Biomass (organic matter) can be used to provide heat, make fuels, chemicals and other products, and generate electricity. Wood, the largest source of bioenergy, has been used to provide heat for thou¬sands of years. But there are many other types of biomass—such as wood, plants, residue from agriculture or forestry, and the organic compo¬nent of municipal and industrial wastes—that can now be used to produce fuels, chemicals and power.

Hydrogen is the simplest element; an atom consists of only one proton and one electron. It is also the most plentiful element in the universe. Despite its simplicity and abundance, hydrogen doesn't occur naturally as a gas on the earth—it is always combined with other elements. Water, for example, is a combination of hydrogen and oxygen (HO). Hydrogen is also found in many organic compounds, notably the "hydrocarbons" that make up many ol our fuels, such as gasoline, natural gas, methanol and propane. Hydrogen is high in energy, yet an engine that burns pure hydrogen produces almost no pollution. NASA has used liquid hydrogen since the 1970s to propel the space shuttle and other rockets into orbit. Hydrogen can be made by separating it from hydrocarbons by applying heat, a process known as "reforming" hydro¬gen. Currently, most hydrogen is made this way from natural gas. An electrical current can also be used to separate water into its components of oxygen and hydrogen.

Hydropower (also called hydroelec¬tric power) facilities in the United Slates can generate enough power to supply 28 million households with electricity, the equivalent of nearly 500 million barrels of oil. Flowing water creates energy that can be captured and turned into electricity. This is called hydropower. Hydropower is currently the largest source of renewable power, generating nearly 10 percent of the electricity used in the United States.

The ocean contains two types of energy: thermal energy from the suns heat and mechanical energy from the tides and waves. Oceans cover more than 70 percent ol the earth's surface, making them the world's largest solar collectors. The sun warms the surface water a lot more than the deep ocean water, and this temperature difference stores thermal energy.

Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds. A barrage (clam) is typically used to convert tidal energy into elec¬tricity by forcing the water through turbines, activating a generator.

For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs, float systems that drive hydraulic pumps and oscillating water column systems that use the waves to compress air within a container. The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.

Sunlight—solar energy—can be used to generate electricity, provide hot water, and to heat, cool and light buildings. Photovoltaic (solar cell) systems convert sunlight directly into electricity. A solar or PV cell consists of semiconducting material that absorbs the sunlight. The solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity A power plant can also use a concentrating solar power system, which uses the sun's heat to generate electricity. The sunlight is collected and focused with mirrors to create a high-intensity heat source. This heat source produces steam or mechanical power to run a generator that creates electricity.

Wind turbines capture the wind's energy with two or three propeller-like blades, which are mounted on a rotor, to generate electricity. The turbines sit high atop towers, taking advantage of the stronger and less turbulent wind at 100 feet (30 meters) or more aboveground. A blade acts much like an airplane wing. When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade. The low-pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift. The force of the lift is actually much stronger than the wind's force against the front side of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a pro¬peller, and the turning shaft spins a generator to make electricity.

Geothermal energy is the heat from the earth. Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles beneath the earth's surface and down even deeper to the extremely high temperatures of molten rock called magma.

A geothermal heat pump system consists of a heat pump, an air delivery system (ductwork) and a heat exchanger—a system of pipes buried in the shallow ground near the building. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to provide a free source of hot water. Hot water near the surface of the earth can be used directly for heat. Geothermal reservoirs of hot water can be drilled into for the generation of electricity. Geothermal power plants use the steam from a reservoir to power a turbine/generator while others use the hot water to boil a working fluid that vaporizes and then turns a turbine.

Fossil fuels like coal, oil and natural gas are nonrenewable, that is, they draw on finite resources that will eventually dwindle, becoming too expensive or too environmentally damaging to retrieve.

In contrast, renewable energy resources such as wind, waves and solar energy are constantly replenished and will never run out.