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.

Beaufort scale—A scale of wind forces, described by name and range of velocity, and classified from force 0 to 12, with an extension to 17. The initial (1805) Francis Beaufort wind force scale of 13 classes (0 to 12) did not reference wind speed numbers but related qualitative wind conditions to effects on the sails of a frigate, then the main ship of the Royal Navy, from “just sufficient to give steerage” to “that which no canvas sails could withstand.” Although the Beaufort scale has little use in site assessments, a system of tree flagging observations has been used to estimate prevailing wind directions and levels on the scale over 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.*

Displacement height—The height above ground level where wind speed is theoretically zero based on the effects of ground cover.

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.*

Diurnal—Having a daily cycle or pattern. It may be useful to average many daily cycles of wind speed or wind energy production to understand a typical daily pattern, by month, season, or year.

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.*

Dual-metering—Buying electricity from the utility and selling it to the utility with two different energy rates, typically retail (buying) and wholesale (selling).

Electric cost adjustment—An energy charge (dollars per kilowatt-hour) on a utility bill in addition to the standard rate in the tariff, which is associated with extra costs to purchase fuel, control emissions, construct transmission upgrades, and so on. These various costs may be itemized or rolled into one electric cost adjustment rate. Sometimes referred to as fuel cost adjustment.

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 curve—A diagram showing the annual energy production at different average wind speeds, typically assuming a Rayleigh wind distribution (with a Weibull shape factor of 2.0).

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.*

Flagging—The deformation of local vegetation toward one direction, indicating the prevailing wind direction and relative strength (more formally called Krummholtz formation). Flagging is sometimes used with the Beaufort scale to generate an initial estimate of local site conditions. (Note: flagging does not determine the wind resource, but is a confirming indicator of it. For example, sometimes flagging is the result of sunlight availability, or trimming of tree branches near electrical lines. The assessor needs to understand when flagging is relevant, or when it is a confirming indicator of another condition at the site.)

Frequency distribution—A statistical function presenting the amount of time at each wind speed level for a given data set and location, usually in percent of time or hours per year.

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.*

Geographic information system (GIS) software—GIS software is used for managing map-based information and data. It may also be used to visualize the relationships between terrain, wind data, land-use boundaries, obstacles, and potential wind turbine locations.

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.*

Gross annual energy production—The amount of annual energy (usually in kilowatt-hours) estimated for a given wind turbine at a given location, before adjusting for losses (see net annual energy production).

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.*

Interannual variability—The variation from year to year in average wind speed, distribution, and patterns.

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.

Microturbine—A very small wind turbine, usually under a 1,000 Watt rating, which is appropriate for small energy needs (e.g., for cabins, campers, sailboats, very small communication stations, or other small off-grid loads).

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 annual energy production—The amount of annual energy (usually in kilowatt hours) produced or estimated for a given wind turbine at a given location, after subtracting losses from the gross annual energy production. A variety of losses may be estimated for obstacle wind shadows, turbulence, turbine wake effects, turbine availability, high-wind hysteresis effects, electrical efficiency, blade icing, blade soiling and surface degradation, idling parasitic losses, control errors, low temperature shutdown, utility system maintenance, and other issues specific to a given turbine installation.

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.*

Orography—A branch of physical geography that deals with mountains.

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

Peak demand—The maximum electricity consumption level (in kilowatts) reached during the month or billing period, usually for a 15- or 30-minute duration. The definition of peak demand may vary by electric utility. This is a simplified definition of a complex topic.

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.*

Permitting—The process of obtaining legal permission to build a project, potentially from a number of government agencies, but primarily from the local building department (i.e., the city, county, or state). During this process, a set of project plans is submitted for review to assure that the project meets local requirements for safety, sound, aesthetics, setbacks, engineering, and completeness. The permitting agency typically inspects the project at various milestones for adherence to the plans and building safety standards.

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.

Reactive power—When the voltage and current waveforms for AC power are out of phase the resulting instantaneous power flow is modeled as real power and reactive power. The presence of reactive power increases the instantaneous current flow required to do work. The increase in current flow results in additional line losses. The utility tariff for larger customers may include a charge for reactive power compensation, measured in kilo-volt-amp-reactive.

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.*

Shadow flicker—A moving shadow that occurs when rotating turbine blades come between the viewer and the sun.

Site assessment—The act of evaluating a site to determine a favorable location for a wind turbine, which includes assessing the expected wind resource and potential turbine performance at that location.

Small wind turbine—A wind turbine that has a rating of up to 100-kilowatts, and is typically installed near the point of electric usage, such as near homes, businesses, remote villages, and other kinds of buildings.

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.

Tariff—An official schedule of rates or charges from a utility, usually with different rate schedules by customer classification (e.g., residential, commercial, industrial, farm, or other designation) and/or a service or meter rating for the customer.

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.

Topography—The surface configuration and relief features of an area, such as hills and bluffs, and the detailed mapping and description thereof.

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.

Turbulence intensity—A basic measure of turbulence that is defined by the ratio of the standard deviation of the wind speed to the mean wind speed. For wind energy applications this is typically defined as a 10-minute average wind speed and standard deviation based on 1-second samples. Turbulence intensity is important for wind energy applications because it has implications for both power performance and turbine loading. Experience indicates that it can be a significant issue for small turbines because of their tower height and location around ground clutter, which puts them in the most turbulent area of the atmospheric boundary layer. The effects of turbulence on distributed wind turbines can be seen in both power production and loading

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 shadow—A turbulent and/or low-wind-speed region downwind of (behind) an object such as a building, tower, or trees.

Wind shear—The difference in wind speed and direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across storm fronts and near the coast, and vertical shear seen typically near the surface (though also at higher levels in the atmosphere near upper-level jets and frontal zones aloft).

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.

Zoning—Most land has been delegated to various zones by a region’s local government and building department officials (at the city, county, or state level [occasionally]). The zones control types of land use, such as agricultural, residential, commercial, and industrial, and include subcategories. Each type of zoning carries its own specific permitting restrictions, such as building height and property line offsets (required separation distance).

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