Glossary of Energy-Related Terms

Here you'll find a glossary of energy-related terms.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

Absolute Humidity: The ratio of the mass of water vapor to the volume occupied by a mixture of water vapor and dry air.
Absorbent: A material that extracts one or more substances from a fluid (gas or liquid) medium on contact, and which changes physically and/or chemically in the process. The less volatile of the two working fluids in an absorption cooling device.
Absorber: The component of a solar thermal collector that absorbs solar radiation and converts it to heat, or, as in a solar photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).
Absorption: The passing of a substance or force into the body of another substance.
Absorption Chiller: A type of air cooling device that uses absorption cooling to cool interior spaces.
Absorption Coefficient: In reference to a solar energy conversion devices, the degree to which a substance will absorb solar energy. In a solar photovoltaic device, the factor by which photons are absorbed as they travel a unit distance through a material.
Absorption Cooling: A process in which cooling of an interior space is accomplished by the evaporation of a volatile fluid, which is then absorbed in a strong solution, then desorbed under pressure by a heat source, and then recondensed at a temperature high enough that the heat of condensation can be rejected to a exterior space.
Absorption Refrigeration: A system in which a secondary fluid absorbs the refrigerant, releasing heat, then releases the refrigerant and reabsorbs the heat. Ammonia or water is used as the vapor in commercial absorption cycle systems, and water or lithium bromide is the absorber.
Absorptivity: In a solar thermal system, the ratio of solar energy striking the absorber that is absorbed by the absorber to that of solar energy striking a black body (perfect absorber) at the same temperature. The absorptivity of a material is numerically equal to its emissivity.
Accumulator: A component of a heat pump that stores liquid and keeps it from flooding the compressor. The accumulator takes the strain off the compressor and improves the reliability of the system.
Acid Rain: A term used to describe precipitation that has become acidic (low pH) due to the emission of sulfur oxides from fossil fuel burning power plants.
Active Cooling: The use of mechanical heat pipes or pumps to transport heat by circulating heat transfer fluids.
Active Power: The power (in Watts) used by a device to produce useful work. Also called input power.
Active Solar Heater: A solar water or space-heating system that use pumps or fans to circulate the fluid (water or heat-transfer fluid like diluted antifreeze) from the solar collectors to a storage tank subsystem.
Adiabatic: Without loss or gain of heat to a system. An adiabatic change is a change in volume and pressure of a parcel of gas without an exchange of heat between the parcel and its surroundings. In reference to a steam turbine, the adiabatic efficiency is the ratio of the work done per pound of steam, to the heat energy released and theoretically capable of transformation into mechanical work during the adiabatic expansion of a unit weight of steam.
Adjustable Speed Drive: An electronic device that controls the rotational speed of motor-driven equipment such as fans, pumps, and compressors. Speed control is achieved by adjusting the frequency of the voltage applied to the motor.
Adobe: A building material made from clay, straw, and water, formed into blocks, and dried; used traditionally in the southwestern U.S.
Aerobic Bacteria: Microorganisms that require free oxygen, or air, to live, and that which contribute to the decomposition of organic material in soil or composting systems.
Air: The mixture of gases that surrounds the earth and forms its atmosphere, composed of, by volume, 21 percent oxygen, 78 percent nitrogen.
Air Change: A measure of the rate at which the air in an interior space is replace by outside (or conditioned) air by ventilation and infiltration; usually measured in cubic feet per time interval (hour), divided by the volume of air in the room.
Air Collector: In solar heating systems, a type of solar collector in which air is heated in the collector.
Air Conditioner: A device for conditioning air in an interior space. A Room Air Conditioner is a unit designed for installation in the wall or window of a room to deliver conditioned air without ducts. A Unitary Air Conditioner is composed of one or more assemblies that usually include an evaporator or cooling coil, a compressor and condenser combination, and possibly a heating apparatus. A Central Air Conditioner is designed to provide conditioned air from a central unit to a whole house with fans and ducts.
Air Conditioning: The control of the quality, quantity, and temperature-humidity of the air in an interior space.
Air Diffuser: An air distribution outlet, typically located in the ceiling, which mixes conditioned air with room air.
Air Infiltration Measurement: A building energy auditing technique used to determine and/or locate air leaks in a building shell or envelope.
Airlock Entry: A building architectural element (vestibule) with two airtight doors that reduces the amount of air infiltration and exfiltration when the exterior most door is opened.
Air Pollution: The presence of contaminants in the air in concentrations that prevent the normal dispersive ability of the air, and that interfere with biological processes and human economics.
Air Pollution Control: The use of devices to limit or prevent the release of pollution into the atmosphere.
Air Quality Standards: The prescribed level of pollutants allowed in outside or indoor air as established by legislation.
Air Register: The component of a combustion device that regulates the amount of air entering the combustion chamber.
Air Retarder/Barrier: A material or structural element that inhibits air flow into and out of a building's envelope or shell. This is a continuous sheet composed of polyethylene, polypropylene, or extruded polystyrene. The sheet is wrapped around the outside of a house during construction to reduce air in-and exfiltration, yet allow water to easily diffuse through it.
Air-Source Heat Pump: A type of heat pump that transfers heat from outdoor air to indoor air during the heating season, and works in reverse during the cooling season.
Air Space: The area between the layers of glazing (panes) of a window.
Airtight Drywall Approach (ADA): A building construction technique used to create a continuous air retarder that uses the drywall, gaskets, and caulking. Gaskets are used rather than caulking to seal the drywall at the top and bottom. Although it is an effective energy-saving technique, it was designed to keep airborne moisture from damaging insulation and building materials within the wall cavity.
Air-to-Air Heat Pump: see Air-Source Heat Pump.
Air-to-Water Heat Pump: A type of heat pump that transfers heat in outdoor air to water for space or water heating.
Albedo: The ratio of light reflected by a surface to the light falling on it.
Alcohol: A group of organic compounds composed of carbon, hydrogen, and oxygen; a series of molecules composed of a hydrocarbon plus a hydroxyl group; includes methanol, ethanol, isopropyl alcohol and others.
Algae: Primitive plants, usually aquatic, capable of synthesizing their own food by photosynthesis.
Alternating Current: A type of electrical current, the direction of which is reversed at regular intervals or cycles; in the U.S. the standard is 120 reversals or 60 cycles per second; typically abbreviated as AC.
Alternative Fuels: A popular term for "non-conventional" transportation fuels derived from natural gas (propane, compressed natural gas, methanol, etc.) or biomass materials (ethanol, methanol).
Alternator: A generator producing alternating current by the rotation of its rotor, and which is powered by a primary mover.
Ambient Air: The air external to a building or device.
Ambient Temperature: The temperature of a medium, such as gas or liquid, which comes into contact with or surrounds an apparatus or building element.
Ammonia: A colorless, pungent, gas (NH3) that is extremely soluble in water, may be used as a refrigerant; a fixed nitrogen form suitable as fertilizer.
Amorphous Semiconductor: A non-crystalline semiconductor material that has no long-range order.
Ampere: A unit of measure for an electrical current; the amount of current that flows in a circuit at an electromotive force of one Volt and at a resistance of one Ohm. Abbreviated as amp.
Amp-Hours: A measure of the flow of current (in amperes) over one hour.
Anaerobic Bacteria: Microorganisms that live in oxygen deprived environments.
Anaerobic Digestion: The complex process by which organic matter is decomposed by anaerobic bacteria. The decomposition process produces a gaseous byproduct often called "biogas" primarily composed of methane, carbon dioxide, and hydrogen sulfide.
Anaerobic Digester: A device for optimizing the anaerobic digestion of biomass and/or animal manure, and possibly to recover biogas for energy production. Digester types include batch, complete mix, continuous flow (horizontal or plug-flow, multiple-tank, and vertical tank), and covered lagoon.
Anaerobic Lagoon: A holding pond for livestock manure that is designed to anaerobically stabilize manure, and may be designed to capture biogas, with the use of an impermeable, floating cover.
Anhydrous Ethanol: One hundred percent alcohol; neat ethanol.
Anemometer: An instrument for measuring the force or velocity of wind; a wind gauge.
Angle of Incidence: In reference to solar energy systems, the angle at which direct sunlight strikes a surface; the angle between the direction of the sun and the perpendicular to the surface. Sunlight with an incident angle of 90 degrees tends to be absorbed, while lower angles tend to be reflected.
Angle of Inclination: In reference to solar energy systems, the angle that a solar collector is positioned above horizontal.
Angstrom Unit: A unit of length named for A.J. Angstome, a Swedish spectroscopist, used in measuring electromagnetic radiation equal to 0.000,000,01 centimeters.
Annual Fuel Utilization Efficiency (AFUE): The measure of seasonal or annual efficiency of a residential heating furnace or boiler. It takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which in turn is affected by changes in weather and occupant controls.
Annual Load Fraction: That fraction of annual energy demand supplied by a solar system.
Annual Solar Savings: The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.
Anode: The positive pole or electrode of an electrolytic cell, vacuum tube, etc. (see also sacrificial anode).
Anthracite (coal): A hard, dense type of coal, that is hard to break, clean to handle, difficult to ignite, and that burns with an intense flame and with the virtual absence of smoke because it contains a high percentage of fixed carbon and a low percentage of volatile matter.
Anthropogenic: Referring to alterations in the environment due to the presence or activities of humans.
Antifreeze Solution: A fluid, such as methanol or ethylene glycol, added to vehicle engine coolant, or used in solar heating system heat transfer fluids, to protect the systems from freezing.
Antireflection Coating: A thin coating of a material applied to a photovoltaic cell surface that reduces the light reflection and increases light transmission.
Aperture: An opening; in solar collectors, the area through which solar radiation is admitted and directed to the absorber.
Apparent Day: A solar day; an interval between successive transits of the sun's center across an observer's meridian; the time thus measured is not equal to clock time.
Apparent Power (kVA): This is the voltage-ampere requirement of a device designed to convert electric energy to a non-electrical form.
Appliance: A device for converting one form of energy or fuel into useful energy or work.
Appliance Energy Efficiency Ratings: The ratings under which specified appliances convert energy sources into useful energy, as determined by procedures established by the U.S. Department of Energy.
Appliance Standards: Standards established by the U.S. Congress for energy consuming appliances in the National Appliance Energy Conservation Act (NAECA) of 1987, and as amended in the National Appliance Energy Conservation Amendments of 1988, and the Energy Policy Act of 1992 (EPAct). NAECA established minimum standards of energy efficiency for refrigerators, refrigerator-freezers, freezers, room air conditioners, fluorescent lamp ballasts, incandescent reflector lamps, clothes dryers, clothes washers, dishwashers, kitchen ranges and ovens, pool heaters, television sets (withdrawn in 1995), and water heaters. The EPAct added standards for some fluorescent and incandescent reflector lamps, plumbing products, electric motors, and commercial water heaters and Heating, Ventilation, and Air Conditioning (HVAC) systems. It also allowed for the future development of standards for many other products. The U.S. Department of Energy (DOE) is responsible establishing the standards and the procedures that manufacturers must use to test their models. These procedures are published in the Code of Federal Regulations (10 CFR, Ch. II, Part 430), January 1, 1994 (Federal Register).
Argon: A colorless, odorless inert gas sometimes used in the spaces between the panes in energy efficient windows. This gas is used because it will transfer less heat than air. Therefore, it provides additional protection against conduction and convection of heat over conventional double -pane windows.
Array (Solar): Any number of solar photovoltaic modules or solar thermal collectors or reflectors connected together to provide electrical or thermal energy.
Ash: The non-combustible residue of a combusted substance composed primarily of alkali and metal oxides.
ASHRAE: Abbreviation for the American Society of Heating, Refrigeration, and Air-Conditioning Engineers.
ASTM: Abbreviation for the American Society for Testing and Materials, which is responsible for the issue of many standard methods used in the energy industry.
Asynchronous Generator: A type of electric generator that produces alternating current that matches an existing power source.
Atmospheric Pressure: The pressure of the air at sea level; one standard atmosphere at zero degrees centigrade is equal to 14.695 pounds per square inch (1.033 kilograms per square centimeter).
Atrium: An interior court to which rooms open.
Attic: The usually unfinished space above a ceiling and below a roof.
Attic Fan: A fan mounted on an attic wall used to exhaust warm attic air to the outside.
Attic Vent: A passive or mechanical device used to ventilate an attic space, primarily to reduce heat buildup and moisture condensation.
Audit (Energy): The process of determining energy consumption, by various techniques, of a building or facility.
Automatic Damper: A device that cuts off the flow of hot or cold air to or from a room as controlled by a thermostat.
Automatic (or Remote) Meter Reading System: A system that records the consumption of electricity, gas, water, etc, and sends the data to a central data accumulation device.
Auxiliary Energy or System: Energy required to operate mechanical components of an energy system, or a source of energy or energy supply system to back-up another.
Availability: Describes the reliability of power plants. It refers to the number of hours that a power plant is available to produce power divided by the total hours in a set time period, usually a year.
Available Heat: The amount of heat energy that may be converted into useful energy from a fuel.
Average Demand: The demand on, or the power output of, an electrical system or any of its parts over an interval of time, as determined by the total number of kilowatt-hours divided by the units of time in the interval.
Average Cost: The total cost of production divided by the total quantity produced.
Average Wind Speed (or Velocity): The mean wind speed over a specified period of time.
Avoided Cost: The incremental cost to an electric power producer to generate or purchase a unit of electricity or capacity or both.
Axial Fans: Fans in which the direction of the flow of the air from inlet to outlet remains unchanged; includes propeller, tubaxial, and vaneaxial type fans.
Axial Flow Compressor: A type of air compressor in which air is compressed in a series of stages as it flows axially through a decreasing tubular area.
Axial Flow Turbine: A turbine in which the flow of a steam or gas is essentially parallel to the rotor axis.
Azimuth (Solar): The angle between true south and the point on the horizon directly below the sun.
AWG: The abbreviation for American Wire Gauge; the standard for gauging the size of wires (electrical conductors).
Awning: An architectural element for shading windows and wall surfaces placed on the exterior of a building; can be fixed or movable.

