Cage — The component of an electric motor composed of solid bars (of usually copper or aluminum) arranged in a circle and connected to continuous rings at each end. This cage fits inside the stator in an induction motor in channels between laminations, thin flat discs of steel in a ring configuration.
Calorie — The amount of heat required to raise the temperature of a unit of water, at or near the temperature of maximum density, one degree Celsius (or Centigrade [C]); expressed as a "small calorie" (the amount of heat required to raise the temperature of 1 gram of water one degree C), or as a "large calorie" or "kilogram calorie" (the amount of heat required to raise one kilogram [1,000 grams] of water one degree C); capitalization of the word calorie indicates a kilogram-calorie.
Calorific Value — The heat liberated by the combustion of a unit quantity of a fuel under specific conditions; measured in calories.
Candela — A unit of luminous intensity; the magnitude to the candela is such that the luminance of the total radiator, at the temperature of solidification of platinum, is 60 candelas per square centimeter.
Candle Power — The illuminating power of a standard candle employed as a unit for determining the illuminating quality of an illuminant.
Capability — The maximum load that a generating unit, power plant, or other electrical apparatus can carry under specified conditions for a given period of time, without exceeding its approved limits of temperature and stress.
Capability Margin — The difference between net electrical system capability and system maximum load requirements (peak load); the margin of capability available to provide for scheduled maintenance, emergency outages, system operating requirements and unforeseen loads.
Capacitance — A measure of the electrical charge of a capacitor consisting of two plates separated by an insulating material.
Capacitor — An electrical device that adjusts the leading current of an applied alternating current to balance the lag of the circuit to provide a high power factor.
Capacity — The load that a power generation unit or other electrical apparatus or heating unit is rated by the manufacture to be able to meet or supply.
Capacity (Condensing Unit) — The refrigerating effect in Btu/h produced by the difference in total enthalpy between a refrigerant liquid leaving the unit and the total enthalpy of the refrigerant vapor entering it. Generally measured in tons or Btu/h.
Capacity (Effective, of a motor) — The maximum load that a motor is capable of supplying.
Capacity (Heating, of a material) — The amount of heat energy needed to raise the temperature of a given mass of a substance by one degree Celsius. The heat required to raise the temperature of 1 kg of water by 1 degree Celsius is 4186 Joules.
Capacity Factor — The ratio of the average load on (or power output of) a generating unit or system to the capacity rating of the unit or system over a specified period of time.
Capital Costs — The amount of money needed to purchase equipment, buildings, tools, and other manufactured goods that can be used in production.
Carbon Dioxide — A colorless, odorless noncombustible gas with the formula CO2 that is present in the atmosphere. It is formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by respiration, which is a slow combustion in animals and plants, and by the gradual oxidation of organic matter in the soil.
Carbon Monoxide — A colorless, odorless but poisonous combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (i.e. coal, petroleum) and their products (e.g. liquefied petroleum gas, gasoline), and biomass.
Carbon Zinc Cell Battery — A cell produces electric energy by the galvanic oxidation of carbon; commonly used in household appliances.
Carnot Cycle — An ideal heat engine (conceived by Sadi Carnot) in which the sequence of operations forming the working cycle consists of isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression back to its initial state.
Catalytic Converter — An air pollution control device that removes organic contaminants by oxidizing them into carbon dioxide and water through a chemical reaction using a catalysis, which is a substance that increases (or decreases) the rate of a chemical reaction without being changed itself; required in all automobiles sold in the United State, and used in some types of heating appliances.
Cathedral Ceiling/Roof — A type of ceiling and roof assembly that has no attic.
Cathode — The negative pole or electrode of an electrolytic cell, vacuum tube, etc., where electrons enter (current leaves) the system; the opposite of an anode.
Cathode Disconnect Ballast — An electromagnetic ballast that disconnects a lamp's electrode heating circuit once is has started; often called "low frequency electronic" ballasts.
Cathodic Protection — A method of preventing oxidation of the exposed metal in structures by imposing between the structure and the ground a small electrical voltage.
Caulking — A material used to seal areas of potential air leakage into or out of a building envelope.
Ceiling — The downward facing structural element that is directly opposite the floor.
Ceiling Fan — A mechanical device used for air circulation and to provide cooling.
Cell — A component of a electrochemical battery. A 'primary' cell consists of two dissimilar elements, known as 'electrodes,' immersed in a liquid or paste known as the 'electrolyte.' A direct current of 1-1.5 volts will be produced by this cell. A 'secondary' cell or accumulator is a similar design but is made useful by passing a direct current of correct strength through it in a certain direction. Each of these cells will produce 2 volts; a 12 volt car battery contains six cells.
