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Solar Photovoltaic Cells
Solar energy is a
general term referring to any process that turns sunlight into energy.
Two common forms of solar energy are used today: Solar photovoltaic cells and
solar thermal technology. Solar thermal technology uses the heat
generated from sunlight to create energy. Most commonly, this can be used
to heat water for a house or other projects. Or, with increasing
complexity, it can turn the heat into electricity. Unfortunately, much of
this technology is too expensive and complex to be practical in the
How solar photovoltaic cells work:
A photovoltaic cell uses semiconductor material to transform light into electrical energy. Photons from light hitting the material excite electrons, releasing them from their atoms into the material. Once electrons are excited, they are able to move freely within the material. The semi-conductor then serves to force the electrons in the desired directions. By creating a junction of a p and n type semiconductor, an electrical potential is created. The electrons move from the n-type to the p-type. Meanwhile, the positively charged atoms move from the p-type to the n-type. As a result, the n-type material gains a positive charge and the p-type gains a negative charge. When an electrical circuit connects the p-type and n-type ends, difference in electrical potential is created which results in current.
This type of cell can be manufactured in many different ways. A monocrystalline semiconductor is much like the ideal type described above. It has a pure p type crystal placed on a pure n type semiconductor crystal. This type of cell is the most efficient in terms of turning energy into electricity. But, it is expensive to manufacture because it is costly to produce large crystals of semiconductor material. A far more cost effective material to produce is polycrystalline cells. These consist of small grains of crystals randomly oriented to each other. Because the smaller crystals, much easier to manufacture, are simply placed together it is much cheaper. However, energy is lost as electrons must maneuver between the different crystals. This form of cells results in a lower efficiency. However, because it is the most economically efficient, it is used today.
Another factor important in producing photovoltaic cells is the material used for the semi-conductor. By far the most common material used today is silicon. It is the second most abundant element in the earth’s crust, allowing for an almost limitless supply. Additionally, the energy required to free an electron from a silicon atom is convenient. It is positioned around the infer-red frequency of light allowing any light with energy greater than that to free electrons. Thus, all light in the visible and ultra-violet sphere will free electrons and create electricity. However, any energy of light above that required to free the electrons is wasted as inefficiencies within the cell. Thus, having lower energy thresholds than infer-red, while allowing more light to be absorbed, loses energy from all photons above the threshold for silicon. What occurs here is a trade off – allowing more photons to be absorbed versus losing energy per photon. Silicon absorbs about sixty percent of the light coming into the atmosphere without having too low of a threshold. When this is combined with its cost effectiveness, it becomes the best material to use in photovoltaic cells.
Advantages of photovoltaic cells:
Perhaps the greatest advantage of solar energy is that it is environmentally friendly. The actual operation of the PV systems releases no harmful chemicals into the air. As a result, they are far cleaner than the burning of fossil fuels. However, the cells do create some pollution during the manufacture stage. About 5 to 10 percent of the amount of carbon dioxide avoided from the burning of natural gasses will be released into the air during production. Additionally, silicon dust can be harmful if inhaled – a minor threat that is easily avoided with some precautions. The biggest environmental consideration of PV cells is their disposal. The material contains toxic substances that need to be properly disposed of. This is something that needs to be considered if they are to be manufactured on a large scale. But, in general, because solar cells avoid other emissions that would be required to replace their energy, they are by far one of the cleanest sources of energy available today.
Another advantage of photovoltaic cells is their adaptability. A standard cell produces only .6 to 1.2 volts. As a result, mass systems are created by grouping these cells in series to increase voltage or in parallel to increase current. This makes it is easy to adapt solar cells to ones needs. If small amounts of energy are needed for a house, systems can effectively be built to suit exactly the energy needs of the house. With today’s systems, enough energy can easily be produced to operate lights and television during the night. This allows for dependable energy for houses that are well outside of cities. It is cheaper to buy the cells than to extend the city electrical grid to cover rural houses. Additionally, though not done today, these cells can conceivably be extended to fit the needs of a city. By simply building enough cells, enough electricity could be produced to full mass demands. Another advantage that goes hand in hand with this is that solar cells are low maintenance. In fact, seventy-five percent of the cost of a cell’s lifetime is in the initial manufacturing. As a result, the cells are reliable, not requiring technicians to constantly watch over them, another way they are practical for rural areas.
