Fossil fuels are nonrenewable because they take millions of years to form and cannot be replenished in a short time. Students analyze the benefits and drawbacks of alternative energy sources (specifically ethanol) in this lesson.
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Renewable versus Nonrenewable
The United States uses 18 million barrels of crude oil a day. Without it, many things that we take for granted wouldn’t be able to occur. For example, factory machines would stop working. Cars and airplanes wouldn’t be able to move. Many common goods, including plastics and fertilizers, wouldn’t exist. But oil, natural gas, and coal are all nonrenewable resources because they take millions of years to form and therefore cannot be replenished in a short time.
Fossil fuels pose another challenge. Burning them produces pollutants that are released into the atmosphere. One such pollutant is carbon dioxide, a gas that traps heat in Earth’s atmosphere. This is called the greenhouse effect. The greenhouse effect is essential for life on Earth, but it relies on a balanced amount of carbon in the atmosphere. Carbon dioxide released from fossil fuels upsets the natural balance.
Fossil fuels currently supply about 80 percent of the world’s energy. So scientists and engineers are trying to find energy sources that are renewable—can be replenished in a short period of time—and are less polluting. Examples of renewable energy include solar energy, wind, geothermal energy from Earth’s internal heat, hydropower from water, and biomass (organic material made from plants and animals).
Using Biomass for Energy
Scientists have known for many years that biomass can be used for energy. People first discovered that wood from trees could be burned to produce thermal energy hundreds of thousands of years ago. Like fossil fuels, biomass contains potential chemical energy because it was once living. When biomass such as firewood is burned, that energy is transformed to thermal energy that can be used to do work.
For example, people have been using fire as an energy source for a million years. When a pot of food is put over a fire, such as in the image to the right, the thermal energy is transferred to the pot, and then the food inside the pot through conduction. This transfer of energy causes the pot and the food to warm up.
However, burning wood in a fire isn’t very efficient. A lot of the energy is transferred to the atmosphere. Because of the conservation of energy, the heat that is warming the air is not warming the pot or the food. When some energy is transferred out of the system and into the air, it means there is less energy available to do work—in this case, warming the pot and the food.
Producing Ethanol
In the last hundred years or so, scientists discovered that many plants have sugars that can be converted to usable energy. Ethanol is a renewable fuel that is made from the sugars found in plant matter such as corn and sugarcane. Wheat, barley, and potatoes can also be used to make ethanol.
Ethanol is made by fermentation—a chemical reaction in animal or plant matter brought about by microscopic yeasts, bacteria, or molds. Fermentation results in the breakdown of complex organic substances into simpler substances. Fermentation is a common process. If you have ever eaten bread, cheese, chocolate, or yogurt, you have had fermented food.
Yeast are microscopic organisms that get energy by eating simple sugars, such as glucose, that are found in most plants. The yeast chemically break down the sugar into substances that it can use. When yeast and sugar combine, a chemical reaction occurs that produces ethanol (ethyl alcohol) and carbon dioxide.
Ethyl alcohol can be blended with gasoline or used by itself as a fuel. Since 1978, Americans have driven more than two trillion miles (80,000 trips around the world) on ethanol-blended gasoline. When combined with gasoline, ethanol helps the fuel burn more cleanly. Some estimates say ethanol produces 25 percent fewer greenhouse gas emissions than fossil fuels.
Benefits and Drawbacks of Ethanol
But there is some controversy over the widespread use of ethanol. The United States currently produces about 2 billion gallons of ethanol each year, primarily from corn. Critics worry that the focus on fuel reduces the amount of land that is available to grow corn for food. It also requires a lot of energy to grow corn. In addition, farmers rely on fertilizers, which are often made from fossil fuels. If people do not replant corn or other biomass sources as fast as they use them, biomass energy becomes a nonrenewable energy source.
There is also debate over the amount of energy needed to actually produce the ethanol. A gallon of pure ethanol contains 34 percent less energy than a gallon of gasoline. Some studies suggest that it takes more energy to produce ethanol than the amount of energy that ethanol provides. Other studies show the opposite, so the research is ongoing.
Scientists are exploring alternative sources of biomass, looking at plant material that is not a food source. Cellulosic ethanol is a promising area of research, although much more still needs to be learned. Cellulosic ethanol comes from sources that are not used for food, such as corn stalks, switchgrass, and wood chips. Plant cell walls are made of cellulose, which is a type of sugar molecule. Cellulosic ethanol is more complicated, and therefore more expensive, to make than it is to make ethanol from corn.
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