The National Academies

The National Academies: What You Need To Know About Energy

What You Need To Know About Energy


Algal Biodiesel

A type of diesel fuel made by chemically processing oils from algae.

All-Electric Vehicle (EV)

A vehicle powered entirely by electricity stored in on-board batteries and without access to the use of gasoline or diesel fuel as an alternative energy source. Batteries are recharged by plugging them into an electricity source while the vehicle is parked.

America's Energy Future (AEF)

A project undertaken by the National Academy of Sciences, the National Academy of Engineering, and the National Research Council to evaluate current contributions and likely future impacts of existing and new energy technologies. More information about the project, including a roster of committee members, can be found on the AEF website.


Biologically-derived butanol, a four-carbon-atom alcohol (as opposed to ethanol, which is a two-carbon-atom alcohol), intended for use as an automotive transportation fuel. It is currently produced from the sugars and starches found in commodity crops using genetically engineered microorganisms.


A type of diesel fuel made from biological material such as vegetable oil or animal fat.


Liquid fuels typically derived from harvested plant material, used primarily for transportation. These are different from fossil fuels, which are derived from transformed organic material residing in the Earth’s crust for millions of years.


A renewable energy source consisting of non-fossil biological material. This includes wood and wood byproducts, municipal waste, methane from landfills, and fuel from agricultural crops.

British Thermal Unit

A unit of measure for the energy content of fuels. One Btu is the amount of energy needed to raise a pound of water by one degree Fahrenheit.

Cap and Trade

An approach to control emissions of a given pollutant by setting a limit, or "cap," on the total amount permitted to be released. Industries, utilities, and other groups are required to purchase, or may be given, "credits" that establish allowable quantities of emissions. Unused credits may be sold in a resulting market for trading emissions allowances.

Carbon Debt

The amount of carbon emissions that exceed sustainable carbon sinks. A carbon debt could be assigned to any entity to which carbon emissions can be attributed, such as an individual, a firm, a group, a country, or an industry. Attribution of emissions and estimation of sustainable carbon sinks are complications to calculating this metric.

Carbon Dioxide (CO2)

A colorless, odorless, non-poisonous gas consisting of one carbon and two oxygen atoms. A by-product of fossil fuel combustion and other industrial processes, it is considered a greenhouse gas because it traps heat (infrared energy) radiated from Earth within the atmosphere. For this reason, CO2 is believed to be a major contributor to human-induced climate change.

Carbon Sequestration

The act of capturing gaseous atmospheric carbon, usually in the form of CO2, and placing it into a carbon sink through either biological or physical processes.

Carbon Tax

An approach to limiting emissions of carbon dioxide, a major contributor to human-induced climate change, by establishing a tax on goods and services that is commensurate with the amount of CO2 released in their creation and delivery.

Cellulosic Ethanol

An advanced type of biofuel that is produced by breaking down and using the cellulose compound found in trees and grasses.

Chlorofluorocarbon Refrigerant Fluid (CFC)

A compound of carbon, chlorine, and fluorine (and possibly hydrogen) used as a refrigerant fluid. CFCs originally replaced more toxic and reactive chemicals, but are now being phased out as refrigerants because of their ozone-depleting properties in the upper atmosphere.

Clean Coal

Coal burned in power plants equipped with technologies that greatly reduce the emission of pollutants such as sulfur, nitrogen, mercury, and, potentially, carbon dioxide.

Climate Change

The process of shifting from one prevailing state in regional or global climate to another. Often used interchangeably with “global warming,” scientists sometimes also use the term to refer to periods of climatic cooling. Climate change is typically the preferred term over “global warming” because it helps convey that there are climate changes in addition to rising temperatures.

Coal Gasification

The process of converting coal into a gas before it is burned. The gas, called syngas, makes it easier to separate CO2 as a relatively pure gas before power is generated.

Compact Fluorescent Lamp (CFL)

A device that emits light due to electronic excitation of mercury atoms within a lamp. The mercury atoms lose their excitation energy by emitting an ultraviolet photon, which is converted to visible light in the fluorescent coating of the bulb. CFLs are much more efficient in converting electrical energy to light energy than incandescent bulbs.

