The Sun is an enormous, effective, and far-away (some 150 million km from Earth) nuclear fusion reactor that can supply the Earth with energy now, and for several billions of years to come. Sunlight, or solar energy, emitted from the Sun is the most abundant energy source. At any given time, sunshine delivers to Earth as light and heat about 10 000 times more energy than the entire world is consuming. Solar radiation, before entering the Earth's atmosphere, has a power density of 1370 W m-2 . Of all the sunlight that passes through the atmosphere annually, only about half reaches the Earth's surface; the other half is scattered or reflected back to space by clouds and the atmosphere, or absorbed by atmospheric gases such as CO2 and water vapor, the atmosphere, and clouds. As 71% of our planet is covered with water, most of that energy that makes it to the surface is absorbed by the oceans. On a clear day, the radiation received on the Earth's surface around noon is about 1000 W m-2 . The amount of energy received on the surface (called insolation) is usually measured in kWh m-2. Annual average insolation varies from maxima in hot desert areas to minima in polar regions. Seasonal variations are more important in regions far from the equator, as differences between winter and summer day lengths are more pronounced. In the extreme case, at the north pole, insolation is near zero during the six months of polar winter when the sun never rises. In the United States, the highest insolations with a daily average in excess of 6 kWh m-2, can be found in the dry and most of the time cloudless South-West. This includes states such as Arizona, New Mexico and Nevada, as well as Southern California and especially the Mojave Desert, where major solar energy technologies are tested. Using the current solar technology, an area of 160 X 160 km in this region could generate as much energy as the entire United States currently consumes . The enormous flux of inexhaustible (at least on the human time-scale) solar energy holds a tremendous potential for providing humanity with clean and sustainable energy. As with the wind, real interest in utilizing the Sun's energy arose in the aftermath of the oil crises of the 1970s. Today, the various ways in which solar energy can be used to generate electricity, provide hot water, and heat or cool our buildings are at different stages of technological development.
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