The World Bank definition for alternative versus renewable energy differs. It uses a hierarchical strategy to categorize energy forms. Alternative energy represents the broadest category of energies that are not based on fossil fuels or nuclear energy. These energy forms are thought to produce fewer greenhouse gases than the fossil fuels they are intended to replace. Within this broad category, there are two subcategories of energy sources or strategies for greenhouse gas reduction: renewable energy and improved energy efficiencies.
Renewable energies can be further sorted into energies used to generate electricity and energies used for transportation. While there is certainly overlap between these two categories (for example, solar energy can generate electricity for distributed use in homes and it can also be used to power vehicles), it makes sense to examine the efficacy of renewable energies by starting with these two categories.
Electricity is produced by finding a way to convert kinetic energy (energy associated with movement) into electrical energy. A generator typically does this. A generator consists of a rotating magnet, called a rotor, and a stationary coil of copper wires known as a stator. By turning the rotor continually past the stator, an electrical current is generated. The challenge is to find a source of energy to move the rotor. In simplified form, a rotor is attached to some type of turbine or propeller that is forced to turn by some external force, usually water, steam, or air. Fossil fuels such as coal, natural gas, and oil, and nuclear fission can be used to generate heat. This heat is used to convert water into steam within a confined space. As steam is created, it expands, creating pressure. This pressure can be directed at a turbine, and when released in a controlled manner, can cause the turbine to turn.
The problem with using fossil fuels for generating electricity is that the combustion of fossil fuels to create the heat needed to rotate turbines creates carbon dioxide and other greenhouse gases. Furthermore, these forms of energy are nonrenewable, and the extraction of fossil fuels through mining and oil drilling itself requires considerable amounts of fossil fuels. The process also transforms the landscape in profoundly destructive ways.
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