Photovoltaic Energy - Research Paper Example

The advancement in technology has led to the manufacture of several devices that require power to run. This has created strains on national power grids and has resulted to rationing of power supply in some countries. Most of the countries are dependent on hydro-electric power, efforts to diversify the power sources being a common phenomenon. Most countries in the world are now resorting to solar energy and wind energy to supplement their energy sources. Others have resorted to nuclear energy but this has erupted hot debates about the global security. The US government has commissioned the department of energy to provide reliable, clean and affordable solar power to the US citizens by carrying out research activities in Photovoltaic systems (eereenergy par1). Photovoltaic Technology Photovoltaic technology utilizes the energy of the sun and is clean and affordable to the users. The photovoltaic energy contributes a little to environment pollution in its use as opposed to many other energy sources. The advantage of using photovoltaic energy is that it can be used in a wide range of devices and products from small electric systems to large electric systems (American Solar Energy Society 13). There is no limit to the type of electrical device that may be driven using photovoltaic energy. Through use of photovoltaic energy, many governments have been able to substantially meet the power needs of most of their citizens. The founder of the photo electric effect was Edmund Bequerel, a French physicist. He first noted the photoelectric effect in 1839 when he discovered that certain materials produce small electric current when they are exposed to sunlight. In 1905, there were contributions in the photoelectric world by Albert Einstein who in a more detailed way described the photoelectric light and the nature of light. This forms the basis of photovoltaic technology. There have been several contributions from other scientist with the progress in time making it a more reliable source of power (Goetzberger & Hoffmann 133). Importance of Photovoltaic Energy Photovoltaic energy has several advantages in its use including commercial and environmental benefits. Photovoltaic energy has no or little environmental impacts according to the American Solar Energy Society. It is a clean source of power and is not detrimental to the natural environmental. There are no pollutants emitted to the environment by photovoltaic equipment thus leaving the environment unpolluted. Its production also does not lead to decline of any environmental resource but utilizes what would have gone to waste if it was not utilized, solar energy according to Garg and Prakash (14). The production of photovoltaic energy produces little air pollution if any and no hazardous waste to the environment. This new technology produces energy from light in a quiet environment and does not require gaseous or liquid fuels combustion. The mass production of photovoltaic energy has been used to boost energy supplies by the national grid. This has resulted to power security as most of the users can now access power for the better part of the day and all week long. The introduction of solar energy as a component of the national grid has resulted to more power supply to power consumers. Figure 1: Photovoltaic cells used to supply energy to national grid The source of photovoltaic energy is sunlight which is free and abundant (American Solar Energy Society 21). This results in the production of cheap and affordable power. The emergence of photovoltaic energy as a source of energy has created job opportunities. The production of voltaic energy has created employment opportunities to engineers, contractors and accountants. The sector has absorbed part of the population who otherwise would be jobless. This has resulted to strong economy through reduction in trade deficit and improves the living standards of the locals. The production of photovoltaic energy removes the burden of cost and uncertainties brought by politically volatile regions. From the fact that photovoltaic energy can be integrated in the national grid, can be national grid independent and can be produced domestically, it has ensured that power consumers are able to run their electrical devices even when the hydro-electric power source has problems. The systems in the national power grid deteriorate naturally and there are inadequate maintenance activities. The systems are always complex and proper functioning is interdependent. This has been very costly when there is disruption of the power supply. Photovoltaic energy is more reliable and requires little maintenance thus reducing the maintenance costs. The initial set up cost is low thus can be produced domestically. This has broadened the consumer base, strengthened the economy and contributed to trade deficit minimization. The photovoltaic energy has been used to mitigate power black outs in big companies as well as domestically. Figure 2 below shows the photovoltaic arrangement. Figure 2: Photovoltaic Arrangement Production of Photovoltaic Energy The photovoltaic energy as mentioned earlier, is the use of photovoltaic cells commonly referred to as solar cells to convert sunlight into electricity. The photovoltaic cell is manufactured from silicon alloys and is usually non-mechanical. The major component of sunlight is photons or commonly referred to as solar energy particles. The photons usually hold a certain amount of energy that is directly related to the solar spectrum wavelength. On striking a photovoltaic cell surface, the photons may either be reflected, absorbed or pass right through. The absorbed photons provide the energy for electricity production and when there is enough absorbed photons, the electrons from the semi conductor material are dislodged from the atoms (Soga 35). During manufacture, the front surface is subjected to special treatment such that it becomes more receptive to free electrons thus the electrons can migrate naturally to the surface of the semiconductor material as shown below. Figure 2: The p and n junctions of the solar cells Imbalance in holes, protons and electrons leads to a potential difference in any semiconductor material. This is the technique used in production of photovoltaic energy. As the electrons migrate to the surface of the semiconductor material, holes are formed. Electrons carry negative charges and when they migrate towards the front surface of the semiconductor cell, there results a charge imbalance between the front and the back cell surfaces. This creates a potential difference just like in the terminals of a dry cell. When the two surfaces are connected using a conductor and an external load, there is electricity flows. The basic unit in a photovoltaic system is the PV cell which may vary in size. The photovoltaic cell may range from 1cm to 10cm across. The energy produced by one cell is