Renewable energy sources are fuels that can be naturally replenished, such as wind, solar, biomass, geothermal, and other sources. These non-hydro renewables generate 6.2 percent of the nation's electricity. The U.S. Energy Information Administration (EIA) projects that non-hydro renewables will generate 10.6 percent of our nation’s electricity in 2040.
The Role of Renewables
Expanding the use of renewable energy sources is an important part of meeting the electric power industry's goal of reducing greenhouse gas (GHG) emissions. To increase the share of renewables in the nation’s fuel mix, electric companies support tax credits and increased funding for research and development, in addition to renewable energy programs in the states. In fact, 30 states and the District of Columbia have established renewable electricity standards (RES), also referred to as renewable portfolio standards (RPS).
Generating Renewable Electricity
Electricity is generated from renewable sources in the following ways:
- Wind is largely determined by weather patterns, the surface of the Earth, and bodies of water. The flow of air masses is harvested by wind turbines that capture the kinetic energy in surface wind and transform it into mechanical or electrical energy.
- Solar power uses photovoltaic (PV) cells convert sunlight directly into electricity. When sunlight strikes a PV cell, electrons are dislodged, creating an electrical current.
- Biomass fuel is burned in a boiler to produce high-pressure steam. This steam is introduced into a steam turbine, where it flows over a series of aerodynamic turbine blades, causing the turbine to rotate. The turbine is connected to an electric generator, so as the steam flow causes the turbine to rotate, the electric generator turns and electricity is produced.
- Geothermal energy is generated from hydrothermal (hot water/steam) resources. There are two types of processes currently in use: steam and binary. Steam plants use very hot (more than 300° F) steam and hot water resources (as found at The Geysers plants in northern California-the largest geothermal electricity producer in the world). The steam either comes directly from the resource, or the very hot, high-pressure water is depressurized ("flashed") to produce steam. The steam then turns turbines, which drive generators that generate electricity.
Binary plants use lower-temperature, but much more common, hot water from geothermal reservoir resources (100° F - 300° F). The hot water is passed through a heat exchanger in conjunction with a secondary (hence, "binary plant") fluid with a lower boiling point (usually a hydrocarbon such as isobutane or isopentane). The secondary fluid vaporizes, which turns the turbines, which drive the generators.
Renewable Energy Technologies
Wind research energy focuses on two goals: increasing the technical viability of wind systems, and increasing the use of wind power in the marketplace. Emerging technologies include those that use low wind-speeds as well as technologies that reduce the cost of electricity generated from wind systems.
Solar research is concentrating on reducing the cost of the photovoltaic system and on the use of solar-energy technologies to produce hydrogen, including solar-thermal and direct-conversion approaches.
Biomass research is investigating development of transportation fuels, commodity chemicals, and combined heat and power technologies for the utility market. Research areas include fermentative organisms and other means for biological conversion of sugars, developing products from the lignin that will be generated along with sugars, catalytic conversion of synthesis gas to valuable chemicals, and fermentation of synthesis gas to ethanol.
Geothermal plants pose special challenges: non-condensable gases and minerals in the geothermal fluid, the need for a greater amount of heat rejection, the use of hydrocarbon fluids, and lack of cool water to cause condensation. The development of new technologies that will improve heat-exchange efficiency, lower the equipment-damaging effects of the sometimes corrosive geothermal fluid, and improve the plant's condensing capability, thus, increasing the efficiency and reducing the operating cost.