Cellulase — An enzyme complex, produced by fungi and bacteria, capable of decomposing cellulose into small fragments, primarily glucose.
Cellulose — The fundamental constituent of all vegetative tissue; the most abundant material in the world.
Cellulose Insulation- A type of insulation composed of waste newspaper, cardboard, or other forms of waste paper.
Central Heating System — A system where heat is supplied to areas of a building from a single appliance through a network of ducts or pipes.
Central Power Plant — A large power plant that generates power for distribution to multiple customers.
Central Receiver Solar Power Plants — Also known as "power towers," these use fields of two-axis tracking mirrors known as heliostats. Each heliostat is individually positioned by a computer control system to reflect the sun's rays to a tower-mounted thermal receiver. The effect of many heliostats reflecting to a common point creates the combined energy of thousands of suns, which produces high-temperature thermal energy. In the receiver, molten nitrate salts absorb the heat energy. The hot salt is then used to boil water to steam, which is sent to a conventional steam turbine-generator to produce electricity.
Cetane Number — A measure of a fuel's (liquid) ease of self-ignition.
Char — A byproduct of low-temperature carbonization of a solid fuel.
Charcoal — A material formed from the incomplete combustion or destructive distillation (carbonization) of organic material in a kiln or retort, and having a high energy density, being nearly pure carbon. (If produced from coal, it is coke.) Used for cooking, the manufacture of gunpowder and steel (notably in Brazil), as an absorbent and decolorizing agent, and in sugar refining and solvent recovery.
Charge Carrier — A free and mobile conduction electron or hole in a semiconductor.
Charge Controller — An electronic device that regulates the electrical charge stored in batteries so that unsafe, overcharge conditions for the batteries are avoided.
Chemical Energy — The energy liberated in a chemical reaction, as in the combustion of fuels.
Chemical Vapor Deposition (CVD) — A method of depositing thin semiconductor films used to make certain types of solar photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.
Chiller — A device for removing heat from a gas or liquid stream for air conditioning/cooling.
Chimney — A masonry or metal stack that creates a draft to bring air to a fire and to carry the gaseous byproducts of combustion safely away.
Chimney Effect — The tendency of heated air or gas to rise in a duct or other vertical passage, such as in a chimney, small enclosure, or building, due to its lower density compared to the surrounding air or gas.
Chlorofluorocarbon (CFC) — A family of chemicals composed primarily of carbon, hydrogen, chlorine, and fluorine whose principal applications are as refrigerants and industrial cleansers and whose principal drawback is the tendency to destroy the Earth's protective ozone layer.
Circuit — A device, or system of devices, that allows electrical current to flow through it and allows voltage to occur across positive and negative terminals.
Circuit Breaker — A device used to interrupt or break an electrical circuit when an overload condition exists; usually installed in the positive circuit; used to protect electrical equipment.
Circuit Lag — As time increases from zero at the terminals of an inductor, the voltage comes to a particular value on the sine function curve ahead of the current. The voltage reaches its negative peak exactly 90 degrees before the current reaches its negative peak thus the current lags behind by 90 degrees.
Circulating Fluidized Bed — A type of furnace or reactor in which the emission of sulfur compounds is lowered by the addition of crushed limestone in the fluidized bed thus obviating the need for much of the expensive stack gas clean-up equipment. The particles are collected and recirculated, after passing through a conventional bed, and cooled by boiler internals.
Clean Power Generator — A company or other organizational unit that produces electricity from sources that are thought to be environmentally cleaner than traditional sources. Clean, or green, power is usually defined as power from renewable energy that comes from wind, solar, biomass energy, etc. There are various definitions of clean resources. Some definitions include power produced from waste-to-energy and wood-fired plants that may still produce significant air emissions. Some states have defined certain local resources as clean that other states would not consider clean. For example, the state of Texas has defined power from efficient natural gas-fired power plants as clean. Some northwest states include power from large hydropower projects as clean although these projects damage fish populations. Various states have disclosure and labeling requirement for generation source and air emissions that assist customers in comparing electricity characteristics other than price. This allows customers to decide for themselves what they consider to be "clean." The federal government is also exploring this issue.
Cleavage of Lateral Epitaxial Films for Transfer (CLEFT) — A process for making inexpensive Gallium Arsenide (GaAs) photovoltaic cells in which a thin film of GaAs is grown atop a thick, single-crystal GaAs (or other suitable material) substrate and then is cleaved from the substrate and incorporated into a cell, allowing the substrate to be reused to grow more thin-film GaAs.
Clerestory — A window located high in a wall near the eaves that allows daylight into a building interior, and may be used for ventilation and solar heat gain.
Climate — The prevailing or average weather conditions of a geographic region.
