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.