B

Backdrafting: The flow of air down a flue/chimney and into a house caused by low indoor air pressure that can occur when using several fans or fireplaces and/or if the house is very tight.
Backup Energy System: A reserve appliance; for example, a stand-by generator for a home or commercial building.
Bacteria: Single-celled organisms, free-living or parasitic, that break down the wastes and bodies of dead organisms, making their components available for reuse by other organisms.
Baffle: A device, such as a steel plate, used to check, retard, or divert a flow of a material.
Bagasse: The fibrous material remaining after the extraction of juice from sugarcane; often burned by sugar mills as a source of energy.
Baghouse: An air pollution control device used to filter particulates from waste combustion gases; a chamber containing a bag filter.
Balance-of-System: In a renewable energy system, refers to all components other than the mechanism used to harvest the resource (such as photovoltaic panels or a wind turbine). Balance-of-system costs can include design, land, site preparation, system installation, support structures, power conditioning, operation and maintenance, and storage.
Balance Point: An outdoor temperature, usually 20 to 45 degrees Fahrenheit, at which a heat pump's output equals the heating demand. Below the balance point, supplementary heat is needed.
Baling: A means of reducing the volume of a material by compaction into a bale.
Ballast: A device used to control the voltage in a fluorescent lamp.
Ballast Efficacy Factor: The measure of the efficiency of fluorescent lamp ballasts. It is the relative light output divided by the power input.
Ballast Factor: The ratio of light output of a fluorescent lamp operated on a ballast to the light output of a lamp operated on a standard or reference ballast.
Band Gap: In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.
Band Gap Energy: The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.
Barrel (petroleum): 42 U.S. gallons (306 pounds of oil, or 5.78 million Btu).
Basal Metabolism: The amount of heat given off by a person at rest in a comfortable environment; approximately 50 Btu per hour (Btu/h).
Baseboard Radiator: A type of radiant heating system where the radiator is located along an exterior wall where the wall meets the floor.
Baseload Capacity: The power output of a power plant that can be continuously produced.
Baseload Demand: The minimum demand experienced by a power plant.
Baseload Power Plant: A power plant that is normally operated to generate a base load, and that usually operates at a constant load; examples include coal fired and nuclear fueled power plants.
Basement: The conditioned or unconditioned space below the main living area or primary floor of a building.
Base Power: Power generated by a power generator that operates at a very high capacity factor.
Batch Heater: This simple passive solar hot water system consists of one or more storage tanks placed in an insulated box that has a glazed side facing the sun. A batch heater is mounted on the ground or on the roof (make sure your roof structure is strong enough to support it). Some batch heaters use "selective" surfaces on the tank(s). These surfaces absorb sun well but inhibit radiative loss. Also known as bread box systems or integral collector storage systems.
Batch Process: A process for carrying out a reaction in which the reactants are fed in discrete and successive charges.
Batt/Blanket: A flexible roll or strip of insulating material in widths suited to standard spacings of building structural members (studs and joists). They are made from glass or rock wool fibers. Blankets are continuous rolls. Batts are pre-cut to four or eight foot lengths.
Battery: An energy storage device composed of one or more electrolyte cells.
Battery Energy Storage: Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.
Beadwall&##8482;: A form of movable insulation that uses tiny polystyrene beads blown into the space between two window panes.
Beam Radiation: Solar radiation that is not scattered by dust or water droplets.
Bearing Wall: A wall that carries ceiling rafters or roof trusses.
Benefits Charge: The addition of a per unit tax on sales of electricity, with the revenue generated used for or to encourage investments in energy efficiency measures and/or renewable energy projects.
Bimetal: Two metals of different coefficients of expansion welded together so that the piece will bend in one direction when heated, and in the other when cooled, and can be used to open or close electrical circuits, as in thermostats.
Binary Cycle: Combination of two power plant turbine cycles utilizing two different working fluids for power production. The waste heat from the first turbine cycle provides the heat energy for the operation of the second turbine, thus providing higher overall system efficiencies.
Binary Cycle Geothermal Plants: Binary cycle systems can be used with liquids at temperatures less than 350 F (177 C). In these systems, the hot geothermal liquid vaporizes a secondary working fluid, which then drives a turbine.
Bin Method: A method of predicting heating and/or cooling loads using instantaneous load calculation at different outdoor dry-bulb temperatures, and multiplying the result by the number of hours of occurrence of each temperature.
Biochemical Oxygen Demand: The weight of oxygen taken up mainly as a result of the oxidation of the constituents of a sample of water by biological action; expressed as the number of parts per million of oxygen taken up by the sample from water originally saturated with air, usually over a period of five days at 20 degrees centigrade. A standard means of estimating the degree of contamination of water.
Bioconversion: The conversion of one form of energy into another by the action of plants or microorganisms. The conversion of biomass to ethanol, methanol, or methane.
Bioenergy: The conversion of the complex carbohydrates in organic material into energy.
Biogas: A combustible gas created by anaerobic decomposition of organic material, composed primarily of methane, carbon dioxide, and hydrogen sulfide.
Biogasification or biomethanization: The process of decomposing biomass with anaerobic bacteria to produce biogas.
Biomass: As defined by the Energy Security Act (PL 96-294) of 1980, "any organic matter which is available on a renewable basis, including agricultural crops and agricultural wastes and residues, wood and wood wastes and residues, animal wastes, municipal wastes, and aquatic plants."
Biomass Energy: Energy produced by the conversion of biomass directly to heat or to a liquid or gas that can be converted to energy.
Biomass Fuel: Biomass converted directly to energy or converted to liquid or gaseous fuels such as ethanol, methanol, methane, and hydrogen.
Biomass Gasification: The conversion of biomass into a gas, by biogasification (see above) or thermal gasification, in which hydrogen is produced from high-temperature gasifying and low-temperature pyrolysis of biomass.
Biophotolysis: The action of light on a biological system that results in the dissociation of a substrate, usually water, to produce hydrogen.
Blackbody: An ideal substance that absorbs all radiation falling on it, and reflecting nothing.
Blower: The device in an air conditioner that distributes the filtered air from the return duct over the cooling coil/heat exchanger. This circulated air is cooled/heated and then sent through the supply duct, past dampers, and through supply diffusers to the living/working space.
Blower Door: A device used by energy auditors to pressurize a building to locate places of air leakage and energy loss.
Blown In Insulation (see also Loose Fill): An insulation product composed of loose fibers or fiber pellets that are blown into building cavities or attics using special pneumatic equipment.
Boiler: A vessel or tank where heat produced from the combustion of fuels such as natural gas, fuel oil, or coal is used to generate hot water or steam for applications ranging from building space heating to electric power production or industrial process heat.
Boiler Feedwater: The water that is forced into a boiler to take the place of that which is evaporated in the generation of steam.
Boiler Horsepower: A unit of rate of water evaporation equal to the evaporation per hour of 34.5 pounds of water at a temperature of 212 degrees Fahrenheit into steam at 212 degrees F.
Boiler Pressure: The pressure of the steam or water in a boiler as measured; usually expressed in pounds per square inch gauge (psig).
Boiler Rating: The heating capacity of a steam boiler; expressed in Btu per hour (Btu/h), or horsepower, or pounds of steam per hour.
Bone (Oven) Dry: In reference to solid biomass fuels, such as wood, having zero moisture content.
Bone Dry Unit: A quantity of (solid) biomass fuel equal to 2,400 pounds bone dry.
Booster Pump: A pump for circulating the heat transfer fluid in a hydronic heating system.
Boot: In heating and cooling system distribution ductwork, the transformation pieces connecting horizontal round leaders to vertical rectangular stacks.
Boron: The chemical element commonly used as the dopant in solar photovoltaic device or cell material.
Bottled Gas: A generic term for liquefied and pressurized gas, ordinarily butane, propane, or a mixture of the two, contained in a cylinder for domestic use.
Bottoming-cycle: A means to increase the thermal efficiency of a steam electric generating system by converting some waste heat from the condenser into electricity. The heat engine in a bottoming cycle would be a condensing turbine similar in principle to a steam turbine but operating with a different working fluid at a much lower temperature and pressure.
Brayton Cycle: A thermodynamic cycle using constant pressure, heat addition and rejection, representing the idealized behavior of the working fluid in a gas turbine type heat engine.
Bread Box System: This simple passive solar hot water system consists of one or more storage tanks placed in an insulated box that has a glazed side facing the sun. A bread box system is mounted on the ground or on the roof (make sure your roof structure is strong enough to support it). Some systems use "selective" surfaces on the tank(s). These surfaces absorb sun well but inhibit radiative loss. Also known as batch heaters or integral collector storage systems.
Brine: Water saturated or strongly impregnated with salt.
British Thermal Unit (Btu): The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.
Building Energy Ratio: The space-conditioning load of a building.
Building Envelope: The structural elements (walls, roof, floor, foundation) of a building that encloses conditioned space; the building shell.
Building Heat-Loss Factor: A measure of the heating requirements of a building expressed in Btu per degree-day.
Building Orientation: The relationship of a building to true south, as specified by the direction of its longest axis.
Building Overall Energy Loss Coefficient-Area Product: The factor, when multiplied by the monthly degree-days, that yields the monthly space heating load.
Building Overall Heat Loss Rate: The overall rate of heat loss from a building by means of transmission plus infiltration, expressed in Btu per hour, per degree temperature difference between the inside and outside.
Bulb: The transparent or opaque sphere in an electric light that the electric light transmits through.
Bulb Turbine: A type of hydro turbine in which the entire generator is mounted inside the water passageway as an integral unit with the turbine. These installations can offer significant reductions in the size of the powerhouse.
Bulk Density: The weight of a material per unit of volume compared to the weight of the same volume of water.
Burner Capacity: The maximum heat output (in Btu per hour) released by a burner with a stable flame and satisfactory combustion.
Burning Point: The temperature at which a material ignites.
Bus (electrical): An electrical conductor that serves as a common connection for two or more electrical circuits; may be in the form of rigid bars or stranded conductors or cables.
Busbar: The power conduit of an electric power plant; the starting point of the electric transmission system.
Busbar Cost: The cost of producing electricity up to the point of the power plant busbar.
Bypass: An alternative path. In a heating duct or pipe, an alternative path for the flow of the heat transfer fluid from one point to another, as determined by the opening or closing of control valves both in the primary line and the bypass line.

