The selection of a specific turbine for a hydropower plant depends primarily on two factors:
Water Head: The vertical distance the water falls from the reservoir to the turbine.
Flow Rate: The volume of water passing through the turbine per unit of time.
Turbines are broadly classified into two main categories: Impulse Turbines and Reaction Turbines.
1. Impulse Turbines
Impulse turbines work by using the velocity of water to move the runner (the rotating part of the turbine). A high-speed jet of water strikes the turbine's buckets or blades, causing them to spin. These are best suited for high head and low flow rate applications.
a) Pelton Turbine (or Pelton Wheel)
- Description: This is the most common type of impulse turbine. It consists of a wheel (runner) with a series of spoon-shaped buckets mounted on its edge. One or more high-pressure nozzles shoot jets of water onto these buckets, causing the wheel to rotate.
- Best For: Very high head (250m to 1800m) and low flow rates.
- Analogy: Think of a modern, highly efficient water wheel.
b) Turgo Turbine
- Description: A variation of the Pelton turbine. The jet of water strikes the runner at an angle (typically 20°) and flows across the blades, exiting on the opposite side. This design allows it to handle a higher flow rate than a Pelton turbine of the same diameter.
- Best For: Medium to high head (50m to 250m) and a wider range of flow rates than the Pelton.
c) Cross-flow Turbine (also known as Michell-Banki or Ossberger Turbine)
- Description: This turbine has a drum-shaped runner with curved vanes. Water flows through the blades twice: first from the outside into the center, and then from the center back to the outside. It is known for its simple construction and good efficiency over a wide range of flow conditions.
- Best For: Low to medium head (3m to 200m) and highly variable flow rates, making it ideal for smaller, remote, or micro-hydro installations.
2. Reaction Turbines
Reaction turbines are fully submerged in water and generate power from both the pressure and the velocity of the moving water. As water flows through the turbine, its pressure and velocity decrease, imparting energy to the runner. These are best suited for low to medium head and high flow rate applications.
a) Francis Turbine
- Description: This is the most widely used turbine in the world. Water enters the turbine radially (from the sides) and exits axially (downwards). It has a complex design with fixed runner blades and adjustable "guide vanes" that control the water flow, allowing it to operate efficiently over a wide range of conditions.
- Best For: Medium head (40m to 600m) and medium to large flow rates. It is the workhorse of the hydropower industry.
b) Kaplan Turbine
- Description: This turbine looks like a ship's propeller. It has adjustable runner blades and adjustable guide vanes, allowing it to maintain high efficiency even with varying head and flow rates. Water flows through the runner axially.
- Best For: Low head (2m to 40m) and very high flow rates. Commonly used in run-of-the-river power plants.
c) Propeller Turbine
- Description: A simpler version of the Kaplan turbine where the runner blades are fixed (not adjustable). It is less expensive but is only efficient within a narrow range of flow conditions.
- Best For: Low head and a consistent, high flow rate.
d) Bulb/Tubular Turbine
- Description: This is a special configuration of a Kaplan or Propeller turbine where the turbine and generator are sealed in a waterproof bulb-shaped casing placed directly in the water stream. This design minimizes civil works and is highly efficient.
- Best For: Very low head (2m to 20m) and high flow rates, such as in tidal barrages or run-of-the-river projects.
Summary Table for Quick Comparison
| Turbine Type | Category | Head Range | Flow Rate | Key Feature |
|-------------------|----------|-----------------|-----------------|-------------------------------------------|
| Pelton | Impulse | High (250m+) | Low | Spoon-shaped buckets hit by water jets. |
| Turgo | Impulse | Medium-High | Low to Medium | Angled jet, handles more flow than Pelton.|
| Cross-flow | Impulse | Low to Medium | Low to Medium | Simple, good for variable flow. |
| Francis | Reaction | Medium (40-600m)| Medium to High | Most common, highly versatile. |
| Kaplan | Reaction | Low (2-40m) | High to Very High| Propeller-like with adjustable blades. |
| Propeller | Reaction | Low | High (Constant) | Simpler version of Kaplan with fixed blades.|
| Bulb/Tubular | Reaction | Very Low (2-20m)| High to Very High| Generator and turbine in a submerged pod. |
