Primary fuels: Coal 30% Nuclear 5% Oil 40% Natural gas 20% Hydroelectric 5% These proportions vary from country to country and with time. Which are likely to increase and which decrease and why?

The rate of fuel consumption has increased 10 fold in the last 100 years - a similar rise in the next 100 years would give a 100 fold increase in total! What could be the consequences - how could it be avoided / achieved? Is solar energy an answer?

Primary fuels are in their natural state. Fossils fuels are primary fuels. Most fossil fuels formed over millions of years ( peat is a fossil fuel, wood a biofuel ). We use them at a faster rate than they are being replaced so they are finite - non-renewable. They will run out. Renewables include solar energy, hydroelectricity, wind power and biofuels. Secondary fuels such as electricity and petrol are produced from primary fuels.

Fissile materials are finite but will last so long that they are sometimes referred to as renewable in the not so popular press! They are materials which contain a proportion of elements which can undergo controlled nuclear fission. i.e. fuel for a nuclear reactor.

Energy stores - Fissile materials, fossil fuels and biofuels store energy conveniently. It is considerably less efficient and more difficult to store solar energy. Is this really true for fissile materials?

Geothermal Energy, unlike all but tidal energy, does not originate from the Sun. A fluid (usually water) extracts heat from the Earth. The source needs to be close to the surface and hot enough to be of some use. There are not very many. The source of the energy is thought to be nuclear reactions in the Earth. (From the estimated age of the Earth, it is calculated that it should be colder than it is. The extra heat is thought to come from nuclear reactions in the Earth.) Where does tidal energy come from?

The value on the Earth’s surface will vary with: weather - absorption in the atmosphere longitude - spread out over a larger area at higher longitudes time of day - spread out over a larger area in the morning and evening time of year - the sun will be higher in the sky in summer giving a higher intensity (note that the Sun is further away from the Earth in our winter!)

weather - absorption in the atmosphere

longitude - spread out over a larger area at higher longitudes

time of day - spread out over a larger area in the morning and evening

time of year - the sun will be higher in the sky in summer giving a higher intensity (note that the Sun is further away from the Earth in our winter!)