C

Cage

The component of an electric motor composed of solid bars (of usually copper or aluminum) arranged in a circle and connected to continuous rings at each end. This cage fits inside the stator in an induction motor in channels between laminations, thin flat discs of steel in a ring configuration.

Calorie

The amount of heat required to raise the temperature of a unit of water, at or near the temperature of maximum density, one degree Celsius (or Centigrade [C]); expressed as a "small calorie" (the amount of heat required to raise the temperature of 1 gram of water one degree C), or as a "large calorie" or "kilogram calorie" (the amount of heat required to raise one kilogram [1,000 grams] of water one degree C); capitalization of the word calorie indicates a kilogram-calorie.

Calorific Value

The heat liberated by the combustion of a unit quantity of a fuel under specific conditions; measured in calories.

Candela

The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Candle Power

The illuminating power of a standard candle employed as a unit for determining the illuminating quality of an illuminant.

Capability

The maximum load that a generating unit, power plant, or other electrical apparatus can carry under specified conditions for a given period of time, without exceeding its approved limits of temperature and stress.

Capability Margin

The difference between net electrical system capability and system maximum load requirements (peak load); the margin of capability available to provide for scheduled maintenance, emergency outages, system operating requirements and unforeseen loads.

Capacitance

A measure of the electrical charge of a capacitor consisting of two plates separated by an insulating material.

Capacitor

An electrical device that adjusts the leading current of an applied alternating current to balance the lag of the circuit to provide a high power factor.

Capacity

The load that a power generation unit or other electrical apparatus or heating unit is rated by the manufacture to be able to meet or supply.

Capacity (Condensing Unit)

The refrigerating effect in Btu/h produced by the difference in total enthalpy between a refrigerant liquid leaving the unit and the total enthalpy of the refrigerant vapor entering it. Generally measured in tons or Btu/h.

Capacity (Effective, of a motor)

The maximum load that a motor is capable of supplying.

Capacity (Heating, of a material)

The amount of heat energy needed to raise the temperature of a given mass of a substance by one degree Celsius. The heat required to raise the temperature of 1 kg of water by 1 degree Celsius is 4186 Joules.

Capacity Factor

The ratio of the average load on (or power output of) a generating unit or system to the capacity rating of the unit or system over a specified period of time.

Capital Costs

The amount of money needed to purchase equipment, buildings, tools, and other manufactured goods that can be used in production.

Carbon Dioxide

A colorless, odorless noncombustible gas with the formula CO₂ that is present in the atmosphere. It is formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by respiration, which is a slow combustion in animals and plants, and by the gradual oxidation of organic matter in the soil.

Carbon Monoxide

A colorless, odorless but poisonous combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (i.e. coal, petroleum) and their products (e.g. liquefied petroleum gas, gasoline), and biomass.

Carbon Zinc Cell Battery

A cell produces electric energy by the galvanic oxidation of carbon; commonly used in household appliances.

Carnot Cycle

An ideal heat engine (conceived by Sadi Carnot) in which the sequence of operations forming the working cycle consists of isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression back to its initial state.

Catalytic Converter

An air pollution control device that removes organic contaminants by oxidizing them into carbon dioxide and water through a chemical reaction using a catalysis, which is a substance that increases (or decreases) the rate of a chemical reaction without being changed itself; required in all automobiles sold in the United State, and used in some types of heating appliances.

Cathedral Ceiling/Roof

A type of ceiling and roof assembly that has no attic.

Cathode

The negative pole or electrode of an electrolytic cell, vacuum tube, etc., where electrons enter (current leaves) the system; the opposite of an anode.

Cathode Disconnect Ballast

An electromagnetic ballast that disconnects a lamp's electrode heating circuit once is has started; often called "low frequency electronic" ballasts.

Cathodic Protection

A method of preventing oxidation of the exposed metal in structures by imposing between the structure and the ground a small electrical voltage.

Caulking

A material used to seal areas of potential air leakage into or out of a building envelope.

Ceiling

The downward facing structural element that is directly opposite the floor.

Ceiling Fan

A mechanical device used for air circulation and to provide cooling.

Cell

A component of a electrochemical battery. A 'primary' cell consists of two dissimilar elements, known as 'electrodes,' immersed in a liquid or paste known as the 'electrolyte.' A direct current of 1-1.5 volts will be produced by this cell. A 'secondary' cell or accumulator is a similar design but is made useful by passing a direct current of correct strength through it in a certain direction. Each of these cells will produce 2 volts; a 12 volt car battery contains six cells.

Cellulase

An enzyme complex, produced by fungi and bacteria, capable of decomposing cellulose into small fragments, primarily glucose.

Cellulose

The fundamental constituent of all vegetative tissue; the most abundant material in the world.

Cellulose Insulation

A type of insulation composed of waste newspaper, cardboard, or other forms of waste paper.

Central Heating System

A system where heat is supplied to areas of a building from a single appliance through a network of ducts or pipes.

Central Power Plant

A large power plant that generates power for distribution to multiple customers.

Central Receiver Solar Power Plants

Also known as "power towers," these use fields of two-axis tracking mirrors known as heliostats. Each heliostat is individually positioned by a computer control system to reflect the sun's rays to a tower-mounted thermal receiver. The effect of many heliostats reflecting to a common point creates the combined energy of thousands of suns, which produces high-temperature thermal energy. In the receiver, molten nitrate salts absorb the heat energy. The hot salt is then used to boil water to steam, which is sent to a conventional steam turbine-generator to produce electricity.

Cetane Number

A measure of a fuel's (liquid) ease of self-ignition.

Char

A byproduct of low-temperature carbonization of a solid fuel.

Charcoal

A material formed from the incomplete combustion or destructive distillation (carbonization) of organic material in a kiln or retort, and having a high energy density, being nearly pure carbon. (If produced from coal, it is coke.) Used for cooking, the manufacture of gunpowder and steel (notably in Brazil), as an absorbent and decolorizing agent, and in sugar refining and solvent recovery.

Charge Carrier

A free and mobile conduction electron or hole in a semiconductor.

Charge Controller

An electronic device that regulates the electrical charge stored in batteries so that unsafe, overcharge conditions for the batteries are avoided.

Chemical Energy

The energy liberated in a chemical reaction, as in the combustion of fuels.

Chemical Vapor Deposition (CVD)

A method of depositing thin semiconductor films used to make certain types of solar photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

Chiller

A device for removing heat from a gas or liquid stream for air conditioning/cooling.

Chimney

A masonry or metal stack that creates a draft to bring air to a fire and to carry the gaseous byproducts of combustion safely away.

Chimney Effect

The tendency of heated air or gas to rise in a duct or other vertical passage, such as in a chimney, small enclosure, or building, due to its lower density compared to the surrounding air or gas.

Chlorofluorocarbon (CFC)

A family of chemicals composed primarily of carbon, hydrogen, chlorine, and fluorine whose principal applications are as refrigerants and industrial cleansers and whose principal drawback is the tendency to destroy the Earth's protective ozone layer.

Circuit

A device, or system of devices, that allows electrical current to flow through it and allows voltage to occur across positive and negative terminals.

Circuit Breaker

A device used to interrupt or break an electrical circuit when an overload condition exists; usually installed in the positive circuit; used to protect electrical equipment.

Circuit Lag

As time increases from zero at the terminals of an inductor, the voltage comes to a particular value on the sine function curve ahead of the current. The voltage reaches its negative peak exactly 90 degrees before the current reaches its negative peak thus the current lags behind by 90 degrees.

Circulating Fluidized Bed

A type of furnace or reactor in which the emission of sulfur compounds is lowered by the addition of crushed limestone in the fluidized bed thus obviating the need for much of the expensive stack gas clean-up equipment. The particles are collected and recirculated, after passing through a conventional bed, and cooled by boiler internals.

Clean Power Generator

A company or other organizational unit that produces electricity from sources that are thought to be environmentally cleaner than traditional sources. Clean, or green, power is usually defined as power from renewable energy that comes from wind, solar, biomass energy, etc. There are various definitions of clean resources. Some definitions include power produced from waste-to-energy and wood-fired plants that may still produce significant air emissions. Some states have defined certain local resources as clean that other states would not consider clean. For example, the state of Texas has defined power from efficient natural gas-fired power plants as clean. Some northwest states include power from large hydropower projects as clean although these projects damage fish populations. Various states have disclosure and labeling requirement for generation source and air emissions that assist customers in comparing electricity characteristics other than price. This allows customers to decide for themselves what they consider to be "clean." The federal government is also exploring this issue.

