As the world population increases, so does the demand for transportation. Automobiles, being the most common means of transportation, are one of the main sources of pollution. Therefore, in order to meet the needs of the society and to protect the environment, scientists began looking for a new solution to this problem. Before they suggested any answers, the scientists first looked at all aspects surrounding the issue.
In 1994, the United States, for the first time ever in history, imported more that fifty percent of its oil. The cost for importing oil rose to about a billion dollars a day. It is important to understand that about sixty three percent of the oil that the United States consumes is used in the automotive industry. Essentially, the American automotive market depends on imported petroleum. Any disruption between the countries the US imports from can cause the automotive industry to shake, as was seen in the 1970s oil embargo. Another aspect of the problem is the worldís rapid use of its limited resources, such as petroleum. According to the NESEA, if current levels of oil consumption continue, the remaining supply of affordable oil may be depleted in less than fifty years. Although there are many political solutions to this problem, there is also another approach--examining other forms of energy upon which cars may operate.
Perhaps the most notable aspect is air pollution. Cars contribute anywhere from 60% - 90% of all air pollution in urban areas. Every gallon that a car uses makes 22 pounds of carbon dioxide, which in turn adds to the greenhouse effect and other hazards like air pollution. Although technology has succeeded in achieving drastic reductions in tailpipe emissions, the number of cars on the roads has dramatically increase within the recent past years and the total air pollution from tail pipe emissions continues to be a concern. Still, many cars older than 1980 are still on the road without any improvements to the catalyst in the tail pipe control. This drastically affects the urban environment and as well as society's well-being; "60% of Americans live in areas whose air quality fails to meet the standards set by the federal government" (O'Brien, 38). Also, it is very expensive to deal with air pollution control. For example, air pollution costs $9.4 billion per year in South Coast basin of California alone, in health costs. (PS Enterprises).
Solutions for many of such conditions as these makes industry look towards Alternative Fueled Vehicles (AFVs). An AFV is a vehicle that relies on energy other than gasoline, and can either be zero-emission or lower emission. Both the federal government and many automobile manufacturers have invested money, time, and manpower into the research of AFVs in order to satisfy both the consumer demands and environmental regulations. Consequently, research has led to significant development of solar and electric vehicles. Although solar and electric vehicles have the potential to reduce harmful pollutants, they need further research to overcome the limitations that are preventing their widespread use in the near future.
In order to evaluate the viability of solar vehicles, one must understand the fundamental principles governing solar vehicles, which rely on solar cells. These cells date back to the 1950's. The introduction of doped semiconductors marked the beginning of the modern solar cell. Scientists found that adding impurities (dopants) in a semiconductor affected the density of free electrons. There are two main types of semiconductors, n-type semiconductors and p-type semiconductors. The first type, n-type, is formed when an element such as silicon from group IVA in the periodic table is doped with an element such as phosphorus from group VA. Phosphorus, like other elements from group VA, has five valence electrons. When introduced in silicon, it forms a negatively charged semiconductor with an extra electron which can easily be dislodged. The second type, p-type, is formed by doping an element from group IVA with an element from group IIIA such as aluminum or boron. Aluminum, unlike phosphorus, has only three valence electrons in its outer shell. Thus when silicon is doped with aluminum a positively charged semiconductor is created with missing electrons or "holes". These holes behave like electrons, but have a positive charge.
A solar cell or photovoltaic cell is created by joining a p-type and n-type semiconductor. When they are brought together, electron carriers are free to move around. Since the have opposite charges, carriers move toward each other. In the process they form an electric field called a gradient, similarly to the electric field formed within a parallel plate capacitor. This electric field is also responsible for the direction of the electric current.
Solar energy is produced when sunlight strikes the photovoltaic cell. Atoms are stormed by photons and forced to give up electrons. These negative electrons together with the positive electrons the leave behind (holes) travel down the gradient. In the process, an emf current is formed and sent down one side to a circuit and back to the other side. This energizes any electrical or battery found along the way.
Developing solar cells to produce electricity has several big challenges, especially in solar cars. The first obstacle that has remained all intrusive in this quest is predicting the sun's availability. The second obstacle is to find an effective method of capturing, converting, and storing the sunís energy when it is available. Then finally, the last obstacle is to make this solar energy competitively priced to compete with current cheaper energy sources. These three dilemmas are the barriers to creating a reliable energy supply from something as prehistoric as the sun, for vehicles to use.
Forecasting the availability of the sun is fundamental to producing solar-powered vehicles. Making a vehicle that stops running when the sun goes down is not very convenient. Some of the sun's attributes can be precisely predicted. The rotation of the Earth on its axis, or diurnal variation, is predicted with exactitude. The tilt of the Earth's axis, produces a seasonal variation that is likewise predictable. The last predictable variation is an annual one that is due to the elliptical orbit of the Earth about the sun. On the flip-side of this, there are variations that are just as non-predictable as the weather. One variation, for example are the clouds. Meteorologists cannot predict exactly when there will be clouds or rain storms. Things like pollution, dust, or haze are also variations that cannot be precisely predicted. Once the sun does get through the atmosphere, the next challenge is to effectively utilize this solar energy. This is where the solar cells on the vehicle come into play.
