Our research group is interested in alternative energy because it will become necessary in the very near future. In particular we have chosen to focus on solar




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НазваниеOur research group is interested in alternative energy because it will become necessary in the very near future. In particular we have chosen to focus on solar
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Photovoltaics

Our research group is interested in alternative energy because it will become necessary in the very near future. In particular we have chosen to focus on solar technology because we feel it is one of the most promising alternatives available and it is very sustainable. For our benefit we chose to interview Professor John Abelson from the Materials Science and Engineering department, who gave us interesting insight into the field of solar technology application.

Manipulating the energy provided by the sun for human purposes is an old practice beginning with conversion from light energy to heat energy. The little child who takes a magnifying glass to burn holes in leaves or to melt toy soldiers is engaging in one of the oldest uses of the sun’s energy, as it is estimated that the use of magnifying glasses to create fire first started in the 7th century B.C. (History of Solar). Although the story is unproven, it is said that in 212 B.C. the Greek scientist Archimedes directed the soldiers of the Greek army to use bronze shields to reflect the sun at enemy ships offshore, setting them aflame (History of Solar). The beginning of the era of solar energy as we tend to think of it today, the process of converting light energy to electrical energy, began in 1839 when the French scientist Alexandre Edmond Becquerel discovered the photoelectric effect. He had been experimenting with an electrode placed in an electrolyte solution and saw that a voltage difference developed as light struck the electrode (Lenardic).

Although Becquerel’s discovery was a significant scientific milestone, no practical use was made of it for decades. The very first solar cell to ever be created was made by Charles Fritts in 1883 and consisted of the semiconductor selenium and a very thin layer of gold (Bellis). Since it converted less than 1 percent of the sun’s rays into electricity, it was incapable of actually powering electrical devices. Richard Ohl is credited with being the first to use silicon as the semiconductor material in solar cells, and his method was applied with success by researchers at Bell Laboratories, beginning the first of three “generations” in solar cell technology. In 1954 the researchers developed a silicon-based solar cell capable of achieving 6% efficiency under direct sunlight (Bellis). Five years later Hoffman Electronics released a commercially available cell which broke into the territory of 10% efficiency. Since then efficiency has increased to levels as high as 40.8%, attained in September of 2008 by scientists working at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL).

This first generation of solar technology defined by bulky silicon-based solar cells is still an active research field, though two new generations of solar cells began to emerge in the late 90s and the beginning of the current millennium. Second generation cells are defined by their design which is aimed at cutting production costs associated with traditional, first-generation cells. To accomplish this, researchers have found ways to use less semiconductor material and to use new semiconductor materials other than silicon. Additionally, they are using methods to create cells which require less energy – as traditional cells require high temperature processing. Third generation cells are similar to those of the second generation, but are distinguished by a concentration on increasing their relatively low efficiency (~10%) to values in the 30 to 60 % range (Green).

In order to better understand these advances being made in photovoltaic technology, it is vital that we first understand how such devices function. All solar power works by converting the energy carried by photons into electrical energy, but there are several different mechanisms for doing so. One large scale mechanism uses the conventional steam turbine system. A large field of light-concentrating mirrors, referred to as Heliostats, concentrates and reflects the sunlight into a tower filled with water; the water is heated into steam and turns a turbine to generate electricity. Another, newer, method of solar power uses a large glass greenhouse with a tower in the middle to create convection currents. The air under the greenhouse is heated above the ambient temperature, causing it to rise through the tower and turn a turbine, generating electricity. Both of these solar power technologies are useful for large scale power production, but are not yet efficient enough to replace coal burning or nuclear power plants, nor are they useful for small scale power generation (Kolb).

The most common form of solar power generation is the photovoltaic cell. The photovoltaic effect is achieved by doping p-type silicon. Doping refers to the process of introducing impurities into the crystal lattice of the semiconductor; in this case the dopant is another type of semiconductor, n-type. P-type silicon is created by a different doping process. Pure silicon is doped with another atom, usually aluminum or boron. The doping material takes weakly bound electrons from the silicon atoms, causing some of the silicon atoms to become positively charged. These ionized silicon atoms are referred to as “holes”. N-type silicon is created using a similar doping process, but replacing the dopant with phosphorous or arsenic. These atoms have extra weakly-bound electrons which are transferred to the silicon atoms, giving the silicon excess charge carriers (Wurfel).

In a photovoltaic cell, the location where p-type and n-type silicon come into contact with each other is called a p-n junction. At this p-n junction the electron concentration gradient created by the imbalance of charges causes electrons to flow from the n-type silicon into the p-type silicon. This cannot happen indefinitely because the diffusion of electrons between the two types of silicon creates a depletion region at the junction where there are no mobile charge carriers; the silicon directly on each side of the junction has either filled a hole or lost an electron to fill a hole. The charge difference at the junction creates an electric field which causes the junction to act like a diode since charge can only flow one way. The electrons can only flow from the p-type silicon to the n-type silicon and the holes move oppositely.