Climate Change — A term used to describe short and long-term affects on the Earth's climate as a result of human activities such as fossil fuel combustion and vegetation clearing and burning.
Close Coupled — An energy system in which the fuel production equipment is in close proximity, or connected to, the fuel using equipment.
Closed Cycle — A system in which a working fluid is used over and over without introduction of new fluid, as in a hydronic heating system or mechanical refrigeration system.
Closed-Loop Geothermal Heat Pump Systems — Closed-loop (also known as "indirect") systems circulate a solution of water and antifreeze through a series of sealed loops of piping. Once the heat has been transferred into or out of the solution, the solution is recirculated. The loops can be installed in the ground horizontally or vertically, or they can be placed in a body of water, such as a pond. See horizontal ground loop, vertical ground loop, slinky ground loop, and surface water loop for more information on the different types of closed-loop geothermal heat pump systems.
Closed-Loop Biomass — As defined by the Comprehensive National Energy Act of 1992 (or the Energy Policy Act; EPAct): any organic matter from a plant which is planted for the exclusive purpose of being used to produce energy." This does not include wood or agricultural wastes or standing timber.
Codes — Legal documents that regulate construction to protect the health, safety, and welfare of people. Codes establish minimum standards but do not guarantee efficiency or quality.
Coefficient of Heat Transmission (U-Value) — A value that describes the ability of a material to conduct heat. The number of Btu that flow through 1 square foot of material, in one hour. It is the reciprocal of the R-Value (U-Value = 1/R-Value).
Coefficient of Performance (COP) — A ratio of the work or useful energy output of a system versus the amount of work or energy inputted into the system as determined by using the same energy equivalents for energy in and out. Is used as a measure of the steady state performance or energy efficiency of heating, cooling, and refrigeration appliances. The COP is equal to the Energy Efficiency Ratio (EER) divided by 3.412. The higher the COP, the more efficient the device.
Coefficient of Utilization (CU) — A term used for lighting appliances; the ratio of lumens received on a flat surface to the light output, in lumens, from a lamp; used to evaluate the effectiveness of luminaries in delivering light.
Coincidence Factor — The ratio of the coincident, maximum demand or two or more loads to the sum of their noncoincident maximum demand for a given period; the reciprocal of the diversity factor, and is always less than or equal to one.
Coincident Demand — The demand of a consumer of electricity at the time of a power supplier's peak system demand.
Cofiring — The use of two or more different fuels (e.g. wood and coal) simultaneously in the same combustion chamber of a power plant.
Cogeneration — The generation of electricity or shaft power by an energy conversion system and the concurrent use of rejected thermal energy from the conversion system as an auxiliary energy source.
Cogenerator — A class of energy producer that produces both heat and electricity from a single fuel.
Coil — As a component of a heating or cooling appliance, rows of tubing or pipe with fins attached through which a heat transfer fluid is circulated and to deliver heat or cooling energy to a building.
Cold Night Sky — The low effective temperature of the sky on a clear night.
Collector — The component of a solar energy heating system that collects solar radiation, and that contains components to absorb solar radiation and transfer the heat to a heat transfer fluid (air or liquid).
Collector Efficiency — The ratio of solar radiation captured and transferred to the collector (heat transfer) fluid.
Collector Fluid — The fluid, liquid (water or water/antifreeze solution) or air, used to absorb solar energy and transfer it for direct use, indirect heating of interior air or domestic water, and/or to a heat storage medium.
Collector Tilt — The angle that a solar collector is positioned from horizontal.
Color Rendering or Rendition — A measure of the ability of a light source to show colors, based on a color rendering index.
Color Rendition (Rendering) Index (CRI) — A measure of light quality. The maximum CRI value of 100 is given to natural daylight and incandescent lighting. The closer a lamp's CRI rating is to 100, the better its ability to show true colors to the human eye.
Color Temperature — A measure of the quality of a light source by expressing the color appearance correlated with a black body.
Combined-Cycle Power Plant — A power plant that uses two thermodynamic cycles to achieve higher overall system efficiency; e.g.: the heat from a gas-fired combustion turbine is used to generate steam for heating or to operate a steam turbine to generate additional electricity.
Combustion — The process of burning; the oxidation of a material by applying heat, which unites oxygen with a material or fuel.
Combustion Air — Air that provides the necessary oxygen for complete, clean combustion and maximum heating value.
Combustion Chamber — Any wholly or partially enclosed space in which combustion takes place.
Combustion Gases — The gaseous byproducts of the combustion of a fuel.
Combustion Power Plant — A power plant that generates power by combusting a fuel.
Combustion Turbine — A turbine that generates power from the combustion of a fuel.
