Two non-solar renewable energy sources are shown in the above figure. One is the motion of the ocean tides, principally driven by the gravitational pull of the moon, the source of tidal energy. The other is geothermal energy from the Earth’s interior, which manifests itself in heat emerging from volcanoes and hot springs, and in heat from hot rocks.
The Sun: principal source of renewable energies
The sun is the ultimate source of virtually all the Earth’s renewable energies. It releases huge amounts of energy as solar radiation, just a small part of which is intercepted by the Earth.
Looking at the figure below which summarises the origins and magnitudes of the Earth’s renewable energy sources, it is clear that the principal source is solar radiation which totals some 5.4 million EJ per year.
The sun radiates huge quantities of energy into the surrounding space, and the tiny fraction intercepted by the earth’s atmosphere 150 million kilometers (km) away is nonetheless around 5.4 million EJ per year. About one third of this is radiated back to space, but this still leaves around 3.8 million EJ per year available for use on Earth. This is about 8,000 times humanity’s present rate of use of fossil and nuclear fuels.
The Sun’s radiation is due to its surface temperature of 6,000 degrees Celsius (°C), maintained by continuous nuclear fusion reactions between hydrogen atoms within its interior. This is an enormous but relatively slow process, and the sun should continue to supply power for another five billion years.