Cleavage of Lateral Epitaxial Films for Transfer (CLEFT)

A process for making inexpensive Gallium Arsenide (GaAs) photovoltaic cells in which a thin film of GaAs is grown atop a thick, single-crystal GaAs (or other suitable material) substrate and then is cleaved from the substrate and incorporated into a cell, allowing the substrate to be reused to grow more thin-film GaAs.

Clerestory

A window located high in a wall near the eaves that allows daylight into a building interior, and may be used for ventilation and solar heat gain.

Climate

The prevailing or average weather conditions of a geographic region.

Climate Change

A term used to describe short and long-term affects on the Earth's climate as a result of human activities such as fossil fuel combustion and vegetation clearing and burning.

Close Coupled

An energy system in which the fuel production equipment is in close proximity, or connected to, the fuel using equipment.

Closed Cycle

A system in which a working fluid is used over and over without introduction of new fluid, as in a hydronic heating system or mechanical refrigeration system.

Closed-Loop Geothermal Heat Pump Systems

Closed-loop (also known as "indirect") systems circulate a solution of water and antifreeze through a series of sealed loops of piping. Once the heat has been transferred into or out of the solution, the solution is recirculated. The loops can be installed in the ground horizontally or vertically, or they can be placed in a body of water, such as a pond. See horizontal ground loop, vertical ground loop, slinky ground loop, and surface water loop for more information on the different types of closed-loop geothermal heat pump systems.

Closed-Loop Biomass

As defined by the Comprehensive National Energy Act of 1992 (or the Energy Policy Act; EPAct): any organic matter from a plant which is planted for the exclusive purpose of being used to produce energy." This does not include wood or agricultural wastes or standing timber.

Codes

Legal documents that regulate construction to protect the health, safety, and welfare of people. Codes establish minimum standards but do not guarantee efficiency or quality.

Coefficient of Heat Transmission (U-Value)

A value that describes the ability of a material to conduct heat. The number of Btu that flow through 1 square foot of material, in one hour. It is the reciprocal of the R-Value (U-Value = 1/R-Value).

Coefficient of Performance (COP)

A ratio of the work or useful energy output of a system versus the amount of work or energy inputted into the system as determined by using the same energy equivalents for energy in and out. Is used as a measure of the steady state performance or energy efficiency of heating, cooling, and refrigeration appliances. The COP is equal to the Energy Efficiency Ratio (EER) divided by 3.412. The higher the COP, the more efficient the device.

Coefficient of Utilization (CU)

A term used for lighting appliances; the ratio of lumens received on a flat surface to the light output, in lumens, from a lamp; used to evaluate the effectiveness of luminaries in delivering light.

Coincidence Factor

The ratio of the coincident, maximum demand or two or more loads to the sum of their noncoincident maximum demand for a given period; the reciprocal of the diversity factor, and is always less than or equal to one.

Coincident Demand

The demand of a consumer of electricity at the time of a power supplier's peak system demand.

Cofiring

The use of two or more different fuels (e.g. wood and coal) simultaneously in the same combustion chamber of a power plant.

Cogeneration

The generation of electricity or shaft power by an energy conversion system and the concurrent use of rejected thermal energy from the conversion system as an auxiliary energy source.

Cogenerator

A class of energy producer that produces both heat and electricity from a single fuel.

Coil

As a component of a heating or cooling appliance, rows of tubing or pipe with fins attached through which a heat transfer fluid is circulated and to deliver heat or cooling energy to a building.

Cold Night Sky

The low effective temperature of the sky on a clear night.

Collector

The component of a solar energy heating system that collects solar radiation, and that contains components to absorb solar radiation and transfer the heat to a heat transfer fluid (air or liquid).

Collector Efficiency

The ratio of solar radiation captured and transferred to the collector (heat transfer) fluid.

Collector Fluid

The fluid, liquid (water or water/antifreeze solution) or air, used to absorb solar energy and transfer it for direct use, indirect heating of interior air or domestic water, and/or to a heat storage medium.

Collector Tilt

The angle that a solar collector is positioned from horizontal.

Color Rendering or Rendition

A measure of the ability of a light source to show colors, based on a color rendering index.

Color Rendition (Rendering) Index (CRI)

A measure of light quality. The maximum CRI value of 100 is given to natural daylight and incandescent lighting. The closer a lamp's CRI rating is to 100, the better its ability to show true colors to the human eye.

Color Temperature

A measure of the quality of a light source by expressing the color appearance correlated with a black body.

Combined-Cycle Power Plant

A power plant that uses two thermodynamic cycles to achieve higher overall system efficiency; e.g.: the heat from a gas-fired combustion turbine is used to generate steam for heating or to operate a steam turbine to generate additional electricity.

Combustion

The process of burning; the oxidation of a material by applying heat, which unites oxygen with a material or fuel.

Combustion Air

Air that provides the necessary oxygen for complete, clean combustion and maximum heating value.

Combustion Chamber

Any wholly or partially enclosed space in which combustion takes place.

Combustion Gases

The gaseous byproducts of the combustion of a fuel.

Combustion Power Plant

A power plant that generates power by combusting a fuel.

Combustion Turbine

A turbine that generates power from the combustion of a fuel.

Commercial Building

A building with more than 50 percent of its floor space used for commercial activities, which include stores, offices, schools, churches, libraries, museums, health care facilities, warehouses, and government buildings except those on military bases.

Commercial Sector

Consists of businesses that are not engaged in transportation or manufacturing or other types of industrial activities. Standard Industrial Classification (SIC) codes for commercial establishments are 50 through 87, 89, and 91 through 97.

Comfort Zone

A frequently used room or area that is maintained at a more comfortable level than the rest of the house; also known as a "warm room."

Commissioning

The process by which a power plant, apparatus, or building is approved for operation based on observed or measured operation that meets design specifications.

Compact Fluorescent

A smaller version of standard fluorescent lamps which can directly replace standard incandescent lights. These lights consist of a gas filled tube, and a magnetic or electronic ballast.

Complete Mix Digester

A type of anaerobic digester that has a mechanical mixing system and where temperature and volume are controlled to maximize the anaerobic digestion process for biological waste treatment, methane production, and odor control.

Composting

The process of degrading organic material (biomass) by microorganisms in aerobic conditions.

Composting Toilet

A self-contained toilet that use the process of aerobic decomposition (composting) to break down feces into humus and odorless gases.

Compound Paraboloid Collector

A form of solar concentrating collector that does not track the sun.

Compressed Air Storage

The storage of compressed air in a container for use to operate a prime mover for electricity generation.

Compressed Natural Gas (CNG)

Natural gas (methane) that has been compressed to a higher pressure gaseous state by a compressor; used in CNG vehicles.

Compression Chiller

A cooling device that uses mechanical energy to produce chilled water.

Compressor

A device used to compress air for mechanical or electrical power production, and in air conditioners, heat pumps, and refrigerators to pressurize the refrigerant and enabling it to flow through the system.

Concentrating (Solar) Collector

A solar collector that uses reflective surfaces to concentrate sunlight onto a small area, where it is absorbed and converted to heat or, in the case of solar photovoltaic (PV) devices, into electricity. Concentrators can increase the power flux of sunlight hundreds of times. The principal types of concentrating collectors include: compound parabolic, parabolic trough, fixed reflector moving receiver, fixed receiver moving reflector, Fresnel lense, and central receiver. A PV concentrating module uses optical elements (Fresnel lense) to increase the amount of sunlight incident onto a PV cell. Concentrating PV modules/arrays must track the sun and use only the direct sunlight because the diffuse portion cannot be focused onto the PV cells. Concentrating collectors for home or small business solar water heating applications are usually parabolic troughs that concentrate the sun's energy on an absorber tube (called a receiver), which contains a heat-transfer fluid.

Condensate

The liquid resulting when water vapor contacts a cool surface; also the liquid resulting when a vaporized working fluid (such as a refrigerant) is cooled or depressurized.

Condensation

The process by which water in air changes from a vapor to a liquid due to a change in temperature or pressure; occurs when water vapor reaches its dew point (condensation point); also used to express the existence of liquid water on a surface.

Condenser

The device in an air conditioner or heat pump in which the refrigerant condenses from a gas to a liquid when it is depressurized or cooled.

Condenser Coil

The device in an air conditioner or heat pump through which the refrigerant is circulated and releases heat to the surroundings when a fan blows outside air over the coils. This will return the hot vapor that entered the coil into a hot liquid upon exiting the coil.

Condensing Furnace

A type of heating appliance that extracts so much of the available heat content from a combusted fuel that the moisture in the combustion gases condenses before it leaves the furnace. Also this furnace circulates a liquid to cool the furnace's heat exchanger. The heated liquid may either circulate through a liquid-to-air heat exchanger to warm room air, or it may circulate through a coil inside a separate indirect-fired water heater.

Condensing Unit

The component of a central air conditioner that is designed to remove heat absorbed by the refrigerant and transfer it outside the conditioned space.

Conditioned Space

The interior space of a building that is heated or cooled.

Conduction

The transfer of heat through a material by the transfer of kinetic energy from particle to particle; the flow of heat between two materials of different temperatures that are in direct physical contact.

Conduction Band

An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge.

Conductivity (Thermal)

This is a positive constant, k, that is a property of a substance and is used in the calculation of heat transfer rates for materials. It is the amount of heat that flows through a specified area and thickness of a material over a specified period of time when there is a temperature difference of one degree between the surfaces of the material.

Conductor

The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.

Conduit

A tubular material used to encase and protect one or more electrical conductors.

Congressional (Energy) Committees:

House Subcommittee on Energy and Environment — This committee has legislative jurisdiction and general and special oversight and investigative authority on all matters relating to energy and environmental research and development and demonstration.

House Water and Power Committee — This committee has oversight over the generation and marketing of electric power from federal water projects by federally charted or Federal RPM authorities, measures and matters concerning water resources planning, compacts relating to use and apportionment of interstate waters, water rights or power movement programs, measures and matters pertaining to irrigation and reclamation projects and other water resources development programs.

Senate Committee on Energy and Natural Resources — This committee has jurisdiction on: coal production, distribution and utilization; energy policy; energy research, conservation, and development; hydroelectric power; irrigation; mineral conservation; nonmilitary development of nuclear energy; solar energy systems; and over territorial possessions, including trusteeships of the United States.

Senate Subcommittee on Energy Research, Development, Production and Regulation — This committee has jurisdiction on the oversight and legislative responsibilities for: coal, nuclear, and non-nuclear energy commercialization projects; DOE National Laboratories; global climate change; new technologies research and development; commercialization of new technologies including, solar energy systems; Federal energy conservation programs; energy information; and power provider policy.

Connected Load

The sum of the ratings of the electricity consuming apparatus connected to a generating system.

Connection Charge

An amount paid by a customer for being connected to an electricity supplier's transmission and distribution system.

Conservation

To reduce or avoid the consumption of a resource or commodity.

Conservation Cost Adjustment

A means of billing electric power consumers to pay for the costs of demand side management/energy conservation measures and programs. (See also Benefits Charge.)