Commercial Building — A building with more than 50 percent of its floor space used for commercial activities, which include stores, offices, schools, churches, libraries, museums, health care facilities, warehouses, and government buildings except those on military bases.
Commercial Sector — Consists of businesses that are not engaged in transportation or manufacturing or other types of industrial activities. Standard Industrial Classification (SIC) codes for commercial establishments are 50 through 87, 89, and 91 through 97.
Comfort Zone — A frequently used room or area that is maintained at a more comfortable level than the rest of the house; also known as a "warm room."
Commissioning — The process by which a power plant, apparatus, or building is approved for operation based on observed or measured operation that meets design specifications.
Compact Fluorescent — A smaller version of standard fluorescent lamps which can directly replace standard incandescent lights. These lights consist of a gas filled tube, and a magnetic or electronic ballast.
Complete Mix Digester — A type of anaerobic digester that has a mechanical mixing system and where temperature and volume are controlled to maximize the anaerobic digestion process for biological waste treatment, methane production, and odor control.
Composting — The process of degrading organic material (biomass) by microorganisms in aerobic conditions.
Composting Toilet — A self-contained toilet that use the process of aerobic decomposition (composting) to break down feces into humus and odorless gases.
Compound Paraboloid Collector — A form of solar concentrating collector that does not track the sun.
Compressed Air Storage — The storage of compressed air in a container for use to operate a prime mover for electricity generation.
Compressed Natural Gas (CNG) — Natural gas (methane) that has been compressed to a higher pressure gaseous state by a compressor; used in CNG vehicles.
Compression Chiller — A cooling device that uses mechanical energy to produce chilled water.
Compressor — A device used to compress air for mechanical or electrical power production, and in air conditioners, heat pumps, and refrigerators to pressurize the refrigerant and enabling it to flow through the system.
Concentrating (Solar) Collector — A solar collector that uses reflective surfaces to concentrate sunlight onto a small area, where it is absorbed and converted to heat or, in the case of solar photovoltaic (PV) devices, into electricity. Concentrators can increase the power flux of sunlight hundreds of times. The principal types of concentrating collectors include: compound parabolic, parabolic trough, fixed reflector moving receiver, fixed receiver moving reflector, Fresnel lense, and central receiver. A PV concentrating module uses optical elements (Fresnel lense) to increase the amount of sunlight incident onto a PV cell. Concentrating PV modules/arrays must track the sun and use only the direct sunlight because the diffuse portion cannot be focused onto the PV cells. Concentrating collectors for home or small business solar water heating applications are usually parabolic troughs that concentrate the sun's energy on an absorber tube (called a receiver), which contains a heat-transfer fluid.
Condensate — The liquid resulting when water vapor contacts a cool surface; also the liquid resulting when a vaporized working fluid (such as a refrigerant) is cooled or depressurized.
Condensation — The process by which water in air changes from a vapor to a liquid due to a change in temperature or pressure; occurs when water vapor reaches its dew point (condensation point); also used to express the existence of liquid water on a surface.
Condenser — The device in an air conditioner or heat pump in which the refrigerant condenses from a gas to a liquid when it is depressurized or cooled.
Condenser Coil — The device in an air conditioner or heat pump through which the refrigerant is circulated and releases heat to the surroundings when a fan blows outside air over the coils. This will return the hot vapor that entered the coil into a hot liquid upon exiting the coil.
Condensing Furnace — A type of heating appliance that extracts so much of the available heat content from a combusted fuel that the moisture in the combustion gases condenses before it leaves the furnace. Also this furnace circulates a liquid to cool the furnace's heat exchanger. The heated liquid may either circulate through a liquid-to-air heat exchanger to warm room air, or it may circulate through a coil inside a separate indirect-fired water heater.
Condensing Unit — The component of a central air conditioner that is designed to remove heat absorbed by the refrigerant and transfer it outside the conditioned space.
Conditioned Space — The interior space of a building that is heated or cooled.
Conduction — The transfer of heat through a material by the transfer of kinetic energy from particle to particle; the flow of heat between two materials of different temperatures that are in direct physical contact.
Conduction Band — An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge.
Conductivity (Thermal) — This is a positive constant, k, that is a property of a substance and is used in the calculation of heat transfer rates for materials. It is the amount of heat that flows through a specified area and thickness of a material over a specified period of time when there is a temperature difference of one degree between the surfaces of the material.
Conductor — The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.
Conduit — A tubular material used to encase and protect one or more electrical conductors.
Congressional (Energy) Committees:
House Subcommittee on Energy and Environment — This committee has legislative jurisdiction and general and special oversight and investigative authority on all matters relating to energy and environmental research and development and demonstration.
