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Solar energy
Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most o
2、f the available renewable energy on earth. Only a minuscule fraction of the available solar energy is used.
Solar power
Solar power is the generation of electricity from sunlight. This can be direct as with photovoltaics (PV), or indirect as with concentrating solar power (CSP), where the suns e
3、nergy is focused to boil water which is then used to provide power. Solar power has the potential to provide over 1,000 times total world energy consumption in 2008, though it provided only 0.02% of the total that year. If it continues to double in use every two to three years, or less, it would bec
4、ome the dominant energy source this century. The largest solar power plants, like the 354 MW SEGS, are concentrating solar thermal plants, but recently multi-megawatt photovoltaic plants have been built. Completed in 2008, the 46 MW Moura photovoltaic power station in Portugal and the 40 MW Waldpole
5、nz Solar Park in Germany are characteristic of the trend toward larger photovoltaic power stations. Much larger ones are proposed, such as the 100 MW Fort Peck Solar Farm, the 550 MW Topaz Solar Farm, and the 600 MW Rancho Cielo Solar Farm.
Solar power is amazing. On average, every square meter of
6、Earths surface receives 164 watts of solar energy. In other words, you could stand a really powerful (150 watt) table lamp on every square meter of Earths surface and light up the whole planet with the Suns energy! Or, to put it another way, if we covered just one percent of the Sahara desert with s
7、olar panels, we could generate enough electricity to power the whole world. Thats the good thing about solar power: theres an awful lot of it—much more than we could ever use.
But theres a downside too. The energy the Sun sends out arrives on Earth as a mixture of light and heat. Both of these are
8、incredibly important—the light makes plants grow, providing us with food, while the heat keeps us warm enough to survive—but we cant use either the Suns light or heat directly to run a television or a car. We have to find some way of converting solar energy into other forms of energy we can use more
9、 easily, such as electricity. And thats exactly what solar panels do.
Solar cell
A solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture ener
10、gy from sunlight such as solar panels and solar cells, while the term photovoltaic cell is used when the light source is unspecified. Assemblies of cells are used to make solar panels, solar modules, or photovoltaic arrays. Photovoltaics is the field of technology and research related to the applica
11、tion of solar cells in producing electricity for practical use. The energy generated this way is an example of solar energy.
History of solar cells
The development of the solar cell stems from the work of the French physicist Antoine-Csar Becquerel in 1839. Becquerel discovered the photovoltaic ef
12、fect while experimenting with a solid electrode in an electrolyte solution; he observed that voltage developed when light fell upon the electrode. About 50 years later, Charles Fritts constructed the first true solar cells using junctions formed by coating the semiconductor selenium with an ultrathi
13、n, nearly transparent layer of gold. Frittss devices were very inefficient, transforming less than 1 percent of the absorbed light into electrical energy.
By 1927 another metalsemiconductor-junction solar cell, in this case made of copper and the semiconductor copper oxide, had been demonstrated. B
14、y the 1930s both the selenium cell and the copper oxide cell were being employed in light-sensitive devices, such as photometers, for use in photography. These early solar cells, however, still had energy-conversion efficiencies of less than 1 percent. This impasse was finally overcome with the deve
15、lopment of the silicon solar cell by Russell Ohl in 1941. In 1954, three other American researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller, demonstrated a silicon solar cell capable of a 6-percent energy-conversion efficiency when used in direct sunlight. By the late 1980s silicon cells, as
16、well as those made of gallium arsenide, with efficiencies of more than 20 percent had been fabricated. In 1989 a concentrator solar cell, a type of device in which sunlight is concentrated onto the cell surface by means of lenses, achieved an efficiency of 37 percent due to the increased intensity o
17、f the collected energy. In general, solar cells of widely varying efficiencies and cost are now available.
Structure
Modern solar cells are based on semiconductor physics -- they are basically just P-N junction photodiodes with a very large light-sensitive area. The photovoltaic effect, which ca
18、uses the cell to convert light directly into electrical energy, occurs in the three energy-conversion layers.
The first of these three layers necessary for energy conversion in a solar cell is the top junction layer (made of N-type semiconductor ). The next layer in the structure is the core of the
19、 device; this is the absorber layer (the P-N junction). The last of the energy-conversion layers is the back junction layer (made of P-type semiconductor).
As may be seen in the above diagram, there are two additional layers that must be present in a solar cell. These ar
20、e the electrical contact layers. There must obviously be two such layers to allow electric current to flow out of and into the cell. The electrical contact layer on the face of the cell where light enters is generally present in some grid pattern and is composed of a good conductor such as a metal.
