Types of Hydropower and their Structural Development

Kathmandu : As part of our weekly series on ‘Urja Gyan’, published every Friday, this week’s topic covers the different types of hydropower. Last week, we explained what  is hydropower ? Today, we aim to provide some general information on its types, structures, and the equipment involved.

1. Types of Hydropower

Hydropower is primarily classified into four types as follows:

(a) Run-of-River (RoR)

In this type of project, no dam is constructed to store water. The electricity is produced based on the natural flow of the river. Most private sector projects in Nepal fall under this category. The Upper Trishuli-3A (60 MW), built by the Nepal Electricity Authority (NEA), is an example of a run-of-river project. Other private sector projects like Sunkoshi, Mai Khola, and Super Dordi are also classified as RoR.

(b) Peaking Run-of-River (PRoR)

This type involves the construction of a dam that stores water for a few hours daily. The stored water is used to meet electricity demand during peak hours. Projects of this type are designed to store water for about 2 to 6 hours. Notable examples include NEA's 144 MW Kali Gandaki 'A' (6 hours peaking), 70 MW Middle Marsyangdi, and 69 MW Marsyangdi.

(c) Reservoir

In a reservoir-based project, a large dam is built to store water, and electricity is produced as per demand. A prominent example is the Kulekhani Hydropower Project, where water from nine streams is collected in the Kulekhani reservoir to generate electricity for three power plants: Kulekhani-1 (60 MW), Kulekhani-2 (32 MW), and Kulekhani-3 (14 MW). Since Kulekhani-2 and Kulekhani-3 use the water from Kulekhani-1, this is referred to as a ‘cascade’ system.

(d) Pumped Storage

Pumped storage is another form of energy storage that uses a reservoir. These projects use the height difference between two reservoirs to generate electricity. Water is released from the upper to the lower reservoir through turbines to generate power, and then pumped back to the upper reservoir when necessary. This system is considered one of the best energy storage options. Although no such projects exist in Nepal yet, plans are underway to develop a 200 MW pumped storage project as part of the 870 MW Dudhkoshi Reservoir project. Other potential sites, such as Kulekhani-Sisneri and Rupa-Begnas in Pokhara, have also been identified.

2. Key Structures of Hydropower

Dam: Dams are built to store water. The size and type of the dam depend on the topography. A higher dam stores more water and increases the head, which boosts the power capacity of the plant. However, taller dams may cause flooding and other challenges.

Spillway: A spillway is a structure built to release excess water from the dam to prevent damage to the surrounding infrastructure.

Reservoir: The stored water behind a dam is called the reservoir. It collects rainwater and is used for electricity generation during dry periods.

Trash Rack: Trash racks are filters installed at the intake to prevent debris, such as tree branches or dead animals, from entering the waterway system and causing damage to turbines.

Headrace Tunnel: This tunnel directs water from the reservoir to the penstock.

Forebay: A small reservoir before the penstock that balances sudden load changes by providing or storing additional water.

Penstock: A pipe that carries water from the forebay to the turbine. It is usually made of steel, though fiber glass is used for smaller plants.

Turbine: This device converts the energy of the water into mechanical power.

Draft Tube: A draft tube is installed at the exit of reaction turbines (like Francis turbines). It helps convert the residual energy of water into mechanical power and discharges the water back into the river.

Generator: The generator, connected to the turbine.

Power Transformer: A power transformer is a device that increases the generated voltage (step-up) to transmit electricity over long distances. By stepping up the voltage, transmission losses are reduced, and consumers receive high-quality electricity.

Tailrace Tunnel: After the water flows through the penstock and turns the turbine, it exits through a channel called the tailrace tunnel or water discharge tunnel. If the water flows out through a canal and returns to the river, it is called a tailrace canal. If it exits through a tunnel, it is referred to as a tailrace tunnel.

Intake Structure: At the intake structure, equipment like trash racks and gates are installed to control the flow of water from the forebay or reservoir into the penstock. All these components together are known as the intake structure.

Desander: During the rainy season, water carries a large quantity of sand particles, which can erode and wear down turbine components. To mitigate this, a desander or settling basin is built to separate sand and other particles from the water before it reaches the turbine.

Multiple-Choice Questions and Answers

1. Which hydropower plant operates as a RoR project?

(a) Marsyangdi
(b) Panauti
(c) Both (a) and (b)
(d) Kulekhani-3
Answer: (b) Panauti

2. According to the Alternative Energy Promotion Centre, what is the maximum capacity of a pico-hydropower plant?

(a) 20 kW
(b) 10 kW
(c) 40 kW
(d) 100 kW
Answer: 10 kW

3. What material is typically used to construct penstocks in hydropower plants?

(a) Cast iron
(b) Mild steel
(c) PVC
(d) Concrete
Answer: Mild steel

(Note: The penstock for the 2.5 MW Sunkoshi Small Hydropower Plant in Sindhupalchok is made of fiberglass, but generally, mild steel is used.)

4. Which hydropower plant was the first built by the Nepal Electricity Authority through its subsidiary, using domestic investment?

(a) Middle Marsyangdi
(b) Devighat
(c) Chilime
(d) Gandak
Answer: Chilime

5. What is the device called that generates electricity in a power plant?

(a) Turbine
(b) Governor
(c) Voltage Regulator
(d) Alternator
Answer: Alternator

6. What type of project is referred to as a reservoir-based project?

(a) One where water cannot be stored
(b) One where water can be stored for a short period
(c) One where water can be used continuously
(d) One where water and machinery can be stored for a long time
Answer: One where water and machinery can be stored for a long time.