Topic 6.9 Notes – Hydroelectric Power

What Hydroelectric Power Is

Hydroelectric power uses moving water to spin a turbine connected to a generator, which makes electricity.

The basic dam setup below gives you the big picture before you get into the energy changes.

The energy changes happen in a chain:

• Gravitational potential energy sits in water held at a higher elevation.
• As water moves downhill, that becomes kinetic energy.
• Flowing water spins a turbine, creating mechanical energy.
• The generator turns that mechanical energy into electrical energy.

A couple of APES reminders matter here:

• Renewable means the water source is naturally replenished by the hydrologic cycle. Water is not burned like coal or oil.
• Hydropower does not only mean dams. Any system using moving water to turn a turbine counts.
• Clean in this course means no direct air pollution or waste during generation. It does not mean no environmental impact.

That last point sets up the whole topic. Hydropower avoids fuel combustion, but the way humans control water can strongly change ecosystems.

Main Types of Hydroelectric Systems

Reservoir dam systems

A dam built across a river creates a reservoir. The stored water sits higher than the river below, creating head, which means vertical drop.

Water moves through:

1. reservoir
2. intake
3. penstock
4. turbine
5. generator
6. river below the dam

Because water is stored, reservoir systems give the most control over when electricity is generated. That matters on grids when demand changes quickly.

They can also provide other services:

• flood control
• irrigation water
• drinking water
• navigation
• recreation

Run-of-river and small-stream systems

These systems use a river’s natural flow, or divert part of that flow through a turbine.

• They have little or no water storage
• They usually flood less land than large reservoir projects
• Their output depends more on current streamflow

So if drought hits or the dry season lowers flow, electricity production drops more directly.

Tidal energy

Tidal energy uses moving seawater to turn turbines.

Two forms you should know:

• underwater tidal turbines
• tidal barrages built across an estuary or bay

Tides are driven by the gravitational pull of the Moon and Sun. That makes tidal energy highly predictable, but it only works in suitable coastal places.

What Determines Hydroelectric Output and How It Works

The biggest factors are:

• flow rate of water
• head or vertical drop
• efficiency of the turbine and generator

General rule:

• more flowing water + greater head = more electricity

For a reservoir system, the sequence is straightforward:

1. Water is stored behind the dam.
2. Water is released through the intake and penstock.
3. Moving water spins the turbine.
4. The generator produces electricity.
5. Water returns downstream.

Reservoir systems can respond quickly to changing demand because operators can release stored water when needed. Run-of-river systems have less control because they store little water. Tidal systems are predictable, but there are fewer places to build them.

Advantages of Hydroelectric Power

Hydropower’s main advantage is that it generates electricity without burning fuel.

That means no direct release of:

• sulfur dioxide
• nitrogen oxides
• particulate matter
• mercury
• carbon dioxide from combustion

It also produces:

• no coal ash
• no radioactive waste from generation

Other advantages:

• renewable because river flow and tides are replenished
• low operational greenhouse gas emissions compared with fossil fuels
• reliable and controllable in reservoir systems
• can support flood control, water supply, irrigation, navigation, and recreation

This is the kind of evidence you’d use on an FRQ when justifying hydropower as an environmental solution.

Environmental Costs and Trade-Offs

The core APES trade-off is simple. Hydropower avoids air pollution from combustion, but the structures used to control water can damage ecosystems and communities.

Habitat and river disruption

Reservoirs can flood:

• terrestrial habitats
• riparian habitats
• wetlands
• river habitats

Dams also fragment rivers and block fish migration.

• Pacific Northwest dams and salmon migration is the classic example.
• Common mitigation includes fish ladders, fish elevators, bypass channels, and intake screens.

A fish ladder like the one shown here is designed to help migratory fish move past a dam in stages.

Fish ladder at a dam

Changes downstream

Dams change natural river patterns.

• Altered flow regimes can disrupt wetlands, flood pulses, and species life cycles.
• Sediment gets trapped in reservoirs, which reduces reservoir capacity and deprives downstream habitats of sediment and nutrients.
• Released water may have different temperature and dissolved oxygen levels than natural river water.

Human and economic costs

• Construction is expensive
• Large projects use a lot of land
• Communities, farmland, forests, and cultural sites may be flooded or displaced
• Reservoirs can increase evaporation losses
• Dam failure can cause catastrophic flooding

Type comparisons and examples

Illustrative examples you should know:

• Hoover Dam on the Colorado River
• Three Gorges Dam on the Yangtze River