Cooling Degree Days

A value representing the amount of fuel required for building cooling, calculated by summing the difference of each day’s average temperature above 65 degrees Fahrenheit.

Corporate Average Fuel Economy (CAFE)

Federal standards that stipulate a target average fuel economy rating (typically expressed in miles per gallon, or mpg) to be met by passenger vehicles by a certain date. The most recent version of the CAFE law, pending passage by Congress, requires new cars, SUVs, and light trucks to average 35.5 mpg by 2016.

Deep Saline Aquifer

A geologic feature consisting of permeable rocks, in which the pores in the rocks are filled with water that has a high dissolved salt content.

Distributed Generation

The generation of electricity from a number of small power plants located close to consumers as an alternative to concentrating power generation in large, centralized facilities located in remote areas. This places less stress on transmission lines and reduces the chance of widespread blackouts.


The capacity for doing work; usable power (as heat or electricity); the resources for producing such power.

Energy Content

The total amount of energy stored within a given quantity of fuel.

Energy Conversion

The transformation of energy from one form to another. For example, when coal (chemical energy) is burned, it produces heat (thermal energy) that is then captured and used to turn a generator (mechanical energy), which transforms the energy into electricity (electrical energy).

Energy Efficiency

The achievement of using less energy without reducing the benefit provided by the end-use service. Energy efficiency is exemplified in a wide variety of applications—from improved lightbulbs and refrigeration to less energy-intensive industrial and manufacturing processes.

Energy Independence and Security Act of 2007

An act of Congress regarding the energy policy of the United States that was primarily focused on automobile fuel economy, development of biofuels, and energy efficiency in public buildings and lighting.

Energy Information Administration (EIA)

An agency within the Department of Energy (DOE) that provides policy-neutral data, forecasts, and analyses to promote sound policymaking, efficient markets, and public understanding regarding energy and its interaction with the economy and the environment.

Energy Intensity

A measure of a nation’s energy efficiency, typically represented through the amount of energy used per unit of gross domestic product (GDP).

Ethanol (or Ethyl Alcohol)

Best known as the type of alcohol found in alcoholic beverages, it is produced in large quantities through the fermentation of the sugars and carbohydrates in agricultural crops and blended with gasoline as an alternative to conventional oil-based fuels for motor vehicles.


A heavy disk that stores and transmits rotational mechanical energy.

Fossil Fuels

Fuels formed in the Earth’s crust over millions of years from decomposed organic matter. The most widely known fossil fuels are petroleum (oil), coal, and natural gas.


Relating to the heat produced inside Earth.


One billion watts, a watt being a unit of measure of power, or how fast energy is used. Gigawatts are typically used to describe very large quantities of power, such as the power carried by a major section of the U.S. electrical grid.

Global Warming

A term used to describe the phenomenon of Earth’s rising average near-surface temperature. Although such fluctuations have occurred in the past due to natural causes, the term is most often used today to refer to current warming trends. Most scientists have concluded that this is very likely due to the observed increase in human-generated greenhouse gas concentrations in the atmosphere.

Greenhouse Gas

A gas which, like a greenhouse window, allows sunlight to enter and then prevents heat from escaping—in this case, from Earth’s atmosphere. The most common greenhouse gases are water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbons, and ozone (O3).


The network connecting electricity producers to consumers. The boundaries of the grid can be drawn differently but may include electricity generators, high power transmission wires, lower power distribution wires, and end users such as homes and businesses as well as the regulatory and market structures that impact electricity transactions. The grid is a physical infrastructure transmitting electricity and is also an economic entity that responds to supply and demand communicated through prices.

Gross Domestic Product (GDP)

The total value of goods and services produced in a country.

Heating Degree Days

A value representing the amount of fuel required for building heating, calculated by summing the difference of each day's average temperature below 65 degrees Fahrenheit.

Hydraulic Fracturing ("Fracking")

The process of pumping fluid under pressure underground to fracture rock containing oil or gas. Hydraulic fracturing creates fissures that increase the surface area in order to allow greater quantities of hydrocarbons to flow out of a reservoir.

