While natural gas is formed primarily of methane, it can also include ethane, propane, butane and pentane. The composition of natural gas can vary widely but in its purest form it is mainly composed of methane. Usually found under the earth, this gas is usually associated with oil deposits. Natural gas is another form of fossil fuel, like coal and oil, and is essentially the remains of plants and animals. Use of natural gas as an energy source comprises 24% of the total energy consumption in the US for the year 2000.
Natural gas, with its varied usage and has many different applications for industrial, commercial and residential sectors, is vital component of the nations energy supply. PHOTOVOLTAICS Photovoltaic literally means light electricity, which was derived from the Greek words photo and volt. Photovoltaic materials and devices basically generate electricity by converting light or solar energy into electricity that can be utilized by devices requiring electrical power. It was discovered by a French physicist Edmond Becquerel on1839 while conducting an experiment involving electrolytic cell connected by two metals as electrodes.
Since then harnessing of solar electricity has been developed and used (Solar_Energy_and_Technology_Program, 2005). Since the discovery of photoconductivity, solar energy has been harnessed to generate electricity. On 1873, Wilby Smith identified Selenium to demonstrate photoconductivity. Photovoltaic property was also observed Adams and Day to be exhibited by Selenium while in its solid form. The first solar cell was created by Charles Fritts on 1883, which was made from Selenium wafers (Corrosion_Doctors, 2007).
PV was originally developed to provide power to devices on places that would require a renewable source of energy like in outer space. With the development of PV devices or solar devices, PV cells have been used in providing power to spacecrafts and devices. The first reported usage of PV cells was during 1959 for the Vanguard 1 satellite using silicon wafers in the solar device. This satellite lasted for 8 years in operation (Corrosion_Doctors, 2007). Since Vanguard 1 other spacecrafts and satellite were reported to utilize also PV technology in providing power to manned and unmanned crafts and satellites.
PV generation is gaining increased importance as renewable source due to its advantages like absence of fuel cost, no noise or wear due to absence of moving parts and little maintenance. The world market for photovoltaic exceeded 200 MW in 1999 could rise to 650 MW in 2005 and 1800 MW in 2010 provided that installed costs for grid-connected PV drop to less than US $4/ Wp in 2005 and $3 in 2010. The world grid-connected market grew to 110 MW in 2000, 400 MW in 2005 and will grow to 700 MW in 2010 provided the installed costs will decrease to $3/W by 2010.
The following tables and graphs show a general idea of how the PV world market is growing and how the prices of PV modules/Wp is dropping since the year 1990 up until now and in the upcoming future. PV Cells PV cells or solar cells, composed of semiconductors that are made from crystalline solids, which permits electrical conduction when connected to two metals, are devices that converts solar or radiant energy into electrical energy. This thin wafer of semiconductor is chemically treated to produce negative charge on one side and positive charge on the other side.
A p-n junction separates and connects the two oppositely charged sides. The process of converting sunlight into electricity is explained in three processes 1. Sunlight is absorbed by the thin semiconductor wafer. 2. Positive and negative charges are generated and are separated in the different parts of the cell. The movement of these particles generates voltage within the cell. 3. And, the electrical current generated by the separation of the charges is transferred to the intended device via the electrical terminals connected to the cell.
When sunlight is absorbed by the solar cell, electron hole pairs are generated, and if their recombination is prevented they can reach the junction where they are separated. The electrons are moved into the front side or negative side of the cell. Commercial solar cells usually designate the negative side in the front part. The back or other side of the cells is designated as the positive side. When these two are connected electrical current flows between through wires connected to the electrical device. The current that flows is usually proportional on the intensity of the sunlight that the solar cell captures (Elwell & Komp, 2007).
The back contact or positive side is usually made of continuous layer of metal but the front contact, on the other hand is usually made into thin fingers. The front contacts are evenly spaced to allow the as much sunlight to reach the positive side of the cell. The cell is covered with an anti reflection coating to maximize the absorption of sunlight. A thin protective coating covers the cell to allow cleaning and maintenance (Elwell & Komp, 2007). An example of a single solar cell structure is shown in figure 1. PV cells are manufactured in many shapes and sizes. Some are even smaller than a postage stamp.
While there are individual cells that are many inches across. Interconnected PV cells form a PV module. Modules are interconnected to form a PV array. The module is designed to contain a number of Silicon cells interconnected as a series of thin layers. This is purposely intended to protect the solar cells from the ambient and as well as generate a higher voltage compared to a single cell, which delivers less than 1 volt. Arrays varies in the size that is needed dependent upon the amount of sunlight that the location is receiving and the needed power generated by the array depending the intended usage.
The array is not just composed of the PV modules, which comprises the majority of the PV system, it also includes the electrical connection, mounting hardware, power-conditioning equipment and batteries that store the generated electricity intended for use during the times when the sun is not available or shining (Solar_Energy_and_Technology_Program, 2005). Crystalline silicon solar cells are used in more than half of all solar electric devices. Like most semiconductor devices, they include a positive layer (on the bottom) and a negative layer (on the top) that create an electrical field inside the cell.
When a photon of light strikes a semiconductor, it releases electrons (see animation). The free electrons flow through the solar cells bottom layer to a connecting wire as direct current (DC) electricity. Some solar cells are made from polycrystalline silicon, which consists of several small silicon crystals. Polycrystalline silicon solar cells are cheaper to produce but somewhat less efficient than single-crystal silicon (Solar Electricity, 2004). A simple silicon solar cell can power a watch or calculator. However, it produces only a tiny amount of electricity.
Connected together, solar cells form modules that can generate substantial amounts of power. Modules are the building blocks of solar electric systems, which can produce enough power for a house, a rural medical clinic, or an entire village. Large arrays of solar electric modules can power satellites or provide electricity for utilities. Solar Electric System Components (Solar Electricity, 2004). PV cells can be made from different semiconductor materials but the most commonly used in manufacturing is crystalline silicon.
Crystalline silicon has gained popularity since it was the first material used in the earliest successful PV devices. Silicon has 14 electrons and its orbital arrangements allows up to four electrons to be given, shared or accepted. These outermost electrons, known as valence electron, play a vital part in photoelectric effect of solar cells. Solar cell technology benefited greatly from the high standard of silicon technology developed originally for transistors and later for integrated circuits. This applied also to the quality and availability of single crystal silicon of high perfection.
In the first years, only Czochralski (Cz) grown single crystals were used for solar cells. This material still plays an important role. As the cost of silicon is a significant proportion of the cost of a solar cell, great efforts have been made to reduce these costs. One technology, which dates back to the 1970s, is block casting which avoids the costly puling process. Silicon is melted and poured into a square SiO/SiN coated graphite crucible. Controlled cooling produces a polycrystalline silicon block with a large crystal grain structure.