Glossary: D

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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

DC Distribution (DCD)

A module in the power solution that distributes DC power to the loads. It also provides protection for the load cables.

DC Distribution - Fused Version (DCF)

A DC Distribution module that uses fuses for protection.

DC Distribution - Miniature Circuit Breaker (MCB) version (DCM)

A DC Distribution module that uses miniature circuit breakers for protection.

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, 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, 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.

DCD

DC Distribution: A module in the power solution that distributes DC power to the loads. It also provides protection for the load cables.

DCF

DC Distribution - Fused Version: A DC Distribution module that uses fuses for protection.

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 Connection

A method of connecting a three-phase source or load in series for a closed circuit (3-wire, plus ground).

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-hours rated cell results in a 25% depth of discharge. Under certain conditions, such as discharge rates lower than that used to rate the cell, depth of discharge can exceed 100%.

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 Input

An input which recognizes an open-circuit and short-circuit.

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.

Digital Output

A voltage free relay contact.

Diode

A semiconductor device that allows electrical current in only one direction.

Direct Current (DC)

A type of current which never reverses its direction. Since the current flows in only one direction, the average value of the current cannot be zero unless the current has stopped flowing.

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.

DMM

Digital Multimeter: 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.

DOD

Depth of Discharge: 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).

Double-Conversion

A UPS design in which the primary power path consists of a rectifier and inverter. Double-conversion isolates the output power from all input anomalies such as low voltage, surges and frequency variations by converting AC to DC to AC. See Online UPS.

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.

Dry Contact

Isolated contacts through which the end user supplies an external circuit. Dry contact UPSs provide basic communication capabilities such as monitoring and shutdown.

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.