Glossary of Terms - D

• Daily Peak - The maximum amount of energy or service demanded in one day from a company or utility service.
• 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.
• Day Trade - The purchase and sale of a futures or an options contract on the same day. Dealer Tank Wagon Price (DTW) The price, usually of gasoline, offered by the majors which is branded and delivered to the service station on a cost, insurance, and freight basis.
• Decentralized (Energy) System - Energy systems supply individual, or small-groups, of energy loads.
• 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.
• 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.
• Dekatherm - Ten therms, 1 million British thermal units.
• Delivered - Often regarded as synonymous with cost, insurance, and freight in the international cargo trade, its terms differ from the latter in a number of ways. Generally, the seller's risks are greater in a delivered transaction because the buyer pays on the basis of landed quality/quantity. Risk and title are borne by the seller until such time as the commodity, such as oil, passes from shipboard into the connecting flange of the buyer's shore installation. The seller is responsible for clearance through customs and payment of all duties. Any in-transit contamination or loss of cargo is the seller's liability. In delivered transactions, the buyer pays only for the quantity of oil actually received in storage.
• Delivering Party - The entity supplying the capacity and/or energy to be transmitted at Point(s) of Receipt.
• Delivery - The term has distinct meaning when used in connection with futures contracts. Delivery generally refers to the changing of ownership or control of a commodity under specific terms and procedures established by the exchange upon which the contract is traded. Typically, except for energy, the commodity must be placed in an approved warehouse, precious metals depository, or other storage facility, and be inspected by approved personnel, after which the facility issues a warehouse receipt, shipping certificate, demand certificate, or due bill, which becomes a transferable delivery instrument. Delivery of the instrument usually is preceded by a notice of intention to deliver. After receipt of the delivery instrument, the new owner typically can take possession of the physical commodity, can deliver the delivery instrument into the futures market in satisfaction of a short position, or can sell the delivery instrument to another market participant who can use it for delivery into the futures market in satisfaction of his short position or for cash, or can take delivery of the physical himself. The procedure differs for energy contracts. Bona fide buyers or sellers of the underlying energy commodity can stand for delivery. If a buyer or seller stands for delivery, the contract is held through the termination of trading. The buyer and seller each file a notice of intent to make or take delivery with their respective clearing members who file them with the Exchange. Buyers and sellers are randomly matched by the Exchange. The delivery payment is based on the contract's final settlement price.
• Delivery Month - The month specified in a given futures contract for delivery of the actual physical spot or cash commodity.
• Delivery Notice - A notice presented through an exchange's clearinghouse by a clearing member announcing the intention to deliver the actual commodity in satisfaction of a contract obligation.
• Delivery Point(s) - Location(s) designated by an exchange at which delivery may be made in fulfillment of contract terms.
• Delta - The sensitivity of an option's value to a change in the price of the underlying futures contract, also referred to as an option's futures-equivalent position. Deltas are positive for calls, and negative for puts. Deltas of deep in-the-money options are approximately equal to one; deltas of at-the-money options are 0.5; and deltas of deep out-of-the-money options approach zero.
• Delta Neutral Spread - A spread where the total delta position on the long side and the total delta on the short side add up to approximately zero.
• Demand (electric) - The rate at which electric energy is delivered to or by a system, part of a system, or a piece of equipment. Demand is expressed in kW, kVA, or other suitable units at a given instant or over any designated period of time. The primary source of "demand" is the power-consuming equipment of the customers.
• 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. It is measured in kilowatts.
• Demand Charge - A charge for the maximum rate at which energy is used during peak hours of a billing period. That part of a utility service charged for on the basis of the possible demand as distinguished from the energy actually consumed.
• Demand Controller - An electrical, mechanical, or electromechanical device or system that monitors the customer demand and causes that demand to be leveled and/or limited.
• Demand Power - see Peak Power
• Demand Ratchet - This is the minimum billing demand based upon a given percentage of the actual demand use, recorded during the last eleven months of demand history.
