Introduction
The British Government
and the EU demands that the quantity electricity generated using fossil fuels
be greatly reduced. The "green" alternatives such as wind wave and
solar power are Dependant on climatic conditions and tidal power has great
difficulty in generating continually over a 24 hour period. This is a real
problem to the electricity supply companies who need to ensure that the demand
for electricity can always be met This is no difficulty at present, since the
quantity of "Green " electricity produced is not a significant
percentage of the total, but as the number of wind farms increase, this will change.
In many countries the majority of the power generation relies on steam
turbines. These are highly efficient BUT inflexible. Basically they have to be
kept spinning and they can not be quickly shut down or started.
A turbine is a rotary engine that extracts energy from a
fluid flow and converts it into useful work.
Hydro electricity a
reliable form of renewable energy. Water turbines are highly efficient and
easily controlled to provide power as and when it is needed. In addition,
currently the only system available to store large quantities of electrical
power, is pumped storage. This involves pumping water into a high level
reservoir. This can happen when the demand for electricity is low, at night
for-instance. When the demand is high the supply can be rapidly increased by
running the stored water through Turbines.
Turbines can be
divided into two basic basic types. These are Impulse Turbines and Reaction Turbines.
In these the whole of the available energy of
the water is converted to Kinetic Energy before the water acts on the moving
parts of the turbine. In this type of turbine the cups or wheel passages are
never entirely filled with water. To achieve this the turbine must be mounted
slightly above the tail race.
In Reaction Turbines the rotation is mainly
achieved by the reaction forces created by the acceleration of the water in the
runner. The basic principle is the same as a rotating lawn sprinkler in which
water enters the arms of the sprinkler at low velocity and leaves through the
jets at high velocity. The exact manner in which this acceleration is achieved
in Turbines depends upon the whether the runner is of the Propeller, Kaplin,
Francis or Deriaz type.
The great variety possible in the geometry of
the runner, makes Reaction Turbines suitable for a wide range of applications.
In all types it is normal for a fraction of the hydraulic pressure to be
converted into velocity as it passes through the inlet structure which consists
of a spiral casing and a gate apparatus leading to the runner. The power from
the water (Pressure and Velocity) is then converted into mechanical power in
the runner. It is usual for the exit pressure from the runner to be below
atmospheric. This is achieved by using a Draft Tube.
Propeller and Kaplan have axial flow runners and are used for the lower heads. They
are particular suitable for large installations. At low heads (Below 25 ft.)
the running speed can be twice that of a Francis Turbine but even so the running
speed of high output propeller turbines is less than 100 r.p.m. The fixed vane
design of the propeller turbine does not lead to flexibility and the machines
need to be run at or near their optimum output. At below 75% load the
efficiency falls rapidly. To overcome this problem the
was developed. It is essentially a propeller
turbine with variable blade angles. An interesting variation on the traditional
design uses a horizontal shaft and an electric generator mounted in a metal
shell which sits in the water-flow. Such machines are particularly suitable for
very low heads and were developed for a French Tidal Power scheme.
The Francis Turbine is probably the most
commonly used type of Turbine. It can operate from very low heads of about 10
ft. up to about 2000 ft. Turbines operating at these heads must have a large
output since the low water quantities and the size of the water passages within
the runner make construction difficult. At very low heads Propeller Turbines
are usually a more economic solution.
A development of the Kaplan Turbine with a
variable pitch design that improves the efficiency under less than full load at
medium heads has been developed and is called the Deriaz Turbine after its inventor.
In these there is a pressure which in some
cases amounts to half the head in the clearance space between the guide vanes
and the wheel vanes.
The velocity with which the water enters the
wheel is due to the difference between the pressure due to the head and the
pressure in the clearance space.
