Environmental energy

Environmental energy
The flows of energy passing continuously as renewable energy through the
Earth are shown in Figure 1.2. For instance, total solar flux absorbed at
sea level is about 1_2×1017W. Thus the solar flux reaching the Earth’s
surface is ?20MW per person; 20MW is the power of ten very large
diesel electric generators, enough to supply all the energy needs of a town
of about 50 000 people. The maximum solar flux density (irradiance)
perpendicular to the solar beam is about 1kWm?2; a very useful and
easy number to remember. In general terms, a human being is able to
intercept such an energy flux without harm, but any increase begins to
cause stress and difficulty. Interestingly, power flux densities of ?1kWm?2
begin to cause physical difficulty to an adult in wind, water currents or
waves.
However, the global data of Figure 1.2 are of little value for practical
engineering applications, since particular sites can have remarkably different
environments and possibilities for harnessing renewable energy. Obviously
flat regions, such as Denmark, have little opportunity for hydro-power but
may have wind power. Yet neighbouring regions, for example Norway, may
have vast hydro potential. Tropical rain forests may have biomass energy
sources, but deserts at the same latitude have none (moreover, forests must
not be destroyed so making more deserts). Thus practical renewable energy
systems have to be matched to particular local environmental energy flows
occurring in a particular region.