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Solar Glossary

In a photovoltaic device, the material that readily absorbs photons to generate charge carriers
(free electrons or holes).

Alternating Current (AC):
A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole that can accept a free electron.

Air Mass (sometimes called air mass ratio):
Equal to the cosine of the zenith angle or that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).

Ambient Temperature:
The temperature of the surrounding area.

Amorphous Semiconductor:
A non-crystalline semiconductor material that has no longrange order.

Amorphous Silicon:
A thin-film, silicon photovoltaic cells having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.

Ampere (amp):
A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

Ampere-Hour (Ah/AH):
A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

Angle of Incidence:
The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.

The positive electrode in an electrochemical cell (battery). Also, the earth or ground in a cathodic protection system. Also, the positive terminal of a diode.

Antireflection Coating:
A thin coating of material applied to a solar cell surface that reduces the light reflection and increases light transmission.

See photovoltaic (PV) array.

Array Current:
The electrical current produced by a photovoltaic array when it is exposed to sunlight.

Array Operating Voltage:
The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load.

Balance of System:
Represents all components and costs other than the photovoltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.

Band Gap:
In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.

Band Gap Energy (Eg):
The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.

Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

Battery Available Capacity:
The total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state of charge, age, and cut-off voltage.

Battery Capacity:
The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

Battery Cycle Life:
The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.

Battery Energy Capacity:
The total energy available, expressed in watt-hours (Kilowatt hours), which can be withdrawn from a fully charged cell or battery. The energy capacity of a given cell varies with temperature, rate, age, and cut-off voltage. This term is more common to system designers than it is to the battery industry where capacity usually refers to ampere-hours.

Battery Energy Storage:
Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

Battery Life:
The period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.

BIPV (Building-Integrated Photovoltaic):
A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.

Blocking Diode:
A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.

Bypass Diode:
A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.

Cadmium Telluride (CdTe):
A polycrystalline thin-film photovoltaic material.

Cell (battery):
A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably). Also see photovoltaic (PV) cell.

Cell Barrier:
A very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell. The barrier inhibits the movement of electrons from one layer to the other, so that higher-energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell. Also called depletion zone or space charge.

Cell Junction:
The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone.

Charge Controller:
A component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status.

Chemical Vapor Deposition (CVD):
A method of depositing thin semiconductor films used to make certain types of photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

A photovoltaic module, which includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.

Conduction Band (or conduction level):
An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge.

The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.

Contact Resistance:
The resistance between metallic contacts and the semiconductor.

A unit that converts a direct current (dc) voltage to another dc voltage.

Copper Indium Diselenide (CuInSe2, or CIS):
A polycrystalline thin-film photovoltaic material (sometimes incorporating gallium (CIGS) and/or sulfur).

Crystalline Silicon:
A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.

See electric current.

Current at Maximum Power (Imp):
The current at which maximum power is available from a module.

Cutoff Voltage:
The voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery.

The discharge and subsequent charge of a battery.

Czochralski Process:
A method of growing large size, high quality semiconductor crystals by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.

DC-to-DC Converter:
Electronic circuit to convert direct current voltages (e.g.,Photovoltaic module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker.

Deep-Cycle Battery:
A battery with large plates that can withstand many discharges to a low state-of-charge.

Deep Discharge:
Discharging a battery to 20% or less of its full charge capacity.

Depth of Discharge (DOD):
The ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example, the removal of 25 ampere-hours from a fully charged 100-ampere-hours-rated cell results in a 25% depth of discharge. Under certain conditions, such as discharge rates lower than that used to rate the cell, depth of discharge can exceed 100%.

Depletion Zone:
Same as cell barrier. The term derives from the fact that this
Microscopically thin region is depleted of charge carriers (free electrons and hole).

Design Month:
The month having the combination of insolation and load that requires the maximum energy from the photovoltaic array.

Diffuse Insolation:
Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.

Diffuse Radiation:
Radiation received from the sun after reflection and scattering by the atmosphere and ground.

Diffusion Furnace:
Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material.

Diffusion Length:
The mean distance a free electron or hole moves before recombining with another hole or electron.

An electronic device that allows current to flow in one direction only. See blocking diode and bypass diode.

Direct Beam Radiation:
Radiation received by direct solar rays. Measured by a Pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.

Direct Current (DC):
A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220-volt household appliances, DC must be converted to alternating current, its opposite.

Direct Insolation:
Sunlight falling directly upon a collector. Opposite of diffuse insolation.

Discharge Rate:
The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).

