Solar Energy

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Generalities The Sun is the center of the solar system. Its weight is with 740 times bigger than the weight of all the planets. Its enormous weight creates the gravitation which attracts the other objects around him. The sun permanently emanates energy in some different forms: visible-light, invisibleinfrared razes, ultraviolet, X and Gamma, radio and plasma waves. The running of energy which becomes part of the interplanetary environment and it’s taken over by the solar system it’s called solar wind. The Sun’s surface permanently changes, light and dark spots are frequently formed and then they disappear. Many times gases violently explode from the surface. Between the inexhaustible resources, with a big energetic power, with big possibilities of being used on the country’s territory, there is the Sun, too. The incident solar energy on Earth’s surface is known as bright energy which can be transformed in thermal or electric energy. Annually, the Sun transmits Earth energy of about 450.000 mld. Tcc, from which: 180.000 mld. Tcc reflects at the level of the atmosphere and cloud’s layers, 45.000 mld. Tcc reflects at the soil’s level, 75.000 mld. Tcc are absorbed by the atmosphere (from which about 20.000 mld. Tcc creates wind), 80.000 mld. Tcc generates the evaporation of the seas and 75.000 mld. Tcc represents the incident solar energy on Earth’s surface (including the

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“Ion Creanga” National College from Bucharest photosynthesis processes) meaning 3000 times’ mankind’s energetic consumption in the year 2000. A huge quantity of solar energy reaches Earth’s surface every year. This energy can be captivated and used under the form of heat in thermo-solar applications, or it can be transformed in electricity with the help of photovoltaic cells (PC).

How does the Sun produce energy? The Sun is a sphere with a diameter of about 1.4 million km, formed of gases at very high temperatures (the interior temperature of the Sun is of about 15 million K). This huge temperature, combined with a pressure of 70 billion times bigger then the Earth’s creates the perfect conditions for the fusion reactions. While these radiations migrate from the center to the exterior of the solar sphere, they react with different elements from the Sun’s interior and transform in low energy radiations. Through this way, the Sun produced energy for about 5 billion years and will continue to do so for another 4-5 billion years.

How is the energy transported on Earth? Earth rotates around the Sun at a distance of about 150 million km. The radiations extend their self at the speed of 300.000 km per hour, light’s speed. The necessary time to reach Earth is of about 8 minutes.

The thermo-solar energy The “thermo-solar” technologies use the heat of solar razes to produce hot water, electric energy and to warm buildings. The thermo-solar applications extend theirselves from a simple residential water heating system to large stations of electric energy.

The history of thermo-solar energy

Through the history people used the sun’s heat for different things. Today the thermosolar energy is used in almost every climate as a cheap and sure source of energy. People used the solar razes for different utilities (for ages) but the real concept of thermo-solar energy appeared in 1767 when the scientist Horace de Saussure invented the

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“Ion Creanga” National College from Bucharest first solar collector or “the hot box”. The famous astrologer Sir John Hershel used, in the year 1830, these “hot boxes” for cooking during his expedition in the South of Africa. The thermo-solar energy became very important in some parts of Africa for cooking and for the distillation of water. The solar heating started its development when Clarence Kemp created the first commercial system for heating water in 1891. In 1987, almost 30% of the houses in Pasadena, California (U.S.A.) had a thermosolar heating system. The solar heating of the water “bloomed”(in U.S.A.) during the years when the cost of energy was expensive (the 70s). Because of the fact that the heating of tha water in many residents can be 40% of the total energy consummation, the solar heating plays an important role in many countries. For example, almost 1.5 million of buildings from Tokyo, Japan and over 30% of buildings from Israel have solar heating water systems. The thermo-solar energy can also be used indirect for the alimentation of a turbine, producer of electricity. This method is very efficient and competitive. The first commercial application of this system appeared at the beginning of the 80s. In U.S.A. this industry is coordinated by the American Department of Energy and grew very much thanks to the projects initiated by it. The thermo-solar energy is produced with the help of technologies which use solar radiation for producing steams. The water heating systems of little dimensions use collectors with flat platter in order to “captivate” the sun’s heat, while the electric factories which are supplied by the thermosolar energy use more complex procedures for the “captivation” of the radiations.

The utilization of the solar energy for the producing of heat of low temperature The professionals think that the best method through which the solar energy would bring an important contribution in the primary energy balance, will be its utilization as heat of low temperature. The solar collector is the main element of an installation for the utilization of solar energy in order to produce heat.

The transformation of thermo-solar energy in electric energy The thermo-solar power stations produce electricity by using a turbine supplied by the steams produced by the boiling of a liquid under the solar radiations.

“Captivation” systems (thermo-solar energy) The thermo-solar power stations use many methods for the “captivation” of solar razes: 1. Systems with central receptor – these systems concentrate the sun’s razes to a central collector by some mirrors placed radial.

2. Systems with tubs – the tubs are made of curbed mirrors which concentrate the sun’s razes on some pipes filled with a liquid. This liquid can reach very high temperatures, for example in the stations from the South of California it can reach 400C.

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3. Systems with parable – use a parable which concentrate the sun’s razes towards a collector placed in the focal center of it.