• 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-Side Management - The planning, implementation, and monitoring of utility activities designed to encourage consumers to modify patterns of electricity usage, including the timing and level of electricity demand. It refers only to energy and load-shape modifying activities that are undertaken in response to utility-administered programs. It does not refer to energy and load-shape changes arising from the normal operation of the marketplace or from government-mandated energy efficiency standards. Demand-Side Management (DSM) covers the complete range of load-shape objectives, including strategic conservation and load management, as well as strategic load growth.
• Demonstration - The application and integration of a new product or service into an existing or new system. Most commonly, demonstration involves the construction and operation of a new electric technology interconnected with the electric utility system to demonstrate how it interacts with the system. This includes the impacts the technology may have on the system and the impacts that the larger utility system may have on the functioning of the technology.
• Departing Member - A member consumer served at retail by an electric cooperative corporation that hs given notice of intent to receive generation services from another source or that is otherwise in the process of changing generation suppliers. These persons shall nonetheless remain members of the electric distribution cooperative corporation for purposes of distribution service.
• 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.
• Depletable Energy Sources - This includes: 1) electricity purchased from a public utility and 2) energy obtained from burning coal, oil, natural gas or liquefied petroleum gasses.
• Depletion Zone - Same as cell barrier. The term derives from the fact that this microscopically thin region is depleted of charge carriers (free electrons and holes).
• Depository or Warehouse Receipt - A document issued by a bank or warehouse indicating ownership of a commodity stored in a bank depository or warehouse. In the case of many commodities deliverable against futures contracts, transfer of ownership of an appropriate depository receipt may effect contract delivery.
• Depreciation, Straight-line - Straight-line depreciation takes the cost of the asset less the estimated salvage value and allocates the cost in equal amounts over the asset's estimated useful life.
• 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 service. The process of deregulating the electric power industry was initiated by the Energy Policy Act of 1992. (See also Restructuring)
• Derivative - Financial instrument derived from a cash market commodity, futures contract, or other financial instrument. Derivatives can be traded on regulated exchange markets or over-the-counter. For example, futures contracts are derivatives of physical commodities, options on futures are derivatives of futures contracts.
• 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 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.
• Designated Agent - An agent that acts on behalf of a transmission provider, customer or transmission customer as required under the tariff.
• 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.
• Diesel Fuel - Distillate fuel oil used in compression-ignition engines. It is similar to home heating oil, but must meet a cetane number specification of 40 or more.
• 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).
• Differentials - Price differences between classes, grades, and locations of different stocks of the same commodity.
• Diffuse Insolation - Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.
• 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 Furnace - Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material.
• Digester (Anaerobic) - A device in which organic material is biochemically decomposed (digested) by anaerobic bacteria to treat the material and/or to produce biogas.
• 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 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 Gain - In direct-gain buildings, sunlight directly enters the home through the windows and is absorbed and stored in massive floors or walls. These buildings are elongated in the east-west direction, and most of their windows are on the south side. The area devoted to south windows varies throughout the country. It could be as much as 20% of the floor area in sunny cold climates, where advanced glazings or moveable insulation are recommended to prevent heat loss at night. These buildings have high insulation levels and added thermal mass for heat storage.
• Direct Isolation - Sunlight falling directly upon a collector. Opposite of diffuse insolation.
• Direct Load Control - Refers to program activities that can interrupt consumer load at the time of annual peak load by direct control of the utility system operator by interrupting power supply to individual appliances or equipment on consumer premises. This type of control usually involves residential consumers. Direct Load Control excludes Interruptible Load and Other Load Management effects. (Direct Load Control, as defined here, is synonymous with Direct Load Control Management reported to the North American Electric Reliability Council on the voluntary Office of Energy Emergency Operations Form OE-411, Coordinated Regional Bulk Power Supply Program Report, with the exception that annual peak load effects are reported here and seasonal (i.e., summer and winter) peak load effects are reported on the OE-411).
