Renewable energy resources, Study Guides, Projects, Research of Physics

Download Renewable energy resources and more Study Guides, Projects, Research Physics in PDF only on Docsity! INTRODUCTION Background information Renewable energy sources are becoming more and more popular, regarding the pollution and non-sustainability of common energy sources. With increasing human population, a question arises, what is going to be the next reliable energy source after the disappearance of fossil fuels? One of most abundant resources is solar energy, which manifests itself directly, as solar irradiance, or indirectly as wind energy and biomass energy. When it comes to the efficiency of energy transformation, a couple of things need to be distinguished. There are two distinct types of energy that can be produced: electrical energy and thermal energy. Electrical energy, mostly because of its ability to be easily transferred to work, is more valuable than thermal energy. The most efficient way to obtain electrical energy is from direct solar irradiance via photovoltaic cells (PV cell). Although the overall efficiency of PV cells ranges from about 5 % - 20 %, it is still higher than the total indirect efficiency when it comes to wind and biomass efficiency. However, it has been shown that the overall efficiency of photovoltaic cells drops drastically with an increase in temperature. The rate of decrease ranges from 0.25 % to 0.5 % per degree Celsius, depending on the cell material used. Especially for concentrated PV cells, which use concentrated sunlight to produce larger amounts of power, and reduce the cost of generally expensive PV equipment, it has been observed that high temperatures greatly decrease the working life of the whole PV system. The photovoltaic effect was experimentally demonstrated first by French physicist Edmond Becquerel. In 1839, at age 19, he built the world's first photovoltaic cell in his father's laboratory. Willoughby Smith first described the "Effect of Light on Selenium during the passage of an Electric Current" in a 20 February 1873 issue of Nature. In 1883 Charles Fritts built the first solid state photovoltaic cell by coating the semiconductor selenium with a thin layer of gold to form the junctions; the device was only around 1% efficient. In 1888 Russian physicist Aleksandr Stoletov built the first cell based on the outer photoelectric effect discovered by Heinrich Hertz in 1887.[5] In 1905 Albert Einstein proposed a new quantum theory of light and explained the photoelectric effect in a landmark paper, for which he received the Nobel Prize in Physics in 1921.[6] Vadim Lashkaryov discovered p-n-junctions in Cu2O and silver sulphide protocells in 1941.[7] Russell Ohl patented the modern junction semiconductor solar cell in 1946[8] while working on the series of advances that would lead to the transistor. The first practical photovoltaic cell was publicly demonstrated on 25 April 1954 at Bell Laboratories.[9] The inventors were Daryl Chapin, Calvin Souther Fuller and Gerald Pearson.[10] Solar cells gained prominence with their incorporation onto the 1958 Vanguard I satellite. General description A solar cell or photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect. Sometimes, the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the source is unspecified. Assemblies of cells are used to make solar panel, solar modules, or photovoltaic arrays. Photovoltaic’s is the field of technology and research related to the application of solar cells for solar energy. Cells, modules, panels and systems Multiple solar cells in an integrated group, all oriented in one plane, constitute a solar photovoltaic panel or solar photovoltaic module. Photovoltaic modules often have a sheet of glass on the sun-facing side, allowing light to pass while protecting the semiconductor wafers. Solar cells are usually connected in series and parallel circuits or series in modules, creating an additive voltage. Connecting cells in parallel yields a higher current; however, problems such as shadow effects can shut down the weaker (less illuminated) parallel string (a number of series connected cells) causing substantial power loss and possible damage because of the reverse bias applied to the shadowed cells by their illuminated partners. Strings of series cells are usually handled independently and not connected in parallel, though as of 2014, individual power boxes are often supplied for each module, and are connected in parallel. Although modules can be interconnected to create an array with the desired peak DC voltage and loading current capacity, using independent MPPTs (maximum power point trackers) is preferable. Otherwise, shunt diodes can reduce shadowing power loss in arrays with series/parallel connected cells. Types of solar cells Monocrystalline solar cells Monocrystalline solar panel is made by growing a single crystal. Because these crystals are usually an oval shape, monocrystalline panels are cut into the distinctive patterns that give them their recognizable appearance: the sliced silicon cells expose the missing corners in the grid-like structure. The crystal framework in a monocrystalline is even, producing a steady blue color and no grain marks, giving it the best purity and highest levels efficiency rating of between 15-24%. Polycrystalline solar cells Polycrystalline solar panel is made by pouring molten silicon into a cast. However, because of this construction method, the crystal structure will form imperfectly, creating boundaries where the crystal formation breaks. This gives the polycrystalline silicon its distinctive, grainy appearance, as the gemstone type pattern highlights the boundaries in the crystal. Because of these impurities in the crystal, polycrystalline silicon efficiency is lower than the monocrystalline solar cells, currently at 13-18%. However, this manufacturing process uses less energy and materials, giving it a significant cost advantage over monocrystalline silicon. When the haze is weak, the skies are clear. The dry air can break the trunks of trees growing in the region. Harmattan haze over Ho, Ghana Cooling methods Conduction Conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. Some substances conduct heat more easily than others. Solids are better conductor than liquids and liquids are better conductor than gases. Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e.g. when you wash your hands in warm or cold water. Convection In liquids and gases, convection is usually the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continuous circulation pattern and heat is transferred to cooler areas. You see convection when you boil water in a pan. The bubbles of water that rise are the hotter parts of the water rising to the cooler area of water at the top of the pan. You have probably heard the expression "Hot air rises and cool air falls to take its place" - this is a description of convection in our atmosphere. Heat energy is transferred by the circulation of the air. Radiation Both conduction and convection require matter to transfer heat. Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object. For example, we feel heat from the sun even though we are not touching it. Heat can be transmitted though empty space by thermal radiation. Thermal radiation (often called infrared radiation) is a type electromagnetic (or light). Radiation is a form of energy transport consisting of electromagnetic waves traveling at the speed of light. No mass is exchanged and no medium is required. Objects emit radiation when high energy electrons in a higher atomic level fall down to lower energy levels. The energy lost is emitted as light or electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels. All objects absorb and emit radiation. ( Here is a java applet showing how an atom absorbs and emits radiation) When the absorption of energy balances the emission of energy, the temperature of an object stays constant. If the absorption of energy is greater than the emission of energy, the temperature of an object rises. If the absorption of energy is less than the emission of energy, the temperature of an object falls.