| Dangling Bonds | A chemical bond associated with an atom on the surface layer of a crystal. The bond does not join with another atom of the crystal, but extends in the direction of exterior of the surface. |
| Day-Ahead Market | The forward market for energy and ancillary services to be supplied during the settlement period of a particular trading day that is conducted by the ISO, the PX, and other Scheduling Coordinators. This market closes with the ISO’s acceptance of the final day-ahead schedule. |
| Day-Ahead Schedule | Day-ahead Schedule A schedule prepared by a Scheduling Coordinator or the ISO before the beginning of a trading day. This schedule indicates the levels of generation and demand scheduled for each settlement period of that trading day. |
| Daylighting | The placement of windows and skylights to provide natural interior lighting and to reduce daytime electrical demand. |
| Daylighting Control | A control system that varies the light output of an electric lighting system in response to variations in available daylight. |
| Daylighting Systems | provide light during the day, significantly reducing the cost of, and heat gain from, electric lighting. |
| Days Of Autonomy | The number of consecutive days that a stand-alone renewable energy system will meet a defined load without additional energy input. |
| Days Of Storage | The number of consecutive days the stand-alone system will meet a defined load without solar energy input. This term is related to system availability. |
| DC (Direct Current) | Direct current. A one-way flow of electrons. Typical sources of direct current are solar-electric cells, rectifiers, and direct current generators. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to AC (alternating current). |
| DC Motor, Brushless | High-technology motor used in centrifugal-type DC submersible pumps and other applications. The motor is filled with oil to keep water out. An electronic system is used to precisely alternate the current, causing the motor to spin. |
| DC Motor, Brush-Type | The traditional DC motor, in which small carbon blocks called “brushes” conduct current into the spinning portion of the motor. They are used in many applications, including DC surface pumps and also in DC submersible diaphragm pumps. Brushes naturally wear down after years of use, and may be replaced. |
| DC Motor, Permanent Magnet | A variable speed motor that uses permanent magnets instead of wound coils. Reduced voltage (in low sun) produces proportionally reduced speed, and causes no harm to the motor. |
| DC-To-DC Converter | Electronic circuit to convert direct current voltages (e.g., photovoltaic module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker. |
| Dealer | A Retailer of PV products and/or PV Systems |
| Deep Cycle Battery | A battery designed to regularly discharge 80% of its capacity before recharging. |
| Deep Discharge | Discharging a battery to less than 20% of it’s capacity. |
| Deep-Cycle Battery | A battery designed to regularly discharge 50 to 80 percent of its capacity before recharging. |
| Degree Day | A quantitative index reflecting demand for energy to heat or cool buildings. Heating and cooling degree days show the difference between the mean daily temperature and a 65°F base. The higher the heating degree days at any location, the colder the winter. The higher the cooling degree days at any location, the hotter the summer. |
| Dehydrate | to free from moisture in order to preserve; to dry fruits, vegetables or lumber, for instance. A factory in in Nevada, for example uses geothermal heat to dehydrate onions and garlic for restaurants. |
| Deintegration | (See disaggregation) |
| Delta T | 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. |
| Demand Bid | Demand Bid A bid into the PX indicating a quantity of energy or an ancillary service that an eligible customer is willing to purchase and, if relevant, the maximum price that the customer is willing to pay. |
| Demand Side Management (DSM) | Planning, implementation, and evaluation of utility-sponsored programs to influence the amount or timing of customers’ energy use. |
| 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 might have on the functioning of the technology. |
| 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. |
| Density | the amount of mass in a given volume of something. Two objects can be the same size, but have different densities because one of the objects has more mass "packed" into the same amount of space. Objects are smaller when they are cold, larger when hot. |
| Depletable Energy Sources | 1) electricity purchased from a public utility 2) energy obtained from burning coal, oil, natural gas or liquefied petroleum gases. |
| 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 hole). |
| Depth Of Discharge (DOD) | the amount of energy withdrawn from a battery or cell expressed as a percentage of its rated capacity. |
| Deregulation | The process of changing policies and laws of regulation in order to increase competition among suppliers of commodities and services. The Energy Policy Act initiated deregulation of the electric power industry in 1992. |
| 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. |
| Design Month | The month in which the combination of insolation and load requires the maximum energy from the array. |
| DHW | Domestic hot water |
| Diesel Oil | Fuel for diesel engines obtained from the distillation of petroleum. It is composed chiefly of aliphatic hydrocarbons. Its volatility is similar to that of gas oil. Its efficiency is measured by cetane number. |
| Differential Controller | An electronic switch that turns off or on based on the difference between two temperatures. In a solar hot water system, the controller measures the temperature at the collector and compares it to the water temperature in a storage tank to turn the pump on or off. |
| Diffuse Insolation | Incident sunlight received indirectly because of scattering due to clouds, fog, particulates, or other obstructions in the atmosphere. The other component of sunlight is Direct. |
| Diffuse Radiation | Radiation received from the sun after reflection and scattering by the clouds, fog, haze, dust or other substances in the atmosphere, and the ground. |
| Diffusion Furnace | Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material. |
| Diffusion Length | The mean distance a free electron or hole moves before recombining with another hole or electron. |
| Digital Multimeter (Dmm) | A device with multiple electrical measurement capabilities, such as voltage, amperage, resistance, etc., commonly usable for both AC and DC circuits. It has a digital display. |
| Diode | A semiconductor device that allows electrical current in only one direction. |
| Diodes | A rectifier that consists of a semi conducting crystal with two terminals and that is analogous in use to an electron tube diode. |
| 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 also Retail Competition) |
| Direct Beam Radiation | Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc. |
| Direct Current | A type of electricity transmission and distribution by which electricity flows in one direction through the conductor. Usually the electricity is a relatively low voltage and high current. Direct current is abbreviated as DC. |