Finally, the
energy available from the sun is massive and endless. “The amount of
sunlight that reaches the continental United States is about 4,000 times more
energy than is used each year” (64). And, according to researches at
Problems that must be overcome
As was already stated, disposal of PV cells presents a problem if they are ever produced on a mass basis. As a result, solutions will have to be created to deal with these toxic elements to prevent them from harming the environment. But, the risk here is far less than what would be gained by reducing the emissions of greenhouse gasses.
Another problem with solar energy is that it is dependent on the weather. Clouds alone significantly reduce the production of solar cells from their maximum potential. Additionally, the energy output by month varies, reaching its peak in June and its minimum in January. As a result, heavy dependence on solar energy would be susceptible to these fluctuations in output which would vary from day to night, day-to-day, and month-to-month. The system simply is not reliable in terms of producing a consistent amount of energy requiring the addition of expensive storage techniques. Another problem with solar energy is that it produces direct current, while the American standard is alternating current. While it is fairly easy to convert current, this is yet another piece of equipment and inefficiency required for solar cells to work.
A greater problem to solar cells is that they are too expensive. The price for the production of solar energy cannot compete with current coal and natural gas power plants on a massive scale. The technology is simply too expensive right now averaging about 10 cents per kilo watt hour. Conventional forms of energy average about 5 cents per kilo watt hour. In order for solar energy to become economically available, many technology advances need to be improved upon. However, because conventional energy relies heavily on natural resources, its costs are unlikely to be reduced. But, solar energy, where costs are almost entirely in production, stands to gain a lot from gains in technology.
Possible Technology in the future
The most promising technology that can come is more efficient solar cells. Current cells, using silicon, are only able to use the energy required to release an electron from a silicon atom. As a result, over half of the potential energy is wasted because the cells fail to use the excess energy higher frequency light provides. As a result, scientists are experimenting with materials that are a synthesis of atoms so they could more effectively use higher energy light. This would increase the output of cells and consequently make electricity cheaper. But, with current technology these cells cannot be made in an economically effective manner.
Additional technology comes from making the cells more complex. The first is moving the cell with the sun, so that they are always perpendicular. This technology would moderately increase the effectiveness of the cells. However, it makes the system more complex requiring more maintenance. As a result, it is currently impractical to do this at local locations. Instead, it can only be used at plants. Another promising technology is magnification. By magnifying light onto a cell, cost and cell surface area can be reduced by a factor of 1000. But, this makes the cells even more susceptible to clouds and light must hit the cell directly at 90 degrees. As a result, this makes using the solar cells far more complex and reduces their application. Once again, if this technology can be expanded to make it more practical, then solar energy will become more practical.
Finally, storage systems for electricity must be improved for PV cells to be more usable on either local levels or national levels. As was already mentioned, the output of a cell varies greatly by the time of day, between days with different weather, and between months. As a result, excess energy has to be created during peak hours, stored, and accessed during the night. However, today batteries are far too costly a matter for doing this. The batteries must be replaced often driving up costs. And, there is no mechanism for storing energy on the mass basis that would be necessary to make solar energy available on a mass production basis. In order for it to become practical, technology here must be improved.
Solar energy has many promising attributes for the future. Because it requires no water, gas, or costly resources to produce it has potential to become inexpensive. Moreover, solar energy is far cleaner than current conventional energy. It has promising chances of reducing carbon dioxide emissions and thus reducing global warming. But, it sill cannot compete economically with current energy means and as a result, more research must be dedicated into the cause in order for its benefits to be fully realized.
Tom’s experiences using Cornell library