Kaplan turbines were first developed in 1913 by Austrian professor Viktor Kaplan. The Kaplan turbine differs from fixed blade propeller turbines because the blades can be rotated in their mountings; the Kaplan turbine is thus a variable pitch turbine. This type of turbine has found widespread application over the past 100 years due to its high operating efficiency even at very low heads of pressure.
Kaplan Turbine Runner
Kaplan turbines convert potential energy to mechanical energy. This type of turbine is classified as a reaction turbine, as it operates within a pressure system and relies upon a continuous body of water from the suction to pressure side of the turbine.
Because of the Kaplan turbine’s ability to operate efficiently at very low heads of pressure, they are particularly well suited to run-of-the-river plants and tidal generation plants; these types of plant belong to the renewable (‘green’) energy sector.
The Kaplan turbine is very similar in design to a ship’s propeller. Compared to other prime movers such as steam turbines or combustion engines, hydro turbines have very low operating speeds, typically less than 400 rpm.
Kaplan turbines can be big. The largest Kaplan turbines in the world are 8.6m in diameter and operate with a nominal head of 34m. Despite this small head of pressure, each turbine generates 230 MW.
A propeller turbine consists of a hub, blades and shaft. A typical runner usually has three to six blades with the total runner diameter ranging from two to 11 metres. With Kaplan turbines, the blades connect to a central hub which houses the mechanisms needed to rotate the blades, but there are no other differences between a fixed blade propeller turbine and a Kaplan turbine.
A spiral case -also known as a scroll case- is used to deliver an even flow of water to the entire runner. Even flow is achieved due to the gradually decreasing cross-sectional area of the casing. As the cross-sectional area decreases, the water velocity remains constant and an even flow of water is delivered to the runner.
The wicket gate directs the water towards the Kaplan runner. The wicket gate is used to start, stop and regulate the flow of water to the runner.
A draft tube converts some of the discharged water’s kinetic energy back into pressure energy. This conversion increases the overall operating efficiency of the turbine.
How Kaplan Turbines Work
Water enters through a pressurised water conductor, known as the penstock. It then flows along the spiral case and through the wicket gate. The wicket gate directs the water flow tangentially across the runner blades and a resultant force is applied to the runner. This force applies itself as torque on the runner shaft, which causes the runner to rotate.
Kaplan Turbine Components
The water then leaves the runner and enters the draft tube where some of its remaining kinetic energy is recovered as pressure energy. Finally, the water is discharged to the tailrace.
Notice that it is only the runner blades which convert the potential and kinetic energy of the water into mechanical energy. This is achieved due to the shape of the blades and the pressure difference created as the water flows along the blade’s surface.
A common shaft connects the runner to a generator, as the runner rotates, so too does the generator rotor. The generator rotor rotates within an electromagnetic field, as the rotor moves through the magnetic field, current is induced in the generator stator windings, at this point the mechanical energy supplied by the Kaplan turbine has been converted to electrical energy. The electrical energy can now be transferred through a national grid to end consumers.
The entire power generation process is continuous, which leads to a constant, renewable and reliable form of power generation.
Compared to other types of hydro turbine, such as the Pelton and Francis turbines, Kaplan turbines are able to operate at very low heads of pressure, and high flow rates. Operating efficiencies exceeding 90% are not uncommon.
Kaplan turbines are reaction turbines, they operate with a full body of water on both the suction and pressure sides of the turbine. Reaction turbines are pressure turbines, whereas impulse turbines are pressure-less.
Water passes over the propeller runner in a parallel direction to the runner shaft. This type of flow is known as axial flow, which is why propeller turbines are classified as axial flow turbines.
Axial Flow Impeller (Runner)
Rotating the Kaplan runner blades changes the angle of attack (the angle at which the blade slices through the water), which increases the turbines efficiency when operating at varying flow rates. The angle of attack is referred to as the pitch, this is where the Kaplan variable pitch propeller (VPP) gets its name.
Varying the pitch also varies the speed of the turbine and consequently the amount of potential energy that can be converted to mechanical energy. Combined with a variable wicket gate, the operating conditions of a Kaplan turbine can be tightly controlled to maximise efficiency.