The energy resources people use today can be divided into two categories: nonrenewable and renewable.

Nonrenewable Resources

Nonrenewable resources cannot be replenished. We have limited supplies of them, and when these supplies are gone we will not have any more.

Fossil fuels were formed from the fossilized remains of tiny plants and animals that lived long ago. Most electricity used in the world is generated from power plants that burn fossil fuels to heat water and make steam. The highly pressurized steam is directed at turbine blades to make them spin.

The three forms of fossil fuels are coal, oil, and natural gas.

• Coal is a hard, black, rock-like substance made up of carbon, hydrogen, oxygen, nitrogen, and sulfur. There are three main types of coal: anthracite, bituminous, and lignite. The precursor to coal, called peat, is still found in many countries and is also used as an energy source. Coal is found in many parts of the U.S. and throughout the rest of the world.

• Oil is a liquid fossil fuel, sometimes also called petroleum. It is found underground within porous rocks. To obtain oil, companies drill down to deposits deep below the earth's surface using oil rigs. More than half of all the oil we use in the U.S. comes from outside our country—most of it from the Middle East.

• Natural gas is made up primarily of a gas called methane. Methane gas is highly flammable and burns very cleanly. Natural gas is usually found underground along with oil. It is pumped up and travels through pipelines to homes and businesses. Natural gas supplies are abundant from sources in the U.S. and Canada.

Nuclear power uses heat released from splitting atoms to convert water into steam that turns turbines. Nuclear power plants rely on uranium, a metal that is mined and specially processed. Fuel rods containing uranium are placed next to each other in a machine called a nuclear reactor. The reactor causes the uranium atoms to split and in so doing, they release a tremendous amount of heat.

Renewable Resources

Renewable energy resources can be replenished in a short period of time, so they will never be all used up. Energy companies around the country are using renewable resources more and more to generate electricity.

Biomass is organic matter, such as agricultural wastes, wood chips, and bark left over when lumber is produced. Biomass can be burned in an incinerator to heat water to make steam, which turns a turbine to make electricity. It can also be converted into a liquid or gas, which can be burned to do the same thing.

Biomass includes energy crops like wood, straw, and other crops grown primarily for use as a fuel. Energy crops are renewable, but some, like trees, take a long time to grow.

Farmers can grow trees on some of their land instead of wheat or other kinds of food. The wood is harvested regularly, cut into small chips and burned to provide heat or run small electric power plants.

Another type of biomass is methane gas, a by-product of decay in landfills. As garbage rots in the ground, it gives off gases that can be collected and burned to produce heat or electricity.

Geothermal energy is steam (or hot water that has been converted to steam) from deep inside the earth. Our planet's interior is very hot—at its core, 4,000 miles deep, temperatures may reach over 9,000° F. This heat is continuously conducted from the earth's core to the surrounding layer of rock, the mantle.

There are some places around the earth where magma (hot molten earth from the mantle) pushes up through cracks into the crust near the earth's surface. Magma can heat nearby rock and water as hot as 700° F. Some of this hot water reaches the earth's surface as hot springs or geysers, and some stays trapped deep underground in cracks and porous rocks. This hot water can be used directly or converted into steam to turn turbines that generate electricity. (The word “geothermal” comes from the Greek words geo, for earth, and therme, for heat. So geothermal means “earth heat.”)

Hydrogen is a colorless, odorless gas. Hydrogen can be converted into electricity through a chemical reaction in a device called a fuel cell. Converting hydrogen into electricity produces no pollution—only water and heat.

Today, NASA uses hydrogen fuel cells to convert hydrogen into electricity for astronauts. There are already some cars that run on hydrogen. (See next page.) In the future, hydrogen will be used to fuel vehicles and airplanes and to provide electricity to buildings.

Hydropower uses the power of falling water to generate electricity. Water that is stored behind a dam is released and directed through tubes to flow against turbine blades and make them turn. Most hydropower facilities are found in hilly or mountainous areas. Hoover Dam is the most famous hydroelectric facility in the country.

Ocean energy is a form of hydropower. Oceans cover more than 70% of the earth's surface, making them the world's largest solar collectors. The ocean stores thermal (heat) energy, which can be used to generate electricity using special turbine generators.

The energy of the ocean's waves and tides can also be used to generate electricity with dams that force ocean water through turbines. This is called tidal energy, or wave power. The world's first wave power station is on the Scottish island of Islay. It generates enough electricity for about 400 homes. Scientists and engineers around the world are working on systems to use the ocean's energy on a large scale.

Every day, more solar energy falls to the earth than all the people on earth could use in 27 years! Special panels of solar cells, or modules, can capture sunlight and convert it directly into electricity. These panels are known as photovoltaic, or PV. (“Photo” is Greek for light, and “voltaic” pertains to electricity.) The electricity they produce can be used right away, fed into the power grid for others to use, or stored in a battery so it is also available on cloudy days.

Another form of solar energy is used for solar hot water collectors, which allow water to be heated by the sun.

Wind power is renewable energy that uses the force of the wind to spin turbines. These spinning turbines generate electricity.

Most wind power is produced at wind farms, which are large groups of turbines in consistently windy locations. A very large wind farm can generate enough electricity for all the homes in a city of about one million people. Small wind turbines can be used for individual homes, businesses, and boats. They can be used to pump water, or the electricity can be stored in large batteries for use at another time.