Water turbines are basically fairly simple systems. They
consist of the following components:
- intake shaft - a tube that connects to the piping
or penstock which brings the water into the turbine
- water nozzle - a nozzle which shoots a jet of water
(impulse type of turbines only)
- runner - a wheel which catches the water as it flows in
causing the wheel to turn
- generator shaft - a steel shaft that connects the runner
to the generator
- generator - a small electric generator that creates the
- exit valve - a tube or shute that returns the water to
the stream it came fro
- powerhouse - a small shed or enclosure to protect the
water turbine and generator from the elements
Impulse vs. Reaction Turbines
Water turbines are also often classified as being either impulse
turbines or reaction turbines. In a reaction turbine the
runners are fully immersed in water and are enclosed in a pressure
casing. The runner blades are angled so that pressure
differences across them create lift forces, like those on aircraft
wings, and the lift forces cause the runner to rotate.
In an impulse turbine the runner operates in air, and is turned by
one or multiple jets of water which make contract with the blade.
A nozzle converts the pressurized low velocity water into a high speed
jet much like you might use with a garden hose nozzle. The nozzle is
aligned so that it provides maximum force on the blades.
Types of Turbines
There are many kinds of micro hydro
turbine designs. Typical microhydro generators have outputs of 10 kilowatts (kW) or less and can generate either DC or AC current depending upon the design. You will often hear water turbines referred to as either Pelton or
Turgo turbines. These terms have to do with the structure of the
water wheel inside the turbine.
here is a Turgo style wheel. A Turgo turbine is an impulse type of
turbine in which a jet of water strikes the turbine blades. The structure of a
wheel is much like that of airplane turbine in which the hub is
surrounded by a series of curved vanes. These vanes catch the water as
it flows through the turbine causing the hub and shaft to turn.
Turgo turbines are designed for higher speeds than Pelton turbines and
usually have smaller diameters.
A Pelton turbine is also an
impulse turbine but in this type of turbine the hub is surrounded by a series of cups
or buckets which catch the water. 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 lips allows the following bucket to move further before
cutting off the jet propelling the bucket ahead of it. This also
permits a smoother entrance of the bucket into the water jet.
A cross-flow turbine, also sometimes called a
Michell-Banki turbine (from the name of the manufacturer) is a turbine
that uses a drum shaped runner much like the wheel on an old paddle
wheel steamboat. A vertical rectangular nozzle is used with this
type of turbine to drive a jet of water along the full length of the
runner. One advantage of this type of turbine is that it can be used
in situations where you have significant flow but not enough head
pressure to use a high head turbine.
The Francis type of turbine is a reaction
type of turbine in which the entire wheel assembly is immersed in
water and surrounded by a pressure casing. In a Francis turbine
the pressure casing is spiral shaped and is tapered to distribute
water uniformly around the entire perimeter of the runner. It
uses guide vanes to ensure that water is fed into the runners at the
A propeller turbine is just what its name implies.
It uses a runner shaped just like a boat propeller to turn the
generator. The propeller usually has six vanes. A
variation of the propeller turbine is the Kaplan turbine in which the
pitch of the propeller blades is adjustable. This type of
turbine is often used in large hydroelectric plants. An
advantage of propeller type of turbines is that they can be used in
very low head conditions provided there is enough flow.
Selecting the Best Type of Turbine
Which type of water turbine is best for a particular situation
often depends on the amount of head (water pressure) you will have in
your location and whether you want to suspend the turbine in the water
(reaction) or whether you want to use jets of water (impulse).
By looking at these factors together you can get some indication of
what type of turbine design will work best: