6.1 Concentrating energy
As far as human needs are concerned, there is a marked difference between 'dilute' and 'concentrated' energy. Water vapour in the atmosphere, for example, has considerable potential energy since a huge mass globally (about 13 × 1015 kg—Smith, 2005) is held high above the Earth's surface. But this potential energy represents a very dilute form of energy; falling rain could not turn a water wheel. It is only when energy can be 'concentrated' that it can be put to good use — in this case by rainfall accumulating in streams and rivers, or being stored in reservoirs at high elevations. The concentration can be expressed colloquially in terms of energy density, which is the amount of energy stored by a resource divided by the volume of the space that it occupies.
Some forms of energy are relatively difficult to concentrate, so have a low energy density, whereas others are easier to concentrate. The energy contained in moving air is rather difficult to concentrate; windmills and wind farms have to be sited where natural factors enhance wind speed and constancy. Solar power has a low energy density, so requires large collecting devices. The potential energy of rain is naturally concentrated and held in mountain lakes; we concentrate this energy artificially when rainwater is stored in a reservoir. This emphasises why fossil fuels are so valuable as they represent naturally concentrated forms of the solar energy that reached the Earth millions of years ago.