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U.S. Department of Energy - Energy Efficiency and Renewable Energy

Solid-State Lighting

Glossary of LED Terms

This page defines common terms relating to solid-state lighting.

Solid-state lighting (SSL) technology uses semi-conducting materials to convert electricity into light. SSL is an umbrella term encompassing both light-emitting diodes (LEDs) and organic light emitting diodes (OLEDs).

Light-emitting diodes (LEDs) are based on inorganic (non-carbon based) materials. An LED is a semi-conducting device that produces light when an electrical current flows through it. LEDs were first developed in the 1960s but were used only in indicator applications until recently.

Organic light-emitting diodes (OLEDs) are based on organic (carbon based) materials. In contrast to LEDs, which are small point sources, OLEDs are made in sheets which provide a diffuse area light source. OLED technology is developing rapidly and is increasingly used in display applications such as cell phones and PDA screens. However, OLEDs are still some years away from becoming a practical general illumination source. Additional advancements are needed in light output, color, efficiency, cost, and lifetime.

General illumination is a term used to distinguish between lighting that illuminates tasks, spaces, or objects from lighting used in indicator or purely decorative applications. In most cases, general illumination is provided by white light sources, including incandescent, fluorescent, high-intensity discharge sources, and white LEDs. Lighting used for indication or decoration is often monochromatic, as in traffic lights, exit signs, vehicle brake lights, signage, and holiday lights.

Luminous efficacy is the most commonly used measure of the energy efficiency of a light source. It is stated in lumens per watt (lm/W), indicating the amount of light a light source produces for each watt of electricity consumed. For white high-brightness LEDs, luminous efficacy published by LED manufacturers typically refers to the LED chip only, and doesn't include driver losses. See more information in the Energy Efficiency section.

Correlated color temperature (CCT) is the measure used to describe the relative color appearance of a white light source. CCT indicates whether a light source appears more yellow/gold/orange or more blue, in terms of the range of available shades of "white." CCT is given in kelvins (unit of absolute temperature). See more information in the Color Quality section.

Color rendering index (CRI) indicates how well a light source renders colors of people and objects, compared to a reference source. See more information in the Color Quality section.

RGB stands for red, green, and blue, the three primary colors of light. When the primaries are mixed, the resulting light appears white to the human eye. Mixing the light from red, green, and blue LEDs is one way to produce white light. The other approach is known as phosphor conversion [see below]. See more information in the Color Quality section.

Phosphor conversion is a method used to generate white light with LEDs. A blue or near-ultraviolet LED is coated with a yellow or multichromatic phosphor, resulting in white light. See more information in the Color Quality section.

U.S. Department of Energy - Energy Efficiency and Renewable Energy

Glossary of Energy-Related Terms

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

Absolute Humidity
The ratio of the mass of water vapor to the volume occupied by a mixture of water vapor and dry air.

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.

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).

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.

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.

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.

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.

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.

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.

The ratio of light reflected by a surface to the light falling on it.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

The non-combustible residue of a combusted substance composed primarily of alkali and metal oxides.

Abbreviation for the American Society of Heating, Refrigeration, and Air-Conditioning Engineers.

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).

An interior court to which rooms open.

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.

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.

The abbreviation for American Wire Gauge; the standard for gauging the size of wires (electrical conductors).

An architectural element for shading windows and wall surfaces placed on the exterior of a building; can be fixed or movable.

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.

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.

A device, such as a steel plate, used to check, retard, or divert a flow of a material.

The fibrous material remaining after the extraction of juice from sugarcane; often burned by sugar mills as a source of energy.

An air pollution control device used to filter particulates from waste combustion gases; a chamber containing a bag filter.

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.

A means of reducing the volume of a material by compaction into a bale.

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.

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.

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.

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.

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.

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.

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.

The conversion of the complex carbohydrates in organic material into energy.

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.

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.

The action of light on a biological system that results in the dissociation of a substrate, usually water, to produce hydrogen.

An ideal substance that absorbs all radiation falling on it, and reflecting nothing.

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.

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.

In heating and cooling system distribution ductwork, the transformation pieces connecting horizontal round leaders to vertical rectangular stacks.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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

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

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

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.

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

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.

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

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.

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

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.

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.

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.

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.

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.


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