House Water and Power Committee — This committee has oversight over the generation and marketing of electric power from federal water projects by federally charted or Federal RPM authorities, measures and matters concerning water resources planning, compacts relating to use and apportionment of interstate waters, water rights or power movement programs, measures and matters pertaining to irrigation and reclamation projects and other water resources development programs.
Senate Committee on Energy and Natural Resources — This committee has jurisdiction on: coal production, distribution and utilization; energy policy; energy research, conservation, and development; hydroelectric power; irrigation; mineral conservation; nonmilitary development of nuclear energy; solar energy systems; and over territorial possessions, including trusteeships of the United States.
Senate Subcommittee on Energy Research, Development, Production and Regulation — This committee has jurisdiction on the oversight and legislative responsibilities for: coal, nuclear, and non-nuclear energy commercialization projects; DOE National Laboratories; global climate change; new technologies research and development; commercialization of new technologies including, solar energy systems; Federal energy conservation programs; energy information; and power provider policy.
Connected Load — The sum of the ratings of the electricity consuming apparatus connected to a generating system.
Connection Charge — An amount paid by a customer for being connected to an electricity supplier's transmission and distribution system.
Conservation — To reduce or avoid the consumption of a resource or commodity.
Conservation Cost Adjustment — A means of billing electric power consumers to pay for the costs of demand side management/energy conservation measures and programs. (See also Benefits Charge.)
Constant Dollars — The value or purchasing power of a dollar in a specified year carried forward or backward.
Constant-Speed Wind Turbines — Wind turbines that operate at a constant rotor revolutions per minute (RPM) and are optimized for energy capture at a given rotor diameter at a particular speed in the wind power curve.
Consumption Charge — The part of a power provider's charge based on actual energy consumed by the customer; the product of the kilowatt-hour rate and the total kilowatt-hours consumed.
Contact Resistance — The resistance between metallic contacts and the semiconductor.
Continuous Fermentation — A steady-state fermentation process.
Contrast — The difference between the brightness of an object compared to that of its immediate background.
Convection — The transfer of heat by means of air currents.
Conventional Fuel — The fossil fuels: coal, oil, and natural gas.
Conventional Heat Pump — This type of heat pump is known as an air-to air system.
Conventional Power — Power generation from sources such as petroleum, natural gas, or coal. In some cases, large-scale hydropower and nuclear power generation are considered conventional sources.
Conversion Efficiency — The amount of energy produced as a percentage of the amount of energy consumed.
Converter — A device for transforming the quality and quantity of electrical energy; also an inverter.
Cooling Capacity — The quantity of heat that a cooling appliance is capable of removing from a room in one hour.
Cooling Degree Day — A value used to estimate interior air cooling requirements (load) calculated as the number of degrees per day (over a specified period) that the daily average temperature is above 65 degrees Fahrenheit (or some other, specified base temperature). The daily average temperature is the mean of the maximum and minimum temperatures recorded for a specific location for a 24 hour period.
Cooling Load — That amount of cooling energy to be supplied (or heat and humidity removed) based on the sensible and latent loads.
Cooling Pond — A body of water used to cool the water that is circulated in an electric power plant.
Cooling Tower — A structure used to cool power plant water; water is pumped to the top of the tubular tower and sprayed out into the center, and is cooled by evaporation as it falls, and then is either recycled within the plant or is discharged.
Coproducts — The potentially useful byproducts of ethanol fermentation process.
Cord (of Wood) — A stack of wood 4 feet by 4 feet by 8 feet.
Coulomb — A unit for the quantity of electricity transported in 1 second by a current of 1 ampere.
Counterflow Heat Exchanger — A heat exchanger in which two fluids flow in opposite directions for transfer heat energy from one to the other.
Covenants — Restrictions on the use of a property.
Crawlspace — The unoccupied, and usually unfinished and unconditioned space between the floor, foundation walls, and the slab or ground of a building.
Creosote — A liquid byproduct of wood combustion (or distillation) that condenses on the internal surfaces of vents and chimneys, which if not removed regularly, can corrode the surfaces and fuel a chimney fire.
Critical Compression Pressure — The highest possible pressure in a fuel-air mixture before spontaneous ignition occurs.
Crystalline Silicon Photovoltaic Cell — A type of photovoltaic cell made from a single crystal or a polycrystalline slice of silicon. Crystalline silicon cells can be joined together to form a module (or panel).
Cubic Foot (of Natural Gas) — A unit of volume equal to 1 cubic foot at a pressure base of 14.73 pounds standard per square inch absolute and a temperature base of 60 degrees Fahrenheit.
Cube Law — In reference to wind energy, for any given instant, the power available in the wind is proportional to the cube of the wind velocity; when wind speed doubles, the power availability increases eight times.
