Solar Electricity


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Solar Electricity

  1. 1. Solar Electricity
  2. 2. Photovoltaic Cells
  3. 3. What does Photovoltaic Mean? <ul><li>Photovoltaic (PV) : </li></ul><ul><ul><ul><li>Photo = light </li></ul></ul></ul><ul><ul><ul><li>Voltaic = produces a voltage </li></ul></ul></ul><ul><ul><ul><li>Photovoltaic : produces a voltage when exposed to light </li></ul></ul></ul><ul><li>PVs converts the energy contained in solar photons into DC electricity </li></ul>
  4. 4. Particles of Light? <ul><li>According to the theory of quantum mechanics light consists of particles or packets of energy called photons called photons </li></ul><ul><li>Each photon has an energy that depends on the color of the light (i.e. wavelength) </li></ul><ul><ul><li>The lower the wavelength), the greater the energy of the photon. </li></ul></ul><ul><li>When a photon strikes an atom, it can interact with the electrons, and the photon’s energy can be absorbed (heat). </li></ul><ul><li>The additional energy can drive an atom’s valence electrons off. </li></ul>
  5. 5. The PV Cell (Solar Cell) <ul><li>Solar cells are devices that convert visible sunlight into electricity. </li></ul><ul><li>As sunlight falls on a bi-layer semi-conductive device, a potential difference is created between the barriers. </li></ul><ul><li>The voltage has the ability to produce a current in an external circuit, thereby “making” useful electricity. </li></ul>
  6. 6. Conductors and Semiconductors <ul><li>In a conductor </li></ul><ul><ul><li>some electrons hold the solid crystal together </li></ul></ul><ul><ul><li>some electrons are free to move around ( free electrons ) </li></ul></ul><ul><li>In a semiconductor </li></ul><ul><ul><li>free electrons only exist once the crystal is energized </li></ul></ul><ul><ul><li>a semiconductor is a conductor at high temperatures and an insulator at low temperatures </li></ul></ul><ul><li>Silicon is the most common semiconductors used in electronic circuits </li></ul>
  7. 7. Review of the Silicon Atom <ul><li>Silicon has an atomic number of 14 and thus has 14 electrons surrounding the nucleus. </li></ul><ul><li>The first two shells are completely full. The outer shell, however, is only half full, having only four valance electrons. </li></ul><ul><li>Recall that the octect rule states that this atom would like to have 4 more valance electrons. </li></ul>Only the valance electrons are show in this diagram . Si
  8. 8. The Silicon Crystal <ul><li>Silicon will chemical bond with other silicon atoms to obtain a total of eight valance electrons in the outer shell. </li></ul><ul><li>This forms a single crystal of silicon. </li></ul><ul><li>Silicon is very stable in pure crystal form. </li></ul><ul><li>Free electrons for electrical current only exist once the crystal is heated </li></ul>Si Si Si Si Si Si Si Si Si
  9. 9. The Doping Process <ul><li>Doping is the adding an impurity to silicon in order to change its internal properties. </li></ul><ul><li>Because the production of energy depends on the separation of positive and negative charges, silicon must be modified. </li></ul><ul><li>The charge carrying behavior of the crystal silicon is changed. </li></ul><ul><li>To create an impurity between the silicon bonds, boron and phosphorus are added through a heating/vapor process. </li></ul>
  10. 10. Boron’s Job <ul><li>Boron has 3 valence electrons. </li></ul><ul><li>When boron is introduced a hole or electron vacancy is present. </li></ul><ul><li>The hole is like a positive charge because it attracts electrons. </li></ul><ul><li>This type of silicon is called P-type due to its positive charge. </li></ul>B Si Si Si Si Si Si Si Si “ Hole”
  11. 11. Phosphorus’ Job <ul><li>Phosphorus has 5 valence electrons. </li></ul><ul><li>Phosphorus adds an extra electron. </li></ul><ul><li>The extra electron causes a negative charge. </li></ul><ul><li>This type of silicon is called N-type due to its negative charge. </li></ul>P Si Si Si Si Si Si Si Si Extra Electron
  12. 12. Doped Silicon Model
  13. 13. Combining the Silicon Types <ul><li>When an n-type and p-type semiconductor are in contact (P-N junction), they create a small potential difference of about 0.5 V </li></ul><ul><li>However, no current will flow unless electrons are supplied to the n side of the junction. This is where photons of light are used to free electrons to start a current. </li></ul>P-N Junction N-type P-type
  14. 14. Piecing it Together <ul><li>A current is produced by the flow of electrons from the n-side. </li></ul><ul><li>If a wire is connected to the N-type silicon, and the other end attached to the P-type region, the electrons will flow through the wire and be absorbed by the boron doped silicon, or P-type. </li></ul>
  15. 15. Solar Cells for the Home <ul><li>Solar generated electricity is dc, which can be used directly for appliances like lights and electric heaters </li></ul><ul><li>For ac appliances, dc can be converted to ac using an inverter </li></ul><ul><li>Extra energy can be stored in batteries or sold back to the electric company through the power grid </li></ul>
