Energy Resources and Efficiency: Revision Notes

Condensed for the final weeks. For the full explanation, use the Energy Resources and Efficiency study guide.

Non-renewable vs renewable

Non-renewable — finite, will run out: coal, oil, natural gas, nuclear fuel. Renewable — replenished naturally: solar, wind, hydroelectric, tidal, wave, geothermal, biofuels.

Comparison table

The syllabus limits “advantages and disadvantages” to three factors only: whether the resource is renewable, when/whether it is available, and its environmental impact. Cost, reliability and public opinion are not creditable — the table below is built on the three permitted factors only.

Source	Renewable?	Availability	Environmental impact
Fossil fuels	No	Available on demand	CO₂ and other pollutants released on burning
Nuclear	No	Available on demand	No CO₂ in use, but produces radioactive waste
Solar	Yes	Only in daylight; weather-dependent	No emissions in use; manufacture has an impact
Wind	Yes	Only when the wind blows	No emissions in use; visual impact on the landscape
Hydroelectric	Yes	Available on demand once the dam is built	No emissions in use; dams flood land and displace habitats
Tidal	Yes	Predictable, but only at certain times (tied to the tides)	No emissions in use; barrages affect estuary ecosystems
Geothermal	Yes	Only where geologically active	No emissions in use
Biofuels	Yes	Available on demand (crop/waste-dependent)	Near carbon-neutral, but combustion still produces pollutants
Waves	Yes	Only in suitable coastal locations; weather-dependent	No emissions in use; can affect marine habitats

“No emissions in use” is the correct phrasing — manufacture still has an impact.

How most power stations work

Fuel burned / nuclear fission
      -> heats water in a BOILER to STEAM
      -> steam turns a TURBINE
      -> turbine drives a GENERATOR
      -> electricity

Wind and hydroelectric skip the heating stage — the turbine is turned directly. Biofuels work the same way as fossil fuels: burning plant or waste material in a boiler produces steam that drives a turbine and generator — the syllabus expects this pattern to be described explicitly, not just assumed from “burning.”

Energy from the Sun

Nearly everything traces back to the Sun: fossil fuels (ancient photosynthesis), wind (uneven heating of the atmosphere), hydroelectric (the water cycle), biofuels (photosynthesis).

Exceptions: nuclear, geothermal and tidal — tidal comes mainly from the Moon’s gravity, geothermal from heat generated by radioactive decay deep within the Earth, and nuclear from fission of a mined fuel — none of these three trace back to solar radiation at all.

Efficiency

efficiency = useful energy output / total energy input  x 100%

The rest is dissipated, usually as thermal energy to the surroundings. It is not destroyed, merely spread out and less useful.

Efficiency can never exceed 100%. If your answer does, you’ve divided the wrong way round.

Worked example: a motor is supplied with 500 J and produces 350 J of useful kinetic energy.
efficiency = (350 / 500) x 100% = 70%

Worked example: a generator has a power input of 2000 W and useful power output of 1600 W.
efficiency = (1600 / 2000) x 100% = 80%

Both formulas — energy-based and power-based — give the same answer for the same device, since power is just energy transferred per second; the ratio of useful to total is unchanged whether it’s measured as a total amount or as a rate. See the Energy Resources and Efficiency study guide for the full reasoning behind each resource’s generation method.

Sankey diagrams (background, not examinable — the syllabus requires only the two efficiency equations, not diagram construction): the width of each arrow is proportional to the energy, drawn to scale; total in = total out, so the arrows leaving must add back up to the single arrow entering.

Exam traps

• Nuclear is non-renewable — uranium is a finite fuel.
• Tidal energy comes chiefly from the Moon, not the Sun.
• Say energy is “dissipated” or “transferred to the surroundings”, never “lost”.
• Renewable ≠ pollution-free; check what the question actually asks.
• Efficiency is a percentage of useful output over total input.
• Bringing in cost, reliability or public opinion when the question asks for advantages/disadvantages — only renewability, availability and environmental impact are creditable.
• Assuming every resource needs a turbine and generator — solar (photovoltaic) converts radiation to electricity directly, with neither.

Self-test

1. Is nuclear power renewable? Explain.
2. A lamp takes 60 J and gives 9 J of light. Find its efficiency.
3. Which three energy resources do not originate from the Sun?
4. Give one advantage and one disadvantage of wind power.
5. Describe the energy transfers in a coal-fired power station.
6. A generator has a power input of 1200 W and a useful power output of 900 W. Find its efficiency.
7. Name the three factors the syllabus permits when comparing advantages and disadvantages of energy resources.
8. Which one named resource can generate electricity without a turbine or generator, and how?

Answers: 1. No — uranium is a finite fuel that will eventually be exhausted, even though it produces no CO₂. 2. (9/60) × 100 = 15%. 3. Geothermal (heat from radioactive decay in the Earth), tidal (the Moon’s gravitational pull), and nuclear (energy from a mined fuel). 4. Advantage: no fuel cost and no emissions in use. Disadvantage: intermittent — no output when the wind does not blow. 5. Chemical energy in coal → thermal energy → kinetic energy of steam and turbine → electrical energy in the generator, with thermal energy dissipated at each stage. 6. (900/1200) × 100 = 75%. 7. Renewable status, availability, and environmental impact. 8. Solar — photovoltaic cells convert radiation directly into electricity, with no boiler, turbine or generator involved.