Energy, power and the units that measure them: Energy in matter: fuels and energy density

4.1 Fuels store energy in chemical bonds

When a fuel burns, carbon and hydrogen atoms in it combine with oxygen from the air to form carbon dioxide and water. The bonds in those products hold less energy than the bonds in the fuel and oxygen did, and the difference is released as heat. Combustion is simply a fast, hot rearrangement of atoms.

Hydrogen and carbon behave differently. Burning hydrogen releases more energy per kilogram than burning carbon, and produces only water. So the more hydrogen a fuel contains relative to carbon, the more energy it gives per kilogram and the less CO₂ it emits per unit of energy. Wood has roughly one hydrogen atom for every carbon atom, plus a lot of oxygen and water that add weight without adding energy. Coal has less than one hydrogen per carbon. Oil has about two. Methane, the main part of natural gas, has four. Pure hydrogen has no carbon at all.

That chemistry explains the long sequence Smil traces in his work on energy transitions. The world moved from wood to coal, then to oil, then increasingly to gas. Each step brought a denser, more hydrogen-rich fuel, easier to transport and burn cleanly, and released less CO₂ per unit of energy. Smil calls the trend decarbonisation, though it has been slow and the absolute amount of carbon burned has kept rising.

Food follows the same chemistry. Carbohydrates and proteins hold about 17 kJ per gram, roughly 4 kilocalories. Fats, which are rich in hydrogen and poor in oxygen, hold about 37 kJ per gram, about 9 kilocalories. Alcohol sits between, at about 29 kJ per gram. Animals store energy as fat for the same reason aircraft carry kerosene: it packs the most energy into the least weight.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Energy in matter: fuels and energy density'
• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M1 'Energy in matter: fuels and energy density'

4.2 Energy density by mass decides what can fly

Gravimetric energy density, energy per kilogram, rises steadily through the fuel sequence. Air-dried wood holds about 15 to 17 MJ per kilogram. Coal ranges from around 10 to 30 MJ per kilogram; Indian thermal coal, high in ash, is often at the lower end, near 13 to 18. Crude oil holds about 42, petrol, diesel and kerosene about 43 to 44, and methane about 50 to 55. Hydrogen tops the list at about 120 MJ per kilogram on a lower heating value basis.

Batteries are far behind. The best lithium-ion cells store around 250 to 300 watt-hours per kilogram, about 0.9 to 1.1 MJ per kilogram, and a full pack with casing and cooling stores less. Kerosene therefore carries roughly 40 to 50 times more energy per kilogram than the best batteries. An electric motor uses its energy two to three times more efficiently than a jet engine, which narrows the gap, but still leaves a factor of roughly 13 to 25 (calculated).

That gap is decisive where weight matters most. A large long-haul aircraft can burn more than 100 tonnes of kerosene on its longest flights. Replacing that with batteries holding the same useful energy would weigh several times more than the aircraft itself. Smil uses this example to argue that long-distance flight is one of the hardest services to electrify, and that claims of imminent battery-powered intercontinental airliners ignore basic physics. Short flights, cars, buses and two-wheelers are a different matter, because they carry less energy and can recharge often.

Energy density by mass is not everything. Hydrogen's 120 MJ per kilogram looks superb until you ask how much space a kilogram takes, which is the subject of the next section.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Energy in matter: fuels and energy density'
• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M1 'Energy in matter: fuels and energy density'

4.3 By volume, gas is a thousand times thinner than oil

Volumetric energy density, energy per litre or per cubic metre, matters wherever space is limited: in fuel tanks, ships, pipelines and storage caverns. Here the ranking changes. Liquids are compact. Crude oil holds about 36 GJ per cubic metre. Natural gas at ordinary pressure holds only about 35 to 38 MJ per cubic metre, roughly a thousand times less.

That thousand-fold gap shapes the whole gas industry. Oil can be poured into tankers, trucks and drums and moved anywhere cheaply. Gas must either be pushed through high-pressure pipelines, compressed into cylinders, as in India's CNG vehicles at about 200 bar, or cooled to minus 162 degrees Celsius to make liquefied natural gas, which shrinks it about 600 times. The LNG chain of liquefaction plants, special tankers and regasification terminals exists to beat this one number, and it consumes roughly 10 to 15% of the gas's energy along the way.

Hydrogen is the extreme case. Per kilogram it beats every fuel, but it is the lightest gas there is, so even compressed to 700 bar it holds only about 5 MJ per litre, less than a sixth of diesel. Liquefied, it must be held at minus 253 degrees Celsius, and the cooling uses a quarter to a third of its energy. This is why proposals to carry hydrogen often convert it into denser carriers such as ammonia or methanol.

The lesson is to ask two questions of any fuel: how much does a kilogram hold, and how much does a litre hold? Different applications care about different answers. A rocket cares about mass. A car cares about both. A ship on a long voyage cares mostly about volume, because cargo space earns money and fuel space does not.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Energy in matter: fuels and energy density'
• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M1 'Energy in matter: fuels and energy density'

See it in the table

Check your understanding

Why does natural gas need pipelines or liquefaction for long-distance trade?

Roughly how many times more energy per kilogram does kerosene hold than today's best lithium-ion cells?

What made each step from wood to coal to oil to gas a move to a 'better' fuel?