Small Wind Guidebook/Glossary of Terms

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Glossary of Terms

Airfoil—The shape of the blade cross-section, which for most modern horizontal-axis wind turbines is designed to enhance the lift and improve turbine performance.

Alternator—An electric generator for producing alternating current. See also generator.*

Ambient—Of the surrounding area or environment; completely surrounding; encompassing. Used to distinguish environmental conditions, e.g. temperature or sound, from what is added by mechanical devices.*

Ampere-hour—A unit for the quantity of electricity obtained by integrating current flow in amperes over the time in hours for its flow; used as a measure of battery capacity.

Anemometer—A device to measure the wind speed.

Authority Having Jurisdiction (AHJ)—The building authority for the area, generally a city or county building department, including its inspectors.*

Availability—A measure of the ability of a wind turbine to make power, regardless of environmental conditions. Generally defined as the time in a period when a turbine is able to make power, expressed as a percentage.*

Average wind speed—The mean wind speed over a specified period of time.

Behind-the-meter / behind-the-fence generation—An electrical generating system connected on the user’s side of a utility meter, primarily for energy usage on site instead of for sale to energy retailers. See also net metering.*

Betz limit—The maximum power coefficient (Cp) of a theoretically perfect wind turbine equal to 16/27 (59.3%) as proven by German physicist Albert Betz in 1919. This is the maximum amount of power that can be captured from the wind. In reality, this limit is never achived because of drag, electrical losses, and mechanical inefficiencies. See also Cp.*

Blades—The aerodynamic surface that catches the wind. See also wing, airfoil, rotor.

Brake—Various systems used to stop the rotor from turning.

Certification—A process by which small wind turbines (100 kW and under) can be certified by an independent certification body to meet or exceed the performance and durability requirements of the American Wind Energy Association (AWEA) Standard.*

Converter—See Inverter.

Corrosivity—A measure of oxidation and/or material degradation.*

Cp—Power coefficient; the ratio of the power extracted from the wind by a wind turbine relative to the power available in the wind. See also Betz limit.*

Cut-in wind speed—The wind speed at which a wind turbine begins to generate electricity.

Cut-out wind speed—The wind speed at which a wind turbine ceases to generate electricity.

Density—Mass per unit of volume.

Direct drive—A blade and generator configuration where the blades are connected directly to the electrical generating device so that one revolution of the rotor equates to one revolution of the electrical generating device.*

Distributed generation—Energy generation projects where electrical energy is generated primarily for on-site consumption. Term is applied for wind, solar, and non-renewable energy.*

Downwind—On the opposite side from the direction from which the wind blows.

Drag—An aerodynamic force that acts in the direction of the airstream flowing over an airfoil.*

Electric utility company—A company that engages in the generation, transmission, and distribution of electricity for sale, generally in a regulated market. Electric utilities may be investor owned, publicly owned, cooperatives, or nationalized entities.*

Energy production—Energy is power exerted over time. Energy production is hence the energy produced in a specific period of time. Electrical energy is generally measured in kilowatt-hours ( kWh). See also power.*

Environmental conditions—Of or pertaining to ambient state of the environment. See also temperature, wind, humidity, corrosivity.*

Furling—A passive protection for the turbine in which the rotor folds up or around the tail vane.

Gearbox—A compact, enclosed unit of gears or the like for the purpose of transferring force between machines or mechanisms, often with changes of torque and speed. In wind turbines, gearboxes are used to increase the low rotational speed of the turbine rotor to a higher speed required by many electrical generators.*

Generator—A machine that converts mechanical energy to electricity. The mechanical power for an electric generator is usually obtained from a rotating shaft. In a wind turbine, the mechanical power comes from the wind causing the blades on a rotor to rotate. See also blade, rotor, stator, alternator.*

Governor—A device used to limit the RPM of the rotor. Limiting RPM serves to reduce centrifugal forces acting on the wind turbine and rotor as well as limit the electrical output of the generating device. Governors can be electrical, also know as “dynamic braking,” or mechanical. Mechanical governors can be “passive,” using springs to pitch the blades out of their ideal orientation, or an offset rotor that pitches out of the wind, or “active” by electrically or hydraulically pitching blades out of their ideal orientation.*

Grid—The utility distribution system. The network that connects electricity generators to electricity users.

