Daylighting: The use of sunlight to supplement or replace electric lighting.
Daylighting Control: A control system that varies the light output of an electric lighting system in response to variations in available daylight.
Days of Supply: The number of days that inventories would satisfy demand. The Energy Information Administration calculates the days of supply by dividing the total inventory of a product by the latest four-week average of product supplied.
Deep Mining: Extraction of coal or minerals at depths greater than 1,000 feet. Coal usually is deep–mined at not more than 1,500 feet.
Deforestation: The removal of forest stands.
Degree Day: A unit, based upon temperature difference and time, used in estimating fuel consumption and specifying nominal annual heating and cooling loads of a building. When the mean temperature is less than 65 degrees Fahrenheit, the heating degree days are equal to the total number of hours that temperature is less than 65 degrees Fahrenheit for an entire year.
Degree Day Normals: The simple arithmetic averages of monthly or annual degree days over a baseline comparison period. International agreement set the baseline comparison period as the mean of a 30–year period, which will be updated at the end of each decade. The “official” period is 1971–2000.
Degree Days, Cooling (CDD): A measure of how warm a location is over a period of time relative to a base temperature, most commonly specified as 65 degrees Fahrenheit. The measure is computed for each day by subtracting the base temperature (65 degrees) from the average of the day's high and low temperatures, with negative values set equal to zero. Each day's cooling degree–days are summed to create a cooling degree day measure for a specified reference period. Cooling degree days are used in energy analysis as an indicator of air conditioning energy requirements or use.
Degree Days, Heating (HDD): A measure of how cold a location is over a period of time relative to a base temperature, most commonly specified as 65 degrees Fahrenheit. The measure is computed for each day by subtracting the average of the day's high and low temperatures from the base temperature (65 degrees), with negative values set equal to zero. Each day's heating degree days are summed to create a heating degree–day measure for a specified reference period. Heating degree days are used in energy analysis as an indicator of space heating energy requirements or use.
Degree Days, Population–Weighted: Heating or cooling degree days weighted by the population in the area in which the degree days are recorded. For example, to calculate the state population–weighted degree days, the state is divided into nine climatically similar divisions that are assigned weights based on the ratio of the population of the division to the total population of the state. Degree day readings for each division are multiplied by the corresponding population weight for each division, and these products are then summed to arrive at the state population–weighted degree day figure.
Deintegration: See Disaggregation.
Delta: A difference in temperature. Often used in the context of the difference between the design indoor temperature and the outdoor temperature.
Demand: The rate at which energy is delivered to loads and scheduling points by generation, transmission or distribution facilities. The level at which electricity or natural gas is delivered to users at a given point in time. Electric demand is expressed in kilowatts.
Demand Billing: The electric capacity requirement for which a large user pays. It may be based on the customer's peak demand during the contract year, on a previous maximum or on an agreed minimum. Measured in kilowatts.
Demand Charge: The sum to be paid by a large electricity consumer for its peak usage level.
Demand Side Management: The methods used to manage energy demand including energy efficiency, load management, fuel substitution and load building. See Load Management.
Density: The mass of a unit volume of a substance.
(U.S.) Department of Energy (US DOE): The federal department established by the Department of Energy Organization Act to consolidate the major federal energy functions into one cabinet–level department that would formulate a comprehensive, balanced national energy policy. The agency is headquartered in Washington, DC.
Deregulation: The elimination of regulation from a previously regulated industry or sector of an industry.
Derivatives: A specialized security or contract that has no intrinsic overall value, but whose value is based on an underlying security or factor as an index. A generic term that, in the energy field, may include options, futures, forwards, etc.
Diesel Fuel: Fuel used for internal combustion in diesel engines. See Distillate Fuel.
Diffuse Radiation: Solar radiation, scattered by water vapor, dust and other particles as it passes through the atmosphere, so that it appears to come from the entire sky. Diffuse radiation is higher on hazy or overcast days than on clear days.
