There are three fundamental so called renewable sources of energy that are available to the world.   The most important of these is the energy received directly from the Sun.   The surface temperature of the sun is around 6000oC.   The energy comes in the form of light, infrared and ultraviolet radiation.

The energy of light is used by plants to convert carbon dioxide and water into organic compounds, notably sugars, by the process called photosynthesis.   This releases oxygen into the atmosphere helping to maintain the balance between this and nitrogen.   Photosynthesis absorbs 100 TWh/y(100 thousand billion watt-hours per year) and removes a 100 million tons of carbon from the atmosphere each year, turning it into bio-mass.    This energy absorption is roughly 7 times the total energy consumption of the world population.    Biomass can be used directly as a fuel, or it can be processed into more useful forms, such as oil or alcohol.   Although this use releases carbon dioxide, the carbon will only have been stored within the last year or so, and so over this time scale the use is effectively carbon neutral.

Solar energy can be used to heat water and can be converted directly to electricity using photovoltaic panels.   The uninhabited equatorial regions of the planet could supply all the worlds demand for electrical energy.   On a local scale, house roofs, even in northern Europe, can provide enough electrical power for household requirements as well as providing hot water.

Solar furnaces, constructed by focussing the suns rays by means of mirrors onto an absorber, can produce any temperature up to several thousands of degrees.   They can be designed for cooking, for melting ores and for separating water into oxygen and hydrogen to supply a possible hydrogen economy.

The second of the sources is gravity.   The moon, in revolving around the Earth pulls the water nearest to it away from the earth and at the same time pulls the Earth away from the water on the other side of the Earth.   The result is a bulge of water that travels around the world following the moon!   The bulge is exceedingly small, but extends over a very large area.   The reason the tides reach the heights they do is complex, but a major factor is shelving of the seabed close to the landmasses. This slows the water down, causing a build up, which can be further amplified if forced into a river mouth.   These tides can also be used to drive generators. It is estimated that there is enough energy available around Britain to supply 20% of UK needs.   A major problem is finding suitable sites.

The last of these sources owes its existence to gravity.   As the Earth formed, the work done by gravity in compressing it into a sphere, caused the interior to heat up.   The insulation of the outer crust of the Earth combined with the heat of radioactive decay maintains the central regions at high temperatures (about 5000oC), as seen by the eruptions of volcanoes.   In some parts of the world, particularly in Yellowstone Park in the US and in Iceland, the molten rock is very close to the surface.   Iceland heats many buildings directly from water pumped through underground pipes.   In the UK and elsewhere there is a significant increase in temperature with depth, approximately 1oC/36m.   A borehole three and a half kilometres deep could boil water continuously and forever!

These three are the primary sources of energy, all other forms of renewable energy being derived in one way or another, from the Sun.   The Sun heats the air causing winds, and the wind forms the waves on the oceans.   The Sun heats the oceans causing currents to flow from the hot regions to the cold.   The resulting evaporation condenses into clouds; water droplets coalesce and fall as rain, forming rivers.   All these effects can be used to generate power.

There are three secondary sources of energy. Some of the radioactive elements that came from the initial supernova can readily undergo fission, which is a process by which atoms are persuaded to split into two or more smaller atoms, releasing nuclear energy, which is mainly heat.   In a nuclear power station uranium is caused to fission and the resulting heat boils water to drive generators.   If one could use every atom in a kilogram of uranium, the power released would be 20,000 MWh. There is no carbon emission in the generation of power, but the construction of a nuclear power station, requiring thousands of tons of concrete and steel, is responsible for a considerable release of carbon.

Fossil fuels, coal, oil and natural gas, are the result of high pressures and temperature applied to biomass over many millions of years.   The deposits were laid down over thousands of years.   We have released a very large proportion of this stored carbon in a little over 100 years!   It is their use that has caused the current problem of global warming.

Hydrogen power is the third source, the hydrogen being obtained either directly by use of solar energy in a furnace, or indirectly by electrolysis using photovoltaic panels.   Under very high pressures and at very high temperatures, of the order of 10 million oC (it’s the process that keeps the Sun burning), hydrogen atoms can be made to join together to make helium.   The mass of the helium is a little less than the mass of hydrogen used to make it, and this mass is turned into energy.   The engineering problems are enormous and a commercial power station has yet to be built.   This is technically not a renewable source, but in practice the supply should outlive the human race!

Hydrogen can also be burnt, forming water and releasing heat, or it can be used in a fuel cell to produce electricity (and water).

In principle there are other secondary fuels.   When silicon is burnt in oxygen, large amounts of heat are released, and the waste product is silicon dioxide, better known as sand!