Glossary
Electrical Terms
Alternating current (AC): Electron (charge) flow in two directions, usually alternating back and forth numerous times per second. The electrical grid provides alternating current.
Alternator: A device that generates AC electricity by passing magnetism past coils of wire.
Ammeter: A device that measures amperage (current).
Ampacity: The maximum amount of current that a wire or device can carry safely, without getting damaged.
Amperage: Also called current, the rate of charge flow in circuits, measured in amperes, or amps (A).
Ampere-hour: Commonly called amp-hour (Ah), a unit for the amount of charge that is transferred by a steady current of one amp for one hour; used to measure battery capacity.
Balance of system (BOS): Electrical components of a wind system besides the turbine and tower, typically including the wiring, voltage clamp, charge controller, batteries, inverter, disconnects and grounding.
Battery: An electrochemical device that stores electrical energy via chemical reactions.
Batteryless grid-tied system: A wind electric system that connects to the grid without batteries, meaning it provides no backup for utility outages. If the grid is down, the system stops working.
Capacity credit: The output of a power source that may be statistically relied upon, expressed as a percentage. At low levels of penetration, the capacity credit of wind is about the same as the capacity factor. As the concentration of wind power on the grid rises, the capacity credit percentage drops.
Capacity factor: Also called load factor, the average expected output of a generator, usually over an annual period. Expressed as a percentage of the nameplate capacity or in decimal form (e.g., 40 percent or 0.40).
Charge: The moving electrons that create electricity.
Charge controller: An electronic device that regulates battery charging and prevents overcharge.
Circuit: An electrical pathway for charges; a complete loop must be present.
Conduit: Tubing that carries electrical wires, often made out of metal or plastic.
Current: Rate of electron (charge) flow, also known as amperage.
Density: Mass per unit of volume.
Depth of discharge (DOD): The level of battery discharge; it’s the inverse of state of charge.
Digital multimeter (DMM): An electronic device that measures voltage, amperage, resistance, and sometimes other electrical parameters.
Direct current (DC): Flow of charge in one direction. Direct current is provided by batteries, solar panels and some small wind turbines, and it is typically used in recreational vehicles, boats, and sometimes small cabins.
Direct drive: When wind turbine blades connect directly to the generator, without going through a gearbox.
Disconnects: Commonly known as breakers, these switches disconnect electrical elements from each other.
Dispatchability: Also called maneuverability, the ability of a given power source to increase and/or decrease output quickly on demand. Wind power is often said to be “highly nondispatchable.”
Dump load: A load designed to dissipate excess energy from a power source, usually controlled by the charge controller. The most common examples are air or water heaters.
Efficiency: The ratio of energy out (or work done) to energy in, expressed as a percentage.
Energy: Work done over time; power (watts) × time (hours) = energy (watt-hours).
Fall-arrest system: Safety features that prevent climbers from falling, even in the case of fainting.
Fixed guyed towers: A tower supported with guy wires.
Frequency: The number of times per second that alternating current (AC) changes direction, measured in hertz (Hz).
Grid: The network that connects electricity generators to electricity users via utility lines and other infrastructure.
Grid-tied: A system that is wired into the grid. It may or may not have batteries for outage protection.
Grounding: Wiring structures to the ground, in order to safely dissipate excess electrical energy (such as from lightning strikes).
Hertz (Hz): The unit of frequency of alternating current (AC) electricity. Must grids are at 50 or 60 Hz.
Intermittency: The extent to which a power source is unavailable; a single wind turbine is highly intermittent, but a large wind farm spread over a wide area will be less so.
Inverter: A device that converts direct current (DC) electricity to alternating current (AC). A device called a rectifier converts AC to DC.
Kilowatt (kW): Rate of power or use of electrical current; also 1,000 watts.
Kilowatt-hour (kWh): A measure of electrical energy, equal to the use of one kilowatt in one hour.
Load: Something that uses electricity, such as an appliance or charging battery, or a whole building.
Maximum power point tracking (MPPT): A system that adjusts the operating voltage of a wind turbine at different speeds to maximize the output and capture as much energy as possible.
Megawatt: A measure of power; also 1,000,000 watts.
Nameplate capacity: The normal maximum output of a generating source.
Ohm (K): The unit of electrical resistance.
Ohm’s Law: A law that states that amperage is equal to voltage divided by resistance (A = V/K).
