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A
Absorber — In a photovoltaic device, the material that
readily absorbs photons to generate charge carriers
(free electrons or holes).
AC — see alternating current.
Activated Shelf Life — The period of time, at a
specified temperature, that a charged battery can be
stored before its capacity falls to an unusable level.
Activation Voltage(s) — The voltage(s) at which a charge
controller will take action to protect the batteries.
Adjustable Set Point — A feature allowing the user to
adjust the voltage levels at which a charge controller
will become active.
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.
Acceptor — A dopant material, such as boron, which has
fewer outer shell electrons than required in an
otherwise balanced crystal structure, providing a hole,
which can accept a free electron.
AIC — See amperage interrupt capability.
Air mass (sometimes called air mass ratio) — Equal to
the cosine of the zenith angle-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 long-range order.
Amorphous Silicon — A thin-film, silicon photovoltaic
cell having no crystalline structure. Manufactured by
depositing layers of doped silicon on a substrate. See
also single-crystal silicon an polycrystalline silicon.
Amperage Interrupt Capability (AIC) — direct current
fuses should be rated with a sufficient AIC to interrupt
the highest possible current.
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.
Ampere Hour Meter — An instrument that monitors current
with time. The indication is the product of current (in
amperes) and time (in hours).
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°.
Annual Solar Savings — The annual solar savings of a
solar building is the energy savings attributable to a
solar feature relative to the energy requirements of a
non-solar building.
Anode — 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 a material
applied to a solar cell surface that reduces the light
reflection and increases light transmission.
Array — 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.
Autonomous System — See stand-alone system.
Availability — The quality or condition of a
photovoltaic system being available to provide power to
a load. Usually measured in hours per year. One minus
availability equals downtime.
Azimuth Angle — The angle between true south and the
point on the horizon directly below the sun.
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B
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.
Barrier Energy — The energy given up by an electron in
penetrating the cell barrier; a measure of the
electrostatic potential of the barrier.
Base Load — The average amount of electric power that a
utility must supply in any period.
Battery — 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 Cell — The simplest operating unit in a storage
battery. It consists of one or more positive electrodes
or plates, an electrolyte that permits ionic conduction,
one or more negative electrodes or plates, separators
between plates of opposite polarity, and a container for
all the above.
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 Photovoltaics) — 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.
Boron (B) — The chemical element commonly used as the
dopant in photovoltaic device or cell material.
Boule — A sausage-shaped, synthetic single-crystal mass
grown in a special furnace, pulled and turned at a rate
necessary to maintain the single-crystal structure
during growth.
Btu (British Thermal Unit) — The amount of heat required
to raise the temperature of one pound of water one
degree Fahrenheit; equal to 252 calories.
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.
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C
Cadmium (Cd) — A chemical element used in making certain
types of solar cells and batteries.
Cadmium Telluride (CdTe) — A polycrystalline thin-film
photovoltaic material.
Capacity (C) — See battery capacity.
Capacity Factor — The ratio of the average load on (or
power output of) an electricity generating unit or
system to the capacity rating of the unit or system over
a specified period of time.
Captive Electrolyte Battery — A battery having an
immobilized electrolyte (gelled or absorbed in a
material)
Cathode — The negative pole or electrode of an
electrolytic cell, vacuum tube, etc., where electrons
enter (current leaves) the system; the opposite of an
anode.
Cathodic Protection — A method of preventing oxidation
of the exposed metal in structures by imposing a small
electrical voltage between the structure and the ground.
Cd — see cadmium.
CdTe — see cadmium telluride.
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 — The process of adding electrical energy to a
battery.
Charge Carrier — A free and mobile conduction electron
or hole in a semiconductor.
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.
Charge Factor — A number representing the time in hours
during which a battery can be charged at a constant
current without damage to the battery. Usually expressed
in relation to the total battery capacity, i.e., C/5
indicates a charge factor of 5 hours. Related to charge
rate.