  16. 16. Advantages of PV Electricity <ul><li>The solar energy captured by photovoltaic (PV) technology is renewable and abundant. </li></ul><ul><li>Solar energy does not cause pollution. However, solar collectors and other associated equipment are manufactured in factories that in turn cause some pollution. </li></ul><ul><li>Solar energy can be used in remote areas where it is too expensive to extend the electricity power grid (distribution/transmission lines). </li></ul><ul><li>PV installations can operate for many years with little maintenance or intervention after their initial set-up. </li></ul>
  17. 17. Disadvantages of PV Electricity <ul><li>EXPENSIVE: about 50–70 cents per kW-hr. </li></ul><ul><li>The functionality of PV power generation varies not only daily, but also seasonally with cloud cover, sun angle, and number of daylight hours. </li></ul><ul><li>Solar cells produce DC which must be converted to AC when used in current existing distribution grids. This incurs an energy loss of 4-12% </li></ul><ul><li>Large areas of land are required to capture the suns energy. Collectors are usually arranged together especially when electricity is to be produced and used in the same location. </li></ul>
  18. 18. Solar Thermal Power Plants
  19. 19. Solar Thermal Power Plants <ul><li>Solar thermal power plants use the sun's rays to heat a fluid, from which heat transfer systems may be used to produce steam. The steam, in turn, is converted into mechanical energy in a turbine and into electricity from a conventional generator coupled to the turbine.   </li></ul><ul><li>The three main types of solar-thermal power systems are: </li></ul><ul><ul><li>Parabolic Trough </li></ul></ul><ul><ul><li>Solar Dish (Parabolic Dish) </li></ul></ul><ul><ul><li>Solar Power Tower (Central Receivers) </li></ul></ul>
  20. 20. Parabolic Trough <ul><li>Parabolic Trough collector has a linear parabolic-shaped reflector that focuses the sun's radiation on a linear receiver. </li></ul><ul><li>The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day. </li></ul>
  21. 21. Parabolic Trough <ul><li>Heat transfer fluid (usually oil or salt water) runs through the receiver to absorb the concentrated sunlight. </li></ul><ul><li>The heat transfer fluid is then used to heat steam in a standard turbine generator. </li></ul><ul><li>Because of its parabolic shape, a trough can focus the sun at 30 to 100 times its normal intensity achieving operating temperatures over 400 ° C. </li></ul>
  22. 22. Parabolic Trough <ul><li>Nine parabolic concentrator facilities have been successfully operating in California's Mojave Desert commercially since 1984 with a combined generating capacity of 354MW </li></ul><ul><li>The installation uses parabolic trough technology along with natural gas to generate electricity. Natural gas is only used when the solar power is insufficient to meet the demand </li></ul><ul><li>The facilities have a total of 936,384 mirrors and cover more than 1,600 acres (6.5 km2). Lined up, the parabolic mirrors would extend over 229 miles (370 km). </li></ul>
  23. 23. Parabolic Trough Power Plant California
  24. 24. Parabolic Dish <ul><li>A parabolic dish is the most powerful type of collector which concentrates sunlight at a single, focal point (receiver).  </li></ul><ul><li>The dish's concentration ratio is much higher that the solar trough, typically over 2,000, with a temperature over 750 ° C. </li></ul><ul><li>However, Parabolic Dishes are not used for large scale electrical power generation.  </li></ul>
  25. 25. Solar Power Tower <ul><li>The solar power tower uses an array of flat, movable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the receiver). </li></ul><ul><li>Early designs used these focused rays to heat water, and used the resulting steam to power a turbine. </li></ul>
  26. 26. Solar Power Tower <ul><li>New designs using liquid sodium in place of water. Sodium is a metal with high heat capacity, which can be used to store the energy before using it to boil water to drive turbines. </li></ul><ul><li>These designs allow power to be generated when the sun is not shining. </li></ul>
  27. 27. Solar Power Tower <ul><li>The advantage of this design above the parabolic trough design is the higher temperature. </li></ul><ul><li>Thermal energy at higher temperatures can be converted to electricity more efficiently and can be more cheaply stored for later use. </li></ul><ul><li>Furthermore, there is less need to flatten the ground area. In principle a power tower can be built on a hillside. </li></ul><ul><li>Construction of large scale Power Tower plants is planed for the western US in the near Future. </li></ul>
  28. 28. Drawbacks of Solar Thermal Power Plants <ul><li>Need areas with a lot of sunlight and open space. </li></ul><ul><li>While the technology has great promise it has not yet been proven to be cost-competitive on a large industrial scale. </li></ul><ul><ul><li>Conventionally generated electricity ranges between 5 and 12 cents per kilowatt hour (the amount of money to get a kilowatt of power for an hour) </li></ul></ul><ul><ul><li>Currently, solar thermal costs around 15 to 17 cents a kilowatt hour. </li></ul></ul>