Overcurrent protection: Protection against high current that may otherwise melt wires or cause fires; typically consists of circuit breakers or fuses.
Parallel: Connecting electrical devices together in parallel paths, with all positives and all negatives wired together; it increases amperage in batteries while voltage remains the same.
Payback: The amount of time it takes energy-generating or energy-saving equipment to pay back its original investment.
Peak power: Maximum wattage a device can generate.
Photovoltaic (PV): Solar-electric panels, commonly called solar panels, that generate electricity directly from sunlight.
Power: The rate of energy generation, transmission, or use; also called wattage.
Power conditioning equipment: Devices like inverters, rectifiers, and controllers that change incoming energy so it’s compatible with the grid or equipment.
Power formula: In electrical terms, power = voltage × amperage × power factor; in wind terms, power = ½ air density × swept area × wind speed cubed.
Rated power: The manufacturer’s claim of energy output, often at the energy peak (and usually at high wind speeds).
Rectifier: A device that converts alternating current (AC) electricity to direct current (DC).
Resistance: A measure of how much an electrical device opposes flow of charge.
Series: Connecting devices together in a daisy-chain fashion, with each positive wired to a negative; it increases voltage while maintaining amperage in batteries.
Setback: The distance between a structure and the property line or a road, often mandated by zoning rules.
State of charge (SOC): Measure of the capacity of battery remaining, as a percentage; it’s the inverse of depth of discharge (DOD).
Step-down: Going from higher to lower voltage in an electrical device.
Tag line: An additional rope used to control items being raised or lowered on a tower (not the line actually doing the lifting).
Volt (V): The unit of electrical force.
Watt (W): A unit of power (wattage), the rate of energy generation, transmission, or use.
Watt-hour (Wh): The unit of electrical energy; one watt-hour is the energy used to create one watt of power for one hour.
Wind Energy–Related Terms
30/500 rule: A rough guideline suggesting that a wind generator’s lowest blade tip should be a minimum of 30 feet above anything within 500 feet to get into a quality wind resource and minimize destructive turbulence (higher still is almost always better).
Air density: Mass per unit volume of air; air is less dense by roughly 3 percent for every 1,000 feet of elevation above sea level. It also varies with temperature and humidity.
Airfoil: The shape of a bird’s feather or an airplane wing, and commonly used for wind turbine blades to optimize the lift/drag ratio and maximize energy production.
Anemometer: A device that measures wind speed, often with cups on a rotating shaft.
Annual energy output (AEO): The amount of energy in kilowatt-hours that a specific wind generator will produce at a specific average wind speed over a year.
Average wind speed: The average recorded wind speed over a set period, usually one year.
Betz limit: The maximum percentage of wind energy (about 60 percent) that a perfect wind generator could capture. Note that a perfect turbine would have an infinite number of infinitely thin blades. However, large turbines are approaching the Betz limit, thanks to advanced materials and active controls.
Blade pitch control: Twisting the blades’ angle of attack into or out of the wind, to either increase energy production or govern by slowing down rotation in high winds, to avoid damage.
Brake (wind): Various systems used to stop the rotor from turning.
Coefficient of performance: Also called power coefficient, the ratio of the power extracted by a wind turbine to the power available in the wind stream. Can never be higher than the Betz limit.
Cube law: Wind power is proportional to the cube of the wind speed (V3).
Cut-in: The wind speed at which a wind generator starts to generate, typically 5 to 9 mph.
Cut-out: The wind speed at which a wind generator stops producing, in order to protect itself from damage in high winds. See also governing.
Downwind: A wind turbine with the rotor on the opposite side of incoming wind (lee side).
Energy curve: A graphic presentation of the energy produced by a wind generator in a range of specific average wind speeds.
Freestanding tower: A self-supporting tower, without guy wires.
Furling: A form of governing in which the rotor folds up or around the tail vane, to reduce exposure of the airfoils to high winds.
Gin pole: A pole used to raise tilt-up towers, or a vertical pole temporarily attached on a nontiltable tower to lift items up.
Governing: Slowing down the rotor in high winds, in order to protect it from damage. This is done through techniques such as furling, blade pitch, or electrical braking.
Guy anchor: The support for guy wires, anchored into the ground.
Guy wire: Steel cable used to hold up guyed towers.
