What is the efficiency of Francis turbine?

Short Answer:

The efficiency of a Francis turbine is the ratio of the useful mechanical power obtained at the shaft to the total hydraulic power supplied by the water. It indicates how effectively the turbine converts water energy into mechanical energy. The efficiency of a well-designed Francis turbine generally lies between 85% and 95%, depending on head, discharge, and turbine design.

In simple words, higher efficiency means the turbine converts more water energy into useful mechanical work. Proper design of blades, guide vanes, and casing helps in achieving maximum energy conversion in Francis turbines.

Detailed Explanation :

Efficiency of Francis Turbine

The efficiency of a Francis turbine represents the performance of the turbine in converting the hydraulic energy of flowing water into useful mechanical power at the turbine shaft. It is an important parameter to measure how efficiently the turbine utilizes the available water head and discharge to generate power.

The Francis turbine is a reaction turbine, which means both pressure energy and kinetic energy of water are used to rotate the runner. It operates efficiently over a wide range of heads (30 m to 600 m) and discharges, making it one of the most widely used turbines in hydroelectric power stations.

Definition of Efficiency

The efficiency of a Francis turbine is defined as:

where,

  • Power supplied by water (Input power) =
  • Power developed by runner (Output power) = torque × angular velocity

Here,
= Density of water (kg/m³)
= Acceleration due to gravity (m/s²)
= Discharge (m³/s)
= Net head (m)

The efficiency shows how much of the total water energy is successfully converted into mechanical power.

Types of Efficiency in Francis Turbine

Efficiency of a Francis turbine can be divided into three main types to understand performance at different stages of energy conversion:

  1. Hydraulic Efficiency (ηₕ):
    It is the ratio of the power delivered to the runner by the water to the power supplied by the water at the inlet.

It depends on the velocity of water, blade angle, and flow conditions inside the runner.

  1. Mechanical Efficiency (ηₘ):
    It is the ratio of the power available at the turbine shaft to the power developed by the runner.

This efficiency considers frictional losses in bearings and mechanical parts.

  1. Overall Efficiency (η₀):
    The overall efficiency is the ratio of the actual power available at the turbine shaft to the total water power supplied at the turbine inlet.

It indicates the total energy conversion efficiency of the turbine.

Typical Efficiency Values

For a properly designed and well-maintained Francis turbine:

  • Hydraulic efficiency: 85% to 90%
  • Mechanical efficiency: 95% to 98%
  • Overall efficiency: 85% to 95%

These high efficiency values make the Francis turbine ideal for medium-head hydroelectric plants.

Factors Affecting Efficiency

  1. Design of Runner Blades:
    The shape and angle of the runner blades play a major role in converting the water energy efficiently. Proper curvature ensures smooth flow and minimizes turbulence.
  2. Guide Vanes and Flow Regulation:
    Guide vanes control the flow direction and discharge entering the runner. Correct adjustment maintains optimal flow angle and reduces hydraulic losses.
  3. Head and Discharge Conditions:
    The turbine operates at maximum efficiency only under the designed head and discharge conditions. Large variations reduce performance.
  4. Frictional and Leakage Losses:
    Losses in bearings, seals, and water passages reduce mechanical and volumetric efficiency. Proper maintenance minimizes these losses.
  5. Cavitation:
    Cavitation occurs when the pressure inside the turbine drops below the vapor pressure of water, forming bubbles that damage the runner blades. Cavitation reduces efficiency and life of the turbine.
  6. Wear and Maintenance:
    Over time, rough surfaces and damaged blades increase energy losses. Regular maintenance ensures smooth operation and high efficiency.

Expression for Efficiency

If,
= input power (W)
and
= output power (W)

Then,

Example:
If water head (H) = 100 m, discharge (Q) = 5 m³/s, and shaft power = 4 MW, then
Input power =
Efficiency =

Improving Efficiency

  • Design blades with smooth, optimized curvature.
  • Use adjustable guide vanes for better flow control.
  • Maintain proper head and discharge conditions.
  • Prevent cavitation by maintaining sufficient submergence.
  • Conduct regular cleaning and lubrication of moving parts.

Advantages of High Efficiency

  • Reduces energy losses.
  • Improves overall power generation.
  • Extends the life of turbine components.
  • Decreases operational costs and maintenance frequency.
Conclusion

The efficiency of a Francis turbine measures how effectively it converts hydraulic energy into mechanical energy. It mainly depends on runner design, flow control, and maintenance. With an overall efficiency range of 85% to 95%, the Francis turbine is one of the most efficient and widely used reaction turbines in hydroelectric power generation. Proper design and operation ensure maximum performance and long service life.