The efficiency of solar cells has been steadily increasing with technological improvements during the past fifty years. Most of these advancements have occurred in photovoltaic cells. Crystalline-silicon modules, which are "interconnected arrays of cells that produce enough electricity to be useful," are from 10 to 13 percent efficient (Zweibel, 364). Experimental silicon cells have reached efficiencies of 24 percent. The efficiency of these cells is very important because of the low concentration or intensity of solar energy. On average, about 5 kilowatt hours of energy falls on a flat level surface one meter square (URL, 2). This averages to about 0.2 kW/m2 in a day. This intensity is relatively low considering the fact that a 100 Watt light bulb has an intensity of 12 kW/m2, and an electric stove has an intensity around 25kW/m2 in each burner. Therefore solar energy systems need more area than most conventional energy sources. This is a major problem because space on a car is very limited.
Most of the present photovoltaic cells are made of crystalline silicon wafers. These cells are the best available method of producing solar energy, yet they are still too expensive. Solar generated electricity currently costs around one dollar per kWh. This is very expensive when compared to the price of coal, hydro, or nuclear generated electricity, which goes for about thirteen cents per kWh. Without a competitive edge economically, the production of photovoltaic cells will remain "a cottage industry--too small to take advantage of economies of scale and automation" (Zweibel, 362). The high production cost of solar energy would put solar powered vehicles in a difficult position, especially when competing against electric vehicles. Even though there are some obstacles to solar power and solar power vehicles, once they are overcome, society will be able to fully exploit the advantages of solar energy.
The advantages of solar energy start with the fact that the sun is environmentally sound. The production of solar energy by solar cars does not do any harm to the environment. Solar cars are emission free, and do not deplete any of the Earth's natural resources. After all, they use a resource that is not even on Earth. This leads to the second advantage--sunlight is free. To use sunlight to power vehicles does not cost the consumer anything. Since no one owns the sun, the consumer does not have to worry about paying for gas or electricity. The third advantage to solar cars is that the energy supply is locally produced. This is good, because in some places it's hard to bring electricity or any other conventional means of producing energy to. In places where there is no gas or power lines, solar energy would be most useful. The last advantage solar cars have is that solar power is generated during peak demand for energy. Most people do the majority of their driving during the day, which is the peak time for electricity production. These advantages put together would certainly outweigh the cost of overcoming the obstacles of solar cars, and give a great outlook to future of solar powered vehicles.
Similar to solar vehicles, vehicles powered by electricity hold great potential for reducing emissions of pollutants. Viewed as "zero emission" transportation, electric vehicles have no tailpipe emissions, though they do contribute to the energy demanded from polluting power plants. In addition, the limitations, high costs, and toxicity of the batteries required for energy storage have prevented their widespread implementation. Nevertheless, there have been numerous breakthroughs that suggest that EVs will become more widely used and will play an increasingly large role in efforts to reduce pollution.
The interest in the development of electric vehicles stems from their ability to reduce certain emissions and transfer tailpipe emissions away from urban areas. According to the California Air Resources Board, "electricity and EVs are by far the lowest emission alternative fuel and clean vehicle option available," (O'Brien, 38) and will reduce emissions by 97% compared to gasoline powered vehicles (PS Enterprises). The change in emissions varies for the different kinds of emissions. The greatest reductions occur for reactive hydrocarbons and carbon monoxide, which would be reduced by 99% and 98% respectively, while oxides of sulfur and particulates would increase by 120% and 250%, respectively (Amann, 73). There are conflicting estimates of changes in nitrogen oxide emissions, ranging from a 92% reduction (PS Enterprises) to a three percent increase (Amann, 73). Another consideration is the source of electricity, which varies depending on location. In regions deriving their electricity from hydropower or nuclear power, the reduction in emissions would be greater than in regions depending on coal-burning plants.
An electric vehicle runs on energy stored in mobile batteries stored in the car. These batteries are recharged by simply plugging the batteries into an electric connector from standard voltage electricity. Typically cars are recharged overnight during off-peak electricity times, which could increase the efficiency of the electricity system since power plants would not have to scale down their production at night (ABCs, 34). Since EVs run on electric motors instead of internal combustion engines, there is no need for motor oil, which frequently leaks into groundwater supplies, and the cars themselves run much more quietly. Currently, the most widely-used battery is a lead-acid battery, which contains lead plates in sulfuric acid. Their popularity comes from their relatively low costs and because they are readily available. A promising alternative is the nickel-cadmium battery, which can operate at a wider range of temperatures and last longer, though their price is about ten times greater than for lead-acid batteries. Batteries can be recharged for about 500 cycles before they must be recycled, which involves draining the acid, purifying it and refilling the battery, a process that results in little waste. Other promising battery components include: nickel-metal-hydride, sodium-sulfur, lithium-metal-disulfide, sodium-nickel-chloride, and lithium-ion (ABCs, 35).