Constant Dollars

The value or purchasing power of a dollar in a specified year carried forward or backward.

Constant-Speed Wind Turbines

Wind turbines that operate at a constant rotor revolutions per minute (RPM) and are optimized for energy capture at a given rotor diameter at a particular speed in the wind power curve.

Consumption Charge

The part of a power provider's charge based on actual energy consumed by the customer; the product of the kilowatt-hour rate and the total kilowatt-hours consumed.

Contact Resistance

The resistance between metallic contacts and the semiconductor.

Continuous Fermentation

A steady-state fermentation process.

Contrast

The difference between the brightness of an object compared to that of its immediate background.

Convection

The transfer of heat by means of air currents.

Conventional Fuel

The fossil fuels: coal, oil, and natural gas.

Conventional Heat Pump

This type of heat pump is known as an air-to air system.

Conventional Power

Power generation from sources such as petroleum, natural gas, or coal. In some cases, large-scale hydropower and nuclear power generation are considered conventional sources.

Conversion Efficiency

The amount of energy produced as a percentage of the amount of energy consumed.

Converter

A device for transforming the quality and quantity of electrical energy; also an inverter.

Cooling Capacity

The quantity of heat that a cooling appliance is capable of removing from a room in one hour.

Cooling Degree Day

A value used to estimate interior air cooling requirements (load) calculated as the number of degrees per day (over a specified period) that the daily average temperature is above 65 degrees Fahrenheit (or some other, specified base temperature). The daily average temperature is the mean of the maximum and minimum temperatures recorded for a specific location for a 24 hour period.

Cooling Load

That amount of cooling energy to be supplied (or heat and humidity removed) based on the sensible and latent loads.

Cooling Pond

A body of water used to cool the water that is circulated in an electric power plant.

Cooling Tower

A structure used to cool power plant water; water is pumped to the top of the tubular tower and sprayed out into the center, and is cooled by evaporation as it falls, and then is either recycled within the plant or is discharged.

Coproducts

The potentially useful byproducts of ethanol fermentation process.

Cord (of Wood)

A stack of wood 4 feet by 4 feet by 8 feet.

Coulomb

A unit for the quantity of electricity transported in 1 second by a current of 1 ampere.

Counterflow Heat Exchanger

A heat exchanger in which two fluids flow in opposite directions for transfer heat energy from one to the other.

Covenants

Restrictions on the use of a property.

Crawlspace

The unoccupied, and usually unfinished and unconditioned space between the floor, foundation walls, and the slab or ground of a building.

Creosote

A liquid byproduct of wood combustion (or distillation) that condenses on the internal surfaces of vents and chimneys, which if not removed regularly, can corrode the surfaces and fuel a chimney fire.

Critical Compression Pressure

The highest possible pressure in a fuel-air mixture before spontaneous ignition occurs.

Crystalline Silicon Photovoltaic Cell

A type of photovoltaic cell made from a single crystal or a polycrystalline slice of silicon. Crystalline silicon cells can be joined together to form a module (or panel).

Cubic Foot (of Natural Gas)

A unit of volume equal to 1 cubic foot at a pressure base of 14.73 pounds standard per square inch absolute and a temperature base of 60 degrees Fahrenheit.

Cube Law

In reference to wind energy, for any given instant, the power available in the wind is proportional to the cube of the wind velocity; when wind speed doubles, the power availability increases eight times.

Current (Electrical)

The flow of electrical energy (electricity) in a conductor, measured in amperes.

Current Dollars

The value or purchasing power of a dollar that has not been reduced to a common basis of constant purchasing power, but instead reflects anticipated future inflation; when used in computations the assumed inflation rate must be stated.

Customer Charge

An amount to be paid for energy periodically by a customer without regard to demand or energy consumption.

Customer Class

Categories of energy consumers, as defined by consumption or demand levels, patterns, and conditions, and generally included residential, commercial, industrial, agricultural.

Cut-In-Speed

The lowest wind speed at which a wind turbine begins producing usable power.

Cut-Out-Speed

The highest wind speed at which a wind turbine stops producing power.

Cycle

In alternating current, the current goes from zero potential or voltage to a maximum in one direction, back to zero, and then to a maximum potential or voltage in the other direction. The number of complete cycles per second determines the current frequency; in the U.S. the standard for alternating current is 60 cycles.

Cycling Losses

The loss of heat as the water circulates through a water heater tank and inlet and outlet pipes.

Cyclone Burner

A furnace/combustion chamber in which finely ground fuel is blown in spirals in the combustion chamber to maximize combustion efficiency.

Czochralski Process

A method of growing large size, high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.

D

Dam: A structure for impeding and controlling the flow of water in a water course, and which increases the water elevation to create the hydraulic head. The reservoir creates, in effect, stored energy.
Damper: A movable plate used to control air flow; in a wood stove or fireplace, used to control the amount and direction of air going to the fire.
Darrius (Wind) Machine: A type of vertical-axis wind machine that has long, thin blades in the shape of loops connected to the top and bottom of the axle; often called an "eggbeater windmill."
Daylighting: The use of direct, diffuse, or reflected sunlight to provide supplemental lighting for building interiors.
Decentralized (Energy) System: Energy systems supply individual, or small-groups, of energy loads.
Declination: The angular position of the sun at solar noon with respect to the plane of the equator.
Declining Block Rate: An electricity supplier rate structure in which the per unit price of electricity decreases as the amount of energy increases. Normally only available to very large consumers.
Decommissioning: The process of removing a power plant, apparatus, equipment, building, or facility from operation.
Decomposition: The process of breaking down organic material; reduction of the net energy level and change in physical and chemical composition of organic material.
De-energize(d): To disconnect a transmission and/or distribution line; a power line that is not carrying a current; to open a circuit.
Deep Discharge: Discharging a battery to 20 percent or less of its full charge capacity.
Degree Day: A unit for measuring the extent that the outdoor daily average temperature (the mean of the maximum and minimum daily dry-bulb temperatures) falls below (in the case of heating, see Heating Degree Day), or falls above (in the case of cooling, see Cooling Degree Day) an assumed base temperature, normally taken as 65 degrees Fahrenheit, unless otherwise stated. One degree day is counted for each degree below (for heating) or above (in the case of cooling) the base, for each calendar day on which the temperature goes below or above the base.
Degree Hour: The product of 1 hour, and usually the number of degrees Fahrenheit the hourly mean temperature is above a base point (usually 65 degrees Fahrenheit); used in roughly estimating or measuring the cooling load in cases where processes heat, heat from building occupants, and humidity are relatively unimportant compared to the dry-bulb temperature.
Dehumidifier: A device that cools air by removing moisture from it.
Demand: The rate at which electricity is delivered to or by a system, part of a system, or piece of equipment expressed in kilowatts, kilovoltamperes, or other suitable unit, at a given instant or averaged over a specified period of time.
Demand Charge: A charge for the maximum rate at which energy is used during peak hours of a billing period. That part of a power provider service charged for on the basis of the possible demand as distinguished from the energy actually consumed.
Demand(ed) Factor: The ratio of the maximum demand on an electricity generating and distribution system to the total connected load on the system; usually expressed as a percentage.
Demand Power: see Peak Power
Demand-Side Management (DSM): The process of managing the consumption of energy, generally to optimize available and planned generation resources.
Demand (Tankless) Water Heater: A type of water heater that has no storage tank thus eliminating storage tank stand-by losses. Cold water travels through a pipe into the unit, and either a gas burner or an electric element heats the water only when needed.
Dendrite: A slender threadlike spike of pure crystalline material, such as silicon.
Dendritic Web Technique: A method for making sheets of polycrystalline silicon in which silicon dendrites are slowly withdrawn from a melt of silicon whereupon a web of silicon forms between the dendrites and solidifies as it rises from the melt and cools.
Department of Agriculture (USDA): A federal government agency involved in rural development, marketing and regulatory programs, food safety, research, education and economics, food, nutrition and consumer service, farm and foreign agricultural services, and natural resources and environment programs.
Department of Energy (DOE): A federal government agency created in 1977, that is entrusted to contribute to the welfare of the United States by providing technical information, and a scientific and educational foundation for technology, policy and institutional leadership to achieve efficiency in energy use, diversity in energy sources, a more productive and competitive economy, improved environmental quality, and a secure national defense.
Dependable Capacity: The load-carrying ability of an electric power plant during a specific time interval and period when related to the characteristics of the load to be/being supplied; determined by capability, operating power factor, and the portion of the load the station is to supply.
Derating: The production of energy by a system or appliance at a level less than its design or nominal capacity.
Deregulation: The process of changing regulatory policies and laws to increase competition among suppliers of commodities and services. The process of deregulating the electric power industry was initiated by the Energy Policy Act of 1992. (See also Restructuring)
