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A turbine converts energy in the form of falling water into rotating shaft power. The selection of the best turbine for any particular hydro site depends on the site characteristics, the dominant ones being the head and flow available. Selection also depends on the desired running speed of the generator or other device loading the turbine. Other considerations such as whether the turbine is expected to produce power under part-flow conditions, also play an important role in the selection. All turbines have a power-speed characteristic. They will tend to run most efficiently at a particular speed, head and flow combination.
A turbine design speed is
largely determined by the head under which it operates. Turbines can be
classified as high head, medium head or low head machines. Turbines are
also divided by their principle way of operating and can be either impulse
or reaction turbines.
The rotating element (called `runner') of a reaction turbine is fully immersed in water and is enclosed in a pressure casing. The runner blades are profiled so that pressure differences across them impose lift forces, like those on aircraft wings, which cause the runner to rotate.
In contrast a impulse turbine runner operates in air, driven by a jet (or jets) of water. Here the water remains at 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.
Impulse turbines are usually
cheaper then reaction turbines because there is no need for a specialist
pressure casing. nor for carefully engineered clearances, but they are
also only suitable for relatively high heads.
Impulse turbines are generally more suitable for micro-hydro applications compared with reaction turbines because they have the following advantages:
A Pelton turbine 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. If a higher running speed and smaller runner are required then the are two further options:
- increasing the number of jets.
- twin runners.
Two runners can be placed on the same shaft either side by side or on opposite sides of the generator. This configuration is unusual and would only be used if the number of jets per runner had already been maximised, but it allow the use of smaller diameter and hence faster rotating runners.
The Turgo turbine 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 exists 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.
Turgo runner blades and water jet
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. At one time they were exclusively made by Gilbert, Gilkes and
Gordon a UK manufacturer who owned the patent rights, but they are now
manufactured in several other countries.
The Ghatta is a traditional Nepalese waterwheel with a vertical axis. The water enter the waterwheel from above. The turbine is made out of wood to enable simple building and repair techniques to be used. A consequent of this design are low efficiency and power output (maximum 12 kW).
Out of this traditional Ghatta the improved
Ghatta was developed. The wooden waterwheel was improved and replaced later
with a steel one with round buckets. This improved the momentum transfer
of the water and doubled power output.
Multi-Purpose Power Unit
The Multi-Purpose Power Unit (MPPU) is chronological situated in between the Ghatta and the improved Ghatta. The name multi-purpose refers to the construction of the MPPU which enables the connection of various machinery to it. The concept of the MPPU is basically the same as that of the improved Ghatta: a vertical axis with a fixed and a rotating grinding stone. Technical complexity, power output and price are in between those of the improved Ghatta and crossflow turbines.
All components are of steel instead of wood, water supply is improved and friction losses are reduced compared to the improved Ghatta.
Design philosophy was to
produce a device as cheap and simple as possible. Special attention was
given to transportability.
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.
Part flow efficiency:
A high part-flow efficiency
can be maintained at less than a quarter of full flow by the arrangement
for flow portioning illustrated in the figure. At low flows, the water
can be channelled through either two-thirds or one third of the runner,
thereby sustaining a relatively high turbine efficiency.
The reaction turbines considered here are the Francis turbine and the propeller turbine. A special case of the propeller turbine is the Kaplan. In all these cases, specific speed is high, i.e reaction turbines rotate faster than impulse turbines given the same head and flow conditions. This has the very important consequences that a reaction turbine can often be compiled directly to an alternator without requiring a speed-increasing drive system. Some manufacturers make combined turbine-generator sets of this sort. Significant cost savings are made in eliminating the drive and the maintenance of the hydro unit is very much simpler. The Francis turbine is suitable for medium heads, while the propeller is more suitable for low heads.
On the whole reaction turbines require more sophisticated fabrication than impulse turbines because they involve the use of larger and more intricately profiled blades together with carefully profiled casings. The extra expenses involved is offset by high efficiency and the advantages of high running speeds at low heads from relatively compact machines.
Fabrication constraints make these turbines
less attractive for use in micro-hydro in developing countries. Nevertheless
because of the importance of low head micro-hydro, work is being undertaken
to develop propeller machines which are simpler to construct. Most reaction
turbines tend to have poor part-flow efficiency characteristics.
Francis turbines can either be volute-cased or open-flume machines. The spiral casing is tapered to distribute water uniformly around the entire perimeter of the runner and the guide vanes feed the water into the runner at the correct angle. The runner blades are profiled in a complex manner and direct the water so that it exits axially from centre of the runner. In doing so the water imparts most of its pressure energy to the runner before leaving the turbine via a draft tube.
The Francis turbine is generally fitted with adjustable guide vanes. These regulate the water flow as it enters the runner and are usually linked to a governing system which matches flow to turbine loading in the same way as a spear valve or deflector plate in a Pelton turbine. When the flow is reduced the efficiency of the turbine falls away.
The basic propeller turbine consists of
a propeller, similar to a ship's propeller, fitted inside a continuation
of the penstock tube. The turbine shaft passes out of the tube at the point
where the tube changes direction. The propeller usually has three to six
blades, three in the case of very low head units and the water flow is
regulated by static blades or swivel gates ("wicket gates") just upstream
of the propeller. This kind of propeller turbine is known as a fixed blade
axial flow turbine because the pitch angle of the rotor blades cannot be
changed. The part-flow efficiency of fixed-blade propeller turbines tend
to be very poor.
Large scale hydro sites make use of more
sophisticated versions of the propeller turbines. Varying the pitch of
the propeller blades together with wicket gate adjustment enables reasonable
efficiency to be maintained under part flow conditions. Such turbines are
know as variable pitch or Kaplan turbines.
Centrifugal pumps can be used as turbines by passing water through them in reverse. Research is currently being done to enable the performance of pumps as turbines to be predicated more accurately.
The potential advantages are the low cost
due to mass production (and in many cases also local production), the availability
of spare parts and the wider dealer/support networks. The disadvantages
are the as yet poorly understood performance characteristics and very poor
part-flow efficiency. Various companies have used pumps as turbines at
various times, but the technology remains unproven and relatively poor