If a photon hits the cell and has enough energy, it excites an electron, allowing it to move freely within the cell. When the electron moves away from the atom it had previously been attached to, it creates a hole. This hole can be filled by other electrons from nearby n-type silicon atoms, allowing holes to move across the cell as well as electrons. However, the electrons can only flow one way across the junction, so there is soon a large negative charge on the n-type silicon and an equally large positive charge on the p-type silicon. Electrical contacts are connected to both sides of the junction, and the charge difference creates a potential difference between the contacts. The solar cell is then connected to a circuit and can provide voltage. As the electrons in the n-type silicon are depleted, the electric field in the middle weakens, and some electrons are able to drift across the junction back to the n-type silicon, replenishing its supplies of electrons and allowing it to continue to generate electricity (Moser).

Scientists have been working diligently for several years now to improve the efficiency of solar cells. The very first solar cell, made from selenium with a thin layer of gold, was only about 1% efficient. The first commercial solar cells made from doped silicon were about 6% efficient. In 1988, 17% efficiency was achieved using a single-junction GaAs solar cell. The theoretical maximum efficiency of a single junction is 33%. The dual junction cell was made with the accidental doping of Ge with a GaAs layer. The first dual junction solar cells that were used on spacecraft had an efficiency of about 20% and dual junction cells would finally reach 22% efficiency. Triple junction solar cells began with 24% efficiency in 2000 and reached 30% efficiency in 2007.

A large concern under scrutiny today is how to increase efficiency in these ways while keeping cells economical. One way to increase efficiency is to try to trap some of the light so the cell can turn it into electricity. A new technique developed by Suniva uses a textured surface and a mirrored layer at the back of the silicon (Moon). This design tries to capture as much of the solar energy as possible by making the light bounce inside the cell for longer than conventional cells. Optics can also be used to concentrate the light in order to gain more efficiency. This allows for the production of a smaller cell using less silicon, which is relatively expensive. Optical concentrators have the side effect of increasing the surface temperature of the solar cell. The cell then has to be cooled with heat sinks or an active cooling system. 1366 Technologies is starting to produce new solar cells that are more efficient using a textured surface combined with two other improvements: smaller silver wires that harvest the current from the silicon and etched collection wires that become a mirror to reflect the light back into the cell (Bullis).

While improving efficiency is obviously a major factor in the development of photovoltaics, scientists are also looking for ways to bring solar technology to the mass markets. One of the biggest challenges facing wide use of solar technology is the cost. With cheaper sources of energy available for mass consumption, such as fossil fuels and nuclear power, solar energy will have to become significantly cheaper before it can reasonably be used on a large enough scale to make a global impact. Right now researchers are exploring a variety of different ways to lower the cost of producing, housing, and maintaining solar cells.

One idea that has been suggested is to use a different type of furnace to produce silicon used in solar cells. Silicon is very abundant; it makes up 25.7% of the Earth’s crust. However, it is found mostly in compounds, and pure silicon is needed in order to make good, efficient solar cells. The silicon has to be at a purity of 99.9999% in order to make silicon wafers. To reach this level of purification, certain reactions must take place at extremely high temperatures. The induction furnace uses an inductor coil to heat the silicon to a molten state and keep it that way to remove impurities. This process would make mass production of solar-grade silicon much cheaper. The current price of silicon of this purity costs roughly $400 per kilogram. Hopefully with processes like these, the price of pure silicon can be brought down, and in turn, the price of solar cells will fall.

In addition to cheapening the process of creating the solar panels themselves, the way in which the solar panels are housed may also prove to be an effective way to lower cost. A cell-producing company, Dow Corning, is going to cheapen the cost of solar cells by using a different substance to protect them on a panel. In opposition to the traditionally-used ethyl vinyl acetate resin, Dow Corning has found a process that can allow solar cells in a panel to be protected by a clear laminate instead. The laminate would give better efficiency and better durability. Also, the production of the cells is cheaper and requires less factory space. This type of solar packing could greatly reduce the cost of solar cells.

Although the efficiency of the solar cells is vital to making solar cells cheaper, the efficiency of the plants themselves may also prove to be a good way to reduce costs. A software company, Magma Design Automation Inc, is designing a version of their previous product, YieldManager, to be used to build solar cells. The program would carefully and constantly make sure that all the silicon wafers being produced are of optimum efficiency, and if they are not, it would have the ability to immediately return the process to its ideal operation. This will keep waste to a minimum, and because the system is automated, the entire process will be done faster and in higher numbers. Both of these will effectively reduce cost of each individual silicon wafer, in turn reducing the cost of each solar cell.