Grid-connected—Small wind energy systems that are connected to the electricity distribution system. These often require a power-conditioning unit that makes the turbine output electrically compatible with the utility grid. See also inverter.*

Guyline—A guyline (or guy wire) supports guyed towers, which are the least expensive way to support a wind turbine. Guyed towers can consist of lattice sections, pipe, or tubing. Because the guy radius must be one-half to three-quarters of the tower height, guyed towers require more space to accommodate them than monopole or self-standing lattice towers.*

Horizontal-axis wind turbine (HAWT)—A wind turbine with a rotor axis that lies in or close to a horizontal plane. Often called a “propeller-style” wind turbine.*

Hub—That component of a wind turbine to which the blades are affixed. See also rotor, blade.*

Hub Height—The distance from the foundation to which the tower is attached to the center of the hub of a HAWT.*

Humidity—A measure of moisture content in the air.*

Induction generator—An asynchronous AC motor designed for use as a generator. Generates electricity by being spun faster than the motor’s standard “synchronous” speed. Must be connected to an already-powered circuit to function (i.e. the grid), but does not require an inverter to produce grid-ready electricity.*

Interconnection standards—Specifies the technical and procedural process by which a customer connects an electricity-generating device to the grid. Such standards include the technical and contractual terms that system owners and utilities must abide by. State public utility commissions typically establish standards for interconnection to the distribution grid, while the Federal Energy Regulatory Commission (FERC) establishes standards for interconnection to the transmission grid. While many states have adopted interconnection standards, some states’ standards apply only to investor-owned utilities and not to municipal utilities or electric cooperatives.*

Intermittency—Stopping or ceasing for a time; alternately ceasing and beginning again. Wind and solar resources are described as intermittent because they change without regard to peoples’ needs or wants.*

International Electrotechnical Commission (IEC)—The international wind-industry standards body.*

Inverter—A device that converts direct current (DC) to alternating current (AC).

kW—Kilowatt, a measure of power for electrical current (1,000 Watts).

kWh—Kilowatt-hour, a measure of energy equal to the use of 1 kilowatt in 1 hour.

Lattice—A structure of crossed wooden or metal strips usually arranged to form a diagonal pattern of open spaces between the strips. Lattice towers, either guyed or freestanding, are often used to support small wind turbines.*

Lift—An aerodynamic force that acts at right angles to the airstream flowing over an airfoil.*

Micrositing—A resource assessment tool used to determine the exact position of one or more wind turbines on a parcel of land to optimize the power production.

Monopole—A freestanding type of tower that is essentially a tube, often tapered.*

MW—Megawatt, a measure of power (1,000,000 Watts).

Nacelle—The body of a propeller-type wind turbine, containing the gearbox, generator, blade hub, and other parts.

Nameplate capacity—The power capacity of a generating device that is typically affixed to the generating device. Nameplate capacity typically, but not necessarily, represents the maximum continuous power output of the generating device.*

Net metering / net billing—For electric customers who generate their own electricity, net metering allows for the flow of electricity both to and from the customer. When a customer’s generation exceeds the customer’s use, electricity from the customer flows back to the grid, offsetting electricity consumed by the customer at a different time during the same billing cycle. In effect, the customer uses excess generation to offset electricity that the customer otherwise would have to purchase at the utility’s full retail rate. Net metering is required by law in most U.S. states, but state policies vary widely. See also behind-the-meter.*

Noise—Generally defined as unwanted sound. Sound power is measured in decibels, dB. Building and planning authorities often regulate sound power levels from facilities. See also sound, electrical noise.*

O & M costs—Operation and maintenance costs.

