Saturday, March 20, 2010

Earth Energy

by Richard Crews
Our planet Earth gets energy from three major sources:

(1) radiation from the Sun
(2) the internal radioactive decay of Uranium and Thorium
(3) the gravitational tug of the Moon and Sun

(1) The Earth receives 174 petawatts of radiant energy from the Sun at the upper atmosphere. (A petawatt is 10^15 watts, so this number is 174,000,000,000,000,000 watts; a medium light bulb delivers about 100 watts.)

Of this, 30% is reflected back into space. The remainder is absorbed by land, clouds (and atmosphere), and oceans. (Some is reflected back, after reaching the surface of the Earth, by water and especially ice and snow, but this is largely absorbed by the atmosphere [and clouds] rather than being lost from the planet into space.)

Since the oceans cover about 71% of the planet's surface, they receive (and absorb) about 85 petawatts. This represents about 570,000 times the current needs of humanity. Most of this is absorbed as heat and goes into making wind and weather patterns, and deep ocean currents. (Only about 1/1,000 of it is captured by the chlorophyl of plants for use by plants and animals.)

(2) The Earth's heat output is about 44 terawatts. (A terawatt is 10^12 watts, so that's 44,000,000,000,000 watts.) Most of this is from the radioactive decay of Uranium and Thorium. In addition, much of it is residual heat from planetary accretion when the Earth first formed; some is from the gravitational effects of the Sun and Moon (solid masses like land can't ebb and flow with tides so they heat up); some, from the electric dynamo effect of the Sun's magnetic field. This energy is principally manifest as volcanoes and earthquakes.

(3) The gravitational tug of the Moon and Sun produces ocean tides (54% from the effects of the Moon, 46% from the Sun), amounting to about 3.75 terawatts (3,750,000,000,000 watts). This is only a tiny fraction (0.00002) of the amount of energy received from the Sun as radiation--but still some 2.8 times humanity's current needs.