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Glossary of technical terms

Acronym Description
Alternative fuels: Vehicle fuels that displace gasoline or diesel. They include methanol, ethanol, biodiesel, electricity, hydrogen, natural gas, synthetic natural gas, and liquefied petroleum gas.
Biogas: Methane and other gases produced during the decomposition of biomass.
Biomass fuels: Living materials or once-living materials (i.e. wood, vegetation or waste from
BTU: British Thermal Unit. The amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit.
Carbon dioxide (CO2): The greenhouse gas whose concentration is most directly affected by human activities. CO2 also serves as the reference to compare all other greenhouse gases. The major source of CO2 emissions is fossil fuel combustion. CO2 emissions are also a product of forest clearing, biomass burning, and non-energy production processes such as cement manufacturing. Atmospheric concentrations of CO2 have been increasing at a rate of about 0.5% per year and are now about 30% above pre-industrial levels.
Clean energy: Electricity that when generated, does not produce pollution.
Cogeneration: Generation in which both electricity and the accompanying heat is captured and used for space heating, water heating, or absorption chillier systems.
Concentrating solar power (CSP): Concentrating solar power refers to technology that generates electricity from the sun's heat, unlike photovoltaic cells, which use light to produce electricity. There are three main types of CSP technology: dish systems, trough systems, and central-receiver systems. For more information, see the Energy Efficiency and Renewable Energy Network of the Department of Energy website or the National Renewable Energy website.
Concentrators: A tool that uses lenses and/or mirrors to focus and enhance the sun's rays. In solar applications, concentrated solar energy is focused to heat a fluid or onto PV cells.
Conventional generation: Conventional electricity generation methods including fossil fuel, nuclear and large hydroelectric resources.
Diesel cycle: The diesel cycle is similar to the Otto cycle except that the ignition spark results from the heat generated during extreme compression of a flammable mixture inside the cylinder, and not a spark plug.
Digester gas: Biogas produced during the decomposition of biomass.
Distributed generation: Electric generation capacity located on or near an end user's facility. Generally these systems are small in comparison to conventional generation plants.
DOE: United States Department of Energy.
Electrolysis: The process of passing electricity through water to separate the hydrogen and oxygen into separate gases.
External combustion: Term used to describe engines in which the working fluid of the engine never comes into direct contact with the heat source, but is heated and cooled through the use of heat exchangers. flat plate solar collector - A type of solar energy system that features a flat (typically glass) face (which serves as the solar transmitting window and a thermal insulator) and a target (either a flat metal plate or a sheet of semi-conductive material) that absorbs the sun's energy.
Federal Energy Regulatory Commission (FERC): FERC is an independent federal regulatory agency that regulates interstate oil, natural gas, and electric matters and oversees environmental issues related to these activities. FERC also licenses and inspects state, municipal, and private hydroelectric projects. For more information visit
Fossil fuel: Any naturally occurring organic fuel, such as petroleum, coal, and natural gas.
Fuel cell: A device that converts the energy of a fuel directly to electricity and heat without combustion.
Generator: A machine that converts mechanical energy into electrical energy.
Genset: A generator set. Generator sets generally include an engine that drives a generator.
Geothermal energy: Geothermal energy is the natural heat of the earth stored deep below the earth's surface. It can be in the form of steam, hot liquid, or hot dry rock. Wells drilled deep into the ground bring steam and hot water to the surface. The steam, or steam produced by the fluids in a heat exchange process, is used to drive a turbine generator to make electricity. Modern technology allows spent geothermal fluids and non-condensable gases to be reinjected back into the ground, eliminating surface disposal and air pollution.
Gigawatt: One-billion watts. (See "watt")
Greenhouse gas: Any gas that absorbs infrared radiation in the atmosphere. Greenhouse gases include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halogenated fluorocarbons (HCdFCs), ozone (O3), perfluorinated carbons (PFCs), and hydroflurocarbons (HFCs).
Grid: An electric utility's system for distributing power.
Hybrid system: For a solarized heat engine, a hybrid engine is one that has been designed to operate with solar power or another heat source, such as natural gas. In principle, a hybrid system could provide power 24 hours per day, using solar energy when the sun is available and another energy source the rest of the time.
Hydrogen Fuel: Hydrogen fuel is simply hydrogen gas, H2, the lightest element in the universe. Hydrogen gas will burn in the presence of an ignition source and air, combining with the oxygen present in the air. The by-product of hydrogen/air combustion is water vapor and heat. Alternately, hydrogen fuel can be combined with oxygen in a fuel cell to produce electricity, heat, and water vapor.
