radiation is electromagnetic radiation in the 0.28…3.0 m wavelength range. The solar spectrum includes a small share of ultraviolet radiation (0.28…0.38 m) which is invisible to our eyes and comprises about 2% of the solar spectrum, the visible light which range from 0.38 to 0.78 m and accounts for around 49% of the spectrum and finally of infrared radiation with long wavelength (0.78…3.0 m), which makes up most of the remaining 49% of the solar spectrum.

The amount of solar radiation reaching the earth’s surface varies greatly because of changing atmospheric conditions and the changing position of the sun, both during the day and throughout the year. Clouds are the predominant atmospheric condition that determines the amount of solar radiation that reaches the earth. Consequently, regions of the nation with cloudy climates receive less solar radiation than the cloud-free desert climates. For any given location, the solar radiation reaching the earth’s surface decreases with increasing cloud cover. Local geographical features, such as mountains, oceans, and large lakes, influence the formation of clouds; therefore, the amount of solar radiation received for these areas may be different from that received by adjacent land areas. For example, mountains may receive less solar radiation than adjacent foothills and plains located a short distance away. Winds blowing against mountains force some of the air to rise, and clouds form from the moisture in the air as it cools. Coastlines may also receive a different amount of solar radiation than areas further inland.
The solar energy which is available during the day varies and depends strongly on the local sky conditions. At noon in clear sky conditions, the global solar irradiation can in e.g. Central Europe reach 1000 W/m2 on a horizontal surface (under very favorable conditions, even higher levels can occur) whilst in very cloudy weather, it may fall to less than 100 W/m2 even at midday.

Both man-made and naturally occurring events can limit the amount of solar radiation at the earth’s surface. Urban air pollution, smoke from forest fires, and airborne ash resulting from volcanic activity reduce the solar resource by increasing the scattering and absorption of solar radiation. This has a larger impact on radiation coming in a direct line from the sun (direct radiation) than on the total (global) solar radiation. On a day with severely polluted air (smog alert), the direct solar radiation can be reduced by 40%, whereas the global solar radiation is reduced by 15% to 25%. A large volcanic eruption may decrease, over a large portion of the earth, the direct solar radiation by 20% and the global solar radiation by nearly 10% for 6 months to 2 years. As the volcanic ash falls out of the atmosphere, the effect is diminished, but complete removal of the ash may take several years.
Solar radiation provides us at zero cost with 10,000 times more energy than is actually used worldwide. All people of the world buy, trade, and sell a little less than 85 trillion (8.5 x 1013 ) kilowatt-hours of energy per year. But that’s just the commercial market. Because we have no way to keep track of it, we are not sure how much non-commercial energy people consume: how much wood and manure people may gather and burn, for example; or how much water individuals, small groups, or businesses may use to provide mechanical or electrical energy. Some think that such non-commercial energy may constitute as much as a fifth of all energy consumed. But even if this were the case, the total energy consumed by the people of the world would still be only about one seven-thousandth of the solar energy striking the earth’s surface per year.
In some developed countries like in the United States people consume roughly 25 trillion (2.5 x 10E13 ) kilowatt-hours per year. This translates to more than 260 kilowatt-hours per person per day – this is the equivalent of running more than one hundred 100 watt bulbs all day, every day. U.S. citizen consumes 33 times as much energy as the average person from India, 13 times as much as the average Chinese, two and a half times as much as the average Japanese, and twice as much as the average Sweden.
Even in such heavy energy consuming countries like USA solar energy falling on the land mass can many times surplus the energy consumed there. If only 1% of land would be set aside and covered by solar systems (such as solar cells or solar thermal troughs) that were only 10% efficient, the sunshine falling on these systems could supply this nation with all the energy it needed. The same is true for all other developed countries. In a certain sense, it is impractical – besides being extremely expensive, it is not possible to cover such large areas with solar systems. The damage to ecosystems might be dramatic. But the principle remains. It is possible to cover the same total area in a dispersed manner – on buildings, on houses, along roadsides, on dedicated plots of land, etc. In another sense, it is practical. In many countries already more than 1% of land is dedicated to the mining, drilling, converting, generating, and transporting of energy. And the great majority of this energy is not renewable on a human scale and is far more harmful to the environment than solar systems would prove to be.

In most places of the world much more solar energy hits a home’s roof and walls as is used by its occupants over a year’s time. Harnessing this sun’s light and heat is a clean, simple, and natural way to provide all forms of energy we need. It can be absorbed in solar collectors to provide hot water or space heating in households and commercial buildings. It can be concentrated by parabolic mirrors to provide heat at up to several thousands degrees Celsius. This heat can be used either for heating purposes or to generate electricity. There exist also another way to produce power from the sun – through photovoltaics. Photovoltaic cells are devices which convert solar radiation directly into electricity.
Solar radiation can be converted into useful energy using active systems and passive solar design. Active systems are generally those that are very visible like solar collectors or photovoltaic cells. Passive systems are defined as those where the heat moves by natural means due to house design which entails the arrangement of basic building materials to maximize the sun’s energy.
Solar energy can be converted to useful energy also indirectly, through other energy forms like biomass, wind or hydro power. Solar energy drives the earth’s weather. A large fraction of the incident radiation is absorbed by the oceans and the seas, which are warmed than evaporate and give the power to the rains which feed hydro power plants. Winds which are harnessed by wind turbines are getting its power due to uneven heating of the air. Another category of solar-derived renewable energy sources is biomass. Green plants absorb sunlight and convert it through photosynthesis into organic matter which can be used to produce heat and electricity as well. Thus wind, hydro power and biomass are all indirect forms of solar energy.

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