Trystan Casey
The Pelton wheel, invented by American Lester Allan Pelton in the 1870s, is a type of impulse turbine used to generate electricity in hydropower plants. It is designed to work with a high hydraulic head and low flow rate, making it suitable for mountainous regions. The Pelton wheel converts the kinetic energy of water into mechanical energy, which is then converted into electrical energy by a generator. The wheel has a series of curved buckets or blades mounted on its periphery that capture the water jet and transfer its momentum to the wheel, spinning it and generating electricity through the connected generator.
| Characteristics | Values |
|---|---|
| Type of turbine | Impulse |
| Energy source | Moving water |
| Water head | 150-2000m |
| Water flow | Low |
| Water velocity | High |
| Runner | Series of curved buckets mounted on the outer periphery of a wheel |
| Bucket shape | Double hemispherical cup or bowl; two halves separated by a high ridge |
| Bucket function | Capture water jet, direct flow backwards, convert kinetic energy to mechanical energy |
| Spear/needle | Controls the flow of water through the nozzle |
| Efficiency | Up to 95% |
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What You'll Learn
The role of impulse energy
The Pelton wheel, named after its inventor Lester Allan Pelton, is an impulse-type water turbine designed in the 1870s. It is a highly efficient mechanism for converting water energy into electricity, particularly in geographical locations with high hydraulic head and low flow rates. This makes it ideal for hydroelectric power generation, a significant form of renewable energy.
The Pelton wheel's unique design allows it to harness the impulse energy of moving water, marking a departure from traditional water wheels that relied on water's dead weight. The wheel consists of a circular disc with specially crafted buckets mounted on its periphery. These buckets, also known as impulse blades, are typically spoon-shaped and fixed side-by-side. Nozzles play a crucial role in enhancing the kinetic energy of the water directed at these buckets.
The high-speed water streams from the nozzles strike the impulse blades, causing the water's velocity to change direction and follow the contours of the blades. This change in velocity results in the impulse energy of the water jet exerting torque on the bucket-and-wheel system. Consequently, the wheel spins, and the impulse energy is transferred to a turbine, generating mechanical energy.
The innovative bucket design, with a divider known as a splitter, ensures that the water jet is split into two equal streams. This design feature eliminates counter-torque by preventing the water jet from pushing against the incoming flow. As a result, the Pelton wheel achieves maximum power and efficiency when the velocity of the water jet is twice that of the rotating buckets. This efficient conversion of impulse energy into mechanical energy is what ultimately enables the Pelton wheel to produce electrical power.
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Bucket and wheel system
The Pelton wheel, invented by American inventor Lester Allan Pelton in the 1870s, is an impulse-type water turbine that extracts energy from the impulse of moving water. Water is directed from the source through a nozzle to form a high-speed water jet. The nozzle enhances the kinetic energy of the water that strikes the buckets or blades connected to the runner, also known as the drive wheel. This nozzle can be controlled by a spear, which can be moved backward and forward in an axial direction to adjust the flow of water.
The bucket and wheel system is a crucial component of the Pelton wheel's operation. The buckets, also known as impulse blades, are typically mounted in pairs and positioned around the outer rim of the drive wheel. The water jet is split into two equal streams, ensuring a smooth and efficient transfer of momentum from the water jet to the turbine wheel. The kinetic energy of the water jet is reduced when it hits the curved buckets, and the spherical shape of the buckets causes the water to change direction and exit at the outer sides of the bucket, decelerated to a low velocity.
The impulse energy of the water jet exerts torque on the bucket-and-wheel system, spinning the wheel. As the water jet hits the blades, the direction of water velocity follows the contours of the blades, causing the wheel to rotate. The water jet's momentum is efficiently transferred to the wheel and then to a turbine, converting the impulse energy into mechanical work. This mechanical energy can then be converted into electrical energy in a hydroelectric power plant.
The geometry of the Pelton wheel is inherently a "low gear" design, making it suitable for low flow and/or high-pressure water sources. The size of Pelton wheels varies, with the smallest being just a few inches across, suitable for tapping power from mountain streams. Larger units, such as the Bieudron Hydroelectric Power Station in Switzerland, can generate over 400 megawatts of power. The efficiency of the Pelton wheel is optimized when the velocity of the water jet is twice the velocity of the rotating buckets.
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Runner and generator
The Pelton wheel, invented by American inventor Lester Allan Pelton in the 1870s, is an impulse-type water turbine that extracts energy from the impulse of moving water. Water is directed from a nozzle in high-speed streams at a series of spoon-shaped buckets, also known as impulse blades, which are mounted around the outer rim of a drive wheel (also called a runner). The impulse energy of the water jet exerts torque on the bucket-and-wheel system, spinning the wheel. The water jet's momentum is transferred to the wheel and hence to a turbine, which converts mechanical energy into electrical energy.