• Direct Utility Cost - A utility cost that is identified with one of the DSM program categories (i.e., Energy Efficiency, Direct Load Control, Interruptible Load, Other Load Management, Other DSM Programs, Load Building).
• 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.
• Dirty Cargo - Those petroleum products which leave significant amounts of residue in tanks. Generally applies to crude oil and residual fuel oil.
• Disaggregation - The functional separation of the vertically integrated utility into smaller, individually owned business units (I.e. generation, dispatch/control, transmission, distribution). The terms "deintegration", "disintegration" and "delimitation" are sometimes used to mean the same thing.
• Discharge Rate - The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.
• Discount - 1) A downward adjustment in price allowed for delivery of stocks of a commodity of lesser than contract grade against a futures contract. 2) Sometimes used to refer to the price differences between futures of different delivery months.
• 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.
• Discount/Interest Rate - The discount rate is used to determine the present value of future or past cash flows. The rate accounts for inflation and the potential earning power of money.
• Discounting - A method of financial and economic analysis used to determine present and future values of investments or expenses.
• Discretionary Account - An arrangement by which the holder of an account gives written power of attorney to someone else, often a broker, to buy and sell without prior approval of the account holder. Often referred to as a "managed account."
• Dispatchability - This is the ability of a generating unit to increase or decrease generation, or to be brought on line or shut down at the request or a utility's system operator.
• Dispatching - To schedule and control the generation and delivery of electric 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.
• Distillate Fuel Oil - A general classification for one of the petroleum fractions produced in conventional distillation operations. It is used primarily for space heating, on-and-off-highway diesel engine fuel (including railroad engine fuel and fuel for agriculture machinery), and electric power generation. Included are Fuel Oils No. 1, No. 2, and No. 4; and Diesel Fuels No. 1, No. 2, and No. 4.
• 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.
• Distributed Power - Generic term for any power supply located near the point where the power is used. Opposite of central power. See 'stand-alone'; 'remote site.'
• Distributed Systems - Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.
• Distribution - The system of wires, switches, and transformers that serve neighborhoods and business, typically lower than 69,000 volts. A distribution system reduces or downgrades power from high-voltage transmission lines to a level that can be used in homes or businesses.
• 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.
• 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.
• Distributive Power - A packaged power unit located at the point of demand. While the technology is still evolving, examples include fuel cells and photovoltaic applications.
• 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 Exchange - An exchange of capacity or energy, or both, between systems whose peak loads occur at different times.
• Diversity Factor - The ratio of the sum of the non-coincident maximum demands of two or more loads to their coincident maximum demand for the same period.
• Divestiture - The stripping off of one utility function from the others by selling (spinning-off) or in most 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.
• Doctor Test - A qualitative method of detecting undesirable sulfur compounds in petroleum distillates; that is, determining whether an oil is sour or sweet.
• DOD - 'Depth of Discharge,' from 100-percent state of charge (SOC), in a battery or battery system.
• 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.
• Donor Level - The level that donates conduction electrons to the system.
• 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 Bottoms - A chart pattern of the price movement of a commodity that shows resistance to a falling market; the inverse of double tops. The price patterns are used by technical analysts to recognize a reversal of a price trend.
• Double Tops - A chart pattern of commodity price movements that depict a rising market which hits resistance at a certain level, retreats, rises again, but still cannot breach the previous resistance point, and falls back again. The price patterns are used by technical analysts to recognize a reversal of a price trend.
• 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.
• Downstream - An industry term referring to commercial oil and gas operations beyond the production phase; oil refining and marketing, and natural gas transmission and distribution.
• 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.
• 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.
• Dry Gas - Gas that does not contain liquid hydro-carbons.
• 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.
• DSM Measure Technology Program - Single devices, equipment, or rates as listed in the Reference Data. A demand-side management program is usually a group of DSM measures or technologies. However, a DSM program could in some cases be a single measure.
• Dynamic Head - The pressure equivalent of the velocity of a fluid.
• Dynamo - A machine for converting mechanical energy into electrical energy by magneto-electric induction; may be used as a motor.
• 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.