CDS Physics · Energy Sources
Sources of Energy
Conventional and non-conventional sources of energy, how a thermal power station makes electricity, what biogas is made of, and how glass traps the Sun's heat.
Why this matters
Five CDS questions from 2017 (II) to 2023 (I), two of them in the 2022 (I) paper. Biogas has been asked twice; the others ask which source is non-conventional, how a thermal power station works, and how a greenhouse traps heat.
Concept 1 of 3: Conventional and non-conventional sources
Conventional sources are the ones we have used on a large scale for a long time: firewood, coal, petroleum, natural gas, and electricity from big dams and thermal plants. Fossil fuels took millions of years to form and will run out. Non-conventional sources are newer ways of using energy that keeps coming back: sunlight, wind, tides, the heat inside the Earth, and biogas. In a thermal power station, burning fuel boils water, the steam spins a turbine, and the turbine turns a generator.
Definition
Two ways to sort energy sources:
- Conventional: firewood, coal, petroleum, natural gas, and electricity from hydroelectric and thermal power stations. The fossil fuels are non-renewable (exhaustible).
- Non-conventional (alternative): solar, wind, tidal, wave, geothermal, biomass and biogas. These are renewable.
- Thermal power station: burns coal, oil or natural gas; the heat turns water into steam; the steam spins a turbine; the turbine turns a generator, which makes electricity.
- Energy chain in a thermal plant: chemical energy, then heat, then kinetic energy of the turbine, then electrical energy.
| Source | Kind |
|---|---|
| Coal | Conventional, non-renewable |
| Petroleum and natural gas | Conventional, non-renewable |
| Solar energy | Non-conventional, renewable |
| Wind, tides and geothermal heat | Non-conventional, renewable |
| Thermal power station | Burns fuel to raise steam that spins a turbine and generatorQ CDS 2023 (I): fuel heats water to steam, which drives the turbine; that is how the station makes electricity. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q90Moderate
Example 1 · Energy Sources · Sources of Energy
Consider the following statements and identify the correct answer using the code given below :
1. In a thermal power station, fuels such as oil, coal or natural gas are used to generate electricity.
2. Fuels are burned to heat water and turn it into steam, which goes through a turbine, which spins and turns, generating electricity.
Code :
Fossil fuels are conventional and exhaustible
Coal, petroleum and natural gas are the classic conventional sources, and they are non-renewable. Sunlight, wind and tides are never used up, which is why they are called non-conventional, renewable sources.
Concept 2 of 3: Biogas
Biogas is made when cow dung, plant waste or sewage rots in a closed tank with no air. Bacteria that live without oxygen break the waste down and give off a gas that is mostly methane, which burns with a clean blue flame. Because there is always more dung and waste, biogas is a renewable fuel, and what is left in the tank makes good manure.
Definition
Biogas facts:
- Made by anaerobic decomposition (in the absence of oxygen) of cow dung, plant waste and sewage, in a biogas plant.
- Composition: mainly methane (about 55 to 75 per cent) with carbon dioxide making up most of the rest, plus traces of hydrogen sulphide and hydrogen.
- Renewable, because the raw material is replaced all the time.
- A good fuel: burns without smoke, leaves no ash, and has a high heating value.
- The slurry left behind is rich in nitrogen and phosphorus and is used as manure.
| Property of biogas | Fact |
|---|---|
| Main gas | Methane, about 55 to 75 per cent |
| Second gas | Carbon dioxide |
| How it is formed | Anaerobic decomposition of dung and waste |
| Renewable or not | RenewableQ CDS 2022 (I): the false statement about biogas is that it is non-renewable. |
| Leftover slurry | Used as manure |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2017 · CDS (II) 2017 — General Knowledge · Q22Easy
Example 2 · Energy Sources · Sources of Energy
What are the main constituents of biogas?
Biogas is methane with carbon dioxide
The two main gases are methane and carbon dioxide. Sulphur dioxide and nitric oxide are not major parts of biogas; hydrogen sulphide is only a trace.
Concept 3 of 3: Solar heating and the greenhouse effect
The Sun is very hot, so most of its radiation is short in wavelength: visible light and short infrared. Glass lets this in. The ground and plants inside a glass house are only warm, so they give out long-wavelength infrared, and glass does not let that out. Energy comes in more easily than it leaves, so the inside heats up. Solar cookers use the same trick.
Definition
Using the Sun's energy:
- Greenhouse effect in a glass house: short-wavelength radiation from the Sun passes through the glass; the long-wavelength infrared given out by the warm ground and plants cannot pass back out, so heat is trapped.
- A hotter body gives out radiation of shorter wavelength; a cooler one, longer.
- Solar cooker (box type): blackened inside to absorb heat, a glass lid to trap it, and a plane mirror to reflect more sunlight in.
- Solar cell: turns sunlight directly into electricity; usually made of silicon.
| Device or effect | How it uses sunlight |
|---|---|
| Glass greenhouse | Lets short-wavelength sunlight in, traps long-wavelength infrared |
| Box-type solar cooker | Black inside to absorb, glass lid to trap heat, mirror to reflect light in |
| Solar cell | Sunlight to electricity directly; made of silicon |
| Solar water heater | Black pipes absorb heat and warm the water |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (I) 2022 — General Knowledge · Q20Moderate
Example 3 · Energy Sources · Sources of Energy
Which one of the following statements is true with regard to a greenhouse ?
Glass lets short waves in and keeps long waves out
Glass lets the Sun's short-wavelength radiation in and blocks the long-wavelength infrared from warm objects inside. It does not block all infrared, and it does not let the long-wave heat back out.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Conventional and non-conventional sources5 rows
| Source | Kind |
|---|---|
| Coal | Conventional, non-renewable |
| Petroleum and natural gas | Conventional, non-renewable |
| Solar energy | Non-conventional, renewable |
| Wind, tides and geothermal heat | Non-conventional, renewable |
| Thermal power station | Burns fuel to raise steam that spins a turbine and generatorQ CDS 2023 (I): fuel heats water to steam, which drives the turbine; that is how the station makes electricity. |
Biogas5 rows
| Property of biogas | Fact |
|---|---|
| Main gas | Methane, about 55 to 75 per cent |
| Second gas | Carbon dioxide |
| How it is formed | Anaerobic decomposition of dung and waste |
| Renewable or not | RenewableQ CDS 2022 (I): the false statement about biogas is that it is non-renewable. |
| Leftover slurry | Used as manure |
Solar heating and the greenhouse effect4 rows
| Device or effect | How it uses sunlight |
|---|---|
| Glass greenhouse | Lets short-wavelength sunlight in, traps long-wavelength infrared |
| Box-type solar cooker | Black inside to absorb, glass lid to trap heat, mirror to reflect light in |
| Solar cell | Sunlight to electricity directly; made of silicon |
| Solar water heater | Black pipes absorb heat and warm the water |
Watch out for (3)
- Fossil fuels are conventional and exhaustible→ Conventional and non-conventional sources
- Biogas is methane with carbon dioxide→ Biogas
- Glass lets short waves in and keeps long waves out→ Solar heating and the greenhouse effect
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.