Wind Power

Betz Coefficient

59.3%. This is the theoretical maximum efficiency at which a wind generator can operate, by slowing the wind down. If the wind generator slows the wind down too much, air piles up in front of the blades and is not used for extracting energy.

Public Utilities Regulatory Policy Act (PURPA)

A 1978 law that requires electric utilities to purchase electricity produced from qualifying power producers that use renewable energy resources or are co-generators.

thrust bearing

A bearing that is designed to handle axial forces along the centerline of the shaft; in a wind generator, the axial force is the force of the wind pushing back against the blades.

wind rose

A circular plot used to portray certain characteristics about wind speed and direction observed at a monitoring location.

guy anchor

A concrete or metal base that secures wind tower guy wires to the ground.

turbine

A device for converting the flow of a fluid (air, steam, water or hot gases) into mechanical motion that can be utilized to produce electricity.

inverter

A device that converts direct current electricity to alternating current, either for stand-alone systems or to supply power to an electric utility system.

windmill

A device that uses wind power to mill grain into flour. Informally used as a synonym for wind generator or wind turbine, and to describe machines that pump water with wind power.

braking system

A device to slow a wind turbine's shaft speed down to safe levels (electrically or mechanically).

wind vane

A device used to measure wind direction.

power curve

A graphic displaying the instantaneous power output of a specific turbine design at various wind speeds; used with wind resource data to determine the potential for electricity generation at a project site.

vain (tail)

A large, flat piece of material used to align a wind turbine rotor correctly into the wind. Usually mounted vertically on the tail boom.

average capacity (capacity factor)

A measure of a wind turbine's productivity, calculated by the amount of power that a wind turbine produces over a set time period, divided by the amount of power that would have been produced if the turbine had operated at full capacity during that same time interval.

green pricing

A practice used by some power providers in which electricity produced from clean, renewable resources is sold at a higher cost than electricity produced from conventional fuels to buyers willing to pay a premium for clean power.

grid-connected system

A residential electrical system, such as solar panels or wind turbines, which is connected to the electric utility system. (serves as a back up)

distributed generation

A small-scale power generation technology that provides electric power at a site closer to customers than a central power plant generation.

guy wire

A strong metal cable or wire that attaches some towers to the ground.

wind power class

A system designed to rate the quality of the wind resource in an area, based on the average annual wind speed. The scale ranges from 1 to 7 with 1 being the poorest wind energy resources and 7 representing exceptional wind energy resources.

wind shear

A term and calculation used to describe how wind speed increases with height above the surface of the earth. The degree of wind shear is a factor of the complexity of the terrain as well as the actual heights measured. Wind shear increases as friction between the wind and the ground becomes greater.

met tower

A tower with a group of instruments (including anemometers and wind vanes) attached that collectively measure various meteorological parameters such as wind speed, wind direction, and temperature at various heights above the ground. The term met is short for meteorological.

variable pitch turbine

A type of wind turbine rotor where the attack angle of the blades can be adjusted either automatically or manually.

gigawatt

A unit of power equal to 1 billion Watts, 1 million kilowatts or 1000 megawatts.

VAWT

A wind generator design in which the rotating shaft (axis of rotation) is perpendicular to the ground and the cups or blades rotate parallel to the ground.

HAWT

A wind turbine design in which the shaft (axis of rotation) is parallel to the ground and blades are perpendicular to the ground.

balancing

Adjusting wind turbine blades' weight and weight distribution through two axes so that all blades are the same (eliminating vibration).

renewable energy certificates

Also known as green tags, certificates representing the environmental attributes of power produced from renewable resources. By separating the environmental attributes from the power, clean power generators are able to sell the electricity they produce to power providers at a competitive market value.

independent power producer

An electricity generator that sells electricity but is not owned by a utility.

grid

An electricity transmission and distribution system.

wind monitoring system

An instrument or group of instruments (including anemometers and wind vanes) that collectively measure various meteorological parameters, such as wind speed, wind direction and temperature at various heights above the ground.

anemometer

An instrument used to measure the velocity, or speed, of the wind.

aggregation

Building several wind energy projects together so that they are treated as one larger project - to improve project economics.

community wind

Locally owned, commercial scale wind projects that optimize local benefits.

net metering

Measuring both consumption and generation of electricity by a small generation facility (such as a small wind system). The net energy produced or consumed is purchased from or sold to the power provider.

hub height

Measuring from the ground, the tower height of the hub, or central part of a HAWT.

downwind turbine

Refers to a HAWT in which the hub and blades point away from the wind direction.

furling

The act of a wind generator yawing out of the wind, either horizontally or vertically, to protect itself from high wind speeds.

payback period

The amount of time required for the savings resulting from a renewable energy system to equal the system cost.

angle of attack

The angle of relative airflow to a wind turbine's blade.

tower

The base structure that supports and elevates a wind turbine rotor and nacelle.

hub

The central part of the wind turbine, which supports the turbine blades on the outside and connects to the low-speed rotor shaft inside the nacelle.

hybrid system

The combination of multiple energy-producing technologies such as photovoltaic solar electric systems combined with small wind turbine systems.

airfoil

The cross section profile of the leeward side of a wind generator blade, designed to reduce drag and maximize lift.

blades

The flat panels on a wind turbine that are connected to a center shaft that converts the push of the wind into a circular motion in a wind turbine.

peak wind speed

The maximum instantaneous wind speed (or velocity) that occurs within a specific time period.

average wind speed (velocity)

The mean wind speed over a specified period of time.

armature

The moving part of an alternator, generator, or motor. In many alternator designs, it carries the magnets and is attached to the blades and hub. Also called a rotor.

availability factor

The percentage of time that a wind turbine is able to operate and is not out of commission due to maintenance or repairs.

wind speed

The rate at which air particles move through the atmosphere, commonly measured with an anemometer.

yaw

The rotation of a HAWT about its tower or vertical axis.

waveform

The shape of the phase power at a certain frequency and amplitude.

nacelle

The structure at the top of the wind turbine tower just behind (or, in some cases, in front of) the wind turbine blades. It houses the key components of the wind turbine, including the rotor shaft, gearbox and generator.

rotor

The visible spinning parts of a wind turbine, including the turbine blades and the hub.

chord

The width of a wind turbine at a given location along the length.

wind resource

The wind energy available for use based on historical wind data, topographic features and other parameters.

rated wind speed

The wind speed at which a wind turbine reaches its nameplate-rated level of power production. For most small wind turbines, this is around 30 to 35 miles per hour.

start-up speed

The wind speed at which a wind turbine rotor starts to rotate. The turbine does not necessarily produce any power until the wind reaches cut-in speed.

cut-in speed

The wind speed at which the turbine blades begin to rotate and produce electricity, typically around 10 mph.

cut-out speed

The wind speed, usually around 55 to 65 mph, at which some wind turbines automatically stop the blades from turning and rotates out of the wind to avoid damage to the turbine.

utility-scale wind

Wind energy projects greater than 100 kilowatts in capacity in which the electricity is sold rather than used on-site. This category includes large arrays of turbines owned by corporations and a single locally-owned wind turbine greater than 100 kilowatts in size.

commercial scale wind

Wind energy projects greater than 100 kilowatts in which the electricity is sold rather than used on-site.