Micro Hydro Power (MHP) Plant - Turbine Types

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The type of turbine that can be used in a micro hydro installation depends on different factors such as, head of water, the volume of flow, and such factors as availability of local maintenance and transport of equipment to the site.

A turbine converts energy from water falling into a rotating shaft power; the selection of a hydro turbine depends on the site characteristics and the head and flow available. The desired running speed of the generator or other devices in the turbine also plays a vital role in the selection process. However other conditions such as weather the turbine is expected to produce power under part-flow conditions could also be considered. All turbines have a power-speed characteristic that will tend to run most efficiently at a particular speed, head and flow combination.[1]

Impulse Turbine and Reaction Turbine

A turbines design speed is largely determined by the head with which it operates. Turbines can either be classified as impulse turbines or reaction turbines.[2] In the impulse turbine, the turbine runner operates in air and is turned by one or multiple jets of water which make contact with the runner blades. On the other side in a reaction turbine, the turbine runner is fully immersed in water and is enclosed in a pressure casing, the runner blades are angled so that pressure differences across them create lift forces, like those on aircraft wings, which cause the runner to rotate.[3]

high head
medium head
low head
impulsive turbines
  • Pelton
  • Turgo
  • cross-flow
  • multi-jet Pelton
  • Turgo
  • cross-flow
reaction turbines

  • Francis
  • propeller
  • Kaplan

The rotating element, also known as the 'runner' of a reaction turbine are fully immersed in water and enclosed in a pressure casing. The runner blades are profiled in a mechanism that pressure differences across them imposes lift forces that make the runners to rotate , like aircraft wings. In contrast, an impulse turbine runner operates in air driven by jets of water. This makes the water to remain in atmospheric pressure before and after making contact with the runner blades. In this case a nozzle converts the pressurised low velocity water into a high speed jet. The runner blades deflect the jet so as to maximise the change of momentum of the water and thus maximising the force on the blades.[5]

Impulse are generally more suitable for micro hydro applications compared with reaction turbines because of the following advantages:

  • They have a greater tolerance to sand and other particles in water.
  • There is a better availability of spare parts
  • There is no pressure seal around the shaft
  • They are easier to fabricate and maintain
  • They have a better part-flow efficiency.

However with the advantages, the main disadvantage for the impulse turbines is that they are mostly unsuitable for low-head sites because of their low specific speeds too great an increase in speed would be required of the transmission to enable coupling to a standard alternator. The crossflow, Turgo and multi-jet Pelton are suitable at medium heads.[5]

Pelton Turbine[6]

It consists of a set of specially shaped buckets mounted on a periphery of a circular disc. . It is turned by jets of water which are discharged from one or more nozzles and strike the buckets. The buckets are split into two halves so that the central area does not act as a dead spot incapable of deflecting water away from the oncoming jet. The cutaway on the lower lip allows the following bucket to move further before cutting off the jet propelling the bucket ahead of it and also permits a smoother entrance of the bucket into the jet. The Pelton bucket is designed to deflect the jet through 165 degrees (not 180 degrees) which is the maximum angle possible without the return jet interfering with the following bucket for the oncoming jet.

In large scale hydro installation Pelton turbines are normally only considered for heads above 150 m, but for micro-hydro applications Pelton turbines can be used effectively at heads down to about 20 m. Pelton turbines are not used at lower heads because their rotational speeds becomes very slow and the runner required is very large and unwieldy. If runner size and low speed do not pose a problem for a particular installation, then a Pelton turbine can be used efficiently with fairly low heads.

Turgo Turbine[6]

This is an impulse machine similar to a Pelton turbine but which was designed to have a higher specific speed. In this case the jets aimed to strike the plane of the runner on one side and exits on the other. Therefore the flow rate is not limited by the discharged fluid interfering with the incoming jet (as is the case with Pelton turbines). As a consequence, a Turgo turbine can have a smaller diameter runner than a Pelton for an equivalent power. With smaller faster spinning runners, it is more likely to be possible to connect Turgo turbines directly to the generator rather than having to go via a costly speed-increasing transmission.

Like the Pelton, the Turgo is efficient over a wide range of speeds and shares the general characteristics of impulse turbines listed for the Pelton, including the fact that it can be mounted either horizontally or vertically. A Turgo runner is more difficult to make than a Pelton and the vanes of the runner are more fragile than Pelton buckets.

Crossflow Turbine[6]

Also called a Michell-Banki turbine a crossflow turbine has a drum-shaped runner consisting of two parallel discs connected together near their rims by a series of curved blades. A crossflow turbine always has its runner shaft horizontal (unlike Pelton and Turgo turbines which can have either horizontal or vertical shaft orientation).
In operation a rectangular nozzle directs the jet onto the full length of the runner. The water strikes the blades and imparts most of its kinetic energy. It then passes through the runner and strikes the blades again on exit, impacting a smaller amount of energy before leaving the turbine. Although strictly classed as an impulse turbine, hydro dynamic pressure forces are also involved and a mixed flow definition would be more accurate.

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