Edge-Defined Film-Fed Growth (EFG):
A method for making sheets of polycrystalline silicon for photovoltaic devices in which molten silicon is drawn upward by capillary action through a mold.

Electric Circuit:
The path followed by electrons from a power source (generator or battery), through an electrical system and back to the source.

Electric Current:
The flow of electrical energy (electricity) in a conductor, measured in amperes.

Electron Volt (eV):
The amount of kinetic energy gained by an electron when accelerated through an electric potential difference of 1 Volt; equivalent to 1.603 x 10^-19; a unit of energy or work.

Epitaxial Growth:
The growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal.

Fermi Level:
Energy level at which the probability of finding an electron is one-half. In a metal, the Fermi level is very near the top of the filled levels in the partially filled valence band. In a semiconductor, the Fermi level is in the band gap.

Fill Factor:
The ratio of a photovoltaic cell's actual power to its power if both current and voltage are at their maxima. A key characteristic in evaluating cell performance.

Float-Zone Process:
In reference to solar photovoltaic cell manufacture, a method of growing a large-size, high-quality crystal whereby coils heat a polycrystalline ingot placed atop a single-crystal seed. As the coils are slowly raised the molten interface beneath the coils becomes a single crystal.

The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

Fresnel Lens:
An optical device that focuses light like a magnifying glass; concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point.

Full Sun:
The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).

Gallium Arsenide (GaAs):
A crystalline, high-efficiency compound used to make certain types of solar cells and semiconductor material.

Gigawatt (GW):
A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 MWs.

Grid-Connected System:
A solar electric or photovoltaic (PV) system in which the PV array acts like a central generating plant, supplying power to the grid.

Grid-Interactive System:
Same as grid-connected system.

Grid Lines:
Metallic contacts fused to the surface of the solar cell to provide a low resistance path for electrons to flow out to the cell interconnect wires.

Harmonic Content:
The number of frequencies in the output waveform in addition to the primary frequency (50 or 60 Hz.). Energy in these harmonic frequencies is lost and may cause excessive heating of the load.

The vacancy where an electron would normally exist in a solid; behaves like a positively charged particle.

The region between an n-layer and a p-layer in a single material, photovoltaic cell.

Hybrid System:
A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.

Hydrogenated Amorphous Silicon:
Amorphous silicon with a small amount of incorporated hydrogen. The hydrogen neutralizes dangling bonds in the amorphous silicon, allowing charge carriers to flow more freely.

Indium Oxide:
A wide band gap semiconductor that can be heavily doped with tin to make a highly conductive, transparent thin film. Often used as a front contact or one component of a heterojunction solar cell.

Infrared Radiation:
Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.

Input Voltage:
This is determined by the total power required by the alternating current loads and the voltage of any direct current loads. Generally, the larger the load, the higher the inverter input voltage. This keeps the current at levels where switches and other components are readily available.

The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour.

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

The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.

I-Type Semiconductor:
Semiconductor material that is left intrinsic, or undoped, so that the concentration of charge carriers is characteristic of the material itself rather than of added impurities.

I-V Curve:
A graphical presentation of the current versus the voltage from a photovoltaic device as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterizes cell performance.

A region of transition between semiconductor layers, such as a p/n junction, which goes from a region that has a high concentration of acceptors (p-type) to one that has a high concentration of donors (n-type).

Junction Box:
A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

Junction Diode:
A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes.

Kilowatt (kW):
A standard unit of electrical power equal to 1000 watts, or to the energy consumption at a rate of 1000 joules per second.

Kilowatt-Hour (kWh):
1,000 thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.

The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material.

Lead-Acid Battery:
A general category that includes batteries with plates made of pure lead, lead--antimony, or lead-calcium immersed in an acid electrolyte.

Light-Induced Defects:
Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

Maintenance-Free Battery:
A sealed battery to which water cannot be added to maintain electrolyte level.

Majority Carrier:
Current carriers (either free electrons or holes) that are in excess in a specific layer of a semiconductor material (electrons in the n-layer, holes in the p-layer) of a cell.

Maximum Power Point (MPP):
The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 volts.

MW (MW):
1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

1,000 kilowatt-hours or 1 million watt-hours.

Minority Carrier:
A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.

Minority Carrier Lifetime:
The average time a minority carrier exists before recombination.

Modified Sine Wave:
A waveform that has at least three states (i.e., positive, off, and negative). Has less harmonic content than a square wave.

See photovoltaic (PV) module.

Module Derate Factor:
A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.

A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.