Progress in developing batteries has been hindered by several key limitations. One limitation has been the relatively low energy density compared to gasoline, a difference caused in part because gasoline uses air for oxidation, while batteries must carry their own oxidizer, such as lead oxide, thereby adding to the weight. Gasoline provides an average 14,000 Wh/kg, while the lead-acid battery contains only about 50 Wh/kg. Another difference is the slower recharging times of batteries that far exceed a quick trip to a gas station. An average EV can be recharged in three to eight hours, depending on the power source and level of discharge (ABCs, 36). Compounding this slow recharging time is the relatively small driving range of EVs between recharging times, about 100 miles on average. The capacities of the battery are seriously limited by weather conditions. At 40°F, a lead-acid battery operates at 75% capacity; at 0°F, this capacity falls to 50% (Guelden). Another current limitation of batteries is that they cost thousands of dollars, and that they must be replaced after a year and a half to two years.
Despite the limitations of batteries, the tremendous efforts devoted to research have resulted in technological breakthroughs in the development of batteries and EV prototypes. The energy density of the batteries has been extended up to 200 Wh/kg in lithium-polymer and zinc bromine batteries, a level far greater than the typical value of 40 to 50 Wh/kg for lead-acid batteries, but still far below the energy density of gasoline (Oman and Gross). The typical limited driving range of 100 miles between recharging has been surpassed by the BMWE2, which traveled 267 miles on a sodium-sulfur battery, (O'Brien, 41) and German postal trucks, which have a maximum range of 186 miles and can obtain a top speed of 66 mph (Oman and Gross, 29). There have also been improvements in recharging times and lifetime of batteries. "Quick-charging" batteries can be recharged on the order of five to fifteen minutes, depending on the level that they have been drained, though these batteries are still highly experimental (ABCs, 36)
As you can see from the information presented, at the current time neither solar nor electric cars are capable of meeting the demands of the general public. An all-solar-powered vehicle is basically out of the question, unless the efficiency of solar cells is greatly improved. In addition, the current cost shows little hope for solar energy as a main source of energy over nuclear, coal burning, or natural fuel burning. Even though solar energy is still an unreliable source of energy, ongoing research and development has demonstrated great potential for solar energy. For example, scientist are now trying to develop thin-film solar cells which would be more economical to produce than creating silicon wafers.
As far as electric cars are concerned, the few that exist have demonstrated great potential as a main form of transportation. However, in order to meet consumer expectations and demands, electric vehicles technology must continue to improve. Currently, battery improvements seem to pose the greatest challenge. For years now, scientist have tried to find a cheap battery that can store energy in a small, dense area and with quick recharging abilities. Their developments in batteries have brought them close to creating competitive electric powered vehicles, but in consumers' eyes gasoline vehicles continue to overshadow the advantages and capabilities of electric power vehicles.
Even with the technology we possess today it is important that we begin to integrate electric-powered vehicles as part of our daily lives. We start by encouraging more households to begin taking part in this transformation by doing thing like maintaining an electric car for short range travel and a gasoline car for long-range travel. Higher gasoline taxes would also encourage the public to seek alternative means of transportation. In general, it is important that we take advantage of the present solar technology. Already, many environmentally active people are finding ways to incorporate solar energy technology into their homes by doing things like placing solar panels on the roof. This energy is then use to cool or heat their homes. In the future, if large amounts of energy are collected, it could be used to power the entire house including the familyís electric automobiles.
In any case, if electric cars are adopted by the public, power plants will have to develop cleaner and environmentally safer means of producing energy that those in existence (nuclear, coal burning, and natural fuel burning). A solution would be to create large solar-power plants, large enough to produce energy to run a cities. Again, solar cells must improve in order for this to occur.
The bottom line is that electric and solar technology will be an essential part of future low-emissions transportation that will be attractive to consumers and better for the environment. The extent or degree of impact will be defined and depend on technological advances of solar cells and batteries.
The ABCís of AFVs--A guide to alternative fuel vehicles. 33-39.
Amann, C. A. "Technical options for energy conservation and controlling environmental impact in highway vehicles". Int. J. of Vehicle Design. Vol. 14, 59-77.
Electronic Universe Project homepage.
Guelden, M. Centre for Photovoltaic Devices and Systems.
Hesse, P. Energy Efficiency and Renewable Energy Clearing house. personal communication.
Northeast Sustainable Energy Association (NESEA) homepage.
Neufville, R. et al. (1996) "The Electric Car Unplugged". Technology Review. Jan, 32-36.
PS Enterprises homepage.
O'Brien, W. (1993). "Electric Vehicles (EVs): A Look Behind the Scenes". IEEE AES Systems Magazine. May, 38-41.
Oman, H. (1993) "8th annual battery conference on advances and applications". IEEE AES Systems Magazine. May, 2-5.
Oman, H. and S. Gross. (1995) "Electric-Vehicles Batteries". IEEE AES Systems Magazine. Feb. 1995.
Solar Energy Stops on Route 66
Zweibel, K. (1993) "Thin-film photovoltaic cells". American Scientist. Vol. 81, 362-369.
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