Hydro-electric plants are classified based on several criteria, with the available water head and their role in the electrical grid (load) being two of the most important.
Here is a detailed classification based on these two factors.
1. Classification Based on Head
The "head" is the vertical distance between the water level in the reservoir (the headrace) and the water level at the turbine outlet (the tailrace). It is a critical factor because the potential power output of a plant is directly proportional to the head.
$Power \propto Head \times Flow \, Rate$
Based on the available head, plants are categorized into three main types:
a) High-Head Plants
- Head Range: Typically above 300 meters (approx. 1000 feet).
- Characteristics:
- They require a relatively small quantity of water to generate a large amount of power due to the very high pressure.
- Often located in mountainous regions.
- Water is carried from a reservoir through a long pipe called a penstock to the powerhouse located at a much lower elevation.
- Turbine Used: Pelton Wheel (Impulse Turbine). This turbine is designed like a wheel with buckets, which is ideal for converting the high kinetic energy of a powerful water jet into rotational motion.
- Example: Bhakra Dam in India, Grande Dixence Dam in Switzerland.
b) Medium-Head Plants
- Head Range: Typically between 30 meters and 300 meters (approx. 100 to 1000 feet).
- Characteristics:
- These are the most common type of hydroelectric plants.
- They usually require a large dam to create a significant reservoir, impounding a river in a valley or canyon.
- Turbine Used: Francis Turbine (Reaction Turbine). This is a highly versatile and efficient turbine that works well for a wide range of head and flow conditions, making it perfect for this category.
- Example: Hoover Dam in the USA, Three Gorges Dam in China.
c) Low-Head Plants
- Head Range: Typically below 30 meters (approx. 100 feet).
- Characteristics:
- These plants require a very large volume of water to compensate for the low head.
- They are often "run-of-the-river" plants, which means they use the natural flow of a river with little or no water storage. A small dam or weir is used to create the necessary head.
- Turbine Used: Kaplan or Propeller Turbine (Reaction Turbine). These turbines look like a ship's propeller and are designed to handle very high flow rates at low pressure efficiently. Kaplan turbines have adjustable blades to maintain efficiency over a range of flow conditions.
- Example: Many plants on the Columbia River in the USA, such as the Bonneville Dam.
2. Classification Based on Load
This classification describes how the plant is operated to meet the electricity demand (load) of the power grid. The demand for electricity is not constant; it varies throughout the day and across seasons.
a) Base Load Plants
- Function: To provide a continuous, steady amount of power to the grid to meet the minimum level of demand.
- Operational Characteristics:
- These plants run continuously, 24/7, at or near their full capacity.
- They must have a reliable and consistent water supply, which usually means they are large reservoir-based plants or large run-of-the-river plants on major rivers with consistent flow.
- Example: Large-scale plants like the Three Gorges Dam or run-of-the-river plants on major rivers often serve as base load suppliers due to their consistent output.
b) Peak Load (or Peaking) Plants
- Function: To operate only during periods of high (peak) electricity demand, such as hot afternoons or evenings.
- Operational Characteristics:
- They must be able to start up, shut down, and change their power output very quickly. Hydroelectric plants are excellent for this role, as they can go from zero to full power in minutes, unlike thermal plants which can take hours.
- They typically operate for only a few hours a day.
- Example: Pumped-Storage Hydroelectric (PSH) plants are the quintessential example.
- How they work: During periods of low demand (e.g., at night), they use cheap electricity from the grid to pump water from a lower reservoir to an upper reservoir. During peak demand, they release this water from the upper reservoir to generate electricity, acting like a giant, rechargeable battery.
c) Intermediate Load Plants
- Function: To cover the demand that is in between the base load and the peak load.
- Operational Characteristics:
- These plants operate for longer durations than peaking plants but not continuously like base load plants.
- Reservoir-based plants with some storage capacity can be operated flexibly to serve as intermediate load plants, adjusting their output as demand changes throughout the day.
Summary Table
| Classification | Head Range | Typical Turbine | Load Role | Key Characteristics |
| :--- | :--- | :--- | :--- | :--- |
| High-Head | > 300 m | Pelton Wheel | Base or Peak | High pressure, low flow; often in mountains. |
| Medium-Head | 30 - 300 m | Francis Turbine | Base or Intermediate | Most common type; requires a large dam and reservoir. |
| Low-Head | < 30 m | Kaplan / Propeller | Base Load | Low pressure, high flow; often run-of-the-river. |
| Pumped-Storage | Varies | Reversible Francis | Peak Load | Acts like a grid-scale battery; highly flexible. |
It's important to note that these classifications are not mutually exclusive. For example, a medium-head plant (classification by head) can be operated as a base load plant (classification by load). A high-head plant could be a traditional power plant or a pumped-storage peaking plant.