Hydroelectric Power

Electric power generated from water turning a generator.

Hydrogen Fuel Cell

An emerging technology which uses a controlled combination of hydrogen and oxygen to generate electrical current, giving off only water vapor as a by-product.

Integrated Gasification Combined Cycle (IGCC)

A technology that converts coal into synthetic gas, or “syngas.” See Coal Gasification.

Intermittent Energy Source

An energy source characterized by output that is dependent on the natural variability of the source rather than the requirements of consumers. Solar energy is an example of an intermittent energy source since it is only available when the sun is shining. Wind is also an intermittent energy source.


One thousand watts, a watt being a unit of measure of power, or how fast energy is used. Kilowatts are typically used to describe intermediate quantities of power, such as power usage in a home.

Kilowatt Hour (kWh)

A unit of measure for energy, typically applied to electricity usage. It is equal to the amount of energy used at a rate of 1000 watts over the course of one hour. One kWh is roughly equal to 3,412 British Thermal Units (Btu).

Light-Emitting Diode (LED)

A device composed of a semiconducting material that emits light upon the application of an electric current. LEDs produce light from electricity more efficiently than either compact fluorescent lights or incandescent lights.


A unit of luminous flux represents the amount of light emitted that is visible to the human eye. In the International System of Units, it is the amount of light a one candela source emits over a square radian angle. It is used in measuring and comparing the amount of light visible to the human eye produced by lamps such as light-emitting diodes, compact fluorescent lights, and incandescent bulbs.

Mechanical Energy

The sum of kinetic energy and potential energy, or the energy stored within a system.


One million watts, a watt being a unit of measure of power, or how fast energy is used. Megawatts are typically used to describe large quantities of power, such as the power output of an electrical generating plant.

Methyl Tertiary-Butyl Ether (MTBE)

An additive to gasoline of the formula (CH3)3COCH3. MTBE increases the octane rating of gasoline, but concerns over toxicity and its detection in soil and groundwater have led MTBE to be banned in some states.

Metric Ton

Also referred to as a metric tonne, it is a measurement of mass equal to 1,000 kilograms, or the mass of one cubic meter of water. This is different from the short ton, a unit of measure commonly used in the United States, which is equal to 2,000 lbs.

National Research Council

A private, nonprofit institution that provides science, technology, and health policy advice under a U.S. congressional charter.

Natural Gas

A gas mixture that occurs naturally in underground deposits. It is composed primarily of methane and may contain other hydrocarbons, carbon dioxide, and hydrogen sulfide. Commonly employed as a fuel for electricity generation, it is also used for space heating, industrial processes, and as a starting material for the manufacture of chemicals and other products.

Natural Gas Combined Cycle (NGCC)

A process which helps improve the efficiency of natural gas power plants by capturing some of the waste heat given off by the combustion of natural gas in order to turn a steam turbine to generate electricity.

Nuclear Energy

Energy whose source is inside the nucleus of atoms.

Nuclear Fission Reaction

The process of an atomic nucleus splitting into two lighter nuclei. The reaction releases energy and sometimes subatomic particles.

Nuclear Fuel

Material that can release energy through a controlled nuclear fission chain reaction.

Nuclear Fusion

The union of two light atomic nuclei to form a heavier nucleus, resulting in the release of enormous amounts of energy.

Nuclear Reactor

An apparatus that contains a controlled nuclear chain reaction and can capture a portion of the energy as heat or mechanical work.

Nuclear Regulatory Commission

An independent agency within the U.S. federal government that regulates commercial nuclear power plants and civilian uses of nuclear materials.

Ocean Acidification

The process by which the hydronium ion concentration increases in the ocean, measured as decreasing pH. One source of ocean acidification is the dissolution of carbon dioxide, which forms carbonic acid with water and then dissociates into hydronium and bicarbonate. Lower pH impacts ocean life, including their soft and hard materials, and also human-built structures in the ocean, such as vessels.