The runner is a circular disc on the periphery of which several buckets are evenly aligned and fixed. The buckets are double hemispherical cups or bowls, each divided into two symmetrical parts by a dividing wall known as a splitter. The runner is designed to fit a 15 mm shaft, and its correct attachment to the generator shaft is important. Stainless steel or galvanised washers of at least 25 mm outside diameter should be used on both sides of the wheel to distribute pressure evenly over the casting. A spring washer is essential, and the nut should be tightened to 6.5 N/m torque (firm with a 160 mm spanner).
The alignment of the water jet is important to achieve maximum efficiency and to prevent possible damage to the blade edge of the runner buckets. The nozzle must be adjusted so that the jet strikes the wheel tangentially 57 mm from the centre of the wheel, following the contours of the blades. The amount of water hitting the runner’s buckets is controlled by a spear in the nozzle, which can be pushed forward or back to decrease or increase the volume of water.
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Hydraulic efficiency
The Pelton wheel, invented by American inventor Lester Allan Pelton in the 1870s, is an impulse-type water turbine that extracts energy from the impulse of moving water. This is in contrast to the traditional overshot water wheel, which relies on the dead weight of water.
The Pelton wheel is a type of hydraulic turbine, and its primary application is in hydropower plants for producing electricity. Pelton wheels are the preferred turbine for hydropower when the accessible water source has a high hydraulic head at low flow rates. The hydraulic head in hydroelectric power plants ranges from 150 meters to 2000 meters.
The efficiency of a Pelton wheel is influenced by several factors, including the design of the turbine blades and the speed ratio between the water jet and the rotating buckets. To achieve maximum efficiency, the turbine is designed to ensure that the velocity of the water jet is twice that of the rotating buckets. This allows for the extraction of almost all the water's impulse energy, resulting in a highly efficient turbine.
The formula for hydraulic efficiency, denoted as ηH, takes into account various parameters such as the velocity of the water jet (Vw1 and Vw2), the speed of the water jet relative to the inlet jet velocity (u), the density of water (ρ), the gravitational acceleration (g), the flow rate (Q), and the hydraulic head (H). The formula for hydraulic efficiency is as follows:
> ηH = \(\frac{R.P}{W.P}~=~\frac{\dot{m} (V_{w_1}~+~V_{w_2})~\times~u}{\rho gQH}\)
By maximizing the hydraulic efficiency of the Pelton wheel, the amount of electricity generated in hydropower plants can be optimized.
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Spear and nozzle
The Pelton wheel is a type of impulse turbine that is used for high heads of water, typically greater than 300 meters. It is the preferred turbine for hydropower when the available water source has a high hydraulic head and low flow rates. The wheel was invented by American Lester Allan Pelton in the 1870s and was designed to work with the small flow found in streams near mines.
The Pelton wheel converts the pressure energy of water into kinetic energy, forming a high-speed water jet that strikes the wheel tangentially, causing it to rotate. This kinetic energy is enhanced by a nozzle, which directs a forceful, high-speed stream of water against a series of spoon-shaped buckets or vanes, also known as impulse blades, mounted around the outer rim of a drive wheel (or runner). The amount of water hitting the runner's buckets is controlled by a spear, a conical needle, that is moved backward and forward inside the nozzle in an axial direction. When the spear is pushed forward, the volume of water striking the runner is reduced, and when it is pushed back, the amount of water increases.
The spear and nozzle work together to optimise the function of the Pelton wheel. Computational fluid dynamics (CFD) is a branch of fluid mechanics that can be used to simulate the interaction of liquids and gases with surfaces, and it can be applied to the nozzle and spear assembly of the Pelton wheel. By computing and comparing different flow coefficients of various geometric configurations, the optimal shape of the nozzle and spear can be determined to ensure the most efficient transfer of momentum from the water jet to the turbine wheel.
The nozzle and spear assembly play a critical role in controlling the water jet and optimising the performance of the Pelton wheel. The spear's movement within the nozzle allows for precise adjustments to the volume of water, ensuring that the wheel rotates efficiently and that the maximum power output is achieved.
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Frequently asked questions
A Pelton wheel is an impulse-type water turbine invented by American inventor Lester Allan Pelton in the 1870s.
The Pelton wheel uses a nozzle to direct high-speed streams of water against a series of spoon-shaped buckets mounted around a drive wheel. The water jet's momentum is transferred to the wheel and hence to a turbine, spinning the wheel.
The bucket, also known as an impulse blade, catches the water jet and directs the flow gradually backwards. The bucket design allows almost all of the kinetic energy from the water to be harnessed as torque prior to discharge.
The Pelton wheel is designed to work with a low ratio of flow to pressure, meaning relatively low flow and/or relatively high pressure. It is best suited for high-head and low-flow-rate applications.
The Pelton wheel converts the kinetic energy of water into mechanical energy, which is then converted into electrical energy by a generator.
