Every energy conversion involves some loss — usually as heat. Understanding energy efficiency and being able to calculate it across single and multi-step conversions is a quantitative skill required in VCE Environmental Science.
| Energy Form | Description | Example |
|---|---|---|
| Chemical | Stored in molecular bonds | Fossil fuels, batteries, food |
| Thermal (heat) | Random kinetic energy of particles | Steam, exhaust gases |
| Mechanical | Movement of objects | Rotating turbine shaft |
| Kinetic | Energy of motion | Spinning blades, flowing water |
| Potential (gravitational) | Energy of position in a gravitational field | Water in a reservoir |
| Electrical | Movement of charged particles | Current in a wire |
| Radiant (light) | Electromagnetic radiation | Sunlight |
| Nuclear | Energy stored in atomic nuclei | Uranium fission |
Energy cannot be created or destroyed — it can only be converted from one form to another.
$$E_{in} = E_{useful} + E_{losses}$$
Implication: 100% of input energy must be accounted for — some becomes useful output, the rest becomes waste (usually heat).
Every energy conversion produces some entropy (disorder). In practice, some energy is always degraded to low-grade heat that cannot be fully recovered for useful work.
Implication: No real energy conversion is 100% efficient. Theoretical maximum efficiency (Carnot efficiency) depends on temperature difference between heat source and sink.
$$\text{Efficiency (\%)} = \frac{\text{Useful energy output}}{\text{Total energy input}} \times 100$$
Or equivalently:
$$\eta = \frac{E_{out}}{E_{in}} \times 100\%$$
A coal plant has a single-step efficiency from chemical to electrical energy of ~35%.
$$\eta = \frac{350}{1000} \times 100 = 35\%$$
A photovoltaic panel with 22% efficiency:
- Input: 1,000 J solar radiation
- Useful electrical output: 220 J
- Losses (reflection, heat): 780 J
$$\eta = \frac{220}{1000} \times 100 = 22\%$$
When energy passes through multiple conversion stages, efficiency compounds:
$$\eta_{total} = \eta_1 \times \eta_2 \times \eta_3 \times \ldots$$
| Step | Efficiency |
|---|---|
| Combustion → steam → electricity (turbine/generator) | 35% |
| Transmission losses (power lines) | 93% |
| End use (electric motor) | 85% |
$$\eta_{total} = 0.35 \times 0.93 \times 0.85 = 0.276 = 27.6\%$$
So only 27.6% of the original chemical energy in coal reaches the end use.
| System | Steps | Overall Efficiency |
|---|---|---|
| Petrol car | Chemical → thermal → mechanical | ~20–25% |
| Electric vehicle | Chemical (coal) → electrical → mechanical | ~35% × 95% × 85% ≈ 28% (from coal) |
| Electric vehicle (solar) | Solar → electrical → mechanical | ~22% × 95% × 85% ≈ 18% |
Even powered by coal, EVs are often more efficient than petrol cars because electric motors are far more efficient than internal combustion engines.
| Source | Energy Conversions | Typical Efficiency |
|---|---|---|
| Coal | Chemical → Thermal → Mechanical → Electrical | ~33–40% |
| Combined-cycle gas | Chemical → Thermal → Mechanical → Electrical (two cycles) | ~55–60% |
| Nuclear | Nuclear → Thermal → Mechanical → Electrical | ~33–37% |
| Hydro | Gravitational potential → Kinetic → Mechanical → Electrical | ~85–92% |
| Wind | Kinetic → Mechanical → Electrical | ~35–45% (Betz limit) |
| Solar PV | Radiant → Electrical | ~18–24% (commercial panels) |
Sankey diagrams visually represent energy flows, with arrow widths proportional to energy amounts. They make efficiency losses visible and allow comparison between systems.
EXAM TIP: VCAA regularly provides energy conversion data and asks students to calculate overall efficiency or identify which step has the greatest losses. Always show your working using the efficiency formula. For multi-step systems, multiply the decimal efficiencies (not percentages). Double-check: efficiency can never exceed 100%.
