4 Renewable energy sources

Fossil and nuclear fuels are often termed non-renewable energy sources. This is because, although the quantities in which they are available may be extremely large, they are nevertheless finite and so will in principle 'run out' at some time in the future.

By contrast, hydropower and bioenergy (from biofuels grown sustainably) are two examples of renewable energy sources – that is, sources that are continuously replenished by natural processes. Renewable energy sources are essentially flows of energy, whereas the fossil and nuclear fuels are, in essence, stocks of energy.

World-wide, there has been a rapid rise in the development and deployment of renewable energy sources during the past few decades, not only because, unlike fossil or nuclear fuels, there is no danger of their 'running out', but also because their use normally entails no (or few) greenhouse gas emissions and therefore does not contribute to global climate change.

Renewable energy sources range from solar power in its various forms, through bioenergy and hydro to wind, wave, tidal and geothermal energy (Figure 27).

Figure 27: The various forms of renewable energy depend primarily on incoming solar radiation, which totals some 5.4 million Exajoules (EJ) per year. Of this, approximately 30 per cent is reflected back into space. The remaining 70 per cent is in principle available for use on Earth, as shown, and amounts to approximately 3.8 million EJ. This is some 10,000 times the current rate of consumption of fossil and nuclear fuels, which in 2000 amounted to some 360 EJ. Two other, non-solar, renewable energy sources are shown in the figure. These are the motion of the ocean tides, caused principally by the moon's gravitational pull (with a small contribution from the sun's gravity); and geothermal heat from the earth's interior, which manifests itself in convection in volcanoes and hot springs, and in conduction in rocks

The general nature and scope of the various 'renewables' can be briefly summarised as follows, beginning with the most important renewable source, solar energy.