Current (Electrical) — The flow of electrical energy (electricity) in a conductor, measured in amperes.
Current Dollars — The value or purchasing power of a dollar that has not been reduced to a common basis of constant purchasing power, but instead reflects anticipated future inflation; when used in computations the assumed inflation rate must be stated.
Customer Charge — An amount to be paid for energy periodically by a customer without regard to demand or energy consumption.
Customer Class — Categories of energy consumers, as defined by consumption or demand levels, patterns, and conditions, and generally included residential, commercial, industrial, agricultural.
Cut-In-Speed — The lowest wind speed at which a wind turbine begins producing usable power.
Cut-Out-Speed — The highest wind speed at which a wind turbine stops producing power.
Cycle — In alternating current, the current goes from zero potential or voltage to a maximum in one direction, back to zero, and then to a maximum potential or voltage in the other direction. The number of complete cycles per second determines the current frequency; in the U.S. the standard for alternating current is 60 cycles.
Cycling Losses — The loss of heat as the water circulates through a water heater tank and inlet and outlet pipes.
Cyclone Burner - A furnace/combustion chamber in which finely ground fuel is blown in spirals in the combustion chamber to maximize combustion efficiency.
Czochralski Process — A method of growing large size, high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.
Dam — A structure for impeding and controlling the flow of water in a water course, and which increases the water elevation to create the hydraulic head. The reservoir creates, in effect, stored energy.
Damper — A movable plate used to control air flow; in a wood stove or fireplace, used to control the amount and direction of air going to the fire.
Darrius (Wind) Machine — A type of vertical-axis wind machine that has long, thin blades in the shape of loops connected to the top and bottom of the axle; often called an "eggbeater windmill."
Daylighting — The use of direct, diffuse, or reflected sunlight to provide supplemental lighting for building interiors.
Decentralized (Energy) System — Energy systems supply individual, or small-groups, of energy loads.
Declination — The angular position of the sun at solar noon with respect to the plane of the equator.
Declining Block Rate — An electricity supplier rate structure in which the per unit price of electricity decreases as the amount of energy increases. Normally only available to very large consumers.
Decommissioning — The process of removing a power plant, apparatus, equipment, building, or facility from operation.
Decomposition — The process of breaking down organic material; reduction of the net energy level and change in physical and chemical composition of organic material.
De-energize(d) — To disconnect a transmission and/or distribution line; a power line that is not carrying a current; to open a circuit.
Deep Discharge — Discharging a battery to 20 percent or less of its full charge capacity.
Degree Day — A unit for measuring the extent that the outdoor daily average temperature (the mean of the maximum and minimum daily dry-bulb temperatures) falls below (in the case of heating, see Heating Degree Day), or falls above (in the case of cooling, see Cooling Degree Day) an assumed base temperature, normally taken as 65 degrees Fahrenheit, unless otherwise stated. One degree day is counted for each degree below (for heating) or above (in the case of cooling) the base, for each calendar day on which the temperature goes below or above the base.
Degree Hour — The product of 1 hour, and usually the number of degrees Fahrenheit the hourly mean temperature is above a base point (usually 65 degrees Fahrenheit); used in roughly estimating or measuring the cooling load in cases where processes heat, heat from building occupants, and humidity are relatively unimportant compared to the dry-bulb temperature.
Dehumidifier — A device that cools air by removing moisture from it.
Demand — The rate at which electricity is delivered to or by a system, part of a system, or piece of equipment expressed in kilowatts, kilovoltamperes, or other suitable unit, at a given instant or averaged over a specified period of time.
Demand Charge — A charge for the maximum rate at which energy is used during peak hours of a billing period. That part of a power provider service charged for on the basis of the possible demand as distinguished from the energy actually consumed.
Demand(ed) Factor — The ratio of the maximum demand on an electricity generating and distribution system to the total connected load on the system; usually expressed as a percentage.
Demand Power — see Peak Power
Demand-Side Management (DSM) — The process of managing the consumption of energy, generally to optimize available and planned generation resources.
Demand (Tankless) Water Heater — A type of water heater that has no storage tank thus eliminating storage tank stand-by losses. Cold water travels through a pipe into the unit, and either a gas burner or an electric element heats the water only when needed.
Dendrite — A slender threadlike spike of pure crystalline material, such as silicon.
Dendritic Web Technique — A method for making sheets of polycrystalline silicon in which silicon dendrites are slowly withdrawn from a melt of silicon whereupon a web of silicon forms between the dendrites and solidifies as it rises from the melt and cools.
