The hydro turbine can be classified into two categories of reaction turbine and impulse turbine according to the principle of their water flow action and structural features. At present, various types of hydraulic turbine produced by manufacturers are mainly as follows:
1. Francis hydro turbine:
The Francis turbine is a type of reaction turbine, a category of turbine in which the working fluid comes to the turbine under immense pressure and the energy is extracted by the turbine blades from the working fluid. A part of the energy is given up by the fluid because of pressure changes occurring in the blades of the turbine, quantified by the expression of degree of reaction, while the remaining part of the energy is extracted by the volute casing of the turbine. At the exit, water acts on the spinning cup-shaped runner features, leaving at low velocity and low swirl with very little kinetic or potential energy left. The turbine's exit tube is shaped to help decelerate the water flow and recover the pressure.
Francis turbine are most widely used among water turbines, is designed for a wide range of heads and flows. Francis turbine cover a head range from 40 to 600 m (130 to 2,000 ft), and their connected generator output power varies from just a few kilowatts up to 1000 MW. Large Francis turbines are individually designed for each site to operate with the given water flow and water head at the highest possible efficiency, typically over 90%.
2. Kaplan hydro turbine
Kaplan turbine is a propeller-type water turbine which has adjustable blades.
Kaplan turbine is most appropriate for operation with a low head and a large amount of discharge, which was not possible with Francis turbines. The head ranges from 10 to 30 metres and the output ranges from 5 to 200 MW. Runner diameters are between 2 and 11 metres (6 ft 7 in and 36 ft 1 in). Turbines rotate at a constant rate.
Owing to adjustable runner blades it offers the significant advantage to give high efficiency even in the range of partial load, and there is little drop in efficiency due to head variation or load. As having adjustable runner blades, the construction of Kaplan turbine becomes naturally a bit complicated. The runner blade operating mechanism consists of a pressure oil head, a runner servomotor, and the blade operating rod inside the shaft etc.
3. Pelton turbine
A Pelton wheel is an impulse-type water turbine invented by American inventor Lester Allan Pelton in the 1870s. The Pelton wheel extracts energy from the impulse of moving water, Many earlier variations of impulse turbines existed, but they were less efficient than Pelton's design.
Pelton wheels are the preferred turbine for hydro-power, where the available water source has relatively high water head at low flow rates.
Nozzles direct forceful, high-speed streams of water against a series of buckets, which are mounted around the outer rim of a runner. As the water jet hits the buckets, the direction of water velocity is changed to follow the contours of the bucket. The impulse energy of the water jet exerts torque on the bucket-and-wheel system, spinning the wheel; In the process, the water jet's momentum is transferred to the turbine runner.
4. Turgo turbine
The Turgo runner is a refinement of the Pelton, where the jets direct water at the runner at an angle. Turgo turbine is an impulse water turbine designed for medium/high head applications. It works with net heads between 60 and 300 m. Compare with Pelton turbine, The Turgo is used especially for situations with high water flows; the design allows for larger jets. Because Turgo can use more water, significant power can be generated with less head.
This results in shorter penstocks, all things being equal. In addition, the Turgo is rugged and can be used as a small and inexpensive AC turbine for the right kind of site, making it a commonly used turbine.