1.7.3 Energy Resources – Efficiency of Energy Transfer
Learning Objective (AO1)
Qualitative understanding of efficiency (AO1):
Define efficiency, write the relevant formulae, and explain why 100 % efficiency is impossible. In the IGCSE syllabus “efficiency” always refers to **energy‑transfer efficiency** – the proportion of the input energy that appears as the intended useful output.
What is Efficiency?
- Definition: Efficiency (\(\eta\)) measures how well a device, process or plant converts the energy supplied to it into the required useful output energy.
- Energy‑based expression (most common in the syllabus):
\[
\eta = \frac{\text{Useful output energy}}{\text{Total input energy}}\times 100\%
\]
- Power‑based expression (convenient when rates are given):
\[
\eta = \frac{P_{\text{out}}}{P_{\text{in}}}\times 100\%
\]
Why Can’t Efficiency Reach 100 %?
- All real processes involve unavoidable losses – heat, sound, friction, electrical resistance, etc.
- The second law of thermodynamics states that in any energy conversion the quality of energy degrades; some energy is always transferred to a less useful form (usually thermal energy that spreads to the surroundings).
- Consequently, every practical device or plant has \(\eta < 100\%\).
Factors That Influence Efficiency (AO2)
- Type of energy conversion – e.g. chemical → thermal, thermal → mechanical, mechanical → electrical.
- Design and construction – quality of materials, precision of moving parts, aerodynamic or hydrodynamic optimisation.
- Operating conditions – load, speed, temperature, pressure, and ambient conditions (e.g. hotter intake air reduces engine efficiency).
- Load‑matching / part‑load performance – many devices are most efficient near their rated (design) load; efficiency falls off at very low or very high loads.
- Age and maintenance – wear, fouling, and poor maintenance increase losses.
Energy Resources Covered in the Syllabus
The following list uses exactly the terminology from the Cambridge IGCSE 0625 syllabus:
- Chemical energy from fossil fuels (coal, natural gas, oil)
- Chemical energy from bio‑fuels
- Solar energy – photovoltaic (PV) panels and solar‑thermal (concentrated) plants
- Wind energy
- Hydroelectric energy (large dams)
- Geothermal energy (binary‑cycle plants)
- Tidal energy (tidal barrages)
- Wave energy (oscillating water columns, etc.)
- Nuclear energy (fission)
Typical Efficiencies of the Syllabus Resources (AO1)
| Energy Resource (as in syllabus) |
Typical Conversion Efficiency
(% of input energy that becomes useful output) |
Key Loss Mechanisms |
| Coal – thermal power station |
30–35 % |
Large waste heat; improved to ≈45 % in super‑critical/ultra‑super‑critical plants. |
| Natural gas – combined‑cycle plant |
50–60 % |
Two‑stage use of exhaust heat (gas turbine + steam turbine). |
| Oil – internal‑combustion engine (car) |
20–25 % |
Most energy lost as heat in exhaust and cooling system. |
| Bio‑fuels – diesel‑engine vehicle |
20–30 % |
Similar loss pattern to fossil‑fuel diesel; additional losses in feed‑stock processing. |
| Solar photovoltaic (PV) panels |
15–22 % |
Band‑gap limitation, reflection and resistive losses. |
| Solar‑thermal (concentrated) plant |
30–35 % |
Heat‑transfer losses, turbine inefficiencies, optical losses in mirrors. |
| Wind turbines |
30–45 % |
Depends on wind speed, blade design, gearbox and generator losses. |
| Hydroelectric (large dam) |
80–90 % |
Mechanical‑to‑electrical conversion is very efficient; minor turbine and generator losses. |
| Geothermal (binary‑cycle plant) |
10–20 % |
Low‑temperature heat source limits maximum (Carnot) efficiency. |
| Tidal power (tidal barrage) |
35–45 % |
Losses in turbines and in water‑level fluctuations. |
| Wave power (oscillating water column) |
25–40 % |
Multiple conversion steps (wave → air pressure → electricity) introduce losses. |
| Nuclear power (fission) |
33–37 % |
Heat from fission drives steam turbines; similar losses to fossil‑fuel steam plants. |
Everyday Qualitative Examples
- Incandescent light bulb: ~5 % of the electrical energy appears as visible light; ~95 % becomes heat.
- Human muscle (e.g., cycling): ~20–25 % of the chemical energy in food is converted into mechanical work; the rest is released as body heat.
- Electric kettle: Almost all electrical energy is turned into heat, but only about 70 % of that heat actually raises the water temperature because of losses to the surrounding air.
- Mobile‑phone charger: Typical charger efficiency is 80–90 %; the remainder is lost as heat in the transformer and circuitry.
Why Efficiency Matters (AO1)
- Economic: Higher efficiency reduces the amount of fuel or electricity required, lowering running costs.
- Environmental: Less wasted energy means lower emissions of CO₂, SOₓ, NOₓ and reduced resource consumption.
- Practical: Efficient devices are often smaller, lighter and have longer service lives.
Suggested Diagram
A schematic of a generic device showing:
- Input energy (arrow labelled “Input”)
- Useful output energy (arrow labelled “Useful output” – e.g., mechanical work, electricity, heat for a purpose)
- Losses (one or more arrows labelled “Heat, sound, friction, etc.”) – indicating energy that does not contribute to the intended output.
Quick Revision Checklist
- State the definition of efficiency and write both the energy‑based and power‑based formulas.
- Explain, using the second law of thermodynamics, why 100 % efficiency cannot be achieved.
- List the main factors that affect the efficiency of a device or plant.
- Recall the typical efficiency ranges for each of the energy resources listed above.
- Describe at least two ways in which improving efficiency benefits the economy and the environment.
Link to the Wider Topic – Energy Resources
When evaluating a resource you must consider more than just its efficiency. A complete assessment includes:
- Renewability and availability
- Environmental impact (emissions, land use, visual impact)
- Economic factors (capital cost, operating cost, fuel cost)
Higher efficiency generally improves these aspects, but a holistic assessment is required for each resource.