in the range of 1 to 2 watts but that is inadequate for running any electrical device. To increase the energy tapped, the energy from the cells are interconnected into a weather tight module package. The packaged modules are further connected to form a photovoltaic array ranging from one to thousands of modules. Several modules can be interconnected to arrive at a desirable power output. The power output and performance of a photovoltaic cells array is dependent on sunlight thus climatic conditions highly influence the amount of energy derived from the PV array at a particular time (Garg & Prakash 390). It should be noted however that the PV cells are only 10 percent efficient in conversion of solar energy to electricity but research is been conducted to increase efficiency to around 20 percent. For there to be induction of the built-in electric field in a photo voltaic cell, the cell has two layers of different semiconductor materials in contact with each other. One of the layers is N-type semi conductor while the other is p-type. The n-type has negative electrical charge while the p-type has positive electrical charge. Both the materials are neutral but the n-type material has excess electrons and p-type excess holes. When the two semi conductor materials are sandwiched together, a p-n junction is created thus generating an electric field. When the p-type and n-type materials get into contact, there is flow of excess electrons from the n-type semiconductor to the p-type semiconductor. This result to positive charge build up on the n-type interface and negative charge build up on the p-type interface. This creates a semiconductor that behaves like a battery. To make photovoltaic cells capable of producing electricity, doping is carried out on the semiconductor materials. Doping is the process of adding a different element or another element on the silicon semiconductor (American Solar Energy Society 34). The size of a PV array is dependent on several factors; amount of available sunlight in a particular geographical location, consumer needs and the cost. The array modules are the major components of a PV system which includes the electrical connections, equipments for power conditioning, mounting hardware and batteries. The batteries are used for energy storage for use when there is no sunshine. The PV arrays produce DC electricity and the larger the surface area of the photovoltaic array the larger the direct current electricity. The interconnection between the modules can either be in series or parallel so as to produce the desired current voltage combination. Most of the photovoltaic devices use a single interface or junction to realize electric field in a semiconductor. The logic behind a single interface operation is that only photons possessing energy greater or equal to the band gap in the cell material can displace an electron for the electric circuit. The response of the photovoltaic single junction cell is limited to the sun’s spectrum possessing energy greater than or equating to the band gap (Sonnenenergie 22). The photons possessing energy less than the band gap has no effect on the electrons. To avoid this limitation, the manufacturers of photovoltaic cells can use more than one cell having more than one band gap and junction in generating power from the cells. The term to refer this type of setting is multi-junction, cascade or tandem cells. Multi junction cells are more efficient in sunlight to electricity conversion thus capable of producing higher electricity compared to single junction cells. A multi junction cell comprises of a stack of single junction cells ranked in terms of the band gaps order. As per the arrangement the topmost cell is responsible for capturing the high energy photons. The rest of the photons are passed to the underlying single junction cells possessing lower band gaps for absorption (sciencenasa par 4). The multi junction device is shown in the below diagram Figure 3: The multi junction device Today’s researchers are concentrating more on the gallium arsenide material in the manufacture of multi junction cells. The cells have shown improved efficiency of up to 35 percent under concentrated sunlight. The amorphous silicon and copper indium diselenide are also being studied for the manufacture of multi junction cells. The above multi junction device uses gallium indium phosphide at the top cell, and a tunnel junction that aids in electron flow between the cells. The bottommost cells are made of gallium arsenide as noted by (Garg & Prakash 45) Energy Loss in a Solar Cell The visible light just forms a section of the electromagnetic spectrum which is made up of different wavelengths thus energy levels. From the fact that the light reaching the PV cells has a wide range of energy, some of them possess energy inadequate to change the electron hole pair Garg & Prakash 52). The energies will just pass through the cells as if it was a transparent material. On the contrary other photons have more energy than the required one thus is reflected back to the atmosphere. The band energy of a semiconductor material determines whether the electromagnetic spectrum is to be absorbed, reflected or pass straight through to the other side. If a photon possesses more energy than required to break the band gap, then the extra energy is lost. The photon energy should be approximately equal to the one required to break any electron-hole pair (Neville 177). The band gap also plays a major role in determining the energy derived from the PV cells thus reducing the band gap may have severe effects on the energy produced compared to the lost energy. The band gap determines the voltage of the electric field and if it is low the effects of absorbing more photons is neutralized. The optimal band gap is approximately 1.4 electron vote for any cell made from the same semiconductor material. Figure 4: Solar power for home use Works cited American Solar Energy Society, Solar today (Volume 18), Colorado, American Solar Energy Society, 2004. Eere.energy.gov, Photovoltaics, viewed on 9th August 2010 from http://www1.eere.energy.gov/solar/photovoltaics.html .2010. Garg H. & Prakash P. Solar energy: fundamentals and applications, New Delhi, Tata McGraw-Hill, 2000. Goetzberger A., Hoffmann V. U., Photovoltaic solar energy generation (illustrated edition), Springer, Springer Publishers, 2005. pg 133. Neville R. C., Solar energy conversion: the solar cell (2nd edition, illustrated), Amsterdam, Elsevier, 1995. Science.nasa.gov, How do Photovoltaics Work? by Gil Knier viewed on 9th August 2010 from http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/ 2010. Soga T., Nanostructured materials for solar energy conversion (illustrated edition), Amsterdam, Elsevier, 2006. Sonnenenergie D. G., Planning and installing photovoltaic systems: a guide for installers, architects and engineers (2nd edition, illustrated), London, Earthscan, 2008. Read More