Direct Access: The ability of a retail customer to purchase commodity electricity directly from the wholesale market rather than through a local distribution utility. See Retail Competition.
Direct Current (DC): Electricity that flows continuously in the same direction.
Direct Energy Conversion: Production of electricity from an energy source without transferring the energy to a working fluid or steam. For example, photovoltaic cells transform light directly into electricity. Direct conversion systems have no moving parts and usually produce direct current.
Direct Radiation: Radiation that has traveled a straight path from the sun, as opposed to diffuse radiation.
Direct Solar Gain: Solar energy collected from the sun (as heat) in a building through windows, walls, skylights or other similar structures.
Directly Conditioned Space: See Conditioned Space.
Disaggregation: The functional separation of the vertically integrated utility into smaller, individually owned business units (i.e., generation, dispatch/control, transmission and distribution). The terms “deintegration”, “disintegration”, and “delamination” are sometimes used to mean the same thing. See Divestiture.
Dissolved Gas: Natural gas that can be developed for commercial use, and which is found mixed with oil in naturally occurring underground formations.
Distillate Fuel Oil: Lighter fuel oils distilled off during the refining process and used primarily for space heating, on– and off–highway diesel engine fuel (e.g., railroad engine fuel and fuel for agricultural machinery) and electric power generation.
Distributed Generation: A distributed generation system involves small amounts of generation located on a utility's distribution system for the purpose of meeting local (substation level) peak loads and/or displacing the need to build additional (or upgrade) local distribution lines.
Distribution: The delivery of electricity to the retail customer's home or business through low voltage distribution lines.
Distribution System (Electric Utility): The substations, transformers and lines that convey electricity from high–power transmission lines to ultimate consumers. See Grid.
Distribution Utility (Disco): The regulated electric utility entity that constructs and maintains the distribution wires connecting the transmission grid to the final customer. The Disco can also perform other services such as aggregating customers, purchasing power supply and transmission services for customers, billing customers and reimbursing suppliers and offering other regulated or non–regulated energy services to retail customers. The “wires” and “customer service” functions provided by a distribution utility could be split so that two totally separate entities are used to supply these two types of distribution services.
Divestiture: The stripping off of one utility function from the others by selling (spinning–off) or in some other way changing the ownership of the assets related to that function. Most commonly associated with spinning–off generation assets so they are no longer owned by the shareholders that own the transmission and distribution assets. See Disaggregation .
Dose: The amount of ionizing radiation energy absorbed per unit mass of irradiated material at a specific location, such as a part of a human body.
Double Glazing: Windows having two sheets of glass with an airspace between.
Dry Bulb Temperature: A measure of the sensible temperature of air.
Dry Hole: A drilled well hole that does not yield gas and/or oil quantities or condition to support commercial production; also applied to gas that has been produced and from which liquid components have been removed.
Dry Natural Gas: Natural gas which remains after: 1) the liquefiable hydrocarbon portion has been removed from the gas stream (i.e., gas after lease, field, and/or plant separation); and 2) any volumes of nonhydrocarbon gases have been removed where they occur in sufficient quantity to render the gas unmarketable. Note: Dry natural gas is also known as consumer–grade natural gas. The parameters for measurement are cubic feet at 60 degrees Fahrenheit and 14.73 pounds per square inch absolute.
Dual–Duct System: A central plant heating, ventilation and air conditioning (HVAC) system that produces conditioned air at two temperatures and humidity levels. The air is then supplied through two independent duct systems to the points of usage where mixing occurs.
Dual–fuel or Bi–fuel Vehicle: Refers to a vehicle capable of operating on two different fuels, in distinct fueling systems, such as compressed natural gas and gasoline.
Dual–paned (Double–glazed): Two panes of glass or other transparent material, separated by a space.
Duct: A passageway made of sheet metal or other suitable material used for conveying air or other gas at relatively low pressures.