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.
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 ™ — 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.
Ethanol — Ethyl alcohol (C2H5OH)
Air Conditioning Units
