Glossary: Technologies / Fuels
Definitions of clean and renewable technologies and fuels, as
well as fossil fuels.
Anaerobic digestor gas: Anaerobic digestion is the breakdown of organic material without the use of oxygen (as opposed to aerobic digestion). When anaerobic digestion occurs, it produces gas that can be used to generate energy. The creation of landfill gas is a common form of anaerobic digestion.
Bioenergy: Energy or electricity produced using biomass as a fuel.
Biodiesel: Biodiesel is a type of biofuel made by combining animal fat or vegetable oil (such as soybean oil or recycled restaurant grease) with alcohol and can be directly substituted for diesel as a stand-alone fuel (called B100, for 100% biodiesel) or be used as an additive (called B20, for 20% bio-diesel). Biodiesel can be used in vehicles (newer cars, usually 1994 or later, are required for B100) and is beginning to be used in on-site electricity generation and heating applications.
Biofuel: Biofuels are renewable liquid fuels made from plant matter rather than fossil fuels. Today’s primary biofuels are ethanol and biodiesel. Biofuels can help reduce air toxics emissions, greenhouse gas buildup, and dependence on imported oil, while supporting United States agriculture.
Biomass: A type of renewable fuels that includes trees and other crops and residues, solid waste, sewage, and liquid fuels derived from agricultural products. Some of the common energy sources derived from biomass are landfill gas, anaerobic digester gas, methane, and biofuels including biodiesel, bio-oil, and ethanol. Biomass gasification is an emerging clean energy technology. See the Bioenergy section of the MTC website for more details.
Biomass gasification: This is a highly efficient process for converting woody biomass (wood chips, pellets, and other wood residues) into energy that can then be converted into electricity.
Bio-oil: Solid biomass can be converted into a carbon-rich liquid which can be used to produce chemicals and fuels. This liquid, or bio-oil, is produced through a process called pyrolysis, in which the biomass is broken down into liquid in an oxygen-free, high-temperature environment.
Clean energy: Clean energy can be generally defined as energy from renewable sources such as biomass, wind, or solar power. The goal of clean energy is to have a low environmental impact, with low or zero emissions, and a minimal impact on the physical surroundings. Hydropower can be defined as clean energy due to zero emissions, but today's hydropower still often has substantial impacts on aquatic ecosystems. Waste-burning and wood-burning plants that capture emissions can be clean energy generators. Fossil fuels do not provide clean energy because of their emissions and environmental impacts. Learn more about clean energy technologies.
Coal: Coal is a fossil fuel that currently provides about half of the country’s electricity. Coal power plants create more emissions per unit of generated electricity than other fuels, and are required to install pollution control devices to curb pollution. Like natural gas and oil, coal is a nonrenewable resource because it cannot be replenished on a human time scale.
Concentrating solar power (CSP): Concentrating solar power plants collect the sun’s energy through different mirror configurations, converting the high-temperature heat collected into electricity through use of a generator. There are three different types of CSP systems: trough systems, power tower systems, and solar dish/engine systems. Each system uses a different method for collecting solar energy. To learn more about CSP, visit the U.S. Department of Energy’s Concentrating Solar Power website.
Ethanol: A biofuel derived from grain and corn that can be used instead of or as an additive to gasoline. Ethanol is primarily used in transportation applications.
Fossil fuels: Fossil fuels (oil, coal, and natural gas) come from the long-term decomposition of plant and animal matter from millions of years ago. These fossil fuels are the main sources of energy used by Americans today to generate electricity, heat, and fuel for transportation. Because fossil fuels cannot be replenished on a human time scale once they are extracted and burned, they are a non-renewable resource. The byproducts of fossil fuel combustion, including carbon dioxide (CO2) and methane, are emissions that increase the “greenhouse” effect that causes global climate change. These emissions also cause many direct air pollution problems like poor air quality and acid rain. Read more about fossil fuels’ environmental impacts.
Fuel cells: A fuel cell is an electrochemical device used to create electricity. Much like a battery, it converts chemical energy to electrical energy. But unlike a typical battery, which holds a limited fuel supply in a sealed container, a fuel cell uses an ongoing supply of fuel to create a continuous flow of electricity. Fuels like natural gas and methane gas are used to produce hydrogen and oxygen. The hydrogen and oxygen are then fed to two terminals in the fuel cell to cause a chemical reaction that produces electricity with heat and water as byproducts. Learn more about fuel cells.
Hydropower (Hydroelectricity): Hydropower, or hydroelectricity, is a clean energy technology that uses moving water to produce electricity. In a hydroelectric system, water flows downstream through a hydraulic turbine that spins and in turn rotates adjacent generators to transform the rotational energy into electricity. When the water exits the turbine it is returned to the stream or riverbed. Hydraulic turbines are generally located near dams that increase the height from which water falls to increase the potential for energy generation. Learn more about hydropower.
