Solar energy

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Solar energy
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Introduction

• Solar energy is energy directly from the Sun.
  This energy drives the climate and weather and
  supports virtually all life on Earth. Heat and
  light from the sun, along with solar-based
  resources such as wind and wave power,
  hydroelectricity and biomass, account for most of
  the available flow of renewable energy.
• Solar energy technologies harness the sun's
  energy for practical ends. These technologies
  date from the time of the early Greeks, Native
  Americans and Chinese, who warmed their buildings
  by orienting them toward the sun. Modern solar
  technologies provide heating, lighting,
  electricity and even flight.

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Types of technologies

• There are many technologies for harnessing
  solar energy.
• Solar energy can be converted into other forms
  of energy, such as heat and electricity.
  Applications span through the residential,
  commercial, industrial, agricultural and
  transportation sectors. Solar energy can be used
  to produce food, heat, light and electricity.
• In the 1830s the British astronomer John
  Herschel used a solar thermal collector box (a
  device that absorbs sunlight to collect heat) to
  cook food during an expedition to Africa. Today,
  people use the sun's energy for lots of things.
• The flexibility of solar energy is manifest in
  a wide variety of technologies such as cars,
  calculators,etc.

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Convertions of the solar energy

• Solar energy can be converted to thermal (or
  heat) energy and used to
• - heat water for use in homes, buildings, or
  swimming pools.
• - heat spaces inside greenhouses, homes, and
  other buildings
• Solar energy can be converted to electricity in
  two ways
• - Photovoltaic (PV devices) or solar cells
  change sunlight directly into electricity. PV
  systems are often used in remote locations that
  are not connected to the electric grid.  They are
  also used to power watches, calculators, and
  lighted road signs.
• - Solar Power Plants -  indirectly generate
  electricity when the heat from solar thermal
  collectors is used to heat a fluid which produces
  steam that is used to power generator. Out of the
  15 known solar electric generating units
  operating in the United States at the end of
  2006, 10 of these are in California, and 5 in
  Arizona. No statistics are being collected on
  solar plants that produce less than 1 megawatt of
  electricity, so there may be smaller solar plants
  in a number of other states.

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SOLAR THERMAL HEAT

• Solar thermal(heat) energy is often used for
  heating swimming pools, heating water used in
  homes, and space heating of buildings. Solar
  space heating systems can be classified as
  passive or active.
• Passive space heating is what happens to your
  car on a hot summer day. In buildings, the air
  is circulated past a solar heat surface(s) and
  through the building by convection (i.e. less
  dense warm air tends to rise while more dense
  cooler air moves downward) . No mechanical
  equipment is needed for passive solar heating.
• Active heating systems require a collector to
  absorb and collect solar radiation.   Fans or
  pumps are used to circulate the heated air or
  heat absorbing fluid.  Active systems often
  include some type of energy storage system.
• Solar collectors can be either nonconcentrating
  or concentrating.
• 1) Nonconcentrating collectors have a
  collector area (i.e. the area that intercepts the
  solar radiation) that is the same as the absorber
  area (i.e., the area absorbing the radiation).
  Flat-plate collectors are the most common and are
  used when temperatures below about 200o degrees F
  are sufficient, such as for space heating.
• 2) Concentrating collectors where the area
  intercepting the solar radiation is greater,
  sometimes hundreds of times greater, than the
  absorber area. 

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SOLAR THERMAL POWER PLANTS

• 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.   Solar thermal
  power generation works essentially the same as
  generation from fossil fuels except that instead
  of using steam produced from the combustion of
  fossil fuels, the steam is produced by the heat
  collected from sunlight. Solar thermal
  technologies use concentrator systems due to the
  high temperatures needed to heat the  fluid.  The
  three main types of solar-thermal power systems
  are
• - Parabolic trought the most common type of
  plant.
• - Solar dish
• - Solar power tower

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• The most common is parabolic troughs long,
  curved mirrors that concentrate sunlight on a
  liquid inside a tube that runs parallel to the
  mirror.
• The liquid, at about 300 degrees Celsius, runs
  to a central collector, where it produces steam
  that drives an electric turbine.
• Parabolic dish concentrators are similar to
  trough concentrators, but focus the sunlight on a
  single point. Dishes can produce much higher
  temperatures, and so, in principle, should
  produce electricity more efficiently. But because
  they are more complicated, they have not
  succeeded outside of demonstration projects. A
  more promising variation uses a stirling engine
  to produce power. Unlike a cars internal
  combustion engine, in which gasoline exploding
  inside the engine produces heat that causes the
  air inside the engine to expand and push out on
  the pistons, a stirling engine produces heat by
  way of mirrors that reflect sunlight on the
  outside of the engine. These dish-stirling
  generators produce about 30 kilowatts of power,
  and can be used to replace diesel generators in
  remote locations.
• The third type of concentrator system is a
  central receiver . One such plant in California
  features a "power tower" design in which a
  17-acre field of mirrors concentrates sunlight on
  the top of an 80-meter tower. The intense heat
  boils water, producing steam that drives a
  10-megawatt generator at the base of the tower.
  The first version of this facility, Solar One,
  operated from 1982 to 1988 but had a number of
  problems. Reconfigured as Solar Two during the
  early to mid-1990s, the facility is successfully
  demonstrating the ability to collect and store
  solar energy efficiently. Solar Twos success has
  opened the door for further development of this
  technology.

