Renewable energy means an energy source that can be continually replaced, such as solar energy and plant materials, where the energy is obtained from the sun during

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photosynthesis. However, to allow indefinite use, renewable sources should not be depleted faster than the source can renew itself.

There have been a number of definitions for sustainability. One definition was ‘development that meets the needs of the present without compromising the ability of future generations to meet their needs’ (Glasby, 2003). It has also been defined as ‘to prolong the productive use of our natural resources over time, while at the same time retaining the integrity of their bases, thereby enabling their continuity’ (de Paula and Cavalcanti, 2000).

Sustainable development focuses on the long term, using scientific developments to allow a switch from the use of finite resources to those which can be renewed. Sustainability has also become a political movement involving groups working to save the environment.

Another term used for non-fossil energy sources is ‘carbon-neutral’, which means that either the energy production yields no carbon dioxide, such as solar and nuclear power, or the process only releases carbon dioxide previously fixed in photosynthesis (Fig. 1.15). In determining the carbon dioxide reduction for renewable energy sources, life-cycle analysis will determine the fossil fuel input into the production of the fuel and carbon dioxide produced. These points must be taken into consideration when the carbon dioxide savings are determined, and when applied to biofuels many are less than 100% carbon-neutral.

Conclusions

It is clear that the world’s energy demand will continue to increase in developed coun­tries and more particularly in developing countries such as China and India. The pattern of fossil fuel use is also changing with coal being replaced with gas for electricity generation. At the same time, renewable sources of energy are being devel­oped, in particular biogas and wind power. It is clear that the supply of fossil fuels is finite, considering how it was produced, but the discussion centres around how long the stocks will last and the extent of the fossil fuel reserves. The world’s dependence on a constant supply of energy means that whatever the estimate of the fossil fuel reserves, renewable sources need to be introduced as rapidly as possible.