» Annual Energy Production:

The calculated or measured energy delivered by a wind turbine generator (WTG) or small wind turbine (SWT).

Alternating current (AC):

An electrical current whose direction reverses periodically, usually many times per second. The world's electrical systems use AC current rated at 120 V and 60 cycles per second (North America and others) or 230 V and 50 Hz (Europe and others).

» Ampere-hour:

A unit for the quantity of electricity supplied or used at some fixed (usually dc) voltage. Obtained by integrating current flow in amperes over time in hours. Multiplying by the voltage gives the energy exchanged. Used as a measure of battery capacity.

» Annual mean wind speed:

calculated as the average of short periods, e.g., 10-minute averages.

» Average wind speed:

The mean wind speed over a specified period of time. For example, for assessing a horizontal axis wind turbine power curve, (i) the unperturbed wind speed is measured at hub height and then averaged within successive periods of 10 minutes, and (ii) the wind turbine power is similarly averaged within concurrent periods.

» Annual Energy Production:

The calculated or measured energy delivered by a wind turbine generator (WTG) or small wind turbine (SWT).

» Betz limit:

The theoretical maximum amount of energy that a perfect wind turbine can extract from the wind approaching its rotor, which Betz calculated as 59% of wind kinetic energy,. In practice, efficient wind turbines have efficiencies of about 70% of this Betz criterion.

» Brake:

A device capable of reducing or stopping the rotor speed.

» Budget Energy:

The budgeted energy for a wind plant is usually defined per month.

» Capacity Factor:

The percent of actual energy for a wind turbine versus the rated maximum power over a specific time period.

» Commercial Availability (CA):

Actual Energy / (Actual Energy + Commercial Lost Energy)

» Data Capture:

The percent of time that data is available for a turbine or meteorological station.

» Direct current (DC):

An electrical current flowing in one direction only (i.e., not AC), defined as from positive to negative voltage,. Typical sources of DC current are batteries of electrochemical cells, photovoltaic cells, and rectifiers of AC. DC current has to be transformed to AC at mains voltage through an inverter for most conventional household appliances.

» Direct drive:

A wind turbine whose rotor shaft is directly connected to the electrical generator shaft, i.e., without an intermediate gearbox.

» Downwind:

The direction in which the wind is blowing, e.g., the leeside, is downwind.

» Equivalent Availability Factor (EAF):

The percent of time that a turbine is available to produce energy due to planned and unplanned outages that are within the plant's control. Excludes non-operating conditions (i.e., high wind shutdown, untwisting, high or low ambient temperature) and outages outside of the plant's control, such as force majeure, curtailment, utility outages, and wake management. Base hours are the total hours for the time period, less any missing data hours. EAF = 100 * (1 - (Plant Planned Outage Hours + Plant Unplanned Outage Hours) / Base Hours)

» Equivalent Forced Outage Rate (EFOR):

The percent of time that a turbine is unavailable to produce energy due to forced outage conditions that are within the plant's control. Excludes planned outages, non-operating conditions (i.e., high wind shutdown, untwisting, high or low ambient temperature), and outages outside of the plant's control, such as force majeure, curtailment, utility outages, and wake management. Base hours are the total hours for the time period, less any missing data hours. EFOR = 100 * Plant Forced Outage Hours / Base Hours

Expected Energy:

The expected energy is based on actual wind speed and the theoretical power curve, adjusted for actual air density. When using the turbine anemometer wind speed, free-stream adjustments shall be applied as necessary.

» Free wheeling mode:

Condition of a wind turbine that is rotating a little over its rated rpm and not generating shaft power. This is only achieved with controlled feathering or stalling regulation systems.

» Gearbox:

A mechanical assembly with an input and an output shaft to house gears. The gearbox is positioned between the main shaft and the electrical generator, thus matching the rotor speed (say, 60 rpm) to the recommended speed of the generator (say, 1,500 rpm).

» Gross Availability (GA):

Actual Energy / (Actual Energy + Gross Lost Energy)

» Hub:

The central fixture of a HAWT rotor, to which the blades of a wind turbine are attached. If the blades are firmly bolted to the hub, the rotor is called 'fixed pitch'. However, blades can have a degree of freedom to turn (pitch) along their longitudinal axis by interposing a rotating device, ball bearings, slewing rings, or turntables. The rotor is then called 'variable pitch'.

» Idling:

Condition of a wind turbine that is rotating slowly and not producing power.

» Inverter:

An electronic device that converts direct current (DC) to alternating current (AC) at the required frequency (e.g., 50 Hz for grid electricity in Europe.)

» kW Kilowatt:

1000 watts is a measure of power (energy per unit time).

» kWh Kilowatt Hour:

1000 Watts for one hour; energy unit (energy per unit time, multiplied by time = energy). The energy dissipated by using one kW in one hour.

» Lost Energy:

The expected energy is lost while a turbine is unavailable to produce energy and there is sufficient wind for generation.

» Main shaft:

The shaft connecting the rotor hub to the gearbox or, with direct drive, to the generator.

» Nacelle:

The body (including frame and cover) that contains the main bearings, brakes, gearbox, generator, pitch controllers, hydraulic equipment, etc. of a wind turbine.

» On-Line Efficiency:

The percent of actual energy produced at the turbine is divided by the expected energy for the turbine when the turbine is online.

» Park Efficiency:

The percent of actual energy delivered to the interconnect divided by expected energy.

» Power coefficient:

The ratio of the power extracted by a wind turbine to the power available in the wind stream ahead of the rotor area at a specified wind speed.

» Power curve:

A graph showing a wind turbine's power output as a function of wind speed.

Rated output capacity:

The output power of a wind machine operating at the rated wind speed.

» Rotor:

The main rotating drive mechanism of a wind turbine includes the blades and blade hub.

» Rotor diameter:

The outer diameter of the rotor. That is approximately twice the blade length.

» Slip ring assembly:

A device for making electrical connections between stationary and rotating parts, e.g., in generators, for connecting monitoring sensors to a rotor, or for connecting mains cables from a rotating nacelle to within a tower. Usually it includes a bronze or other conducting alloy circular component over which carbon conducting brushes move with minimal friction.

» Swept area:

The area swept by the turbine rotor is, for common HAWT, A=pR2, where R is the radius of the rotor.

» Small Wind Turbine (SWT):

According to the IEC 61400 standard, SWTs are wind turbines of swept area less than 200 m2, i.e., HAWT less than 16 m in diameter. However, this definition is not universal.

» Turbine Efficiency:

The percent of actual energy produced at the turbine is divided by the expected energy for the turbine.

» Turbulence:

Change in wind speed and direction over periods of seconds. Turbulence is an intrinsic property of wind that is increased by obstacles.

» Turbulence intensity:

Ratio of the standard deviation of the wind speed to the mean wind speed, as determined from the same set of measured data on wind speed for a specified period of time.

» Upwind:

On the same side as the direction from which the wind is blowing: windward.

» VAWT:

Vertical-axis wind turbine.

» Wind shear:

The increase in wind speed above the ground as a function of height. A logarithmic expression is sometimes used to characterize wind shear.

» Yaw:

• The movement of the tower top (nacelle) of a HAWT turns the turbine rotor with respect to the oncoming wind.