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PHOTOVOLTAIC ENERGY

• In 1839, French scientist Edmund Becquerel
  discovered that certain materials would give off
  a spark of electricity when struck with sunlight.
  This photoelectric effect was used in primitive
  solar cells made of selenium in the late 1800s.
  In the 1950s, scientists at Bell Labs revisited
  the technology and, using silicon, produced solar
  cells that could convert four percent of the
  energy in sunlight directly to electricity.
  Within a few years, these photovoltaic (PV) cells
  were powering spaceships and satellites.
• The most important components of a PV cell are
  two layers of semiconductor material generally
  composed of silicon crystals. On its own,
  crystallized silicon is not a very good conductor
  of electricity, but when impurities are
  intentionally addeda process called dopingthe
  stage is set for creating an electric current.
  The bottom layer of the PV cell is usually doped
  with boron, which bonds with the silicon to
  facilitate a positive charge (P). The top layer
  is doped with phosphorus, which bonds with the
  silicon to facilitate a negative charge (N).
• The surface between the resulting p-type and
  n-type semiconductors is called the P-N
  junction . Electron movement at this surface
  produces an electric field that only allows
  electrons to flow from the p-type layer to the
  n-type layer. When sunlight enters the cell,
• its energy knocks electrons loose in both
  layers. Because of the opposite
• charges of the layers, the electrons want to
  flow from the n-type layer to
• the p-type layer, but the electric field at the
  P-N junction prevents this from
• happening. The presence of an external
  circuit,however, provides the
• necessary path for electrons in the n-type layer
  to travel to the p-type
• layer. Extremely thin wires running along the
  top of the n-type layer provide
• this external circuit, and the electrons flowing
  through this circuit provide
• the cells owner with a supply of electricity.
  Most PV systems consist of
• individual square cells averaging about four
  inches on a side. Alone, each
• cell generates very little power (less than two
  watts), so they are often grouped together as
  modules. Modules can then be grouped into larger
  panels encased in glass or plastic to provide
  protection from the weather, and these panels, in
  turn, are either used as separate units or
  grouped into even larger arrays.

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The clasification of the solar cells

• The three basic types of solar cells made from
  silicon are single-crystal, polycrystalline, and
  amorphous.
• Single-crystal cells are made in long cylinders
  and sliced into round or hexagonal wafers. While
  this process is energy-intensive and wasteful of
  materials, it produces the highest-efficiency
  cellsas high as 25 percent in some laboratory
  tests. Because these high-efficiency cells are
  more expensive, they are sometimes used in
  combination with concentrators such as mirrors or
  lenses. Concentrating systems can boost
  efficiency to almost 30 percent. Single-crystal
  accounts for 29 percent of the global market for
  PV.
• Polycrystalline cells are made of molten
  silicon cast into ingots or drawn into sheets,
  then sliced into squares. While production costs
  are lower, the efficiency of the cells is lower
  tooaround 15 percent. Because the cells are
  square, they can be packed more closely together.
  Polycrystalline cells make up 62 percent of the
  global PV market.
• Amorphous silicon (a-Si) is a radically
  different approach. Silicon is essentially
  sprayed onto a glass or metal surface in thin
  films, making the whole module in one step. This
  approach is by far the least expensive, but it
  results in very low efficienciesonly about five
  percent. A number of exotic materials other
  than silicon are under development, such as
  gallium arsenide (Ga-As), copper-indium-diselenide
  (CuInSe2), and cadmium-telluride (CdTe). These
  materials offer higher efficiencies and other
  interesting properties, including the ability to
  manufacture amorphous cells that are sensitive to
  different parts of the light spectrum. By
  stacking cells into multiple layers, they can
  capture more of the available light. Although
  a-Si accounts for only five percent of the global
  market, it appears to be the most promising for
  future cost reductions and growth potential.
  Solar cell are also being used in developing
  countries. Solar panels can power a 17" b/w TV, a
  radio or a fan. Some electric lighting systems
  provide sufficient current for up to 10 hours of
  lightning each evening. Locally produced car
  batteries can provide up to 5 nights of energy
  for a 8 watt DC fluorescent light. The new Mazda
  929, uses solar cells to activate a fan to
  ventilate the car when the car is idle and parked
  during a sunny hot day.