Horizontal axis wind turbine (HAWT): A wind generator that spins on a horizontal shaft, like most available wind turbines do today.
Hub: The center of a wind turbine’s rotor, where the blades are connected to the shaft.
Hub height: The height above the ground at the hub.
Instantaneous wind speed: Wind speed at a given moment.
Nacelle: The body of a propeller-type wind turbine, usually containing the gearbox (if present), alternator, and other parts.
Net metering: A system of utility interconnection that credits a user for renewable energy produced, often at the same rate the utility charges and up to the level of the building’s usage.
Nose cone: Fiberglass or metal cowling in front of a wind generator’s rotor (optional).
Off-grid system: A wind electric system not connected to the utility grid.
Peak wind speed: Maximum wind speed that a device experiences.
Power coefficient: See coefficient of performance.
Power curve: A plot of the instantaneous wattage of a wind generator in various wind speeds. Manufacturers like to show these off, but they aren’t very useful for customers.
Power law method: A way to calculate the increase in wind speed with height. The formula is V/Vo = (H/Ho), where Vo is the wind speed at the original height, V is the wind speed at the new height, Ho, is the original height, H is the new height, and μ is the surface roughness exponent.
Power (wind): In the wind industry, power is a function of ½ air density (ρ) × swept area (A) × wind speed cubed (V3) or P = ½ ρAV3.
Rated output capacity: The output power of a wind generator operating at the rated wind speed.
Rated wind speed: The wind speed at which a wind generator’s rated power output is measured.
Renewable energy: Energy generated from “clean,” naturally reoccurring sources like the wind, sun, falling water, and biomass (plant and animal materials).
Rotor: The part of a wind turbine (or generator or alternator) that rotates. In the case of a wind turbine, this includes the blades and hub.
Slip rings: Rings that form an electrical connection between the generator and the tower, while allowing the turbine to rotate around 360 degrees, without breaking anything. Slip rings are usually made of brass and graphite brushes, which make a sliding contact.
Start-up: The wind speed at which a turbine starts spinning, typically before cut-in (when the generator starts producing electricity).
Survival wind speed: The maximum wind speed equipment will “survive” without permanent damage.
Swept-area: The area (in square feet or square meters) that a wind generator’s blades sweep. For HAWTs, this is equal to pi (π) times the blade rotor’s radius (r) squared, or πr2.
Tail boom: A rod that extends behind the wind turbine to support the tail vane (on upwind designs).
Tail vane: On upwind designs, the sheet of metal, wood, or plastic on a tail boom that keeps the rotor facing into the wind.
Tilt-up tower: A tower that’s typically not climbable but can be lowered to the ground using cables, lifting gear, and a gin pole.
Tip speed ratio (TSR): The speed at the tip of the rotor blade as it moves through the air divided by the wind velocity.
Tower adapter: The hardware that connects the wind generator to its tower.
Turbulence: Randomized movement of air, frequently in the form of eddies, caused by obstacles. Turbulence decreases the amount of energy available to a turbine.
Upwind: A wind turbine with the rotor on the same side as the incoming wind (windward).
Vertical axis wind turbine (VAWT): A wind turbine that spins on a vertical shaft. The earliest recorded windmills were vertical, though horizontal designs have been dominant for more than a century. New VAWTs have enjoyed considerable interest recently, although they remain controversial.
Wind farm: A group of wind turbines, also known as a wind power plant.
Wind generator: A mechanical and electrical device that produces electricity from moving air, also called a wind turbine.
Wind rose: A graphical presentation of which directions a site’s wind comes from.
Wind shear: A sudden change in the speed or direction of wind. Many in the small wind industry use this term to refer to the rate of increase in wind speed as you move away from the ground, though that can also be described as the wind gradient.
Yaw: Rotation of the turbine around the tower, which allows the turbine to stay into the wind.
Yaw bearing: The bearing (typically a ball bearing) that allows a wind generator to turn and face the wind.
Conversions, Abbreviations, and Acronyms
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If you have a smart phone, conversions can now be done with free abbreviations tools:
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• For an iPhone: www.abbreviations.com/iphone.aspx
Relative to a reference height of 10 m (33 ft).
Adapted from Characteristics of the Wind by Walter Frost and Carl Aspliden in Wind Turbine Technology, and Windenergie: Theorie, Anwendung Messung by Jens-Peter Molly.