Particulate Matter

Extremely small particles of solid or liquid droplets suspended in either a liquid or gas. Particulate matter is a common emission from the combustion of fossil fuels and can increase the risk of health problems. Examples include dust, smoke, aerosols, and other fine particles.

Photovoltaic (PV) Cell

Sometimes referred to as a solar cell, a device that utilizes the photoelectric effect to convert incident sunlight directly into electricity. This can be distinguished from solar thermal energy, which is sometimes used to create electricity indirectly.

Plug-In Hybrid Electric Vehicle (PHEV)

A vehicle that contains a gasoline powered engine as well as batteries that can be charged when plugged into an electric power source. The vehicle typically runs on battery power until the charge has been depleted and then uses the gasoline engine for extended range.

Primary Energy

Energy that has not undergone transformation to another form. This may include fuels like natural gas or oil, or may include other forms of energy, such as solar or wind energy.

Process Heating

The application of heat to an industrial process.

Public-Private Sector Partnership (PPP)

A contractual agreement between a public agency (local, state, or federal) and a private-sector entity to deliver a service or product to the general public. For example, the FutureGen project is a collaboration of the U.S. Department of Energy and members of the coal industry to develop a near-zero emissions coal-fired power plant.

Pulverized Coal

Coal that has been crushed into a fine dust prior to combustion, allowing the coal to burn more rapidly and efficiently.


A unit of measure used to describe very large quantities of energy, such as the annual energy output of the United States. One quad is equal to one quadrillion—that is, one million billion, or 1015—Btu.


A reaction that converts methane or other hydrocarbons and water into hydrogen and carbon monoxide. It is the largest source of hydrogen on Earth today.

Renewable Energy Resource

An energy source that is naturally replenished. Examples include biomass, wind, geothermal, hydro, and solar energy.


A device on top of a power plant smokestack that removes some of the pollutants from the effluent stream.

Secondary Energy Resource (or Source)

A source of energy that is dependent on a primary source of energy for its power. Since the production of electricity, for example, is dependent on the use of fossil fuels, nuclear power, or renewable sources, it is referred to as a secondary energy source.

Small Modular Reactor

A nuclear fission reactor that is typically less than 300 megawatt electrical and is manufactured in a standard process to minimize the costs and risks associated with larger nuclear reactors.

Smart Grid

An electric grid that is able to use two-way communication and computer processing to provide increased reliability and efficiency. Smart grids may be able to automate and control more functions than the current electric grid.


A photochemical haze that is produced when sunlight reacts with the emissions of hydrocarbons and oxides of nitrogen in the atmosphere. Primarily caused by excess automobile exhaust, it is a form of air pollution that can be threatening to human health.

Solar Energy

Radiant energy from the Sun.

Solar Thermal Generation

Electricity generated from heat produced by solar energy.


Material left behind after a plant has been harvested, such as leaves and stalks.


Sustaining the supply of energy and materials needed to support current levels of consumption, making them available where most needed, and addressing the environmental problems resulting from their extraction, consumption, and disposal.


A mixture of carbon monoxide, hydrogen, and sometimes other gases that can react to form higher hydrocarbons, natural gas, or methanol. Syngas is short for synthesis gas.

Thermal Expansion

A material’s change in volume, area, or length upon heating.

Tight oil

Petroleum present in underground formations with low permeability, such as oil present in shales or sandstones.

U.S. Department of Energy

A federal agency within the executive branch of the U.S. federal government responsible for the advancement of the national, economic, and energy security of the United States.

U.S. Department of Transportation

A federal agency within the executive branch of the U.S. federal government responsible for ensuring a fast, safe, efficient, accessible, and convenient transportation system.

U.S. Energy Information Administration (EIA)

An agency within the U.S. Department of Energy that provides policy-neutral data, forecasts, and analyses to promote sound policy making, efficient markets, and public understanding regarding energy and its interaction with the economy and the environment.


A unit of measure for power, or how fast energy is used. One watt of power is equal to one ampere (a measure of electric current) moving at one volt (a measure of electrical force).

Wind Farm

A collection of wind turbines used to generate electricity for transmission to and distribution on the electric grid.

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