Lecture 28 Hydro : Part 2

Summary

The lecture explains how a hydropower plant converts the energy of water into electricity, describes its main components, and classifies plants by capacity, head, and construction type.

How hydropower works

A typical dam- or reservoir-based plant stores water at an elevation. As water flows toward the turbine, its potential energy becomes kinetic energy. The turbine converts that energy into mechanical energy, and a generator converts the mechanical energy into electrical energy. A transformer raises the voltage before electricity is sent to transmission lines.

The power available depends chiefly on flow rate (discharge, Q) and head (H), as well as the system’s efficiency. A high head can compensate for a lower flow, and a large flow can make a low-head plant productive. A plant with both high head and high flow can generate substantial power.

Main plant components

• Dam or intake structure: Holds back water to form a reservoir and establish the head needed to supply the plant.
• Gates and valves: Regulate how much water enters the water-conveyance system.
• Trash rack: Screens out large debris at the intake to help protect the penstock and turbine.
• Penstock: A large pipe, tunnel, or conduit that carries water to the turbine, often under pressure.
• Turbine: Converts the energy of moving water into mechanical rotation. Turbine choice depends on head and flow; examples include Pelton, Francis, and Kaplan turbines.
• Generator: Uses the turbine’s rotating shaft to produce alternating-current electricity.
• Transformer: Steps up the generated voltage for transmission.
• Draft tube: Carries water away from the turbine and helps recover pressure while reducing the velocity of discharged water.
• Tailrace: Channel or tunnel that conveys discharged water back to the river or lower reservoir.
• Spillway: Releases excess reservoir water to prevent overtopping and potential damage to the dam.
• Fishway: Provides aquatic life with a route past the dam without entering the penstock and turbine.

Classification by capacity

The lecture gives these approximate categories:

• Pico: Less than 5 kW
• Micro: 5–100 kW
• Mini: 101 kW to 2 MW
• Small: 2–25 MW
• Large: More than 25 MW

Classification by head

• Low head: Less than 30 m
• Medium head: 30–100 m
• High head: More than 100 m

These cutoffs vary among countries and references; some sources use a higher upper limit for medium head. Low-head plants are commonly associated with barrages on rivers or canals, while high-head plants are often located in hilly or mountainous areas.

Head and power calculations

• Gross head is the vertical difference between the water level at the intake or forebay and the downstream water level at the tailrace. It represents the maximum potential head before losses.
• Net head is the gross head minus losses in the canal, penstock, joints, bends, and other parts of the system. The power equation uses net head.

Lecture example:

• Forebay water level: 1,200 m above sea level
• Tailwater level: 850 m above sea level
• Gross head: 1,200 − 850 = 350 m
• Total head losses: 15 m
• Net head: 350 − 15 = 335 m

Classification by construction features

1. Valley dam / reservoir-based plant

   • A dam impounds a river and stores a large volume of water, creating a substantial head.
   • This is a widely used configuration for large-scale hydropower.
   • It typically has medium-to-high head and seasonal to multi-year storage.
   • Construction costs and environmental impacts can be high, including land inundation and ecosystem changes.

2. Run-of-river plant

   • Uses the flow of a river, often with a low weir that diverts some water through a powerhouse and returns it downstream.
   • Requires at least a small head; the lecture gives an approximate range of 2–50 m.
   • Has little or no storage, so output can fall when river levels are low.
   • Generally involves less flooding than a large reservoir project.

3. Diversion-canal plant

   • A weir diverts part of a river into a relatively low-slope, often artificial canal leading to a powerhouse.
   • The water is returned to the river or continues downstream through the canal.
   • The lecture gives an approximate head range of 5–30 m.
   • Storage is negligible, and local flow patterns can be affected.

4. High-head diversion plant

   • Diverts a stream at high elevation, often in mountainous terrain, and conveys it through a canal or tunnel to a forebay, then down a penstock to the powerhouse.
   • A desilting basin can remove sand and other heavy sediment before water enters the conveyance system.
   • The large vertical drop can produce significant power even with relatively low flow.
   • The lecture describes the head as typically starting around 200 m and extending to over 1,000 m. Storage is generally limited to small settling or forebay areas.

5. Pumped-storage hydropower (PSH)

   • Connects an upper and a lower reservoir using a hydraulic passage and reversible pump-turbines.
   • Pumping mode: During off-peak hours, surplus electricity pumps water from the lower to the upper reservoir.
   • Generating mode: During peak demand, water flows back down to generate electricity.
   • It consumes energy overall but provides a way to store energy and support grid flexibility. The lecture gives a typical head range of 100–600 m and notes its high capital cost.

Overall design considerations

The lecture contrasts large reservoirs, which offer substantial storage but can have major environmental footprints, with run-of-river and diversion projects, which generally have smaller footprints but little storage. It presents pairing lower-impact river projects with pumped storage as one way to provide both steady generation and flexibility for the grid.

Speakers or sources featured

• One lecturer or instructor, whose name is not identified in the subtitles.
• The lecture refers generally to books and other references when noting that hydropower classification thresholds can vary; no specific source is named.