The place for renewable energies
The overall size of the flow of energy to and from the earth and its atmosphere is vast, mainly driven by the power of the sun. The amount of solar energy incident on the earth annually is equivalent to 160 times the energy stored in the world’s proven reserves of fossil fuels, and equivalent to more than 15.000 times the world’s annual use of fossil and nuclear fuels and hydro power.
The issue is thus not one of availability of solar energy, but of the practicality of converting it to forms suitable for human use.
Solar energy can be converted into useful energy using various technologies. It can be absorbed in solar ‘collectors’ to provide space and water heating. Buildings can be designed with ‘passive solar’ features to allow solar energy to contribute to the heating/cooling requirements. Small solar collectors are widely used to supply domestic hot water. It can be concentrated with simple parabolic or funnel mirrors to provide heat at up to several thousand degrees Celsius, and these high temperatures may then be used either for heating or cooking purposes or to generate electricity. Solar radiation can also be converted directly into electrical energy using photovoltaic devices (solar ‘cells’).
The term ‘renewable energy’ is defined as ‘energy obtained from continuous or repetitive currents of energy recurring in the natural environment’ (Twidel and Weir, 1986), therefore ‘energy flows which are replenished at the same rate as they are “used”’ (Sørensen, 1979). In broader terms it can be referred to as the use of any energy storage reservoir which is being refilled at rates comparable to that of extraction.
The ultimate sources of most of this energy are the sun, gravity and the earth’s rotation. The main channels through which this energy can be extracted for human usage, using current technology are: biomass, hydroelectricity, tidal power, wind energy, wave energy, geothermal energy, solar photovoltaic and solar thermal energy.
* Energy as a tool for sustainable development.'
..European Commission and UNDP. 1999