Understanding Acting Head in Hydro-Electric Plants

A hydro-electric power plant converts the potential energy of water stored at a height into electrical energy using a turbine connected to a generator. The performance of the turbine depends significantly on the 'head' of the water, which is the height difference between the water source and the turbine outlet. However, the effective head that actually acts on the turbine, also known as the acting head or effective head, is influenced by various components in the system.

Components and Their Role in Hydro-Electric Plants

Let's look at the function of each component listed in the options:

• Penstock: This is a pipe or conduit that carries water under pressure from the reservoir or surge tank to the turbine. Its primary function is to convey water safely and efficiently, minimizing head loss due to friction. It utilizes the available gross head but doesn't actively increase the acting head on the water wheel beyond what is available from the source.
• Draft tube: This is a diverging cone-shaped tube that connects the outlet of a reaction turbine (like a Francis or Kaplan turbine) to the tailrace (the channel where water flows out after passing through the turbine). Its main purpose is to recover the kinetic energy of the water leaving the turbine and convert it into pressure energy. By creating a negative pressure (below atmospheric) at the turbine outlet, the draft tube effectively increases the pressure difference across the turbine, thereby increasing the acting head and allowing the turbine to be placed above the tailrace level without losing head.
• Surge tank: This is a vertical cylinder or reservoir connected to the penstock before the turbine. Its function is to absorb pressure fluctuations caused by sudden changes in water flow demand (e.g., when the turbine load changes and the gates close or open). It protects the penstock from water hammer effects but does not increase the acting head on the water wheel.
• Reservoir: This is a large artificial lake where water is stored. It provides the initial potential energy by creating a significant difference in water level between the reservoir and the power plant (gross head). While essential for providing the initial head, it does not specifically increase the 'acting' head on the water wheel beyond the maximum available gross head minus losses.

Increasing Acting Head with the Draft Tube

The question asks which component is used to increase the acting head on the water wheel. While the reservoir provides the gross head, losses occur in the penstock and other components. The draft tube is unique because it recovers some of the energy that would otherwise be lost at the turbine exit (kinetic energy) and converts it back into usable pressure head. By creating a lower pressure at the turbine outlet than the tailrace pressure, it effectively 'pulls' the water through the turbine and increases the effective pressure difference, thereby increasing the acting head on the water wheel. This recovery of kinetic energy can significantly improve the overall efficiency of the hydro-electric power plant.

Therefore, the draft tube is the component that is specifically designed to increase the acting head on the water wheel by recovering kinetic energy from the turbine outlet.