Desiccant: A material used to desiccate (dry) or dehumidify air.
Desiccant Cooling: To condition/cool air by dessication.
Desiccation: The process of removing moisture; involves evaporation.
Design Cooling Load: The amount of conditioned air to be supplied by a cooling system; usually the maximum amount to be delivered based on a specified number of cooling degree days or design temperature.
Design Heating Load: The amount of heated air, or heating capacity, to be supplied by a heating system; usually the maximum amount to be delivered based on a specified number of heating degree days or design outside temperature.
Design Life: Period of time a system or appliance (or component of) is expected to function at its nominal or design capacity without major repair.
Design Temperature: The temperature that a system is designed to maintain (inside) or operate against (outside) under the most extreme conditions.
Design Tip Speed Ratio: For a wind turbine, the ratio of the speed of the tip of a turbine blade for which the power coefficient is at maximum.
Design Voltage: The nominal voltage for which a conductor or electrical appliance is designed; the reference voltage for identification and not necessarily the precise voltage at which it operates.
Desuperheater: An energy saving device in a heat pump that, during the cooling cycle, recycles some of the waste heat from the house to heat domestic water.
Dewpoint: The temperature to which air must be cooled, at constant pressure and water vapor content, in order for saturation or condensation to occur; the temperature at which the saturation pressure is the same as the existing vapor pressure; also called saturation point.
Difference of Potential: The difference in electrical pressure (voltage) between any two points in an electrical system or between any point in an electrical system and the earth.
Differential Thermostat: A type of automatic thermostat (used on solar heating systems) that responds to temperature differences (between collectors and the storage components) so as to regulate the functioning of appliances (to switch transfer fluid pumps on and off).
Diffuse Solar Radiation: Sunlight scattered by atmospheric particles and gases so that it arrives at the earth's surface from all directions and can not be focused.
Diffusion: The movement of individual molecules through a material; permeation of water vapor through a material.
Diffusion Length: The mean distance a free electron or hole moves before recombining with another hole or electron.
Digester (Anaerobic): A device in which organic material is biochemically decomposed (digested) by anaerobic bacteria to treat the material and/or to produce biogas.
Dimmer: A light control device that allows light levels to be manually adjusted. A dimmer can save energy by reducing the amount of power delivered to the light while consuming very little themselves.
Dip Tube: A tube inside a domestic water heater that distributes the cold water from the cold water supply line into the lower area of the water heater where heating occurs.
Diode: An electronic device that allows current to flow in one direction only.
Direct Access: The ability of an electric power consumer to purchase electricity from a supplier of their choice without being physically inhibited by the owner of the electric distribution and transmission system to which the consumer is connected to. (See also Open Access.)
Direct Beam Radiation: Solar radiation that arrives in a straight line from the sun.
Direct Current: A type of electricity transmission and distribution by which electricity flows in one direction through the conductor; usually relatively low voltage and high current; typically abbreviated as dc.
Direct-Gain: The process by which sunlight directly enters a building through the windows and is absorbed and stored in massive floors or walls.
Direct Solar Water Heater: These systems use water as the fluid that is circulated through the collector to the storage tank. Also known as "open-loop" systems.
Direct Vent Heater: A type of combustion heating system in which combustion air is drawn directly from outside and the products of combustion are vented directly outside. These features are beneficial in tight, energy-efficient homes because they will not depressurize a home and cause air infiltration, and backdrafting of other combustion appliances.
Direct Water Heater: A type of water heater in which heated water is stored within the tank. Hot water is released from the top of the tank when a hot water faucet is turned. This water is replaced with cold water that flows into the tank and down to just above the bottom plate under which are the burners.
Discounting: A method of financial and economic analysis used to determine present and future values of investments or expenses.
Discount Rate: The interest rate at which the Federal Reserve System stands ready to lend reserves to commercial banks. The rate is proposed by the 12 Federal Reserve banks and determined with the approval of the Board of Governors.
Dispatching: To schedule and control the generation and delivery of electric power.
Dispatchability: The ability to dispatch power.
Displacement Power: A source of power (electricity) that can displace power from another source so that source's power can be transmitted to more distant loads.
Distributed Generation: A term used by the power industry to describe localized or on-site power generation.
Distribution: The process of distributing electricity; usually defines that portion of a power provider's power lines between a power provider's power pole and transformer and a customer's point of connection/meter.
Distribution Feeder: (See Feeder)
Distribution Line: One or more circuits of a distribution system on the same line or poles or supporting structures' usually operating at a lower voltage relative to the transmission line.
Distribution System: That portion of an electricity supply system used to deliver electricity from points on the transmission system to consumers.
District Heating: A heating system in which steam or hot water for space heating or hot water is piped from a central boiler plant or electric power/heating plant to a cluster of buildings.
Diversity Factor: The ratio of the sum of the noncoincidental maximum demands of two or more loads to their coincidental maximum demands for the same period.
DOE-2.1: A computer software program that simulates energy consumption of commercial buildings; used for design and auditing purposes.
Dome (Geodesic): An architectural design invented by Buckminster Fuller with a regular polygonal structure based on radial symmetry.
Domestic Hot Water: Water heated for residential washing, bathing, etc.
Donor: In a solar photovoltaic device, an n-type dopant, such as phosphorus, that puts an additional electron into an energy level very near the conduction band; this electron is easily exited into the conduction band where it increases the electrical conductivity over than of an undoped semiconductor.
Dopant: A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).
Doping: The addition of dopants to a semiconductor.
Double-Pane or Glazed Window: A type of window having two layers (panes or glazing) of glass separated by an air space. Each layer of glass and surrounding air space reradiates and traps some of the heat that passes through thereby increasing the windows resistance to heat loss (R-value).
Double Wall Heat Exchanger: A heat exchanger in a solar water heating system that has two distinct walls between the heat transfer fluid and the domestic water, to ensure that there is no mixing of the two.
Downwind Wind Turbine: A horizontal axis wind turbine in which the rotor is downwind of the tower.
Draft: A column of burning combustion gases that are so hot and strong that the heat is lost up the chimney before it can be transferred to the house. A draft brings air to the fire to help keep it burning.
Draft Diverter: A door-like device located at the mouth of a fireplace chimney flue for controlling the direction and flow of the draft in the fireplace as well as the amount of oxygen that the fire receives.
Draft Hood: A device built into or installed above a combustion appliance to assure the escape of combustion byproducts, to prevent backdrafting of the appliance, or to neutralize the effects of the stack action of the chimney or vent on the operation of the appliance.
Drag: Resistance caused by friction in the direction opposite to that of movement (i.e., motion) of components such as wind turbine blades.
Drainback (Solar) Systems: A closed-loop solar heating system in which the heat transfer fluid in the collector loop drains into a tank or reservoir whenever the booster pump stops to protect the collector loop from freezing.
Draindown (Solar) Systems: An open-loop solar heating system in which the heat transfer fluid from the collector loop and the piping drain into a drain whenever freezing conditions occur.
Dry Bulb Temperature: The temperature of the air as measured by a standard thermometer.
Dry Steam Geothermal Plants: Conventional turbine generators are used with the dry steam resources. The steam is used directly, eliminating the need for boilers and boiler fuel that characterizes other steam-power-generating technologies. This technology is limited because dry-steam hydrothermal resources are extremely rare. The Geysers, in California, is the nation's only dry steam field.
Dual Duct System: An air conditioning system that has two ducts, one is heated and the other is cooled, so that air of the correct temperature is provided by mixing varying amounts of air from each duct.
Dual Fuel (or Flex Fuel) Vehicle: A vehicle with an engine capable of operating on two different types of fuels.
Duct(s): The round or rectangular tube(s), generally constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite, located within a wall, floor, and ceiling that distributes heated or cooled air in buildings.
Duct Fan: An axial flow fan mounted in a section of duct to move conditioned air.
Duty Cycle: The duration and periodicity of the operation of a device.
Dynamo: A machine for converting mechanical energy into electrical energy by magneto-electric induction; may be used as a motor.
Dynamic Head: The pressure equivalent of the velocity of a fluid.
Dynamometer: An apparatus for measuring force or power, especially the power developed by a motor.
Dyne: The absolute centimeter-gram-second unit of force; that force that will impart to a free mass of one gram an acceleration of one centimeter per second per second.

