Renewable or Sustainable?
Renewable Energy is energy from sources that are continuously replenished and virtually inexhaustible within human timescales, derived directly or indirectly from the sun, radioactive decay and gravitational energy.
The sun gives us energy as:
- heat - including solar thermal heating and electricity generation,
- photosynthesis - giving us food, and plants we use as fuels,
- photovoltaic effects - giving us electricity from solar panels,
- wind - produced by the heat of the sun making air move via convection,
- waves - produced by the action of wind on water,
- falling water - produced by heat from the sun evaporating water and making it rise before precipitating - which can give hydro-electricity.
Radioactive decay of elements in the Earth's crust gives us geothermal energy.
Gravitational energy - the kinetic energy of the Moon's rotation around the Earth - gives us tidal energy.
By contrast fossil fuels are strictly limited in supply (although the exhaustion of our supplies has been predicted for decades but has yet to happen).
Nuclear fission energy also depends on limited supplies of substances such as Uranium and Thorium, although supplies are probably sufficient for centuries or even millenia if used with appropriate technologies. The raw materials for some forms of nuclear fusion, if it can be made to work, are so plentiful as to be practically unlimited.
Not all forms of renewable energy are sustainable: the most widely-used form of renewable energy is biomass, in the form of wood and crop residues, widely used in the developing world as cooking fuel and increasingly used in the developed world for space heating and electricity generation. Demand for wood for fuel causes deforestation or, at best, hinders the radical reforestation we need for global carbon sequestration. (Use of biomass fuels for cooking also cause sickness and death amongst those - especially women and children - exposed to smoke from cooking fires.)
Damming rivers for hydroelectricity generally has bad consequences for ecology and biodiversity, and can result in significant emissions of greenhouse gases when seasonal changes of water level result in vegetation growing around the margins of a reservoir when the water level falls, which then gets submerged when the level rises and the now-submerged vegetation decays releasing methane.
Sustainable Energy is energy that we can use without compromising our own, and future generations', survival. Principally this means energy sources with effectively zero emissions of CO2 and other greenhouse gases. Such sources include some renewables, as well as nuclear energy, and the use of fossil fuels and biomass with Carbon Capture and Storage.