Charge Rate — The current applied to a cell or battery
to restore its available capacity. This rate is commonly
normalized by a charge control device with respect to
the rated capacity of the cell or battery.
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.
Cleavage of Lateral Epitaxial Films for Transfer (CLEFT)
— A process for making inexpensive Gallium Arsenide (GaAs)
photovoltaic cells in which a thin film of GaAs is grown
atop a thick, single-crystal GaAs (or other suitable
material) substrate and then is cleaved from the
substrate and incorporated into a cell, allowing the
substrate to be reused to grow more thin-film GaAs.
Cloud Enhancement — The increase in solar intensity
caused by reflected irradiance from nearby clouds.
Combined Collector — A photovoltaic device or module
that provides useful heat energy in addition to
electricity.
Concentrator — 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.
Conductor — 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.
Conversion Efficiency — See photovoltaic (conversion)
efficiency.
Converter — 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.
Current — 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.
Cycle — The discharge and subsequent charge of a
battery.
Czochralski Process — A method of growing large size,
high quality semiconductor crystal by slowly lifting a
seed crystal from a molten bath of the material under
careful cooling conditions.
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D
Dangling Bonds — A chemical bond associated with an atom
on the surface layer of a crystal. The bond does not
join with another atom of the crystal, but extends in
the direction of exterior of the surface.
Days of Storage — The number of consecutive days the
stand-alone system will meet a defined load without
solar energy input. This term is related to system
availability.
DC — See direct current.
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%.
Dendrite — A slender threadlike spike of pure
crystalline material, such as silicon.
Dendritic Web Technique — A method for making sheets of
polycrystalline silicon in which silicon dendrites are
slowly withdrawn from a melt of silicon whereupon a web
of silicon forms between the dendrites and solidifies as
it rises from the melt and cools.
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.
Diode — 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 — The withdrawal of electrical energy from a
battery.
Discharge Factor — A number equivalent to the time in
hours during which a battery is discharged at constant
current usually expressed as a percentage of the total
battery capacity, i.e., C/5 indicates a discharge factor
of 5 hours. Related to discharge rate.
Discharge Rate — The rate, usually expressed in amperes
or time, at which electrical current is taken from the
battery.
Disconnect — Switch gear used to connect or disconnect
components in a photovoltaic system.
Distributed Energy Resources (DER) — A variety of small,
modular power-generating technologies that can be
combined with energy management and storage systems and
used to improve the operation of the electricity
delivery system, whether or not those technologies are
connected to an electricity grid.
Distributed Generation — A popular term for localized or
on-site power generation.
Distributed Power — Generic term for any power supply
located near the point where the power is used. Opposite
of central power. See stand-alone systems.
Distributed Systems — Systems that are installed at or
near the location where the electricity is used, as
opposed to central systems that supply electricity to
grids. A residential photovoltaic system is a
distributed system.
Donor — In a photovoltaic device, an n-type dopant, such
as phosphorus, that puts an additional electron into an
energy level very near the conduction band; this
electron is easily exited into the conduction band where
it increases the electrical conductivity over than of an
undoped semiconductor.
Donor Level — The level that donates conduction
electrons to the system.
Dopant — 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).
Doping — The addition of dopants to a semiconductor.
Downtime — Time when the photovoltaic system cannot
provide power for the load. Usually expressed in hours
per year or that percentage.
Dry Cell — A cell (battery) with a captive electrolyte.
A primary battery that cannot be recharged.
Duty Cycle — The ratio of active time to total time.
Used to describe the operating regime of appliances or
loads in photovoltaic systems.
Duty Rating — The amount of time an inverter (power
conditioning unit) can produce at full rated power.
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E
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 returning to the source.
Electric Current — The flow of electrical energy
(electricity) in a conductor, measured in amperes.
Electrical grid — An integrated system of electricity
distribution, usually covering a large area.