E

Earth Berm: A mound of dirt next to exterior walls to provide wind protection and insulation.
Earth Cooling Tube: A long, underground metal or plastic pipe through which air is drawn. As air travels through the pipe it gives up some of its heat to the soil, and enters the house as cooler air.
Earth-Coupled Ground Source (Geothermal) Heat Pump: A type of heat pump that uses sealed horizontal or vertical pipes, buried in the ground, as heat exchangers through which a fluid is circulated to transfer heat.
Earth Sheltered Houses: Houses that have earth berms around exterior walls.
Earth-Ship: A registered trademark name for houses built with tires, aluminum cans, and earth.
Easement: An incorporated right, liberty, privilege, or use of another entity's property, distinct from ownership, without profit or compensation; a right-of-way.
Eccentric: A device for converting continuous circular motion into reciprocating rectilinear motion.
Economizer: A heat exchanger for recovering heat from flue gases for heating water or air.
Edge-Defined Film-Fed Growth (EFG): A method for making sheets of polycrystalline silicon (for solar photovoltaic devices) in which molten silicon is drawn upward by capillary action through a mold.
Efficacy: The amount of energy service or useful energy delivered per unit of energy input. Often used in reference to lighting systems, where the visible light output of a luminary is relative to power input; expressed in lumens per Watt; the higher the efficacy value, the higher the energy efficiency.
Effective Capacity: The maximum load that a device is capable of carrying.
Efficiency: Under the First Law of Thermodynamics, efficiency is the ratio of work or energy output to work or energy input, and cannot exceed 100 percent. Efficiency under the Second Law of Thermodynamics is determined by the ratio of the theoretical minimum energy that is required to accomplish a task relative to the energy actually consumed to accomplish the task. Generally, the measured efficiency of a device, as defined by the First Law, will be higher than that defined by the Second Law.
Efficiency (Appliance) Ratings: A measure of the efficiency of an appliance's energy efficiency.
Elasticity of Demand: The ratio of the percentage change in the quantity of a good or service demanded to the percentage change in the price.
Electrical Energy: The energy of moving electrons.
Electrical Charge: A condition that results from an imbalance between the number of protons and the number of electrons in a substance.
Electrical System: All the conductors and electricity using devices that are connected to a source of electromotive force (or generator).
Electrical System Energy Losses: A measure of the amount of energy lost during the generation, transmission, and distribution of electricity.
Electric Circuit: The path followed by electrons from a generation source, through an electrical system, and returning to the source.
Electric Energy: The amount of work accomplished by electrical power, usually measured in kilowatt-hours (kWh). One kWh is 1,000 Watts and is equal to 3,413 Btu.
Electric Furnace: An air heater in which air is blown over electric resistance heating coils.
Electricity Generation: The process of producing electricity by transforming other forms or sources of energy into electrical energy; measured in kilowatt-hours.
Electricity Grid: A common term referring to an electricity transmission and distribution system.
Electric Rate: The unit price and quantity to which it applies as specified in a rate schedule or contract.
Electric Rate Schedule: A statement of the electric rate(s), terms, and conditions for electricity sale or supply.
Electric System: The physically connected generation, transmission, and distribution facilities and components operated as a unit.
Electric System Loss(es): The total amount of electric energy loss in an electric system between the generation source and points of delivery.
Electric Power Plant: A facility or piece of equipment that produces electricity.
Electric Power Sector: Those privately or publicly owned establishments that generate, transmit, distribute, or sell electricity.
Electric Power Transmission: The transmission of electricity through power lines.
Electric Resistance Heating: A type of heating system where heat, resulting when electric current flows through an "element" or conductor, such as Nichrome, which has a high resistance, is radiated to a room.
Electric Utility: A corporation, person, agency, authority or other legal entity that owns and/or operates facilities for the generation, transmission, distribution or sale of electricity primarily for use by the public. Also known as a power provider.
Electric Vehicles: A battery-powered electrically driven vehicle.
Electricity Industry Restructuring: The process of changing the structure of the electric power industry from one of guaranteed monopoly over service territories, as established by the Public Utility Holding Company Act of 1935, to one of open competition between power suppliers for customers in any area.
Electrochemical Cell: A device containing two conducting electrodes, one positive and the other negative, made of dissimilar materials (usually metals) that are immersed in a chemical solution (electrolyte) that transmits positive ions from the negative to the positive electrode and thus forms an electrical charge. One or more cells constitute a battery.
Electrode: A conductor that is brought in conducting contact with a ground.
Electrodeposition: Electrolytic process in which a metal is deposited at the cathode from a solution of its ions.
Electrolysis: A chemical change in a substance that results from the passage of an electric current through an electrolyte. The production of commercial hydrogen by separating the elements of water, hydrogen, and oxygen, by charging the water with an electrical current.
Electrolyte: A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell.
Electromagnetic Energy: Energy generated from an electromagnetic field produced by an electric current flowing through a superconducting wire kept at a specific low temperature.
Electromagnetic Field (EMF): The electrical and magnetic fields created by the presence or flow of electricity in an electrical conductor or electricity consuming appliance or motor.
Electromotive Force: The amount of energy derived from an electrical source per unit quantity of electricity passing through the source.
Electron: An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom.
Electronic Ballast: A device that uses electronic components to regulate the voltage of fluorescent lamps.
Electron Volt: The amount of kinetic energy gained by an electron when accelerated through an electric potential difference of 1 Volt; equivalent to 1.603 x 10^-12; a unit of energy or work; abbreviated as eV.
Electrostatic Precipitator: A device used to remove particulate matter from the waste gasses of a combustion power plant.
Ellipsoidal Reflector Lamp: A lamp where the light beam is focused 2 inches ahead of the lamp reducing the amount of light trapped in the fixture.
Emission(s): A substance(s) or pollutant emitted as a result of a process.
Emission Factor: A measure of the average amount of a specified pollutant or material emitted for a specific type of fuel or process.
Emissivity: The ratio of the radiant energy (heat) leaving (being emitted by) a surface to that of a black body at the same temperature and with the same area; expressed as a number between 0 and 1.
Enclosure: The housing around a motor that supports the active parts and protects them. They come in different varieties (open, protected) depending on the degree of protection required.
Endothermic: A heat absorbing reaction or a reaction that requires heat.
End Use: The purpose for which useful energy or work is consumed.
Energize(d): To send electricity through a electricity transmission and distribution network; a conductor or power line that is carrying current.
Energy: The capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.
Energy Audit: A survey that shows how much energy you use in your house or apartment. It will help you find ways to use less energy.
Energy Charge: That part of an electricity bill that is based on the amount of electrical energy consumed or supplied.
Energy Contribution Potential: Recombination occurring in the emitter region of a photovoltaic cell.
Energy Crops: Crops grown specifically for their fuel value. These include food crops such as corn and sugarcane, and nonfood crops such as poplar trees and switchgrass. Currently, two energy crops are under development: short-rotation woody crops, which are fast-growing hardwood trees harvested in 5 to 8 years; and herbaceous energy crops, such as perennial grasses, which are harvested annually after taking 2 to 3 years to reach full productivity.
Energy Efficient Mortgages: A type of home mortgage that takes into account the energy savings of a home that has cost-effective energy saving improvements that will reduce energy costs thereby allowing the homeowner to more income to the mortgage payment. A borrower can qualify for a larger loan amount than otherwise would be possible.
Energy Efficiency Ratio (EER): The measure of the instantaneous energy efficiency of room air conditioners; the cooling capacity in Btu/hr divided by the watts of power consumed at a specific outdoor temperature (usually 95 degrees Fahrenheit).
Energy Density: The ratio of available energy per pound; usually used to compare storage batteries.
Energy Factor (EF): The measure of overall efficiency for a variety of appliances. For water heaters, the energy factor is based on three factors: 1) the recovery efficiency, or how efficiently the heat from the energy source is transferred to the water; 2) stand-by losses, or the percentage of heat lost per hour from the stored water compared to the content of the water: and 3) cycling losses. For dishwashers, the energy factor is defined as the number of cycles per kWh of input power. For clothes washers, the energy factor is defined as the cubic foot capacity per kWh of input power per cycle. For clothes dryers, the energy factor is defined as the number of pounds of clothes dried per kWh of power consumed.
Energy End-Use Sectors: Major energy consuming sectors of the economy. The Commercial Sector includes commercial buildings and private companies. The Industrial Sector includes manufacturers and processors. The Residential Sector includes private homes. The Transportation Sector includes automobiles, trucks, rail, ships, and aircraft.
Energy Guide Labels: The labels placed on appliances to enable consumers to compare appliance energy efficiency and energy consumption under specified test conditions as required by the Federal Trade Commission.
Energy Intensity: The relative extent that energy is required for a process.
Energy Policy Act of 1992 (EPAct): A comprehensive legislative package that mandates and encourages energy efficiency standards, alternative fuel use, and the development of renewable energy technologies. Public Law 102-486, October 24th, 1992. Also authorized the Federal Energy Regulatory Commission (FERC) to order the owners of electric power transmission lines to transmit or "wheel" power for power generators including electric power providers, federal power marketing authorities, and exempt wholesale generators.
Energy Security Act of 1980: Legislation authorizing a U.S. biomass and alcohol fuel program, and that authorized loan guarantees and price guarantees and purchase agreements for alcohol fuel production.
Energy Service Company (ESCO): A company that specializes in undertaking energy efficiency measures under a contractual arrangement whereby the ESCO shares the value of energy savings with their customer.
Energy Storage: The process of storing, or converting energy from one form to another, for later use; storage devices and systems include batteries, conventional and pumped storage hydroelectric, flywheels, compressed gas, and thermal mass.
Enthalpy: A thermodynamic property of a substance, defined as the sum of its internal energy plus the pressure of the substance times its volume, divided by the mechanical equivalent of heat. The total heat content of air; the sum of the enthalpies of dry air and water vapor, per unit weight of dry air; measured in Btu per pound (or calories per kilogram).
Entrained Bed Gasifier: A gasifier in which the feedstock (fuel) is suspended by the movement of gas to move it through the gasifier.
Entropy: A measure of the unavailable or unusable energy in a system; energy that cannot be converted to another form.
Environment: All the natural and living things around us. The earth, air, weather, plants, and animals all make up our environment.
Epitaxial Growth: In reference to solar photovoltaic devices, the growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal.
Equinox: The two times of the year when the sun crosses the equator and night and day are of equal length; usually occurs on March 21st (spring equinox) and September 23 (fall equinox).
Erg: A unit of work done by the force of one dyne acting through a distance of one centimeter.
Ethanol — Ethyl alcohol (C₂H₅OH): A colorless liquid that is the product of fermentation used in alcoholic beverages, industrial processes, and as a fuel additive. Also known as grain alcohol.
Ethyl Tertiary Butyl Ether (ETBE): A chemical compound produced in a reaction between ethanol and isobutylene (a petroleum-derived by-product of the refining process). ETBE has characteristics superior to other ethers: low volatility, low water solubility, high octane value, and a large reduction in carbon monoxide and hydrocarbon emissions.
Eutectic: A mixture of substances that has a melting point lower than that of any mixture of the same substances in other proportions.
Eutectic Salts: Salt mixtures with potential applications as solar thermal energy storage materials.
Evacuated-Tube Collector: A collector is the mechanism in which fluid (water or diluted antifreeze, for example) is heated by the sun in a solar hot water system. Evacuated-tube collectors are made up of rows of parallel, transparent glass tubes. Each tube consists of a glass outer tube and an inner tube, or absorber. The absorber is covered with a selective coating that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space between the tubes to form a vacuum, which eliminates conductive and convective heat loss. Evacuated-tube collectors are used for active solar hot water systems.
Evaporation: The conversion of a liquid to a vapor (gas), usually by means of heat.
Evaporative Cooling: The physical process by which a liquid or solid is transformed into the gaseous state. For this process a mechanical device uses the outside air's heat to evaporate water that is held by pads inside the cooler. The heat is drawn out of the air through this process and the cooled air is blown into the home by the cooler's fan.
Evaporator Coil: The inner coil in a heat pump that, during the cooling mode, absorbs heat from the inside air and boils the liquid refrigerant to a vapor, which cools the house.
Excitation: The power required to energize the magnetic field of a generator.
Exempt Wholesale Generator: An unregulated subsidiary of a power provider that is allowed to generate and sell wholesale power as an independent energy producer, and is exempt from the Public Utility Holding Company Act of 1935.
Exothermic: A reaction or process that produces heat; a combustion reaction.
Expanded Polystyrene: A type of insulation that is molded or expanded to produce coarse, closed cells containing air. The rigid cellular structure provides thermal and acoustical insulation, strength with low weight, and coverage with few heat loss paths. Often used to insulate the interior of masonry basement walls.
Expansion Tank: A tank used in a closed-loop solar heating system that provides space for the expansion of the heat transfer fluid in the pressurized collector loop.
Expansion Valve: The device that reduces the pressure of liquid refrigerant thereby cooling it before it enters the evaporator coil in a heat pump.
External Combustion Engine: An engine in which fuel is burned (or heat is applied) to the outside of a cylinder; a Stirling engine.
Externality: The environmental, social, and economic impacts of producing a good or service that are not directly reflected in the market price of the good or service.
Extruded Polystyrene: A type of insulation material with fine, closed cells, containing a mixture of air and refrigerant gas. This insulation has a high R-value, good moisture resistance, and high structural strength compared to other rigid insulation materials.