Hydrogen Production: Hydrogen that can be produced from a variety of feedstocks utilizing a variety of different process technologies. Feedstock options include fossil resources such as coal, natural gas, and petroleum, and renewable resources such as biomass and water. Process technologies include thermochemical, biological, electrolytic and photolytic.
Independent power producer (IPP): A cogenerator which produces and sells firm power under contract to the utilities.
Internal combustion: Term used to describe heat engines in which the working fluid of the engine is mixed with and comes into direct contact with the heat source and then exhausted after the expansion process.
Kilowatt (kW): One thousand watts. (See "watt")
Kilowatt hours (kWh): One thousand watt hours.
Kyoto Protocol: A Protocol to the United Nations Framework Convention on Climate Change agreed by participants at the Kyoto Summit (Conference of Parties 3) in December 1997. It commits industrialized countries to firm reductions in greenhouse gas emissions. The United States is to reduce emissions by 7% below 1990 levels by the years 2008-2010. The U.S. signed the Protocol in November 1998, but it has not been submitted by the Administration to Congress for ratification due to Congressional concerns about the lack of requirements for emission reductions by developing nations.
Megawatt (MW): One million watts. (See "watt")
Micro-generator: Generating systems less than 1 MW in size.
Micro-turbines: Small combustion turbines in the size of less than 500 kW.
Natural gas: A naturally occurring mixture of hydrocarbon and nonhydrocarbon gases found in porous geological formations beneath the earth's surface, often in association with petroleum. The principal constituent is methane.
NREL: National Renewable Energy Laboratory.
Otto cycle: The Otto Cycle is an idealization of the thermodynamic cycle used in standard spark-ignition internal combustion engines. The cycle consists of four stages: isentropic compression, constant volume heat addition, isentropic expansion, and constant volume heat rejection.
Parabolic collector trough: A system that tracks the path of the sun by pivoting on one axis (typically east-west or north-south), using shiny parabolic troughs to heat the collector fluid that passes through a tube at the focus. (Also sometimes referred to as Linear Focus Concentrating Systems.)
Photovoltaic (PV): A renewable energy technology that converts the sun's light, not its heat, directly into electricity. Sunlight shining on specially treated cells or film produces direct-current (DC) electricity. The solar cells are made of thin layers of material, usually silicon. The layers, after treatment with special compounds, have either too many or too few electrons. When light strikes a sandwich of the different layers, electrons start flowing, and an electric current is produced.
Power conversion unit: A system that converts potential energy in the form of fuels into electricity.
Power tower: A type of concentrating solar power that uses a field of mirrors to track the sun and focus its light onto a single point on a tower. At this focal point, a fluid (typically air, water, or molten salts) is heated and passed through a steam turbine to generate electricity. Also referred to as "central receiver" system.
Public utility commissions (PUCs): State regulatory agencies that provide various forms of guidance and oversight to electric utilities in each commission's respective state.
Renewable energy: Energy from sources that cannot be exhausted: sunshine, water flow, wind and vegetation.
Solar dish concentrator system: A system that uses mirrors clustered in the general shape of a parabolic dish to focus solar energy onto a heat engine that is positioned at the focal point of the mirrors. The heat engine converts solar energy into electricity.
Solar thermal energy systems: Systems that either absorb or reflect solar light in a device to produce heat energy. Examples of solar thermal energy systems are solar dish concentrators, power towers, parabolic collector troughs, or flat plate solar water heater systems.
Stirling cycle: The Stirling cycle is a thermodynamic idealization of an external combustion heat cycle in which the working fluid inside the engine is contained within a closed-loop system. The heating and cooling of the working fluid is achieved through metal heat exchangers, with the fluid never coming in direct contact with the heat source. Key advantages are that none of the internal parts become fouled by the combustion process and that practically any fuel source can be used. An ingenious regenerator helps to capture and use some of the waste heat from a previous cycle to heat the current cycle, making it one of the most efficient engines in use today. The idealized cycle consists of four stages: isothermal expansion and heat addition, constant volume heat rejection into the regenerator, isothermal compression and heat rejection, and constant volume heat recovery from the regenerator.
Trough system: A type of concentrating solar power that consists of mirrored troughs that focus the sun's energy onto a pipe receiver positioned at the focal point of the troughs' reflectors. A special oil solution is circulated through the receiver piping to collect the solar hear energy and then passed through a heat exchanger to generate steam that drives a turbine electric generator.
Watt: Unit of power. One watt is equal to one Newton-meter of work per second.