Electricity — Energy resulting from the flow of charge
particles, such as electrons or ions.
Electrochemical Cell — A device containing two
conducting electrodes, one positive and the other
negative, made of dissimilar materials (usually metals)
that are immersed in a chemical solution (electrolyte)
that transmits positive ions from the negative to the
positive electrode and thus forms an electrical charge.
One or more cells constitute a battery.
Electrode — A conductor that is brought in conducting
contact with a ground.
Electrodeposition — Electrolytic process in which a
metal is deposited at the cathode from a solution of its
ions.
Electrolyte — A nonmetallic (liquid or solid) conductor
that carries current by the movement of ions (instead of
electrons) with the liberation of matter at the
electrodes of an electrochemical cell.
Electron — An elementary particle of an atom with a
negative electrical charge and a mass of 1/1837 of a
proton; electrons surround the positively charged
nucleus of an atom and determine the chemical properties
of an atom. The movement of electrons in an electrical
conductor constitutes an electric current.
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.
Energy — The capability of doing work; different forms
of energy can be converted to other forms, but the total
amount of energy remains the same.
Energy Audit — A survey that shows how much energy used
in a home, which helps find ways to use less energy.
Energy Contribution Potential — Recombination occurring
in the emitter region of a photovoltaic cell.
Energy Density — The ratio of available energy per
pound; usually used to compare storage batteries.
Energy Levels — The energy represented by an electron in
the band model of a substance.
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.
Equalization — The process of restoring all cells in a
battery to an equal state-of-charge. Some battery types
may require a complete discharge as a part of the
equalization process.
Equalization Charge — The process of mixing the
electrolyte in batteries by periodically overcharging
the batteries for a short time.
Equalizing Charge — A continuation of normal battery
charging, at a voltage level slightly higher than the
normal end-of-charge voltage, in order to provide cell
equalization within a battery.
Equinox — The two times of the year when the sun crosses
the equator and night and day are of equal length;
usually occurs on March 21st (spring equinox) and
September 23 (fall equinox).
Extrinsic Semiconductor — The product of doping a pure
semiconductor.
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F
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 were at
their maxima. A key characteristic in evaluating cell
performance.
Fixed Tilt Array — A photovoltaic array set in at a
fixed angle with respect to horizontal.
Flat-Plate Array — A photovoltaic (PV) array that
consists of non-concentrating PV modules.
Flat-Plate Module — An arrangement of photovoltaic cells
or material mounted on a rigid flat surface with the
cells exposed freely to incoming sunlight.
Flat-Plate Photovoltaics (PV) — A PV array or module
that consists of nonconcentrating elements. Flat-plate
arrays and modules use direct and diffuse sunlight, but
if the array is fixed in position, some portion of the
direct sunlight is lost because of oblique sun-angles in
relation to the array.
Float Charge — The voltage required to counteract the
self-discharge of the battery at a certain temperature.
Float Life — The number of years that a battery can keep
its stated capacity when it is kept at float charge.
Float Service — A battery operation in which the battery
is normally connected to an external current source; for
instance, a battery charger which supplies the battery
load< under normal conditions, while also providing
enough energy input to the battery to make up for its
internal quiescent losses, thus keeping the battery
always up to full power and ready for service.
Float-Zone Process — 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 single crystal.
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.
Frequency — The number of repetitions per unit time of a
complete waveform, expressed in Hertz (Hz).
Frequency Regulation — This indicates the variability in
the output frequency. Some loads will switch off or not
operate properly if frequency variations exceed 1%.
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).
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G
Ga — See gallium.
GaAs — See gallium arsenide.
Gallium (Ga) — A chemical element, metallic in nature,
used in making certain kinds of solar cells and
semiconductor devices.
Gallium Arsenide (GaAs) — A crystalline, high-efficiency
compound used to make certain types of solar cells and
semiconductor material.
