Water turbine
A Francis Turbine and generator cut-away view
A water turbine is a rotary engine that takes energy from moving water.
Water turbines were developed in the nineteenth century and were widely used for industrial power prior to electrical grids. Now they are mostly used for electric power generation. They harness a clean and renewable energy source.
| Table of contents |
|
2 Theory of operation 3 Types of water turbines 4 Design and Application 5 Environmental impact 6 See also 7 External links |
History
Water wheels have been used for thousands of years for industrial power. Their main shortcoming is size, which limits the flow rate and head that can be harnessed. They also tend to rotate slower than the machines they power.
The word turbine was coined by the French engineer Claude Bourdin and is derived from the Latin word for "whirling" or a "vortex". Water turbines were developed during the Industrial revolution, using scientific principals and methods, for industrial power. They also made extensive use of new materials and manufacturing methods developed at the time. The main difference between early water turbines and water wheels is a swirl component of the water which passes energy to a spinning rotor.
This additional component of motion allowed the turbine to be smaller than a water wheel of the same power. They could process more water by spinning faster and could harness much greater heads. (Later, impulse turbines were developed which didn't use swirl). Development took about twenty-five years between the first water turbine and a modern design (and mastery of the engineering).
A Francis Turbine runner being installed at the Grand Coulee Dam
Bénoit Fourneyron developed an outward-flow turbine In 1826. This was an efficient machine (~80%) that sent water through a runner with blades curved in one dimension. The stationary outlet also had curved guides.
In 1820, Jean V. Poncelet developed an inward-flow turbine. Inward flow water turbines have a better mechanical arrangement and all modern reaction water turbines are of this design. As the swirling mass of water spins into a tighter rotation, it trys to speed up to conserve energy. This property acts on the runner, in addition to the water's falling weight. Water pressure decreases to zero as it passes through the turbine blades and gives up its energy.
In 1849, James B. Francis improved the inward flow reaction turbine to over 90% efficiency. He also conducted sophisticated tests and developed engineering methods for water turbine design. The Francis turbine, named for him, is the first modern water turbine. It is still the most widely used water turbine in the world today.
The impulse water turbine was invented in 1879 by Lester Pelton. Turgo and Crossflow turbines were later impulse designs.
The Kaplan turbine, a propeller type machine, was invented around 1913 by Victor Kaplan. It was an evolution of the Francis turbine but revolutionized the ability to develop low head hydro sites.
Theory of operation
Water turbines are divided into two groups; reaction turbines and impulse turbines.
Prior to hitting the turbine blades, the water's energy is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades and the turbine doesn't require a housing for operation.
Newton's second law describes the transfer of energy for impulse turbines.
Impulse turbines are most often used in high head applications.
Newton's third law describes the transfer of energy for reaction turbines.
Reaction turbines are most often used in low head applications.
Reaction turbines:
Turbine selection is based mostly on the available water head, and less so on the available flow rate. In general, impulse turbines are used for high head sites, and reaction turbines are used for low head heights.
The specific speed of a of a turbine can also be defined as the speed of an ideal, geometrically similar turbine, which yields one unit of discharge for one unit of head.
The specific speed of a turbine is given by the manufacturer (along with other ratings) and will always refer to the point of maximum efficiency. These allow accurate calculations to be made of the turbine's performance for a range heads and flows.
Image adapted from European CommunityÃÂs 'Layman's Guidebook (on how to develop a small hydro site)'
The specific speed is also the starting point for analytical design of a new turbine. Once the specific speed is known the basic dimensions of the turbine parts can be easily be calculated.
Impulse turbines
Impulse turbines change the direction of flow of the water stream. Water impinges on the turbine's blades which reverses the flow of water. The resulting change in momentum (impulse) causes the turbine to spin. The diverted water flow is left with diminished energy. Reaction turbines
Reaction turbines are acted on by water, which changes pressure as it moves through the turbine. Reaction turbines must be encased to contain the water pressure (or suction). Or they must be fully submerged in the water flow. Pumped storage
Some water turbines are designed for Pumped storage hydroelectricity. They can reverse flow and operate as a pump to fill a high reservoir during off-peak electrical hours, and then revert to a turbine for power generation during peak electrical demand. This type of turbine is similiar to the francis in design.Power
Water is very heavy and it's flow energetic. The power available in dammed water is;
where:
Large industrial water turbines typically operate at efficiencies greater than 90%.Types of water turbines
Design and Application
Typical range of heads
Specific speed
The specific speed, , of a turbine characterizes the turbine's shape in a way that is not related to its size. This allows a new turbine design to be scaled from an existing design of known performance. The specific speed is also the main criteria for matching a specific hydro site with the correct turbine type.
(dimensioned parameter), = rpm
(dimensionless parameter), = angular velocity (radians/second)
Example; Given a flow and head for a specific hydro site, and the rpm requirement of the generator, calculate the specific speed. The result is the main criteria for turbine selection.
Runaway speed
The runaway speed of a water turbine is its speed at full flow, and no shaft load. The turbine will be designed to survive the mechanical forces of this speed. The manufacturer will supply the runaway speed rating.