📖 Chapter 5 of 8 · Green Energy - Basic 2 · Class 7
Chapter 5: What Other Clean Energy Is Hidden Around Us?
In this chapter
- Geothermal energy — heat from deep inside the Earth.
- Tidal energy — power from the rising and falling sea.
- Biogas — cooking gas made from cow dung and waste.
- Meet Stanzin from Ladakh and Kiran from Karnataka.
The Big Idea
Nature hides even more clean energy. Deep inside the Earth it is very hot — this heat is geothermal energy. The Moon’s pull makes sea water rise and fall every day — these are tides, and moving water can spin turbines. Cow dung and kitchen waste slowly rot and give off biogas, which can be burnt for cooking or to make electricity.
Did You Know?
India has hot springs in places like Manikaran in Himachal Pradesh, Tattapani in Chhattisgarh and Puga Valley in Ladakh. Scientists are studying such places to see how their heat can be used to make electricity.
Story Time — Stanzin and the Steaming Valley
Stanzin, age 11 · Leh, Ladakh
Stanzin lives in Leh, high in the mountains of Ladakh. In winter, the temperature can drop far below zero, and families burn a lot of fuel, often brought from far away, just to stay warm.
In summer, Stanzin’s aunt took him to Puga Valley, more than 4,000 metres above the sea. The ground was cold and rocky, but clouds of steam were rising from pools of bubbling water. “Is the ground on fire?” he asked.
An engineer in a yellow hard hat laughed. “Not fire — heat! Deep below us, the rocks are extremely hot. Water seeps down, gets heated, and comes back up as hot springs and steam. This is geothermal energy.”
“Can it make electricity?” Stanzin asked. “That is what we are testing,” she said. “We drill deep wells and study the hot water. One day it could spin turbines and also heat homes — day and night, snow or sunshine.”
Stanzin pulled his cap tighter and smiled. “So the warmest blanket for Ladakh may be hidden under our own mountains!”
Story Time — Kiran’s Gas from Gobar
Kiran, age 10 · Near Mandya, Karnataka
Kiran lives on a sugarcane farm near Mandya in Karnataka. His family has three cows — Gowri, Lakshmi and Nandi. Earlier, his Amma cooked on a wood fire. The smoke made her cough and made Kiran’s eyes water.
Then his Appa built a biogas plant in the backyard. It looks like a small brick dome. Every morning, Kiran helps mix the cows’ dung (gobar) with water and pour it into the plant.
Inside the closed dome, tiny living things called bacteria slowly break down the dung. As they work, they give off a gas. A pipe carries this gas to the kitchen, where it burns with a clean blue flame.
“No smoke, no firewood to carry, and no trees cut down!” Amma said happily as she made ragi mudde for lunch.
The best surprise was the thick, wet leftover that comes out of the plant, called slurry. It is a wonderful natural fertiliser, and Appa spreads it on the sugarcane fields. Kiran patted Gowri and laughed. “Our cows give us milk, cooking gas and manure!”
How Tides Can Make Power
Sea water does not stay still. The Moon’s gravity pulls on it and makes it rise and fall.
- Tides: Most beaches get two high tides and two low tides almost every day, because the Earth spins while the Moon pulls.
- Tidal power: When the tide rushes in and out, it can spin turbines placed in the water, just like wind spins a wind turbine.
- Tides are predictable: We know the times of tides far in advance, which makes planning easier than with wind.
- India’s tides: The Gulf of Khambhat and the Gulf of Kutch in Gujarat, and the Sundarbans in West Bengal, have strong tides. India does not have a big tidal power plant yet, but scientists see these as possible sites.
- Biogas in 4 steps: collect dung and waste → bacteria break it down in a closed tank → gas flows through a pipe → leftover slurry becomes fertiliser.
GOBARdhan: The Government of India’s GOBARdhan programme helps villages turn cattle dung and farm waste into biogas and organic manure, keeping villages cleaner too.
Solved Examples
Think first, then read the answer.
1. What does the word “geothermal” mean?
Show answer
Answer: “Geo” means Earth and “thermal” means heat. So geothermal energy is heat from the Earth.
2. Why can’t every place use geothermal energy easily?
Show answer
Answer: The hot rocks are usually very deep. Only a few places, such as hot-spring areas, have heat close enough to the surface to use easily.
3. Why is biogas better than cooking on firewood?
Show answer
Answer: It burns without smoke, so the kitchen air stays clean; no trees need to be cut; and the leftover slurry is a free fertiliser.
4. What makes the sea rise and fall as tides?
Show answer
Answer: Mainly the pull (gravity) of the Moon on the ocean, together with the spinning of the Earth.
5. A biogas plant needs 25 kg of dung a day. Kiran’s 3 cows give 10 kg each. Is that enough? How much extra is there?
Show answer
Answer: 3 × 10 = 30 kg. Yes, it is enough, with 30 − 25 = 5 kg extra.
✎ Now You Try!
A beach has high tide at 6:00 in the morning and again at about 6:25 in the evening. About how many hours apart are the two high tides?
Check Your Understanding
A. Fill in the blanks
- Heat from deep inside the Earth is called energy.
- The rise and fall of sea water is called a .
- Biogas is made when break down dung and waste.
- Puga Valley is in .
- The leftover from a biogas plant, called , is used as fertiliser.
B. True or False
- The Moon’s pull helps cause tides. ( ___ )
- Biogas burns with a lot of black smoke. ( ___ )
- Every place in India has hot springs. ( ___ )
- Tide times can be known in advance. ( ___ )
C. Choose the correct option
- Kiran’s family cooks with (a) biogas (b) coal (c) petrol
- “Geo” means (a) water (b) Earth (c) air
- The Gulf of Khambhat is known for (a) strong tides (b) deserts (c) snow
D. Answer in two or three sentences
- How could geothermal energy help people in Ladakh?
- Write the four steps of making biogas.
- Why is tidal energy easier to plan than wind energy?
Brain Buster
Ask an elder: did anyone in your family ever cook with firewood or dung cakes? How is cooking different today?
Check your understanding
Tap an answer to see if it is correct. This is just for fun — nothing is saved.
1. What does “geothermal” mean?
- Heat from the Earth
- Water from the sky
- Light from the Moon
Show answer
A. Heat from the Earth
2. What mainly causes the tides?
- The wind
- The pull of the Moon
- Ships
Show answer
B. The pull of the Moon
3. Biogas is made when bacteria break down…
- Plastic and glass
- Dung and waste
- Stones
Show answer
B. Dung and waste
4. What is the leftover slurry from a biogas plant used for?
- Fertiliser for fields
- Petrol for cars
- Paint
Show answer
A. Fertiliser for fields