📖 Chapter 5 of 11 · Green Energy - Advance 1 · Class 8
Chapter 5: How Does Hydropower Work — and What Does It Cost a River?
In this chapter
- Head and flow: the two secrets of hydropower.
- Inside a hydro station: reservoir, penstock, powerhouse.
- Pumped storage: a mountain-sized battery.
- The honest side: people, rivers and climate risks. Meet Pranav and Dechen.
The Big Idea
A hydro plant’s power depends on two things: the head (how far the water falls) and the flow (how much water passes each second). Hydropower is clean, reliable and can start within minutes, which makes it valuable for balancing the grid. But large dams also flood land, move people, and change the life of rivers, so they must be planned with great care.
Did You Know?
Large and small hydropower together give India more than 50 GW of capacity. Pumped storage plants, which pump water uphill when power is spare and release it when power is needed, are being built in many states to store solar energy.
Story Time — Pranav and the Two Lakes
Pranav, age 13 · Rishikesh, Uttarakhand
Pranav’s Dadaji grew up in old Tehri town, which now lies under the waters of the Tehri reservoir. On a trip to the lake, Dadaji pointed at the blue water. “My school, the market and the clock tower are all down there.”
Pranav was shocked. “Why did they flood a whole town?” Dadaji sighed. “To build one of India’s tallest dams. It gives electricity, drinking water to Delhi, and water for farms. The families were moved to New Tehri. Some were happy with their new homes; many still miss the old ones.”
A guide at the viewpoint explained something new. “Tehri now also has a pumped storage plant. When there is spare electricity in the grid — for example at midday, when solar power is plentiful — pumps push water from the lower reservoir back up to the Tehri lake from the smaller Koteshwar lake downstream. In the evening, when everyone switches on lights, the water runs down again through turbines.”
“So it’s like charging a battery with water!” said Pranav. “Exactly,” said the guide. “It uses a little more energy to pump than it gives back, but it makes power available exactly when people need it.”
On the way home, Pranav thought about his grandfather’s lost town and the lights of Delhi. “Clean energy isn’t free,” he said quietly. “Someone always pays a price. We should make sure they are treated fairly.” Dadaji squeezed his hand.
Story Time — Dechen and the Mountain Warning
Dechen, age 12 · Mangan district, Sikkim
Dechen lives in North Sikkim, where the Teesta river rushes through deep valleys. In October 2023, a glacial lake high in the mountains burst after heavy rain. A huge flood roared down the Teesta, damaging roads, bridges, homes and the Teesta-III hydropower dam at Chungthang.
Dechen’s family was safe, but the memory stayed. A year later, her science teacher explained what had happened. “As the climate warms, glaciers melt and some high lakes grow bigger. If the natural wall holding a lake breaks, the water rushes out all at once. Scientists call this a glacial lake outburst flood.”
“Should we stop building dams in the mountains, then?” Dechen asked. Her teacher thought carefully. “Hydropower gives clean energy that India needs. But engineers must now plan for bigger floods, study the lakes upstream, and build early-warning systems so people have time to move to safety.”
Near Dechen’s home, a new siren tower now stands by the river. Her grandmother says it helps her sleep better. “The river gives us power and life,” Ama-la says, “but we must always respect it.”
Dechen wrote her class essay on one sentence: “A good dam is not just strong — it is also wise about the mountains around it.”
Inside a Hydro Station
The reservoir stores water behind the dam. An intake lets it into a steep pipe called the penstock. At the bottom, the fast water spins the turbine, which turns the generator. The water then leaves through the tailrace and returns to the river.
- Storage plants hold large reservoirs and can generate on demand.
- Run-of-river plants use the river’s natural flow with little storage, so they flood less land but depend on the season.
- Hydro turbines can go from standstill to full power in a few minutes, which helps keep the grid steady.
Pumped Storage: a Water Battery
A pumped storage plant has two reservoirs at different heights. It stores energy as water in the upper lake and returns roughly three-quarters of it as electricity later. As India adds more solar power, which peaks at midday, these “water batteries” help supply the evening peak.
🔢 Numbers Corner
Hydro power (kW) ≈ 9.8 × flow (m³/s) × head (m) × efficiency. With 10 m³/s falling 100 m at 90% efficiency: 9.8 × 10 × 100 × 0.9 ≈ 8,820 kW (8.8 MW).
Myth or Fact?
Myth: Hydropower has no effect on the environment because it burns nothing.
Fact: It makes no smoke, but large dams flood forests and villages, block fish migration, trap river sediment and change water flow downstream. Good planning reduces these effects.
🔬 Try it in the Green Energy Lab
Experiment with these ideas in Lab 3: Hydropower Calculator.
Solved Examples
Work it out first, then check the answer.
1. What two factors decide how much power a hydro plant can make?
Show answer
Answer: The head (height the water falls) and the flow (volume of water per second).
2. A plant has a head of 50 m and a flow of 20 m³/s. Estimate its power at 90% efficiency.
Show answer
Answer: 9.8 × 20 × 50 × 0.9 = 8,820 kW ≈ 8.8 MW.
3. A pumped storage plant uses 100 MWh to pump water up and returns 75 MWh. What is its round-trip efficiency?
Show answer
Answer: 75 ÷ 100 × 100 = 75%.
4. Why is pumped storage useful for a grid with lots of solar power?
Show answer
Answer: It can store spare midday solar energy and release it in the evening when demand is high and the Sun has set.
5. Give two social or environmental costs of large dams.
Show answer
Answer: People and villages may be displaced by the reservoir; rivers change, fish migration is blocked and sediment is trapped.
Check Your Understanding
A. Fill in the blanks
- The height through which water falls is called the .
- The steep pipe carrying water to the turbine is the .
- Water leaves a hydro station through the .
- A plant with two reservoirs that stores energy is called storage.
- A sudden flood from a burst mountain lake is a glacial lake flood.
B. True or False
- Run-of-river plants need very large reservoirs. ( ___ )
- Hydro turbines can start within minutes. ( ___ )
- Pumped storage returns more energy than it uses. ( ___ )
- Old Tehri town is now under the reservoir. ( ___ )
C. Choose the correct option
- Hydro power grows if (a) head or flow increases (b) the dam is painted (c) the river is colder
- Round-trip efficiency of pumped storage is roughly (a) 25% (b) 75% (c) 150%
- The Teesta-III dam was damaged by a flood in (a) 2013 (b) 2023 (c) 2033
D. Answer in detail
- Why did Pranav say “clean energy isn’t free”?
- How can early-warning systems make mountain hydropower safer?
- Compare storage and run-of-river hydro plants.
Brain Buster
Pour water from a jug onto a small plastic pinwheel, first from 10 cm high and then from 40 cm high. When does it spin faster? Link this to “head”.
Check your understanding
Tap an answer to see if it is correct. This is just for practice — nothing is saved.
1. Hydro power depends mainly on…
- Head and flow
- Colour and taste of water
- Temperature only
Show answer
A. Head and flow
2. What is a penstock?
- A fish ladder
- The steep pipe to the turbine
- A kind of generator
Show answer
B. The steep pipe to the turbine
3. Pumped storage is most like…
- A battery
- A solar panel
- A fuel tank for cars
Show answer
A. A battery
4. Flow 5 m³/s, head 100 m, 90% efficiency. Power?
- 441 kW
- 4,410 kW
- 44,100 kW
Show answer
B. 4,410 kW
5. Which is a real cost of large dams?
- People may have to move
- They make smoke
- They use coal
Show answer
A. People may have to move