F

Fan: A device that moves and/or circulates air and provides ventilation for a room or a building.
Fan Coil: A heat exchanger coil in which a fluid such as water is circulated and a fan blows air over the coil to distribute heat or cool air to the different rooms.
Fan Velocity Pressure: The pressure corresponding to the outlet velocity of a fan; the kinetic energy per unit volume of flowing air.
Farad: A unit of electrical capacitance; the capacitance of a capacitor between the plates of which there appears a difference of 1 Volt when it is charged by one coulomb of electricity.
Feather: In a wind energy conversion system, to pitch the turbine blades so as to reduce their lift capacity as a method of shutting down the turbine during high wind speeds.
Federal Energy Management Program (FEMP): A program of the U.S. Department of Energy (DOE) that implements energy legislation and presidential directives. FEMP provides project financing, technical guidance and assistance, coordination and reporting, and new initiatives for the federal government. It also helps federal agencies identify the best technologies and technology demonstrations for their use.
Federal Energy Regulatory Commission (FERC): This is an independent regulatory agency within the U.S. DOE that has jurisdiction over interstate electricity sales, wholesale electric rates, natural gas pricing, oil pipeline rates, and gas pipeline certification. It also licenses and inspects private, municipal, and state hydroelectric projects and oversees related environmental matters.
Federal Power Marketing Administrations (PMA): These are separate and distinct organizational agencies within the U.S. DOE that market power at federal multipurpose water projects at lowest possible rates to consumers consistent with sound business principles. There are five PMA's: Alaska Power Administration, Bonneville Power Administration, Southeastern Power Administration, Southwestern Power Administration, Western Area Power Administration.
Feeder: A power line for supplying electricity within a specified area.
Feedstock: A raw material that can be converted to one or more products.
Fenestration: The arrangement, proportion, and design of windows in a building.
Fermentation: The decomposition of organic material to alcohol, methane, etc., by organisms, such as yeast or bacteria, usually in the absence of oxygen.
Fiberglass Insulation: A type of insulation, composed of small diameter pink, yellow, or white glass fibers, formed into blankets or batts, or used in loose-fill and blown-in applications.
Filament: A coil of tungsten wire suspended in a vacuum or inert gas-filled bulb. When heated by electricity the tungsten "filament" glows.
Fill Factor: The ratio of a photovoltaic cell's actual power to its power if both current and voltage were at their maxima. A key characteristic in evaluating cell performance.
Filter (air): A device that removes contaminants, by mechanical filtration, from the fresh air stream before the air enters the living space. Filters can be installed as part of a heating/cooling system through which air flows for the purpose of removing particulates before or after the air enters the mechanical components.
Fin: A thin sheet of material (metal) of a heat exchanger that conducts heat to a fluid.
Finish: Both a noun and a verb to describe the exterior surface of building elements (walls, floors, ceilings, etc.) and furniture, and the process of applying it.
Fire Classification: Classifications of fires developed by the National Fire Protection Association.
Fireplace: A wood or gas burning appliance that is primarily used to provide ambiance to a room. Conventional, masonry fireplaces without energy saving features, often take more heat from a space than they put into it.
Fireplace Insert: A wood or gas burning heating appliance that fits into the opening or protrudes on to the hearth of a conventional fireplace.
Fire-Rating: The ability of a building construction assembly (partition, wall, floor, etc.) to resist the passage of fire. The rating is expressed in hours.
Firewall: A wall to prevent the spread of fire; usually made of non-combustible material.
Firing Rate: The amount of BTUs/hour or kWs produced by a heating system from the burning of a fuel.
First Law of Thermodynamics: States that energy cannot be created or destroyed, but only changed from one form to another. First Law efficiency measures the fraction of energy supplied to a device or process that it delivers in its output. Also called the law of conservation of energy.
Fiscal Year (FY): The U.S. Government's 12-month financial year, from October to September, of the following calender year; e.g.: FY 1998 extends from Oct. 1, 1997 to Sept. 30, 1988.
Flame Spread Classification: A measure of the surface burning characteristics of a material.
Flame Spread Rating: A measure of the relative flame spread, and smoke development, from a material being tested. The flame spread rating is a single number comparing the flame spread of a material with red oak, arbitrarily given the number 100 and asbestos cement board with a flame spread of 0. Building codes require a maximum flame spread of 25 for insulation installed in exposed locations.
Flash-Steam Geothermal Plants: When the temperature of the hydrothermal liquids is over 350 F (177 C), flash-steam technology is generally employed. In these systems, most of the liquid is flashed to steam. The steam is separated from the remaining liquid and used to drive a turbine generator. While the water is returned to the geothermal reservoir, the economics of most hydrothermal flash plants are improved by using a dual-flash cycle, which separates the steam at two different pressures. The dual-flash cycle produces 20% to 30% more power than a single-flash system at the same fluid flow.
Flashing: Metal, usually galvanized sheet metal, used to provide protection against infiltration of precipitation into a roof or exterior wall; usually placed around roof penetrations such as chimneys.
Flashpoint: The minimum temperature at which sufficient vapor is released by a liquid or solid (fuel) to form a flammable vapor-air mixture at atmospheric pressure.
Flat-Black Paint: Nonglossy paint with a relatively high absorptance.
Flat Plate Solar Thermal/Heating Collectors: Large, flat boxes with glass covers and dark-colored metal plates inside that absorb and transfer solar energy to a heat transfer fluid. This is the most common type of collector used in solar hot water systems for homes or small businesses.
Flat Plate Solar Photovoltaic Module: An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.
Flat Roof: A slightly sloped roof, usually with a tar and gravel cover. Most commercial buildings use this kind of roof.
Float-Zone Process: In reference to solar photovoltaic cell manufacture, a method of growing a large-size, high-quality crystal whereby coils heat a polycrystalline ingot placed atop a single-crystal seed. As the coils are slowly raised the molten interface beneath the coils becomes a single crystal.
Floor: The upward facing structure of a building.
Floor Space: The interior area of a building, calculated in square feet or meters.
Flow Condition: In reference to solar thermal collectors, the condition where the heat transfer fluid is flowing through the collector loop under normal operating conditions.
Flow Restrictor: A water and energy conserving device that limits the amount of water that a faucet or shower head can deliver.
Flue: The structure (in a residential heating appliance, industrial furnace, or power plant) into which combustion gases flow and are contained until they are emitted to the atmosphere.
Flue Gas: The gas resulting from the combustion of a fuel that is emitted to the flue.
Fluffing: The practice of installing blow-in, loose-fill insulation at a lower density than is recommended to meet a specified R-Value.
Fluidized Bed Combustion (FBC): A type of furnace or reactor in which fuel particles are combusted while suspended in a stream of hot gas.
Fluorescent Light: The conversion of electric power to visible light by using an electric charge to excite gaseous atoms in a glass tube. These atoms emit ultraviolet radiation that is absorbed by a phosphor coating on the walls of the lamp tube. The phosphor coating produces visible light.
Fly Ash: The fine particulate matter entrained in the flue gases of a combustion power plant.
Flywheel Effect: The damping of interior temperature fluctuations by massive construction.
Foam (Insulation): A high R-value insulation product usually made from urethane that can be injected into wall cavities, or sprayed onto roofs or floors, where it expands and sets quickly.
Foam Board: A plastic foam insulation product, pressed or extruded into board-like forms, used as sheathing and insulation for interior basement or crawl space walls or beneath a basement slab; can also be used for exterior applications inside or outside foundations, crawl spaces, and slab-on-grade foundation walls.
Foam Core Panels: A type of structural, insulated product with foam insulation contained between two facings of drywall, or structural wood composition boards such as plywood, waferboard, and oriented strand board.
Foot Candle: A unit of illuminance; equal to one lumen per square foot.
Foot Pound: The amount of work done in raising one pound one foot.
Force: The push or pull that alters the motion of a moving body or moves a stationary body; the unit of force is the dyne or poundal; force is equal to mass time velocity divided by time.
Forced Air System or Furnace: A type of heating system in which heated air is blown by a fan through air channels or ducts to rooms.
Forced Ventilation: A type of building ventilation system that uses fans or blowers to provide fresh air to rooms when the forces of air pressure and gravity are not enough to circulate air through a building.
Formaldehyde: A chemical used as a preservative and in bonding agents. It is found in household products such as plywood, furniture, carpets, and some types of foam insulation. It is also a by-product of combustion and is a strong-smelling, colorless gas that is an eye irritant and can cause sneezing, coughing, and other health problems.
Fossil Fuels: Fuels formed in the ground from the remains of dead plants and animals. It takes millions of years to form fossil fuels. Oil, natural gas, and coal are fossil fuels.
Foundation: The supportive structure of a building.
Fractional Horse Power Motor: An electric motor rated at less than one horse power (hp).
Frame (Window): The outer casing of a window that sits in a designated opening of a structure and holds the window panes in place.
Framing: The structural materials and elements used to construct a wall.
Francis Turbine: A type of hydropower turbine that contains a runner that has water passages through it formed by curved vanes or blades. As the water passes through the runner and over the curved surfaces, it causes rotation of the runner. The rotational motion is transmitted by a shaft to a generator.
Frequency: The number of cycles through which an alternating current passes per second; in the U.S. the standard for electricity generation is 60 cycles per second (60 Hertz).
Freon: A registered trademark for a cholorfluorocarbon (CFC) gas that is highly stable and that has been historically used as a refrigerant.
Fresnel Lens: An optical device for concentrating light that is made of concentric rings that are faced at different angles so that light falling on any ring is focused to the same point.
Friction Head: The energy lost from the movement of a fluid in a conduit (pipe) due to the disturbances created by the contact of the moving fluid with the surfaces of the conduit, or the additional pressure that a pump must provide to overcome the resistance to fluid flow created by or in a conduit.
Fuel: Any material that can be burned to make energy.
Fuel Cell: An electrochemical device that converts chemical energy directly into electricity.
Fuel Efficiency: The ratio of heat produced by a fuel for doing work to the available heat in the fuel.
Fuel Grade Alcohol: Usually refers to ethanol to 160 to 200 proof.
Fuel Oil: Any liquid petroleum product burned for the generation of heat in a furnace or firebox, or for the generation of power in an engine. Domestic (residential) heating fuels are classed as Nos. 1, 2, 3; Industrial fuels as Nos. 4, 5, and 6.
Fuel Rate: The amount of fuel necessary to generate one kilowatt-hour of electricity.
Full Sun: The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).
Fungi: Plant-like organisms with cells with distinct nuclei surrounded by nuclear membranes, incapable of photosynthesis. Fungi are decomposers of waste organisms and exist as yeast, mold, or mildew.
Furling: The process of forcing, either manually or automatically, a wind turbine's blades out of the direction of the wind in order to stop the blades from turning.
Furnace (Residential): A combustion heating appliance in which heat is captured from the burning of a fuel for distribution, comprised mainly of a combustion chamber and heat exchanger.
Fuse: A safety device consisting of a short length of relatively fine wire, mounted in a holder or contained in a cartridge and connected as part of an electrical circuit. If the circuit source current exceeds a predetermined value, the fuse wire melts (i.e. the fuse 'blows') breaking the circuit and preventing damage to the circuit protected by the fuse.

G

Gallium Arsenide: A compound used to make certain types of solar photovoltaic cells.
Gasification: The process in which a solid fuel is converted into a gas; also known as pyrolitic distillation or pyrolysis. Production of a clean fuel gas makes a wide variety of power options available.
Gasifier: A device for converting a solid fuel to a gaseous fuel.
Gasket/Seal: A seal used to prevent the leakage of fluids, and also maintain the pressure in an enclosure.
Gasohol: A registered trademark of an agency of the state of Nebraska, for an automotive fuel containing a blend of 10 percent ethanol and 90 percent gasoline.
Gasoline: A refined petroleum product suitable for use as a fuel in internal combustion engines.
Gas Turbine: A type of turbine in which combusted, pressurized gas is directed against a series of blades connected to a shaft, which forces the shaft to turn to produce mechanical energy.
Gauss: The unit of magnetic field intensity equal to 1 dyne per unit pole.
Generator: A device for converting mechanical energy to electrical energy.
Geopressurized Brines: These brines are hot (300 F to 400 F) (149 C to 204 C) pressurized waters that contain dissolved methane and lie at depths of 10,000 ft (3048 m) to more than 20,000 ft (6096 m) below the earth's surface. The best known geopressured reservoirs lie along the Texas and Louisiana Gulf Coast. At least three types of energy could be obtained: thermal energy from high-temperature fluids; hydraulic energy from the high pressure; and chemical energy from burning the dissolved methane gas.
Geothermal Energy: Energy produced by the internal heat of the earth; geothermal heat sources include: hydrothermal convective systems; pressurized water reservoirs; hot dry rocks; manual gradients; and magma. Geothermal energy can be used directly for heating or to produce electric power.
Geothermal Heat Pump: A type of heat pump that uses the ground, ground water, or ponds as a heat source and heat sink, rather than outside air. Ground or water temperatures are more constant and are warmer in winter and cooler in summer than air temperatures. Geothermal heat pumps operate more efficiently than "conventional" or "air source" heat pumps.
Geothermal Power Station: An electricity generating facility that uses geothermal energy.
Gigawatt (GW): A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.
Gin Pole: A pole used to assist in raising a tower.
Glare: The discomfort or interference with visual perception when viewing a bright object against a dark background.
Glazing: A term used for the transparent or translucent material in a window. This