Renewable Energy Vocabulary: D-F
A-C • D-F • G-I • J-M • N-P • R-S • T-Z
DAYLIGHTING
The placement of windows and skylights to provide natural interior lighting and to reduce daytime electrical demand.
DAYS OF AUTONOMY
The number of consecutive days that a stand-alone renewable energy system will meet a defined load without additional energy input.
DC
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.
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.
DELTA T
Difference in temperature.
DEPTH OF DISCHARGE (DOD)
The ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example, the removal of 25 ampere-hours from a fully charged 100 ampere-hour rated cell results in a 25 percent depth of discharge. Depth of discharge is the opposite of state of charge (SOC).
DHW
Domestic hot water: refers to any system that provides hot water for household use.
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.
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.
DIRECT CURRENT (DC)
An electrical current that moves in one direction only.
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.
DISCHARGE RATE
The rate at which energy is being drained from a battery.
DISCONNECT
Switch gear used to connect or disconnect components in a system.
DISTRIBUTED GENERATION
Distributed generation is any electricity generating technology installed by a customer or independent electricity producer that is connected at the distribution system level of the electric grid. Learn More
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.
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.
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.
EARTH
Synonymous with “ground.”
EFFICIENCY (PV modules)
The ratio of power output of a photovoltaic cell to the incident power from the sun or simulated sun sources under specified standard insolation conditions. A solar cell that converts 1/10 of the sun’s energy that strikes its surface to electricity has an efficiency of 10 percent.
EFFICIENCY
The effectiveness of a device to convert energy from one form to another, or to transfer energy from one body to another. An electric pump that is 60 percent efficient converts 60 percent of the input energy into work—pumping water. The remaining 40 percent becomes waste heat.
ELECTRICAL POTENTIAL
Same as VOLTAGE.
ELECTRIC CURRENT
The rate at which electrons flow through an electrical conductor, usually measured in amperes (amps).
ELECTRICITY
Energy flow resulting from the flow of charged particles, such as electrons or ions.
ELECTROLYSIS
The breaking down of a chemical compound into simpler compounds or elements by the passage of electricity through the chemical compound. Commonly used to describe the extraction of hydrogen (and oxygen) from water.
ELECTROLYTE
The fluid in a battery, which is the medium of ion transport within an electrochemical cell. It provides a path for electron transfer between the anode and cathode of a battery cell.
ELECTROMAGNETIC RADIATION (EMR)
Magnetic radiation produced by a changing electrical current, such as alternating current (AC).
ELECTRON
A negatively charged particle. The movement of electrons in an electrical conductor constitutes an electric current.
EMBODIED ENERGY
The energy consumed by all of the processes associated with the production of a material. This includes the energy required in mining, transport, manufacturing, administration, use, disposal, etc.
ENERGY
The amount of work that a system or entity can do (potential energy) or is doing (kinetic energy), measured in joules. The product of power and time, measured in watt-hours. 1,000 watt-hours = 1 kilowatt-hour (KWH).
ENERGY DENSITY
A ratio of a battery or cell’s capacity to either its volume or weight. Volumetric energy density is expressed in watt-hours per cubic inch. Weight energy density is expressed in watt-hours per pound.
ENGINE
A machine that converts energy into mechanical force or motion. Sources of energy include heat, chemical reaction, potential energy of elevated water, etc.
EQUALIZATION
The process of restoring all cells in a battery to an equal state-of-charge. For lead-acid batteries, this is a charging process designed to bring all cells to 100 percent state-of-charge.
EQUALIZING CHARGE
A continuation of normal battery charging, at a voltage level slightly higher than the normal end-of-charge voltage, in order to provide cell equalization within a battery.
EQUINOX (SPRING & FALL)
The time when the sun crosses the plane of the earth’s equator, making night and day of equal length all over the earth, occurring about March 21st and September 21st.
EVAPORATION
The process of a liquid changing its state into a gas when heat is added. In the most common occurrence on earth, water evaporation requires 970 BTUs per pound (pint).
FEATHERING
In wind generators, this refers to an adjustment of the blades so that they catch less wind. This can prevent damage to the machine in high winds.
FINISH CHARGE
The final stage of battery charging, when the battery is charged at a slow rate over a long period of time.
FLAGGING
Noticeable deformation of trees from prevailing winds. Flagging is an indication of an effective wind site. Lack of flagging is not necessarily an indication of a poor wind site.
FLAT PLATE COLLECTOR
A solar thermal collector that converts the sun’s radiation into heat on a flat surface. Does not use reflecting surfaces or lens arrangements to concentrate the sun’s energy.
FLOAT CHARGE
A trickle charge to keep a battery fully charged at a safe voltage level with minimal gassing.
FLOAT SERVICE
A battery operation in which the battery is normally connected to an external current source; for instance, a battery charger that supplies the battery load under normal conditions, while also providing enough energy input to the battery to make up for its internal losses, thus keeping the battery always at full charge and ready for service.
FLOW
In hydro-electric terms, flow refers to the quantity of water supplied to a water source or exiting a nozzle per unit of time. Commonly measured in gallons per minute.
FLUORESCENT LIGHT
An electric lamp coated on its inner surface with phosphor and containing mercury vapor. When bombarded with electrons, the vapor emits ultraviolet light that causes the phosphor to emit visible light.
FRANCIS TURBINE
A type of reaction hydro-turbine used in low to medium heads. It consists of fixed vanes on a shaft. Water flows down through the vanes and out sideways.
FREESTANDING TOWER
A wind generator tower with no guy wires. This can be either a lattice tower or a monopole. Freestanding towers are the most expensive type of tower, requiring large excavations and large amounts of concrete.
FRICTION LOSS
Lost energy due to friction.
- In hydro systems, pipe sized too small can lead to serious friction losses.
- In any belt drive system, there will be some losses due to friction.
FULL SUN
The full sun condition is the amount of power density received at the surface of the earth at noon on a clear day—about 1 KW per m^2, or 1 Sun. Lower levels of sunlight are often expressed as 0.5 sun or 0.1 sun.
FURLING
Reducing a wind generator’s swept area to protect it from high winds. Common furling methods are to tilt the rotor (blades) up or sideways out of the wind, or to feather (twist) the blades to degrade the airfoil.
FUSE
An electrical device that is designed to break a circuit by melting an internal conductor when the current in the circuit exceeds the maximum considered safe for the conductors or devices in the circuit.
