Renewable Energy Glossary

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Daylighting

The use of sunlight and/or natural ambient light for daytime lighting needs. Strategies include proper orientation of windows, skylights, and clerestory windows in addition to proper placement of solar tubes, reflective surfaces, and interior glazing to allow for deeper penetration of light throughout the interior space.

Days of Autonomy

The number of consecutive days a battery bank can provide energy to loads without being recharged.

DC

Direct current (DC) is the one-way flow of current, meaning that electrons are always moving in the same direction. It is the type of current produced by PV modules, and stored in batteries.

DC Input Voltage

The DC input voltage of an inverter must match the nominal battery bank voltage, which is typically 12, 24, or 48VDC.

DC-to-DC Converter

A small electronic device installed on PV modules to maximize power production by manipulating the module voltage.

Deep Cycle Batteries

Batteries that have the ability to be discharged up to 80 percent and are commonly used in RE systems.

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

The percentage of battery capacity that has been used. 100% DOD indicates an empty battery.

Derate Factors

Elements that reduce system production, such as non-optimal array tilt angle and orientation, dust, and voltage drop losses.

DHW

Domestic hot water (DHW) is potable hot water for household or human use.

Differential Controller

A thermostat that operates on the difference between two temperatures. In a SWH system, the sensors measure the temperature at the collector and the water temperature in a storage tank and the control turns the pump on and off based on the temperatures.

Diffuse Radiation

Solar radiation that is transmitted to the Earth's surface after significant scattering by the atmosphere due to clouds, haze, or smog.

Digital Multimeters

Electrical meters with LCD or LED numeric screens used to measure values such as voltage, current, and resistance.

Direct Beam Radiation

Solar radiation that is transmitted to the Earth's surface with minimal scattering by (??) the atmosphere--cloudless sunshine.

Direct Current

The one-way flow of current, meaning that electrons are always moving in the same direction. It is the type of current produced by PV modules, and stored in batteries.

Direct Forced Circulation System

An open system without a heat exchanger, typically used in non- freezing climates, that circulates potable water through the collectors.

Direct Gain System

The simplest type of passive solar heating system in which heat is gained through south-facing windows and is absorbed and stored by thermal mass materials during the daytime. At night, the stored heat radiates out, gently warming the space.

Direct Solar Gain

The simplest type of passive solar heating system in which the sun's heat is gained through south-facing windows and is typically absorbed and stored by thermal mass materials during the daytime. At night, the stored heat radiates out, gently warming the space.

Disconnect

A switch used to open and close circuits, shut off power, and service components.

Distributed Generation

Refers to electricity that is produced very near to where it is used, such as a rooftop solar electric system.

Diversion Load

A resistance heater that is energized automatically (by a controller) so as to divert surplus hydro energy and regulate the voltage and speed. The turbine needs a constant total load. If the user load is reduced, then more energy must be diverted to dump. If a battery is connected, the diversion load will also control charging voltage.

DMMs

Digital multimeters (DMMs) are electrical meters with LCD or LED numeric screens and are used measure values such as voltage, current, and resistance.

DOD

Depth of discharge (DOD) is the percentage of battery capacity that has been used. 100% DOD indicates an empty battery.

Downwind Turbine

A turbine with its blades on the downwind side of the tower, such that the wind passes the tower before reaching the blades.

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. The weight of water in the draft tube creates suction in the turbine casing, increasing the relative head of pressure. A draft tube is often bell-shaped so that the water departs slowly with minimal kinetic energy.

Drainback System

An indirect forced circulation system that only fills the collector when the control energizes the collector loop pump. The water that is circulated through the collectors is stored in a drainback reservoir. Draining the collectors provides freeze protection.

Drainback Tank

A reservoir used to hold all the liquid in a collector loop after the collectors and piping have drained to freeze-protect the system.

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.

Dual-Axis Tracker

A tracker that not only follows the sun's path from east to west (azimuth angle), but also follows the altitude angle of the sun as it changes throughout the day and the year.

Dump Load

A resistance heater that is energized automatically (by a controller) so as to divert surplus power away from the battery or inverter, and prevent a potentially harmful excess of voltage in the wind turbine circuit.

Dump Load

A resistance heater that is energized automatically (by a controller) so as to divert surplus hydro energy and regulate the voltage and speed. The turbine needs a constant total load. If the user load is reduced, then more energy must be diverted to dump. If a battery is connected, the diversion load will also control charging voltage.

Duty Cycle

The percentage of time that an appliance is active and drawing power while turned on.

Dynamic Braking

A method of braking that shorts the output (either multi-phase wild AC or DC) in order to stop the wind turbine for service or when not needed. The short circuit current that determines the braking torque varies (with the alternator design) from very decisive to tenuous or ineffective. Dynamic braking is also occasionally used for speed control.

Dynamic Head

The "net" head of pressure at the turbine when water flows down the pipe. This is lower than the "static head" or "gross head" due to head loss in the pipe and fittings.

Dynamic Head

The resistance that occurs between a fluid and the channel through which it travels.

1 A B C D E F G H I J K L M N O P Q R S T U V W Y

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