumps can be used as substitutes for hydraulic turbines. When the action of a pump is reversed, it operates like a turbine. Since pumps are mass produced, you'll find them more readily available and less expensive than turbines. 

However, for adequate pump performance, your microhydro site must have fairly constant head and flow. Pumps are also less efficient and more prone to damage.