Large numbers of silicon atoms bond with each other by means of their valence electrons to form a crystal. These valence electrons, play a very important role in the photovoltaic (PV) effect. Only the outermost four electrons can be given to, accepted from, or shared with other atoms. ![]() In silicon solids, each silicon atom shares one of its four valence electrons with each of four neighboring atoms. The four electrons that orbit the nucleus in the outermost energy levels, valence electrons, are the ones that interact with neighboring atoms to form solids. Electrons that are further from the nucleus have more energy than electrons closer to the nucleus. Silicon is usually found in the form of silicon dioxide, usually known as quartz.Īs depicted in the diagram at left, the silicon atom has 14 electrons. It is also the 7th most common element in the universe. Silicon (Si) is a very abundant non-metallic element which makes up about 26% of the earth's crust and is the the second most abundant element after oxygen. It demonstrates how an "average" daily insolation can be misleading considering winter and summer extremes. Cairo has approximately the same latitude and weather pattern as does Phoenix, Arizona. Shown in the figure above is the insolation by time of day and month of the year for Cairo, Egypt. Typical values are about 200 W/m^2 in Australia, 185 W/m^2 in the United States and 105 W/m^2 in the United Kingdom. However, insolation varies quite at bit from place to place. Because of losses in the atmosphere (about 50%), different angles to the sun from the equator to the poles (averages 50%), and no sunlight during nightime (another 50%), the average "daily insolation" for the average position on earth (ignoring clouds) is about 170 watts per square meter (one-eigth the solar constant). The solar constant varies by up to ☓% per day as the earth orbits the sun. Over the course of a year the average solar "radiance" arriving at the top of the earth's atmosphere is 1,367 watts per square meter - called the "solar constant". The name comes from a combination of the words " incident solar radi ation". On the other hand "insolation" is the amount of radiation "received" on a given surface (usually a square meter) on a plane perpendicular to the sun in a given amount of time (usually a day). Solar radiation "emits" from the sun equally in all directions at frequencies that are visible and non-visable. Radiation is usually short for electromagnetic radiation and radiance is an instantaneous measurement at a distinct point in time. To a scientist, there is a distinct difference. What is the difference between radiance and insolation? To a lay persn, they are probably the same thing. Other stars have enormous sources of energy in the form of high energy X-rays, but lucky for us, our sun releases almost half of its energy as visible light. This is how we know with good accuracy the temperature at the sun's surface. The shape of the spectrum is almost a perfect fit for a heavenly body whose temperature is 5,800 K. Radiation in the infrared region (49%), which we feel as heat, has slightly more total energy than the radiation in the visible region. Radiation in the ultraviolet region (5%), which is not visible, causes the skin to tan and has more energy than that in the visible region. As one can see, a good portion of the sun's radiation is in the visible range (46%). ![]() Shown in the diagram on the left is how the energy from the sun is distributed by wavelength and photon energy. There are enough strong solar regions to one day provide all the energy that is needed by mankind. ![]() The map above shows in maroon, red, and orange the prime regions of the world for generating solar electricity. See the Energy Profiles Page for more general energy information. In total, the sun emits about 2.2 billion times the amount of radiation that is received by the earth. Putting this in another way, the earth absorbs more energy in one hour than the world uses in one year according to physicist Steven Chu, US Energy Secretary and former Director of Lawrence Berkeley National Laboratory. This is about 0.01% of the total yearly energy coming from the sun. ![]() The earth's energy use by mankind is approximately 500 exajoules per year. The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850,000 exajoules (EJ) per year. Of the 49% that does not reach the earth, 30% is reflected back into space and 19% is absorbed by the atmosphere and clouds. The average amount of the sun's radiation that penetrates the atmosphere and reaches the earth is 51% of the total incoming energy as illustrated above.
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