  29. 29. Wind Power
  30. 30. What is Wind? <ul><li>Wind results from the differential heating of the earth’s surface by the sun (i.e. some surface, like land, heat up faster than others, like water). </li></ul><ul><li>Wind results from the atmosphere attempting to balance this indifference of temperatures. </li></ul><ul><li>About one or 2 percent of solar energy is converted to wind energy (which is about 50-100 times more than the energy converted to biomass by all plants on earth </li></ul>
  31. 31. Wind Turbines <ul><li>Wind turbines consist of three main parts: the tower, the rotor, and a box behind the blades, called the nacelle. </li></ul><ul><li>The nacelle houses the generator, which transforms mechanical motion into electricity. </li></ul>
  32. 32. Rotor Blades <ul><li>The rotor area determines how much energy a wind turbine is able to harvest from the wind. </li></ul><ul><li>Since the rotor area increases with the square of the rotor diameter, a turbine which is twice as large will receive 22 = 2 x 2 = four times as much energy. </li></ul><ul><li>Thus, doubling the rotor size quadruples the energy yield, generating 4 times as much electricity </li></ul>
  33. 33. Wind Speed <ul><li>The speed of the wind hitting the rotors affects how much energy a turbine captures. </li></ul><ul><li>Modern wind turbines are designed to work most efficiently at wind speeds between 15 and 35 MPH. </li></ul><ul><li>Because the wind blows stronger than this some of the time, a wind turbine must adapt itself to the prevailing wind speed to operate most efficiently </li></ul>
  34. 34. Size of Turbines <ul><li>The wind blows faster at higher altitudes because of the friction near the surface. </li></ul><ul><li>Doubling the height of a tower increases the expected wind speeds by 10% and the expected power by 34%. </li></ul><ul><li>Doubling the tower height generally requires doubling the diameter as well, increasing the amount of material by a factor of eight. </li></ul>
  35. 35. Size of Turbines <ul><li>Today’s large wind turbines sit on towers up to 300 feet above the ground, with the diameter of the rotor and blades reaching more than 250 feet. </li></ul><ul><li>Lying on the ground, a three-bladed rotor can almost cover a football field! </li></ul>
  36. 36. Wind Turbine Generators <ul><li>Wind power generators convert wind energy (mechanical energy) to electrical energy. </li></ul><ul><li>The generator is attached at one end to the rotor, which provides the mechanical energy. </li></ul><ul><li>At the other end, the generator is connected to the electrical grid. </li></ul><ul><li>The generator needs to have a cooling system to make sure there is no overheating. </li></ul>
  37. 37. Wind Farms <ul><li>Modern wind farms are groups of turbines installed in rows or arrays perpendicular to the prevailing wind direction. </li></ul><ul><li>Wind farms can consist of a few up to several hundred turbines, depending on the size of the project and individual turbines. </li></ul><ul><li>A 100 MW wind farm with between 65 and 150 turbines can provide enough electricity yearly to power as many as 45,000 houses. </li></ul>
  38. 38. Twin Groves Wind Farm <ul><li>Twin Groves Wind Farm in located in central Illinois outside of Bloomington on two prominent glacial moraines in eastern McLean County. </li></ul><ul><li>The wind farm consists of 240 turbines and has an installed capacity of 400 MW - enough to power approximately 60,000 Illinois homes. </li></ul><ul><li>The wind farm obtained commercial operation in March 2007 and was the largest utility scale wind farm east of the Mississippi River upon completion </li></ul>
  39. 39. Twin Groves Wind Farm <ul><li>Each wind turbine stands 214 ft (65.23 m) tall and has three 85 ft (35.91 m) long blades. </li></ul>
  40. 40. Advantages of Wind Power <ul><li>Compared to solar, relatively low cost: 6.1–8.4 cents/kWh </li></ul><ul><li>Once the wind turbine is built the energy it produces does release CO2 or other pollutants. </li></ul><ul><li>Although wind turbines can be very tall each takes up only a small plot of land. This means that the land below can still be used. This is especially the case in agricultural areas as farming can still continue. </li></ul><ul><li>Remote areas that are not connected to the electricity power grid can use wind turbines to produce their own supply. </li></ul>
  41. 41. Disadvantages of Wind Power <ul><li>The strength of the wind is not constant and it varies from zero to storm force. wind works best at night, when demand is low. </li></ul><ul><li>Large wind farms are needed to provide entire communities with enough electricity. </li></ul><ul><li>There are some minor concern over the noise produced by the rotor blades, aesthetic (visual) impacts , and birds and bats having been killed ( avian/bat mortality ) by flying into the rotors. </li></ul>