Applications Hot water can be used at low stair for household utilities or at a high stair for supplying the thermo-solar power stations. The applications at a low stair use, in general, collectors with flat platter, while the power stations use systems of concentrating the sun’s radiations. • • • •

Hot water for household utility Heating a pool Commercial and household utilities Thermo-solar power stations Using tubs, parables and central receptors the thermo-solar power stations concentrate the solar razes towards the collectors which reach very high temperatures (sometimes they can reach 600C). Today, there are many active commercial stations, bigger ones waiting to be built.

The advantages of the thermo-solar power stations • • • •

The producing of electricity and hot water at the same time The stations can be adapted at the aplications for what they’re used for The poluation is very low, even unexistent The thermo-solar stations are built faster then the conventional ones

The Working method of the photovoltaic cells When a pure cristal of silicium(A-B) is lightened by a solar raze, one or more electrons are pushed away from the nucleus(C), in their place appearing „holes”. These „holes” are filled later with other electrons, this process ending with the emitimg of heat.

Tipes of fotovoltaic cells • • • • • •

Conventional Thin Spherical Multijonctional Ploicristaline Without silicium

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Today, the fotvoltaic systems supply thousends of izoltated comunication systems like: radio systems, phones, control systems, etc. The convertion of solar energy in different forms of energy In order to use solar energy, it has to be converted in other forms of energy.

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Photothermic convertion – in this case, there is produced heat placed in water, steam, warm air, other mediums(liquid, solid, gas). This energy can be used direct or it can be converted in electric energy, through thermo-electric stations or by thermoionic effect; it can be used through thermo-chimical transformations or it an be stocked in different solide or liquid mediums. Photomecanical convertion – it refers to the supplying of cosmic ships destined for long interplanetar trips, with the so called „solar sails”(at which, thanks to the interaction between the photons and big reflectant surfaces, which took place after the ship arrived in the „cosmic emptiness”,is produced the propulsie) Photochemical convertion – it can use the Sun in two ways: direct by the lightened excitations of an object’s molecules; indirect by plants (photosynthesis) or by the transformation of animal’s dejection produces. Photoelectric convertion – the direct photoeletric convertion can be produced by using the properties of semiconductor materials from which are made the photovoltaic batteries.

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Method of energy transformation

Solar energy can be transformed for use elsewhere or utilised directly. Direct solar power involves only one transformation into a usable form. For example: • • • • •

Sunlight hits a photovoltaic cell (also called a photoelectric cell) creating electricity. Sunlight hits the dark absorber surface of a solar thermal collector and the surface warms. The heat energy is carried away by a fluid circuit. Sunlight strikes a solar sail on a space craft and is converted directly into a force on the sail which causes motion of the craft. Sunlight strikes a light mill and causes the vanes to rotate, although little practical application has yet been found for this effect. Sunlight is focused on an externally mounted fibre optic cable which conducts sunlight into building interiors to supplement lighting.

Indirect solar power involves more than one transformation to reach a usable form. Many other types of power generation are indirectly solar-powered. Some of these are so indirect that they are often excluded from discussion of solar power:

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Vegetation uses photosynthesis to convert solar energy to chemical energy, which can later be burned as fuel to generate electricity (see biofuel). Energy obtained from oil, coal, and peat originated as solar energy captured by vegetation in the remote geological past and fossilised. Hence the term Fossil fuel. Though strictly solar power, the great time delay between the input of the solar energy and its recovery means these are not normally classified as such. Hydroelectric dams and wind turbines are indirectly powered by solar energy through its interaction with the Earth's atmosphere and the resulting weather phenomena. Energy obtained from methane (natural gas) may be derived from solar energy either as a biofuel or fossil fuel. Some methane derives from the primeval gas cloud which formed the solar system and is therefore not solar in origin.

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Ocean thermal energy production uses the thermal gradients that are present across ocean depths to generate power. These temperature differences are ultimately due to the energy of the sun.

Deployment of solar power Deployment of solar power depends largely upon local conditions and requirements. But as all industrialised nations share a need for electricity, it is clear that solar power will increasingly be used to supply a cheap, reliable electricity supply. Several experimental photovoltaic (PV) power plants of 300 to 600 kW capacity are connected to electricity grids in Europe and the U.S. Other major research is investigating economic ways to store the energy which is collected from the sun's rays during the day

Concentrated Solar Power (CSP) Commercial Developments A company called Stirling Energy Systems (SES) of Phoenix, Arizona is developing a CSP system called The Dish Stirling System that concentrates sunlight on a Stirling Engine to produce electricity. SES recently signed agreements with Southern California Edison and San Diego Gas & Electric to develop large solar arrays in the southern California deserts that utilize their Dish Stirling System to generate large amounts of solar electricity. The initial contracts call for 800 megawatts (MW) of electricity to be produced from Dish Stirling Systems arranged in two large series arrays, with the potential to expand the two arrays to 2,000 MW in the future (an electricity generation scale that is comparable to four coal-fired power plants). Each dish can produce up to 25 kilowatts (a fraction 1/40th of a MW) of electricity; therefore, to achieve their 800 MW goal SES will have to install over 32,000 SES dishes. One advantage of the SES approach is that the dishes operate in a series, and therefore if one dish must be taken out of service for repairs or replacement it will not affect the overall solar dish array operation.

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