Multi-Stage Controller:
A charging controller unit that allows different charging currents as the battery nears full state-of-charge.

A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.

Parallel Connection:
A way of joining solar cells or photovoltaic modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage.

A chemical reaction that eliminates the detrimental effect of electrically reactive atoms on a solar cell's surface.

Peak Power Current:
Amperes produced by a photovoltaic module or array operating at the voltage of the I-V curve that will produce maximum power from the module.

Peak Power Point:
Operating point of the I-V (current-voltage) curve for a solar cell or photovoltaic module where the product of the current value times the voltage value is a maximum.

Peak Watt:
A unit used to rate the performance of solar cells, modules, or arrays; the maximum nominal output of a photovoltaic device, in watts (Wp) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified.

Phosphorous (P):
A chemical element used as a dopant in making n-type semiconductor layers.

An electric current induced by radiant energy.

Photoelectric Cell:
A device for measuring light intensity; used in photometers.

Photoelectrochemical Cell:
A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use.

A particle of light that acts as an individual unit of energy.

Photovoltaic(s) (PV):
Pertaining to the direct conversion of light into electricity.

Photovoltaic (PV) Array:
An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.

Photovoltaic (PV) Cell:
The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.

Photovoltaic (PV) Conversion Efficiency:
The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

Photovoltaic (PV) Device:
A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device. Solar photovoltaic devices are made of various semiconductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multicrystalline, or amorphous forms.

Photovoltaic (PV) Effect:
The phenomenon that occurs when photons, the "particles" in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available.

Photovoltaic (PV) Generator:
The total of all PV strings of a PV power supply system, which are electrically interconnected.

Photovoltaic (PV) Module:
The smallest environmentally protected, essentially planar, assembly of solar cells and ancillary parts, such as interconnections, terminals, (and protective devices such as diodes) intended to generate direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).

Photovoltaic (PV) Panel:
Often used interchangeably with PV module (especially in onemodule systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current).

Photovoltaic (PV) System:
A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.

Physical Vapor Deposition:
A method of depositing thin semiconductor photovoltaic films. With this method, physical processes, such as thermal evaporation or bombardment of ions, are used to deposit elemental semiconductor material on a substrate.

A semiconductor photovoltaic (PV) device structure that layers an intrinsic semiconductor between a p-type semiconductor and an n-type semiconductor; this structure is most often used with amorphous silicon PV devices.

Polycrystalline Silicon:
A material used to make photovoltaic cells, which consist of many crystals, unlike single-crystal silicon.

Power Factor (PF):
The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.

Primary Battery:
A battery whose initial capacity cannot be restored by charging.

P-Type Semiconductor:
A semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity (e.g., boron in silicon).

An instrument used for measuring global solar irradiance.

An instrument used for measuring direct beam solar irradiance. Uses an aperture of 5.7° to transcribe the solar disc.

Rated Battery Capacity:
The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature. See battery capacity.

Reverse Current Protection:
Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See blocking diode.

Ribbon (Photovoltaic) Cells:
A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.

Sealed Battery:
A battery with a captive electrolyte and a resealing vent cap, also called a valve-regulated battery. Electrolyte cannot be added.

Series Connection:
A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.

Series Controller:
A charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery.

Series Resistance:
Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.

Silicon (Si):
A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It's commonly found in sand and quartz (as the oxide).

Sine Wave:
A waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle.

Sine Wave Inverter:
An inverter that produces utility-quality, sine wave power forms.

Solar Constant:
The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 Watts per square meter or 492 Btu per square foot.

Solar Energy:
Electromagnetic energy transmitted from the sun (solar radiation). The Namount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

Solar-Grade Silicon:
Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.

Solar Resource:
The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.

Solar Spectrum:
The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter).

Split-Spectrum Cell:
A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See mulitjunction device.

A process used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate.

The physical material upon which a photovoltaic cell is applied.

Thin-Film Photovoltaic Module:
A photovoltaic module constructed with sequential layers of thin-film semiconductor materials. See amorphous silicon.

Tilt Angle:
The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

Tin Oxide:
A wide band-gap semiconductor similar to indium oxide; used in heterojunction solar cells or to make a transparent conductive film, called NESA glass when deposited on glass.

Tracking Array:
A photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are: (1) one axis where the array tracks the sun east to west; and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.

Two-Axis Tracking:
A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).

Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

Volt (V):
A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

The amount of electromotive force, measured in volts, that exists between two points.

A thin sheet of semiconductor (photovoltaic material) made by cutting from a single crystal or ingot.

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