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.


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.”


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.


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.


The process of removing a power plant, apparatus, equipment, building, or facility from operation.


The process of breaking down organic material; reduction of the net energy level and change in physical and chemical composition of organic material.


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.


A device that cools air by removing moisture from it.


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.


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.


The production of energy by a system or appliance at a level less than its design or nominal capacity.


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)


A material used to desiccate (dry) or dehumidify air.

Desiccant Cooling

To condition/cool air by dessication.


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.


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.


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.


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.


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.


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.


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.


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.


To schedule and control the generation and delivery of electric power.


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.


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 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.


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.


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.


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).


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.


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.


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.


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.


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.


An apparatus for measuring force or power, especially the power developed by a motor.


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.