| Direct Current (DC) | Current where the flow of electrons never changes 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 Expansion | (refrigeration) |
| Direct Gain | In passive solar heating, a direct gain system relies on the sunshine to directly hit the substance or mass being heated. Direct gain systems used today usually rely on a layer(s) of glass to assist in holding the heat within a space where the heat is desirable. |
| Direct Insolation | Sunlight falling directly upon a collector. Opposite of diffuse insolation. |
| Direct Radiation | Light that has traveled in a straight path from the sun (also referred to as beam radiation). An object in the path of direct radiation casts a shadow on a clear day. |
| Direct Solar Gain | Solar energy collected from the sun (as heat) in a building through windows, walls, skylights, etc. |
| Direct Use | use of geothermal water and it’s heat to grow fish, dry vegetable, fruit and wood products, heat greenhouses and city buildings, or provide hot water for spas. |
| Directly Conditioned Space | See conditioned space, directly. |
| 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 "delamination" are sometimes used to mean the same thing. (See also "Divestiture.") |
| Discharge | Withdrawal of electrical energy from a battery. |
| Discharge Factor | A number equivalent to the time in hours during which a battery is discharged at constant current usually expressed as a percentage of the total battery capacity, i.e., C/5 indicates a discharge factor of 5 hours. |
| Discharge Rate | The rate, usually expressed in amperes or time, at which electrical current is taken from the battery. |
| Disconnect | Switch gear used to connect or disconnect components in a photovoltaic system. |
| Dispatch | The operating control of an integrated electric system to: Assign generation to specific generating plants and other sources of supply to effect the most reliable and economical supply as the total of the significant area loads rises or falls. Control operations and maintenance of high-voltage lines, substations and equipment, including administration of safety procedures. Operate the interconnection. Schedule energy transactions with other interconnected electric utilities. |
| Dispatchability | The ability of a generating unit or other source of electric power to vary output. |
| Dispatchable Power | Energy output that can be planned on and typically provides a continuous power output. Solar power and Wind power in not dispatchable without configuration with out some other power or storage mechanism. Hydrocarbon based power plants or nuclear plants are dispatchable. |
| Dissolved Gas | Natural gas that can be developed for commercial use, and which is found mixed with oil in naturally occurring underground formations. |
| Distributed Energy Resources (Der) | A variety of small, modular power-generating technologies that can be combined with energy management and storage systems and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid. |
| Distributed Generation | A popular term for localized or on-site power generation. |
| Distributed Power | Generic term for any power supply located near the point where the power is used. Opposite of central power. See stand-alone systems. |
| 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 process of distributing electricity. Distribution usually refers to the portion of power lines between a utility’s power pole and transformer and a customer’s point of connection. |
| Distribution Network | The overall network formed by the Distribution System. |
| Distribution System | The poles, wires, cables, substations and other equipment required to transport electricity energy from the transmission system to people’s homes. |
| 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. |
| Distributor | Using means a wholesaler of PV products |
| District Heating System | a heating system that provides heat to a large number of buildings all from a central facility. In geothermal district heating systems, one or more wells can serve entire districts. |
| 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 also "Disaggregation.") |
| 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. DOE’s main headquarters are in Washington, D.C. |
| Doldrums | A narrow, virtually windless zone near the Equator, created as heated air rises upward, leaving the ocean’s surface calm and glassy. |
| Donor | In a 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. |
| 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. |
| Downburst | A severe localized downdraft from a thunderstorm. Also called a microburst. |
| Downstream | A term used in the petroleum industry referring to the refining, transportation and marketing side of the business. |
| Downtime | Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage. |
| Downwind | In relation to a wind turbine, the direction away from the source of wind. A downwind turbine has its blades on the downwind side of the tower. |
| Downwind Wind Turbine | A horizontal axis wind turbine in which the rotor is downwind of the tower. |
| Draft Tube | A tube added to the outfall of a hydro turbine to increase energy production by taking advantage of the drop in the tailrace. |
| Drainback System | A solar hot water system that only fills the collector when the temperature differential is appropriate. The water that is circulated through the collectors is stored in a reservoir. Draining the collectors provides freeze protection. |
| Draindown System | A solar hot water system that uses a special draindown valve that redirects the collector fluid and drains it down when the collector system pump is not operating. These systems have been prone to failure and are not recommended. |
| Dry Bulb Temperature | A measure of the sensible temperature of air. |
| Dry Cell | A cell (battery) with a captive electrolyte. A primary battery that cannot be recharged. |
| Dry Cell Battery | A battery that uses a solid paste as an electrolyte. |
| Dry Hole | A drilled well 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. |
| 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-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. |
| Dump | Excess hydropower that cannot be stored or conserved. Also know as SPILL ENERGY. |
| Dump Load | A device to which the wind generators power flows when the batteries are too full to accept more charge, sometimes an electric heating element is used. |
| Dust Devil | A small whirlwind, of a usually short duration, that swirls dust, debris, and sand to great heights. |
| Duty Cycle | The fraction of time a device or load actually uses energy in a unit of time. For example, a load that uses energy for 5 seconds out of every 10 seconds has a 50 percent duty cycle. |
| Duty Rating | The amount of time an inverter (power conditioning unit) can produce at full rated power. |
| Dynamic/Shunt Braking | Dissipating the kinetic energy of rotation either as heat in a braking resistor or bulb, or in a direct short circuit. |
| Dynamo | The first type of large generator developed for a power plant. |
