D

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.

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 drain down 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.