Department of Agriculture (USDA) — A federal government agency involved in rural development, marketing and regulatory programs, food safety, research, education and economics, food, nutrition and consumer service, farm and foreign agricultural services, and natural resources and environment programs.
Department of Energy (DOE) — A federal government agency created in 1977, that is entrusted to contribute to the welfare of the United States by providing technical information, and a scientific and educational foundation for technology, policy and institutional leadership to achieve efficiency in energy use, diversity in energy sources, a more productive and competitive economy, improved environmental quality, and a secure national defense.
Dependable Capacity — The load-carrying ability of an electric power plant during a specific time interval and period when related to the characteristics of the load to be/being supplied; determined by capability, operating power factor, and the portion of the load the station is to supply.
Derating — The production of energy by a system or appliance at a level less than its design or nominal capacity.
Deregulation — The process of changing regulatory policies and laws to increase competition among suppliers of commodities and services. The process of deregulating the electric power industry was initiated by the Energy Policy Act of 1992. (See also Restructuring)
Desiccant — A material used to desiccate (dry) or dehumidify air.
Desiccant Cooling — To condition/cool air by dessication.
Desiccation — The process of removing moisture; involves evaporation.
Design Cooling Load — The amount of conditioned air to be supplied by a cooling system; usually the maximum amount to be delivered based on a specified number of cooling degree days or design temperature.
Design Heating Load — The amount of heated air, or heating capacity, to be supplied by a heating system; usually the maximum amount to be delivered based on a specified number of heating degree days or design outside temperature.
Design Life — Period of time a system or appliance (or component of) is expected to function at its nominal or design capacity without major repair.
Design Temperature — The temperature that a system is designed to maintain (inside) or operate against (outside) under the most extreme conditions.
Design Tip Speed Ratio — For a wind turbine, the ratio of the speed of the tip of a turbine blade for which the power coefficient is at maximum.
Design Voltage — The nominal voltage for which a conductor or electrical appliance is designed; the reference voltage for identification and not necessarily the precise voltage at which it operates.
Desuperheater — An energy saving device in a heat pump that, during the cooling cycle, recycles some of the waste heat from the house to heat domestic water.
Dewpoint — The temperature to which air must be cooled, at constant pressure and water vapor content, in order for saturation or condensation to occur; the temperature at which the saturation pressure is the same as the existing vapor pressure; also called saturation point.
Difference of Potential — The difference in electrical pressure (voltage) between any two points in an electrical system or between any point in an electrical system and the earth.
Differential Thermostat — A type of automatic thermostat (used on solar heating systems) that responds to temperature differences (between collectors and the storage components) so as to regulate the functioning of appliances (to switch transfer fluid pumps on and off).
Diffuse Solar Radiation — Sunlight scattered by atmospheric particles and gases so that it arrives at the earth's surface from all directions and can not be focused.
Diffusion — The movement of individual molecules through a material; permeation of water vapor through a material.
Diffusion Length — The mean distance a free electron or hole moves before recombining with another hole or electron.
Digester (Anaerobic) — A device in which organic material is biochemically decomposed (digested) by anaerobic bacteria to treat the material and/or to produce biogas.
Dimmer — A light control device that allows light levels to be manually adjusted. A dimmer can save energy by reducing the amount of power delivered to the light while consuming very little themselves.
Dip Tube — A tube inside a domestic water heater that distributes the cold water from the cold water supply line into the lower area of the water heater where heating occurs.
Diode — An electronic device that allows current to flow in one direction only.
Direct Access — The ability of an electric power consumer to purchase electricity from a supplier of their choice without being physically inhibited by the owner of the electric distribution and transmission system to which the consumer is connected to. (See also Open Access.)
Direct Beam Radiation — Solar radiation that arrives in a straight line from the sun.
Direct Current — A type of electricity transmission and distribution by which electricity flows in one direction through the conductor; usually relatively low voltage and high current; typically abbreviated as dc.
Direct-Gain — The process by which sunlight directly enters a building through the windows and is absorbed and stored in massive floors or walls.
Direct Solar Water Heater — These systems use water as the fluid that is circulated through the collector to the storage tank. Also known as "open-loop" systems.
Direct Vent Heater — A type of combustion heating system in which combustion air is drawn directly from outside and the products of combustion are vented directly outside. These features are beneficial in tight, energy-efficient homes because they will not depressurize a home and cause air infiltration, and backdrafting of other combustion appliances.
Direct Water Heater — A type of water heater in which heated water is stored within the tank. Hot water is released from the top of the tank when a hot water faucet is turned. This water is replaced with cold water that flows into the tank and down to just above the bottom plate under which are the burners.
Discounting — A method of financial and economic analysis used to determine present and future values of investments or expenses.
