Energy Densities of Storage

Lesson notes

Overview

Fuels pack far more energy per kilogram than batteries. This topic is rated advanced and is number 93 of 93 in Engineering. Deeper models, formal results and harder reasoning.

What you are seeing

The panel above shows a bar chart that compares values on one scale, so differences in size are easy to see. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

Fuels store far more energy per kilogram than batteries: petrol holds about 46 MJ/kg, diesel 45, and hydrogen 120, while a lithium-ion battery stores about 0.9 MJ/kg (250 Wh/kg), roughly fifty times less. Engines are less efficient than electric motors, so the real gap is smaller, about ten to one, but weight remains a challenge for electric aircraft and long-range trucks. Energy density per volume also matters for storage space.

Reading the relationship

The expression "MJ per kg" is a compact summary of the idea. It uses the symbols M, J, p, e, r, k; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.

Try this

Try to explain why the pattern holds, not just what it is. Look for limiting cases and what happens at the edges.

Common pitfalls

Always ask what the safety margin is and which assumption would break the design first. Idealised models hide losses, noise and limits.

How it connects

It builds on “Specific Strength of Materials”, the topic before it in Engineering.

Why it matters

Engineers use this idea to design structures, machines and systems that are safe, efficient and reliable.