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> Solar Spectrum Challenges: Photovoltaics Issue: 2010-3 Section: Green energy

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Solar hot water: in this technology the power systems have two components: a solar panel that gathers energy and a water reservoir. The collector is usually composed of a box with a transparent face towards the sun, and in the box there are tubes that are crossed through by different fluids (water or antifreeze solution).

Solar photovoltaic technology: consisting in the production of electricity of a solar cell when exposed to energy radiations (fig. 4). At this moment there are several variants of photovoltaic cells in various thicknesses, with different composition and different yields (Table 1).

The first step in manufacturing solar cells is the preparation of optimal silicon. The raw silicon, which is in the form of quartz sand or crushed quartz (fig. 5, 6), is placed in an electric arc furnace, where a carbon arc discharge is applied to release the oxygen. The product is CO2 (carbon dioxide) and a 99% purity fused silica. Solar cells are made from silicon ingots, which are obtained by Czochralski method (fig.7).

It is a fairly a simple process that involves inserting a silicon tip in a bath of molten silicon. As it withdraws and turns, it takes the shape of a cylindrical ingot having 100% purity, because the impurities tend to remain in the molten silica bay. The ingots are then cut according to preference: with a circular, rectangular or hexagonal surface (Fig. 8).

In the next step in the process the silicon wafer is doped (treated) with P (phosphorus) and B (boron) in order to have a crystal structure capable of conducting electricity. Boron is introduced into the Czochralski process, and after that the disks are placed in an oven, where they will be heated near to the melting temperature of silicon (1410 °C) in the presence of phosphorus gas.

To make the connection, from the solar cells that produce the power, and the deposit of the current produced, strips of metal are used, usually palladium/silver, nickel or copper. They are obtained by high vacuum thermal.

The next problem to be solved is to increase the capacity of the doped silicon disks to absorb light. Because they are shiny and will reflect 35% of the sunlight, a thin layer deposition of TiO2 (titanium dioxide) will be applied on their surface. On this purpose vacuum thermal vaporization will be used.

This method involves heating metal until evaporation, where the molecules will move within the high vacuum chamber (fig. 9), and will be adsorbed on the desired surface. Solid metal is placed in a container with a very high melting point (usually W), placed between two electrodes. As the electric current flows the metal will heat up and will go from solid to liquid and then to gaseous state, due to the Joule effect.

Because the room is highly vacuumed the molecules have a long free mean path from the container to the target, and because the latter one has a temperature lower than that of the evaporated molecules, they will coat the solar cell’s surface.

After production, the next problem to be solved on PV energy issue is its storage. One method to store this energy is to use Vanadium-redox batteries (fig. 10). This type of battery uses as electrolyte melted salts of vanadium in different oxidation states and sulfuric acid. The electric potential of solar cells is about 0.5-0.6 volts and the output power and intensity is proportional to the surface but also to the solar power radiation.

 

Facing the problems connected with the emission of CO2, more and more countries have started to move from power plants based on fossil fuels to solar power plants and solar farms. At the moment, the world leader at using electricity from solar cells is Germany. Despite the low level of solar irradiation (fig.9) the German government decided to increase the installation surfaces with solar panels by lowering taxes on production and installation. As the equivalent energy produced through solar panels grows by 1000 MW/year, the taxes will continuously decrease by 1-3%.

· In the year 2007, the European Union leaders have started a project named 20-20-20. This project has as main objective that by the year 2020, to achieve 3 targets:

· To reduce the emissions that determine the greenhouse effect by 20% below the 1990 level

· The energy consumption of the EU from renewable energy to be at 20%

· The energy consumption of the EU from classic power plants reduced by 20%

 

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Mihai Sopronyi :: 20/05/2011 :: Pageviews: 7893