Where Does Energy Come From? - The Renaissance of Renewable Energy - page 25

But we're not there yet. While, in theory, all matter contains the same amount of energy,
because of the specific chemical makeup of different molecules, some substances are more
reticentaboutreleasingtheirenergythanothers.Thosethatreleaseitmostreadily-organic
compounds consisting mostly of hydrogen and carbon (hydrocarbons) - are suitable as
combustion fuels. Yet even these fuels release only a tiny fraction (less than 0.001 per cent)
of their embodied energy. Nuclear fission, the reaction that comes closest to unlocking the
potential in Einstein's formula, also only converts 0.5 per cent of the total mass of the
uranium fuel into energy.
The most important characteristic of a fuel is what we call its 'energy density' (see
Table 2.2 ) . This is the amount of available energy in a given unit of a fuel. If we compare
hydrogen and petrol, hydrogen has a higher energy density per unit mass than petrol, but a
much lower energy density per unit volume, even in liquid form. This means that one kilo
of hydrogen releases more energy than one kilo of petrol, but one litre of petrol releases
more energy than a litre of hydrogen (see Figure 2.1 ).

Table 2.2. Energy density of different fuels in Joules per kilogram of mass, plus the
main uses of each fuel and the type of energy involved

Storage material Energy

per kg

Direct uses Energy

type

Enriched uranium
(3.5% U-235)

3,500,000
MJ

Electric power plants (fast
breeder reactor)

Nuclear

Natural uranium
(0.7% U-235)

440,000
MJ

Electric power plants (light
water reactor)

Nuclear

Hydrogen (at 700
bar)

130 MJ Experimental automotive

engines

Chemical

Natural gas (pipeline
pressure)

53 MJ Heating, electric power

plants

Chemical

Petrol (gasoline) 45 MJ Automotive engines Chemical

Diesel 44 MJ Automotive engines Chemical