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Advantages and disadvantages of the photovoltaic
method

• Some advantages of photovoltaic systems are 
• - Conversion from sunlight to electricity is
  direct, so that bulky mechanical generator
  systems are unnecessary.
• - PV arrays can  be installed quickly and in
  any size required or allowed.
• - The environmental impact is minimal,
  requiring no water for system cooling and
  generating no by-products.
• Current drawbacks of photovoltaic cells
• - The use of silicon crystals in the
  Photovoltaic cells makes it expensive. 1)
  silicon crystals are currently assembled manually
• 2) silicon purification is difficult and a
  lot of silicon is
• wasted
• 3) the operation of silicon cells require a
  cooling system, because performance degrades
  at high temperatures.
• However, it has convinced analysts that solar
  cells will become a significant source of energy
  by the end of the century.
• Research is underway for new fabrication
  techniques, like those used for microchips.
  Alternative materials like cadmium sulfide and
  gallium arsenide are at an experimental stage.
  Reduction of cost will depend the economies of
  scale.

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Solar Heat Collectors

• Besides using design features to maximize their
  use of the sun, some buildings have systems that
  actively gather and store solar energy. Solar
  collectors, for example, sit on the rooftops of
  buildings to collect solar energy for space
  heating, water heating, and space cooling. Most
  are large, flat boxes painted black on the inside
  and covered with glass. In the most common
  design, pipes in the box carry liquids that
  transfer the heat from the box into the building.
  This heated liquidusually a water-alcohol
  mixture to prevent freezingis used to heat water
  in a tank or is passed through radiators that
  heat the air.
• Oddly enough, solar heat can also power a
  cooling system. In desiccant evaporators, heat
  from a solar collector is used to pull moisture
  out of the air. When the air becomes drier, it
  also becomes cooler. The hot moist air is
  separated from the cooler air and vented to the
  outside. Another approach is an absorption
  chiller. Solar energy is used to heat a
  refrigerant under pressure when the pressure is
  released, it expands, cooling the air around it.
  This is how conventional refrigerators and air
  conditioners work, and its a particularly
  efficient approach for home or office cooling
  since buildings need cooling during the hottest
  part of the day. These systems are currently at
  work in humid southeastern climates such as
  Florida.

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Future of Solar Energy

• The success of solar power will depend on the
  answer to the following question 'What do you do
  when the sun goes down?'
• The simple answer is to build an auxiliary
  system that will store energy when the sun is
  out.. However, the problem is that such storage
  systems are unavailable today. Simple systems,
  like water pipes surrounded by vacuum, do exist.
  It is based on the concept that provided the
  pipes are insulated, the water will store thermal
  energy.
• The ocean is a natural reservoir of solar power
  and could be used as a source for thermal energy.
  If we can draw warm water from the surface and
  cold water from the depths, an ocean thermal
  plant could operate 24 hours a day. George Claude
  tested this hypothesis as early as 1930 in Cuba.
  Cold water from the pipe and warm water from the
  surface were pumped into a plant on shore. It
  produced 22KW when the water temperatures were
  optimum and 12KW when seasonal current
  fluctuation reduced the efficiency.
• There are also the hybrid systems. Wyoming has
  a system that holds back water on a neighboring
  hydroelectric plant when the wind is blowing,
  which for the time being, runs the turbines. As
  discussed earlier, wind is an indirect form of
  solar energy. Thus the hybrid system is used in
  the fuel saver mode.
• Research on photovoltaic cells will continue.
  Compared to the other options, majority of the
  resources will probably flow into research for
  developing better and more efficient solar cells.
  Parallel to that, more research will be
  undertaken to develop rechargeable batteries that
  will last longer hours.
• Solar energypower from the sunis free and
  inexhaustible. This vast, clean energy resource
  represents a viable alternative to the fossil
  fuels that currently pollute our air and water,
  threaten our public health, and contribute to
  global warming. Failing to take advantage of such
  a widely available and low-impact resource would
  be a grave injustice to our children and all
  future generations.

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Disadvantages of the solar energy

• The major disadvantages of solar energy are
• The amount of sunlight that arrives at the
  earth's surface is not constant. It depends on
  location, time of day, time of year, and weather
  conditions.
• Because the sun doesn't deliver that much
  energy to any one place at any one time, a large
  surface area is required to collect the energy at
  a useful rate.

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Thank you for watching
Golban Oana - Iuliana Vitu
Alexandru - Andreas