A Pelton wheel is a water turbine designed to turn the energy of fast moving water into rotational motion. It is used in hydroelectric power plants where water drops from a large height before reaching the turbine. This type of site is called a high-head site, and it can produce powerful jets even if the water flow rate is not very large.
Pelton wheels matter because they are efficient renewable energy machines for mountainous regions and steep pipelines.
Key Facts
- Hydraulic power available from falling water is P = ρgQH.
- Jet speed from an ideal nozzle is v = sqrt(2gH).
- A Pelton wheel works best at high head and relatively low flow rate.
- The water jet strikes split buckets and reverses direction, transferring momentum to the wheel.
- Turbine efficiency is η = Pout / Pin.
- Electrical power output can be estimated by Pe = ηρgQH.
Vocabulary
- Pelton wheel
- A Pelton wheel is an impulse water turbine that uses high-speed jets to spin a wheel with spoon-shaped buckets.
- Head
- Head is the vertical height difference that gives water gravitational potential energy before it reaches the turbine.
- Nozzle
- A nozzle is a narrow opening that converts water pressure into a fast, focused jet.
- Impulse turbine
- An impulse turbine extracts energy mainly from the momentum of a moving fluid jet rather than from pressure inside the runner.
- Flow rate
- Flow rate is the volume of water passing a point each second, usually measured in cubic meters per second.
Common Mistakes to Avoid
- Treating a Pelton wheel like a low-head turbine is wrong because Pelton wheels are designed for high-speed jets from large height drops, not broad slow flows.
- Ignoring the nozzle is wrong because the nozzle is what turns water pressure into jet speed, which is essential for transferring momentum to the buckets.
- Assuming the water must hit the center of the wheel is wrong because the jet should strike the buckets near the rim to produce a large torque.
- Using mass instead of flow rate in P = ρgQH is wrong because hydroelectric power depends on how much water passes each second, not just on a fixed amount of water.
Practice Questions
- 1 A Pelton turbine has head H = 120 m and flow rate Q = 0.40 m3/s. Using ρ = 1000 kg/m3 and g = 9.8 m/s2, calculate the hydraulic power available.
- 2 Water exits a Pelton nozzle from a head of 80 m. Using v = sqrt(2gH) with g = 9.8 m/s2, estimate the ideal jet speed.
- 3 Explain why a Pelton wheel is better suited for a mountain stream with a large height drop than for a wide, slow river with little height difference.