The following briefly describes renewable energy technologies that might be offered in green pricing programs or competitive markets in your area.
Hydropower is the most mature and largest source of renewable power, producing about 10% of the nation’s electricity. Existing hydropower capacity is about 77,000 megawatts (enough electricity to meet the needs of 35 million households).
Hydropower plants convert the energy in flowing water into electricity. The most common form of hydropower uses dams on rivers to retain large reservoirs of water. Water is released through turbines to generate power. Other hydropower plants, called “run of the river,” do not dam up large amounts of water, but divert water from the river and direct it through a pipeline to a turbine.
Hydropower plants produce no air emissions, but can affect water quality and fish and wildlife habitats. To lessen these impacts, many hydropower projects are diverting a portion of the flow around the dams to mimic the natural flow of the river. Although this helps fish and wildlife, it also reduces the power plant’s output. Fish ladders and other approaches are also being used to assist upstream migration of fish like salmon, and further measures such as improved turbines, offer the promise of reducing fish kills in downstream migration. In addition, low-impact hydropower plants have been designed and are operated to minimize impacts on the river.
Biomass is already this country’s leading non-hydro resource of renewable energy, accounting for more than 7,000 megawatts of installed capacity. Biomass fuels come from a variety of sources. The majority consist of waste from industrial processing—forestry and wood products, agriculture and food byproducts, and other industries such as construction and transportation that must dispose of large quantities of unused biomass.
For utilities and power generating companies with coal power plants, replacing some coal with biomass may represent one of the least costly renewable energy options. As much as 15% of the coal may be replaced with biomass without significant modifications to the existing plant. Biomass has less sulfur than coal; therefore, less sulfur dioxide is released into the air.
Gasification—converting the biomass to a gas and burning it in a gas turbine— is another way to generate electricity from biomass.
Electricity can also be generated using landfill gas, produced by the decay of biomass in landfills. Landfill gas consists largely of methane, which can be burned in a boiler to produce steam for electricity generation or for industrial processes. Thus, this potentially harmful gas can be used in a beneficial way.
Biomass that would go to the landfill can be burned in a power plant instead.
About 2800 megawatts of geothermal electric capacity are produced in the United States annually. Geothermal power plants utilize naturally occurring steam and hot water, which originate under the Earth’s surface. All of the geothermal plants in the United States are in California, Hawaii, Nevada, and Utah.
California already relies on geothermal energy to provide 6% of its electricity.
Geothermal power plants use steam to rotate a turbine, which powers an electric generator. Another growing use of geothermal energy is to heat buildings directly using the hot water. More than 500 megawatts of direct geothermal heating capacity are installed today.
Currently, wind energy capacity amounts to 2500 megawatts in the United States.
Although much of the nation’s wind capacity—about 1600 megawatts—is in California, about half that much capacity was added in the Midwest and Great Plains during the last several years.
Wind energy has been the fastest growing source of energy in the world since 1990, increasing at an average rate of 25% per year, a trend driven largely by dramatic improvements in wind technology.
Wind turbines operate on a simple principle. The energy in the wind is used to rotate blades around a hub. The hub is connected to the main shaft, which spins a generator. Utility scale turbines range in size from 50 kilowatts to one or two megawatts. Small turbines, below 50 kilowatts, are used for homes, farms, ranches, telecommunications, or water pumping.
Photovoltaic (PV) cells—also called solar cells—produce electricity from sunlight. A small, but growing amount of PV generation is connected to the grid in 36 states.
The photovoltaic cell is the basic unit in a PV system. The cell is made from semiconductor materials similar to those used in computer chips. Sunlight is absorbed by the materials, freeing electrons from their atoms, and allowing the electrons to flow through an external circuit to generate electricity. The greater the intensity of the sunlight, the more power generated in the cell.
Since photovoltaic systems use no moving parts to produce electricity, they are durable power systems with low maintenance, high reliability, and low environmental impacts. Because their basic building block, the module, is small, photovoltaic systems are suitable for both large and small electricity supply applications. For example, systems of several hundred kilowatts in size have been built in a number of locations.