Landfill gas: Landfill gas is created when food, wood, and other organic waste in a landfill decomposes under anaerobic – or oxygen-free – conditions. Because landfill gas is about 50 percent methane, it can be used as a source of energy similar to natural gas (which is about 90% methane). Carbon dioxide (CO2) is the other primary component of landfill gas. Since landfill gas is generated continuously, it provides a reliable fuel for a range of energy applications, including heating and electric power generation. Learn more about landfill gas.
Methane gas: Methane is a common, naturally occurring and human-produced gas that can have serious climate change impacts when it is not captured. When captured, it can be used as a fuel. Methane produced by decomposition in landfills and through other human activities can be burned to produce energy for turbines and even fuel cells.
Natural gas: Natural gas is a fossil fuel made of about 50% methane, a potent greenhouse gas. Like coal and oil, natural gas is a nonrenewable resource because it cannot be replenished on a human time scale. According to the U.S. EPA, natural gas power plants provide about 14 percent of the electricity produced in the United States, ranking third behind coal and nuclear power. In New England, its use is much higher, with about 40% of our electricity and the majority of our heating provided by natural gas.
Nuclear energy: Nuclear energy relies on the splitting of uranium atoms in a process called fission, which generates heat for producing steam that then turns a turbine to produce electricity. While nuclear power plants do not emit air pollutants, nuclear wastes and abandoned uranium mines pose health risks from radiation for as long as 250,000 years if not contained properly.
Oil: Oil, a liquid fossil fuel, is used in enormous quantities worldwide. Oil contains carbon, nitrogen, sulfur, mercury, lead, and arsenic, all of which are emitted when oil is burned to produce energy. Advancements have been made in producing cleaner-burning oil; however, its emissions are still significant. Oil is a nonrenewable resource, like coal and natural gas, and oil spills have caused severe damage to natural environments.
Renewable energy: Renewable energy comes from sources that can be replenished on a human time scale, such as biomass (wood), or that are essentially inexhaustible, such as waste and geothermal, wind, and solar energy. Fossil fuels are non-renewable energy sources; there is a finite supply of them. Renewable energy is also often clean energy; it can be generated with few or zero emissions and little to no environmental damage.
Solar heating: Solar heating converts the sun’s power into heat for hot water, space heating, and swimming pools. Passive solar heating uses large windows to let in more light and warmth, while active solar heating uses specially designed mechanical systems to intensify the sun’s heat for use indoors. To learn more about solar heating, visit the U.S. Department of Energy’s Solar Heating website.
Solar hot water: The sun’s power can be used to heat water in homes and commercial buildings, which reduces the need for electricity and fossil fuel use in water heating. Solar energy is collected and used to heat the water, which is then stored in a tank. A conventional system provides extra heat as necessary. Active or passive systems can be installed. To learn more, visit the U.S. Department of Energy’s Solar Water Heating website.
Solar photovoltaics (PV): PV converts sunlight directly into electricity. PV is made from semiconductor materials, and does not create any pollution, noise, or other impacts on the environment. Homes and businesses may incorporate solar panels and arrays as a source of clean energy. Learn more about solar photovoltaics.
Solar photovoltaic array: A series of PV panels can be connected to each other to form an array of any size to provide the electricity needed for a home, office, or larger facility.
Solar photovoltaic cell: A PV cell is the most basic element of a solar photovoltaic system. Each cell is made from semiconductor materials, and creates an electrical charge in reaction to sunlight that can be transformed into a current of electricity.
Solar photovoltaic panel: A PV panel is one or more photovoltaic cells covered with a layer of glass, an anti-reflective sheet or coating, and a backing and frame to provide support for installation. It can be used as a source of clean power for homes and businesses, and multiple panels can be configured into an array.
Solar thermal electricity: See concentrating solar power.
Solar power: The sun's energy can be used to generate electricity, provide hot water, and to heat, cool, and light buildings. This can be achieved using solar photovoltaic panels, concentrating solar power, and passive solar design.
Wind power: Wind power uses the kinetic energy of flowing air to create mechanical energy in a wind turbine that can be transformed into pollution-free electricity. Learn more about wind power and wind turbines.
Wind turbine: Wind turbines are structures that use moving air to generate electricity (wind power) through the use of blades that are easily turned by the wind. This rotating motion is translated inside the turbine into an electric current, which is then interconnected via cables to a nearby electric grid. Learn more about wind turbines and wind power.