At this point we have looked at several advances in photovoltaic technology; we have explored how their efficiency is being improved upon as well as their shift into a more mass marketable technology, but the question arises—just how practical is solar energy? There is not a simple answer to this question since many factors must be considered when debating whether or not the use of solar cells is actually an economical option. These factors, such as the cost and robustness of materials, location, impact on the environment, and government support through funding, all play a major role in the choice to employ solar cell technology (Abelson). These factors mean the choice to use solar energy must truly be considered in a case by case manner.

For example, Germany leads the world in use of photovoltaics (Susman). This might seem curious at first regarding what we know about western European weather and the low average amount of direct sunlight the country receives. So what makes solar energy such a commodity for Germans? Traditionally, electricity is generated at plants that burn very expensive imported fuel; in order to cushion the expense of fuel consumption, the German government has set up programs that reward companies for utilizing alternative energy, thus creating a large demand for solar cells (Susman). As a result of the increased use of alternative energy sources, including solar energy, Germany is able to save money that it would normally spend on expensive fuel imports making solar energy an economical choice for the country.

Where else might photovoltaics prove to be economical? A certain balance of factors must be achieved. With Germany, the location isn’t ideal to maximize the energy output of the photovoltaics, however, the reduction of money the government spends on fuel imports balances out the less than maximum output of the solar cells. We hear of solar energy often being used to generate power in developing countries. Often in off-grid locations such as these it costs less to install solar panels than it would to run lines from a major power plant, so photovoltaics are also economical as a main source of power in smaller, remote areas. However in large on-grid areas, the installation of adequate solar energy systems to replace the power generated by a main plant would be more expensive to implement than to stick with the plant and simply utilize solar energy as a supplemental power source (Abelson).

There are infinitely many cases like these that can be considered, and each has factors unique to itself that must be weighed. The use of photovoltaics to replace traditional polluting power generating methods might seem like a no-brainer, but as we can see, this is not always a practical or economical solution.

We have learned that the photoelectric effect was first discovered about 150 years ago, and has since gone on to inspire scientists to create solar cells using multi-junction semiconductors capable of up to 40.8% efficiency. Work is also under way to make solar technology more accessible to the public and private sectors, which includes increasing efficiency while decreasing costs. We also explored whether or not this strategy would be economical in various situations. Given the economic ramifications we have learned of, solar cells are a good solution when and where they meet the balance between cost and efficiency required by the specific task. Finally, we still have yet to learn what the future of solar cells will look like and just how far they will take us in replacing traditional energy sources.

To learn more about this field, you may wish to take the following courses offered at the university:

  • MSE 304: Electronic Properties of Materials

  • MSE 498: Special Topics: Photovoltaic Cells: Materials, Devices and Performance

Works Cited

Abelson, John R. Personal interview. 14 Oct. 2008.

Bellis, Mary. "History: Photovoltaics Timeline." About.com. 3 Apr. 2008. 21 Oct. 2008 .

Bullis, Kevin. "More Powerful Solar Cells." Technology Review. 27 Mar. 2008. Massachusetts Institute of Technology. 15 Oct. 2008 .

Green, Martin A. "Third generation photovoltaics: solar cells for 2020 and beyond." Physica E: Low-dimensional Systems and Nanostructures 14 (2002): 65-70.

The History of Solar. United States of America. Department of Energy. Energy Efficiency and Renewable Energy. 5 Jan. 2006. 24 Oct. 2008 .

Kolb, Gregory J., Michael Geyer, and Manuel Romero. Concentrating Solar Power in 2001. SolarPACES. Comp. Craig E. Tyner. Almería, Spain: SolarPACES, 2001. 1-16.

Lenardic, Denis, ed. "History of Photovoltaics." PV Resources. 7 Feb. 2008. 22 Oct. 2008 .

Moon, Mariella. "Suniva Promises Cheap and Efficient Solar Cells." Good Clean Tech. 25 Sept. 2008. 12 Oct. 2008 .

Moser, Jacques E. The Basic Physics and Design of III-V Multijunction Solar Cells. Lausanne, Swizterland: Ecole Polytechnique Fédérale de Lausanne, 2006.

"NREL Solar Cell Sets World Efficiency Record at 40.8 Percent." ElectricalEngineer.com. 29 Sept. 2008. 14 Oct. 2008 .

Susman, Gerald I. Evolution of the Solar Energy Industry: Strategic Groups and Industry Structure. 27 July 2008.

Wurfel, Peter. Physics of Solar Cells : From Principles to New Concepts. New York, NY: John Wiley & Sons, Incorporated, 2005.

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