Solar
Solar energy is a renewable resource. It is derived from the sun's rays. Solar energy is converted directly to electricity through solar photovoltaic panels. Solar rays, collected off reflective surfaces, heat an object in a process that creates solar thermal energy. Solar power has grown in popularity since the oil crisis of the 1970's and consumers continue to rely on solar panels for various purposes within the United States today.
Two types of solar power technologies have different potentials of success. Reflective Thermal Panels, which concentrate the sun’s rays to heat an object, need direct sunlight in order to work properly. Flat photovoltaic solar panels can convert reflected sunlight off surfaces, such as the ground or clouds, needing little direct sunlight to work properly.
At the distance of the earth’s orbit from the sun, an average flux of 1,358 watts of solar energy fills every square meter of space above the earth’s atmosphere. At the earth’s surface, this solar flux is reduced to about 990 watts per square meter. The total amount of power received by the earth from the sun daily works out to about 18,000 terawatts while global energy demand today is only about 9 terawatts.
Unfortunately, this energy flux is not quite as useful as it might at first appear. First, much of this solar flux falls on the oceans that cover roughly three quarters of the surface of the planet. The most efficient commercially available solar power collectors are only 28.5% efficient in producing DC electrical power so a space-based solar collector of this type can harvest only 380 watts of power, and when mounted at the surface of the earth a similar type solar array can harvest no more than about 280 watts of power. These highly efficient solar collector cells rely on expensive semiconductors using Germanium, Gallium Arsenide and Gallium Indium Phosphide, each of which absorbs solar energy at specific sets of wavelengths.
A new “Quantum Dot” technology promises efficiency increases by making a single-crystal semiconductor containing Copper, Indium and Selenium that might someday deliver an efficiency as high as 63.5%. Unfortunately most of the inexpensive solar collectors used today still employ multi-crystalline silicon that only delivers efficiencies of 10% to 15%. In addition, these ground-based solar panels can only collect solar energy during the daytime and have their efficiencies greatly reduced by cloud cover and low sun angle.
About forty years ago, physicists at Princeton University began designing architectures for a network of space-based solar power satellites that would collect solar energy in the vacuum of outer space and convert this into microwave energy that would have been beamed to ground-based rectannae on the surface of the earth where it could be used to feed the existing electrical power grid or to inexpensively electrolyze sea water to produce inexpensive hydrogen and oxygen for in green fuel cells to power automobiles. Today, the Defense Advanced Research Projects Agency (DARPA) at the Pentagon continues to pursue research into the feasibility of using this method to provide inexpensive and reliable electrical for military operations around the world.
Today, the average American home uses a little more than 10 kilowatts at peak demand. If it were possible to convert the solar flux arriving at the earth’s surface into electricity with a 63% efficiency using Quantum-Dot technology, each American home could be powered by a solar array measuring no more that about 25 square meters. A space-based network of solar power satellites might someday provide even greater quantities of solar energy to power the existing electrical power grid or to enable a hydrogen-based energy economy.
Proposals have also been made to blanket the deserts of the southwestern United States with commercial-scale solar power collectors, though at the risk of significant environmental impacts on fragile desert ecosystems. For the moment, in most latitudes outside the southwestern United States, solar power remains a supplemental power source that awaits investment in more efficient technologies and architectures.