Obstruction—A general term for any significant object that would disturb wind flow passing through a turbine rotor. Most common examples are homes, buildings, trees, silos, and fences. Topographical features such as hills or cliffs that might also affect wind flow and are not called obstructions.*

Off-grid—Energy-generating systems that are not interconnected directly into an electrical grid. Energy produced in these systems is often used for battery charging.*

Overall height—The total height of a wind turbine from its base at grade to its uppermost extent. See also total height.*

Peak power—The maximum instantaneous power than can be produced by a power-generating system or consumed by a load. Peak power may be significantly higher than average power.*

Power coefficient—The ratio of the power extracted by a wind turbine to the power available in the wind stream.

Power curve—A chart showing a wind turbine’s power output across a range of wind speeds.

Prevailing wind—The most common direction or directions that the wind comes from at a site. Prevailing wind usually refers to the amount of time the wind blows from that particular direction but may also refer to the direction the wind with the greatest power density comes from.*

PUC—Public Utility Commission, a state agency that regulates utilities. In some areas known as Public Service Commission (PSC).

PURPA—Public Utility Regulatory Policies Act (1978), 16 U.S.C. § 2601.18 CFR §292 that refers to small generator utility-connection rules.

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

Rated wind speed—The lowest wind speed at which the rated output power of a wind turbine is produced.

Rotor—The rotating part of a wind turbine, including either the blades and blade assembly or the rotating portion of a generator.

Rotor diameter—The diameter of the circle swept by the rotor.

Rotor speed—The revolutions per minute of the wind turbine rotor.

Setback—In zoning parlance, the distance required between a structure and another structure, property line, utility easement or other demarkation.*

Sound—Pressure waves occurring at a frequency in the audible range of human hearing that are registered as sensory input by the ear. See also noise.*

Start-up wind speed—The wind speed at which a wind turbine rotor will begin to spin. See also Cut-in wind speed.

Stator—The stationary part of a rotary machine or device, especially a generator or motor. Most especially related to the collection of stationary parts in its magnetic circuits. The stator and rotor interact to generate electricity in a generator and to turn the driveshaft in a motor.*

Swept area—The area swept by the turbine rotor, A = π R2, where R is the radius of the rotor. See also rotor diameter.

Temperature—A measure of thermal energy.*

Tip-speed ratio—The speed at the tip of the rotor blade as it moves through the air divided by the wind velocity. This is typically a design requirement for the turbine.

Total height—The height of the wind system from the top of the foundation to which the tower is attached to the tip of a blade extended upwards. See also overall height.*

Tower—A structure designed to support a wind turbine at a substantial height above grade in a wind flow. Typical types include monopole, guyed lattice, and self-supporting lattice designs.*

Turbulence—The changes in wind speed and direction, frequently caused by obstacles.

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

Upwind rotor—A horizontal-axis wind turbine whose propeller is located upwind of the tower; a wind turbine with an architecture such that the wind flow passes through the propeller prior to flowing past the tower.*

Vertical-axis wind turbine (VAWT)—A wind turbine whose rotor spins about a vertical or near-vertical axis.*

Wet stamp—Refers to a specific engineering review of a specific plan or set of drawings by an in-state licensed engineer who subsequently approves the plan or drawings with his/her stamp. A wet stamp implies an original stamped document, not a copy.*

Wind—The movement of an air mass.*

Wind farm—A group of wind turbines, often owned and maintained by one company. Also known as a wind power plant.

Wind rose—A visual means of representing the frequency with which the wind blows from different directions.*

Wind turbine—A mechanical device that converts kinetic energy in the wind into electrical energy.*

Yaw—The movement of the tower top turbine that allows the turbine to stay into the wind.

* This definition was contributed by the Distributed Wind Energy Association (DWEA).