Comparing Energy Sources

This lesson examines how different energy sources compare in terms of efficiency, environmental impact, cost, and practicality. Understanding how to evaluate energy sources is an important skill for the AQA GCSE Combined Science Trilogy (8464) specification.

---

Types of Energy Sources

Energy sources can be classified as renewable or non-renewable:

Category	Definition	Examples
Non-renewable	Will eventually run out; cannot be replaced in a human lifetime	Fossil fuels (coal, oil, natural gas), nuclear fuel (uranium)
Renewable	Can be replenished at the same rate or faster than they are used	Solar, wind, hydroelectric, tidal, geothermal, biomass/biofuels

graph TD
    A["Energy Sources"] --> B["Non-Renewable"]
    A --> C["Renewable"]
    B --> D["Coal"]
    B --> E["Oil"]
    B --> F["Natural Gas"]
    B --> G["Nuclear"]
    C --> H["Solar"]
    C --> I["Wind"]
    C --> J["Hydroelectric"]
    C --> K["Tidal"]
    C --> L["Geothermal"]
    C --> M["Biomass / Biofuels"]

---

Fossil Fuels

Fossil fuels (coal, oil, natural gas) are formed from the remains of ancient organisms over millions of years. They are currently the world's primary energy source.

Advantages

Advantage	Detail
High energy density	Produce large amounts of energy per unit mass
Reliable	Can generate electricity 24/7 regardless of weather
Existing infrastructure	Power stations, pipelines, and supply chains already exist
Relatively cheap	Long-established and widely available

Disadvantages

Disadvantage	Detail
Non-renewable	Finite supply — will eventually run out
CO₂ emissions	Combustion releases carbon dioxide, contributing to climate change
Air pollution	Produces sulfur dioxide (acid rain), nitrogen oxides, particulates
Environmental damage	Mining and drilling cause habitat destruction and pollution

---

Hydrogen as a Fuel

Feature	Detail
Energy density	Very high per unit mass (~142 MJ/kg)
Product of combustion	Water only — $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$2H2​+O2​→2H2​O
Production	Currently mostly from steam reforming of methane (releases CO₂)
Storage	Difficult — highly flammable, needs high-pressure tanks
Use in fuel cells	Converts chemical energy directly to electrical energy

---

Comparing Fuels by Energy Density

Energy density measures how much energy is released per unit mass of fuel.

Fuel	Energy Density (MJ/kg)
Hydrogen	~142
Natural gas (methane)	~56
Petrol	~46
Diesel	~45
Coal	~24
Wood	~16
Ethanol	~27

Exam Tip: Hydrogen has the highest energy density per unit mass of any fuel. However, because hydrogen is a gas at room temperature, it takes up a large volume and is hard to store — this is a key disadvantage.

---

Comparing Environmental Impact

Energy Source	CO₂ Emissions	Other Pollution	Land Use	Reliability
Coal	Very high	SO₂, particulates, ash	Mining damage	Reliable (24/7)
Natural gas	High (less than coal)	NOₓ	Drilling, pipelines	Reliable (24/7)
Oil (petrol/diesel)	High	CO, NOₓ, particulates	Drilling, spills	Reliable
Nuclear	Very low (during operation)	Radioactive waste	Small footprint	Reliable (24/7)
Solar	Zero (during use)	Manufacturing waste	Large area needed	Intermittent (daylight only)
Wind	Zero (during use)	Noise, visual impact	Moderate	Intermittent (wind dependent)
Hydrogen fuel cell	Zero at point of use	Production may release CO₂	Small	Continuous (with fuel supply)
Biomass	Carbon neutral (in theory)	Particulates, CO₂	Farmland needed	Reliable

---

Evaluating Energy Sources — Exam Technique

When asked to evaluate energy sources, you must:

1. Give advantages of each source.
2. Give disadvantages of each source.
3. Compare them fairly.
4. Reach a conclusion — state which is better and why, based on the evidence.

graph LR
    A["State advantages \nof Source A"] --> B["State disadvantages \nof Source A"]
    B --> C["State advantages \nof Source B"]
    C --> D["State disadvantages \nof Source B"]
    D --> E["Compare and \nreach a conclusion"]

Example Evaluation Structure

Question: Evaluate the use of hydrogen fuel cells compared to petrol engines for cars.

Criterion	Hydrogen Fuel Cell	Petrol Engine
Emissions	Water only (at point of use)	CO₂, CO, NOₓ, particulates
Efficiency	~60%	~25%
Fuel availability	Very limited (few H₂ stations)	Widely available
Storage	Difficult (flammable, high pressure)	Easy (liquid at room temp)
Cost	Expensive (platinum catalyst)	Relatively cheap

Conclusion: Hydrogen fuel cells are better for the environment but face practical barriers (cost, infrastructure, storage). Petrol engines are currently more convenient but contribute to climate change. A transition to hydrogen makes sense long-term if these barriers can be overcome.

---

Common Mistakes

Mistake	How to Avoid It
Only giving one side in an evaluation	Always give BOTH advantages AND disadvantages
Not reaching a conclusion	State which source is "better" and why — even if conditional
Saying hydrogen fuel cells are "zero carbon"	They are only zero carbon if hydrogen is produced from renewables
Forgetting about reliability	Solar and wind are intermittent; fossil fuels and nuclear are reliable
Ignoring practical barriers	Always mention infrastructure, cost, and storage where relevant

---

Summary

• Energy sources can be renewable (replenished) or non-renewable (finite).
• Fossil fuels have high energy density and are reliable, but produce CO₂ and air pollutants.
• Hydrogen has the highest energy density per unit mass but is difficult to store.
• Renewable sources produce little or no CO₂ during use but can be intermittent.
• When evaluating, always discuss advantages, disadvantages, and reach a conclusion.
• Consider: energy density, emissions, cost, reliability, infrastructure, and environmental impact.