21、The grid pattern does not cover the entire face of the cell since grid materials, though good electrical conductors, are generally not transparent to light. Hence, the grid pattern must be widely spaced to allow light to enter the solar cell but not to the extent that the electrical contact layer wi
22、ll have difficulty collecting the current produced by the cell. The back electrical contact layer has no such diametrically opposed restrictions. It need simply function as an electrical contact and thus covers the entire back surface of the cell structure. Because the back layer must be a very good
23、 electrical conductor, it is always made of metal.
How do solar cells work
A solar cell is a sandwich of n-type silicon (blue) and p-type silicon (red).
1. When sunlight shines on the cell, photons (light particles) bombard the upper surface.
2. The photons (yellow blobs) carry their energy
24、down through the cell.
3. The photons give up their energy to electrons (green blobs) in the lower, p-type layer.
4. The electrons use this energy to jump across the barrier into the upper, n-type layer and escape out into the circuit.
5. Flowing around the circuit, the electrons make the lamp
25、 light up.
Solar Power - Advantages and Disadvantages
Solar Power Advantages
There are many advantages of solar energy. Just consider the advantages of solar energy over that of oil:
Solar energy is a renewable resource. Although we cannot utilize the power of the sun at night or on stormy,
26、cloudy days, etc., we can count on the sun being there the next day, ready to give us more energy and light. As long as we have the sun, we can have solar energy (and on the day that we no longer have the sun, you can believe that we will no longer have ourselves, either).
Oil, on the other hand,
27、is not renewable. Once it is gone, it is gone. Yes, we may find another source to tap, but that source may run out, as well.
Solar cells are totally silent. They can extract energy from the sun without making a peep. Now imagine the noise that the giant machines used to drill for and pump oil make
28、!
Solar energy is non-polluting. Of all advantages of solar energy over that of oil, this is, perhaps, the most important. The burning of oil releases carbon dioxide and other greenhouse gases and carcinogens into the air.
Solar cells require very little maintenance (they have no moving parts th
29、at will need to be fixed), and they last a long time.
Although solar panels or solar lights, etc., may be expensive to buy at the onset, you can save money in the long run. After all, you do not have to pay for energy from the sun. On the other hand, all of us are aware of the rising cost of oil.
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Solar powered lights and other solar powered products are also very easy to install. You do not even need to worry about wires.
Here are the disadvantages of solar energy:
The initial cost is the main disadvantage of installing a solar energy system, largely because of the high cost of
31、the semi-conducting materials used in building one.
The cost of solar energy is also high compared to non-renewable utility-supplied electricity. As energy shortages are becoming more common, solar energy is becoming more price-competitive.
Solar panels require quite a large area for installat
32、ion to achieve a good level of efficiency.
The efficiency of the system also relies on the location of the sun, although this problem can be overcome with the installation of certain components.
The production of solar energy is influenced by the presence of clouds or pollution in the air.
33、Similarly, no solar energy will be produced during nighttime although a battery backup system and/or net metering will solve this problem.
Development, deployment and economics
Beginning with the surge in coal use which accompanied the Industrial Revolution, energy consumption has steadily transi
34、tioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce. However development of solar technologies stagnated in the early 20thcentury in the face of the increasing availability, ec
35、onomy, and utility of coal and petroleum.
The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies.Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utili
36、zation Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the US (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE).
Between 1970 and 1983 photovoltaic installations grew rapidly, but falli
37、ng oil prices in the early 1980s moderated the growth of PV from 1984 to 1996.Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6GW at the end of 2007,and 14.73GW in 2008.Since 2006 it has been economical for investors to install photovoltaics for
38、free in return for a long term power purchase agreement. 50% of commercial systems were installed in this manner in 2007 and it is expected that 90% will by 2009. Nellis Air Force Base is receiving photoelectric power for about 2.2/kWh and grid power for 9/kWh.
Commercial concentrating solar therma
39、l power (CSP) plants were first developed in the 1980s. CSP plants such as SEGS project in the United States have a levelized energy cost (LEC) of 12–14/kWh.The 11MW PS10 power tower in Spain, completed in late 2005, is Europes first commercial CSP system, and a total capacity of 300MW is expected t
40、o be installed in the same area by 2013.In August 2009, First Solar announced plans to build a 2 GW photovoltaic system in Ordos City, Inner Mongolia, China in four phases consisting of 30 MW in 2010, 970 MW in 2014, and another 1000 MW by 2019. As of June 9, 2009, there is a new solar thermal power station being built in the Banaskantha district in North Gujarat. Once completed, it will be the worlds largest.
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