Tran 1 Solar EnergyAbout 47 percent of the energy that the sun releases to the earth actually reachesthe ground. About a third is reflected directly back into space by the atmosphere. Thetime in which solar energy is available, is also the time we least need it least – daytime. Because the sun’s energy cannot be stored for use another time, we need to convert thesuns energy into an energy that can be stored. One possible method of storing solar energy is by heating water that can beinsulated.
The water is heated by passing it through hollow panels. Black-coated stealplates are used because dark colors absorb heat more efficiently. However, this method only supplies enough energy for activities such as washingand bathing. The solar panels generate low grade heat, that is, they generate lowtemperatures for the amount of heat needed in a day. In order to generate high gradeheat, intense enough to convert water into high-pressure steam which can then be used toturn electric generators there must be another method. The concentrated beams of sunlight are collected in a device called a solar furnace,which acts on the same principles as a large magnifying glass. The solar furnace takes thesunlight from a large area and by the use of lenses and mirrors can focus the light into avery small area. Very elaborate solar furnaces have machines that angle the mirrors andlenses to the sun all day.
This system can provide sizable amounts of electricity and createextremely high temperatures of over 6000 degrees Fahrenheit.Solar energy generators are very clean, little waste is emitted from the generatorsinto the environment. The use of coal, oil and gasoline is a constant drain, economicallyand environmentally.
Will solar energy be the wave of the future? Could the worlds Tran 2requirement of energy be fulfilled by the powerhouse of our galaxy – the sun? Automobiles in the future will probably run on solar energy, and houses will have solarheaters.Solar cells today are mostly made of silicon, one of the most common elements onEarth. The crystalline silicon solar cell was one of the first types to be developed and it isstill the most common type in use today. They do not pollute the atmosphere and theyleave behind no harmful waste products.Photovoltaic cells work effectively even in cloudy weather and unlike solar heaters,are more efficient at low temperatures. They do their job silently and there are no movingparts to wear out.
It is no wonder that one marvels on how such a device would function. To understand how a solar cell works, it is necessary to go back to some basicatomic concepts. In the simplest model of the atom, electrons orbit a central nucleus,composed of protons and neutrons. Each electron carries one negative charge and eachproton one positive charge. Neutrons carry no charge. Every atom has the same numberof electrons as there are protons, so, on the whole, it is electrically neutral.
The electrons have discrete kinetic energy levels, which increase with the orbitalradius. When atoms bond together to form a solid, the electron energy levels merge intobands. In electrical conductors, these bands are continuous but in insulators andsemiconductors there is an energy gap, in which no electron orbits can exist, betweenthe inner valence band and outer conduction band Book 1.Valence electrons help to bind together the atoms in a solid by orbiting 2 adjacentnuclei, while conduction electrons, being less closely bound to the nuclei, are free to movein response to an applied voltage or electric field.
The fewer conduction electrons thereare, the higher the electrical resistively of the material.Tran 3In semiconductors, the materials from which solar sells are made, the energy gapE.g. is fairly small. Because of this, electrons in the valence band can easily be made tojump to the conduction band by the injection of energy, either in the form of heat or lightBook 4. This explains why the high resistively of semiconductors decreases as thetemperature is raised or the material illuminated.
The excitation of valence electrons to the conduction band is best accomplishedwhen the semiconductor is in the crystalline state, i.e. when the atoms are arranged in aprecise geometrical formation or lattice. At room temperature and low illumination,pure or so-called intrinsic semiconductors have a high resistively.
But the resistively canbe greatly reduced by doping, i.e. introducing a very small amount of impurity, of theorder of one in a million atoms.There are 2 kinds of doping. Those which have more valence electrons that thesemiconductor itself are called donors and those which have fewer are termedacceptors Book 2.In a silicon crystal, each atom has 4 valence electrons, which are shared with aneighboring atom to form a stable tetrahedral structure. Phosphorus, which has 5 valenceelectrons, is a donor and causes extra electrons to appear in the conduction band.
Siliconso doped is called n-type Book 5. On the other hand, boron, with a valence of 3, is anacceptor, leaving so-called holes in the lattice, which act like positive charges and renderthe silicon p-typeBook 5. Holes, like electrons, will remove under the influence of an applied voltage but, asthe mechanism of their movement is valence electron substitution from atom to atom, theyare less mobile than the free conduction electrons Book 2. In a n-on-p crystalline silicon Tran 4solar cell, a shadow junction is formed by diffusing phosphorus into a boron-based base.At the junction, conduction electrons from donor atoms in the n-region diffuse into thep-region and combine with holes in acceptor atoms, producing a layer ofnegatively-charged impurity atoms. The opposite action also takes place, holes fromacceptor atoms in the p-region crossing into the n-region, combining with electrons andproducing positively-charged impurity atoms Book 4.
The net result of these movements is the disappearance of conduction electronsand holes from the vicinity of the junction and the establishment there of a reverse electricfield, which is positive on the n-side and negative on the p-side. This reverse field plays avital part in the functioning of the device. The area in which it is set up is called thedepletion area or barrier layerBook 4. When light falls on the front surface, photons with energy in excess of the energygap interact with valence electrons and lift them to the conduction band.
This movementleaves behind holes, so each photon is said to generate an electron-hole pair Book 2.In the crystalline silicon, electron-hole generation takes place throughout thethickness of the cell, in concentrations depending on the irradiance and the spectralcomposition of the light. Photon energy is inversely proportional to wavelength. Thehighly energetic photons in the ultra-violet and blue part of the spectrum are absorbedvery near the surface, while the less energetic longer wave photons in the red and infraredare absorbed deeper in the crystal and further from the junction Book 4. Most areabsorbed within a thickness of 100 m.
The electrons and holes diffuse through thecrystal in an effort to produce an even distribution. Some recombine after a lifetime of theorder of one millisecond, neutralizing their charges and giving up energy in the form ofheat. Others reach the junction before their lifetime has expired. There they are separated Tran 5by the reverse field, the electrons being accelerated towards the negative contact and theholes towards the positive Book 5.
If the cell is connected to a load, electrons will be pushed from the negativecontact through the load to the positive contact, where they will recombine with holes.This constitutes an electric current. In crystalline silicon cells, the current generated byradiation of a particular spectral composition is directly proportional to the irradianceBook 2. Some types of solar cell, however, do not exhibit this linear relationship. The silicon solar cell has many advantages such as high reliability, photovoltaicpower plants can be put up easily and quickly, photovoltaic power plants are quitemodular and can respond to sudden changes in solar input which occur when clouds passby. However there are still some major problems with them.
They still cost too much formass use and are relatively inefficient with conversion efficiencies of 20% to 30%. Withtime, both of these problems will be solved through mass production and newtechnological advances in semiconductors. Physics Essays