Micro-hydro power

 The power available in a river or stream depends on the rate at which the water is flowing, and the height (head) which it falls down. Hydro schemes are sometimes classified into four groups, although there’s no universal agreement on the boundaries between them, and the basic principles of operation are the same for all.

•  Mini: 5 kW to 100 kW

The core of a small hydro scheme is the turbine, which is rotated by the moving water. Different types are used, depending on the head and flow at the site. The turbine rotates a shaft, which is often used to drive an electrical generator.

Pelton turbine (for high head, low flow) consists of a set of small buckets arranged around a wheel onto which one or more jets of water are arranged to impact.

Francis turbine (lower head and higher flow) has a spiral casing that directs the water flow through vanes on a rotor.

Cross-flow or  turbines (even lower head and higher flow) are made as a series of curved blades fixed between the perimeters of two disks to make a cylinder. The water flows in at one side of the cylinder and out of the other, driving the blades around. They are much easier to make than most other designs.

Propeller turbine (very low head and large flow) has fixed blades, like a boat propeller. A more complex version, the Kaplan turbine, has blades that can be adjusted in pitch relative to the flow.

River current turbine, which is like a wind-turbine immersed in water, can be used to extract power from with a large flow in a river, where there is virtually no head.

Most small hydro systems are ‘run-of-river’ which means that they don’t need large dams to store water. However, they do need some water-management systems.

A small dam in the river bed directs the water to a settling tank. This allows silt to settle out of the water, and the clean water to flow into a canal or a pipe to a second settling tank called the ‘forebay’, which is sited above the power house. The canal or pipe can be fairly long, 1 km or more, if a suitable stream is distant from where the power is required. The outlet from the forebay has a screen to trap silt and floating debris. Water flows out into a pipe called the ‘penstock’, which is made as steep as possible to transfer water to the turbine. Water leaving the turbine is led back to the stream through the outlet pipe or ‘tail-race’.