Discount Rate — The interest rate at which the Federal Reserve System stands ready to lend reserves to commercial banks. The rate is proposed by the 12 Federal Reserve banks and determined with the approval of the Board of Governors.
Dispatching — To schedule and control the generation and delivery of electric power.
Dispatchability — The ability to dispatch power.
Displacement Power — A source of power (electricity) that can displace power from another source so that source's power can be transmitted to more distant loads.
Distributed Generation — A term used by the power industry to describe localized or on-site power generation.
Distribution — The process of distributing electricity; usually defines that portion of a power provider's power lines between a power provider's power pole and transformer and a customer's point of connection/meter.
Distribution Feeder (see Feeder)
Distribution Line — One or more circuits of a distribution system on the same line or poles or supporting structures' usually operating at a lower voltage relative to the transmission line.
Distribution System — That portion of an electricity supply system used to deliver electricity from points on the transmission system to consumers.
District Heating — A heating system in which steam or hot water for space heating or hot water is piped from a central boiler plant or electric power/heating plant to a cluster of buildings.
Diversity Factor — The ratio of the sum of the noncoincidental maximum demands of two or more loads to their coincidental maximum demands for the same period.
DOE-2.1 — A computer software program that simulates energy consumption of commercial buildings; used for design and auditing purposes.
Dome (Geodesic) — An architectural design invented by Buckminster Fuller with a regular polygonal structure based on radial symmetry.
Domestic Hot Water — Water heated for residential washing, bathing, etc.
Donor — In a solar photovoltaic device, an n-type dopant, such as phosphorus, that puts an additional electron into an energy level very near the conduction band; this electron is easily exited into the conduction band where it increases the electrical conductivity over than of an undoped semiconductor.
Dopant — A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).
Doping — The addition of dopants to a semiconductor.
Double-Pane or Glazed Window — A type of window having two layers (panes or glazing) of glass separated by an air space. Each layer of glass and surrounding air space reradiates and traps some of the heat that passes through thereby increasing the windows resistance to heat loss (R-value).
Double Wall Heat Exchanger — A heat exchanger in a solar water heating system that has two distinct walls between the heat transfer fluid and the domestic water, to ensure that there is no mixing of the two.
Downwind Wind Turbine — A horizontal axis wind turbine in which the rotor is downwind of the tower.
Draft — A column of burning combustion gases that are so hot and strong that the heat is lost up the chimney before it can be transferred to the house. A draft brings air to the fire to help keep it burning.
Draft Diverter — A door-like device located at the mouth of a fireplace chimney flue for controlling the direction and flow of the draft in the fireplace as well as the amount of oxygen that the fire receives.
Draft Hood — A device built into or installed above a combustion appliance to assure the escape of combustion byproducts, to prevent backdrafting of the appliance, or to neutralize the effects of the stack action of the chimney or vent on the operation of the appliance.
Drag — Resistance caused by friction in the direction opposite to that of movement (i.e., motion) of components such as wind turbine blades.
Drainback (Solar) Systems — A closed-loop solar heating system in which the heat transfer fluid in the collector loop drains into a tank or reservoir whenever the booster pump stops to protect the collector loop from freezing.
Draindown (Solar) Systems — An open-loop solar heating system in which the heat transfer fluid from the collector loop and the piping drain into a drain whenever freezing conditions occur.
Dry Bulb Temperature — The temperature of the air as measured by a standard thermometer.
Dry Steam Geothermal Plants — Conventional turbine generators are used with the dry steam resources. The steam is used directly, eliminating the need for boilers and boiler fuel that characterizes other steam-power-generating technologies. This technology is limited because dry-steam hydrothermal resources are extremely rare. The Geysers, in California, is the nation's only dry steam field.
Dual Duct System — An air conditioning system that has two ducts, one is heated and the other is cooled, so that air of the correct temperature is provided by mixing varying amounts of air from each duct.
Dual Fuel (or Flex Fuel) Vehicle — A vehicle with an engine capable of operating on two different types of fuels.
Duct(s) — The round or rectangular tube(s), generally constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite, located within a wall, floor, and ceiling that distributes heated or cooled air in buildings.
Duct Fan — An axial flow fan mounted in a section of duct to move conditioned air.
Duty Cycle — The duration and periodicity of the operation of a device.
Dynamo — A machine for converting mechanical energy into electrical energy by magneto-electric induction; may be used as a motor.
Dynamic Head — The pressure equivalent of the velocity of a fluid.
Dynamometer — An apparatus for measuring force or power, especially the power developed by a motor.
Dyne — The absolute centimeter-gram-second unit of force; that force that will impart to a free mass of one gram an acceleration of one centimeter per second per second.
Earth Berm
Air Conditioning Units
