Environmental Engineering Reference
In-Depth Information
Figure 2.2 The sources of energy used since 1850
The Earth is bombarded continuously by radiation from the Sun, with an intensity
of 1,363 W/m
2
per year, which corresponds to 173,000,000 W incident on the
whole Earth. Approximately 30% of this radiation is reflected into the atmosphere
and reemitted back to space. The remaining 70% is responsible for producing the
Earth's climate, and 23% of this radiation is consumed in evaporation of the wa-
ter in the oceans, circulation of the water vapor, and water precipitation in what is
called the hydrological cycle. A fraction of the water accumulates in lakes above
sea level or in rivers, from where it runs back to the oceans. In the process it can
generate hydroelectricity. The final 47% of the Sun's energy is absorbed by the at-
mosphere, warming the air, the oceans, and the surface of the Earth (determining
its average temperature). The warming of the air gives rise to winds, which can be
used to generate electricity in wind machines; wind also generates ocean waves. A
