Fuels Old and New
Since the late 1700s, around the time of the Industrial Revolution, most of the world’s energy needs have been supplied by fossil fuels. These fuels, which include oil, coal, and natural gas, are found in Earth’s crust and are made of organic material — decayed plant and some animal matter buried underground millions of years ago. Fossil fuels can be burned to produce heat, a useful form of energy. Many power plants convert this heat to electricity, while the furnaces found in many homes transfer the heat through ducts or radiators to warm the air inside the home.
However, the fossil-fuel supply that we have used so well for so long will eventually run out. The U.S. Department of Energy estimates that we will have to turn to alternative power sources within 250 years.
Fossil fuels are known as nonrenewable energy sources; once they are used up, they can’t be replaced. But there are some energy supplies that will never run out. They’re the “renewable” energy sources, and they include the sun (solar power), moving water (hydropower and tidal power), the wind (wind power), and the heat in Earth’s crust (geothermal energy). Wood is also a renewable energy source, because new trees can be planted to replace those used for fuel.
Clean Energy
An energy source is considered clean if it produces no unwanted byproducts, either during manufacture or consumption. Fossil fuels aren't "clean" energy sources. Though packed with energy, fossil fuels release carbon dioxide and pollutants such as sulfur dioxide when they are burned. About 150 years of burning vast amounts of coal, oil, and natural gas has left Earth's atmosphere with increased levels of CO2, contributing to global warming. Sulfur pollutants have contributed to acid rain.
While the sun, wind, and waves are all clean energy sources, even they aren’t ideal. In the case of wind power, for example, only about 40 percent of the available energy in the wind is converted to useable energy in the form of electricity. There are also environmental impacts: Planners have to take care when choosing sites for wind farms, because flocks of birds can collide with the rapidly-moving blades.