Gassing — The evolution of gas from one or more of the
electrodes in the cells of a battery. Gassing commonly
results from local action self-discharge or from the
electrolysis of water in the electrolyte during
charging.
Gassing Current — The portion of charge current that
goes into electrolytical production of hydrogen and
oxygen from the electrolytic liquid. This current
increases with increasing voltage and temperature.
Gel-Type Battery — Lead-acid battery in which the
electrolyte is composed of a silica gel matrix.
Gigawatt (GW) — A unit of power equal to 1 billion
Watts; 1 million kilowatts, or 1,000 megawatts.
Grid — See electrical grid.
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.
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H
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.
Heterojunction — A region of electrical contact between
two different materials.
High Voltage Disconnect — The voltage at which a charge
controller will disconnect the photovoltaic array from
the batteries to prevent overcharging.
High Voltage Disconnect Hysteresis — The voltage
difference between the high voltag disconnect set point
and the voltage at which the full photovoltaic array
current will be reapplied.
Hole — The vacancy where an electron would normally
exist in a solid; behaves like a positively charged
particle.
Homojunction — 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.
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I
Incident Light — Light that shines onto the face of a
solar cell or module.
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.
Insolation — 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. See diffuse insolation and direct
insolation.
Interconnect — A conductor within a module or other
means of connection that provides an electrical
interconnection between the solar cells.
Intrinsic Layer — A layer of semiconductor material,
used in a photovoltaic device, whose properties are
essentially those of the pure, undoped, material.
Intrinsic Semiconductor — An undoped semiconductor.
Inverter — A device that converts direct current
electricity to alternating current either for
stand-alone systems or to supply power to an electricity
grid.
Ion — An electrically charged atom or group of atoms
that has lost or gained electrons; a loss makes the
resulting particle positively charged; a gain makes the
particle negatively charged.
Irradiance — The direct, diffuse, and reflected solar
radiation that strikes a surface. Usually expressed in
kilowatts per square meter. Irradiance multiplied by
time equals insolation.
ISPRA Guidelines — Guidelines for the assessment of
photovoltaic power plants, published by the Joint
Research Centre of the Commission of the European
Communities, Ispra, Italy.
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.
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J
Joule — A metric unit of energy or work; 1 joule per
second equals 1 watt or 0.737 foot-pounds; 1 Btu equals
1,055 joules.
Junction — 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.
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K
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.
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L
Langley (L) — Unit of solar irradiance. One gram calorie
per square centimeter. 1 L = 85.93 kwh/m2.
Lattice — 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.
Life — The period during which a system is capable of
operating above a specified performance level.
Life-Cycle Cost — The estimated cost of owning and
operating a photovoltaic system for the period of its
useful life.
Light-Induced Defects — Defects, such as dangling bonds,
induced in an amorphous silicon semiconductor upon
initial exposure to light.
Light Trapping — The trapping of light inside a
semiconductor material by refracting and reflecting the
light at critical angles; trapped light will travel
further in the material, greatly increasing the
probability of absorption and hence of producing charge
carriers.
Line-Commutated Inverter — An inverter that is tied to a
power grid or line. The commutation of power (conversion
from direct current to alternating current) is
controlled by the power line, so that, if there is a
failure in the power grid, the photovoltaic system
cannot feed power into the line.
Liquid Electrolyte Battery — A battery containing a
liquid solution of acid and water. Distilled water may
be added to these batteries to replenish the electrolyte
as necessary. Also called a flooded battery because the
plates are covered with the electrolyte.
Load — The demand on an energy producing system; the
energy consumption or requirement of a piece or group of
equipment. Usually expressed in terms of amperes or
watts in reference to electricity.
Load Circuit — The wire, switches, fuses, etc. that
connect the load to the power source.
Load Current (A) — The current required by the
electrical device.
Load Resistance — The resistance presented by the load.
See resistance.
Low Voltage Cutoff (LVC) — The voltage level at which a
charge controller will disconnect the load from the
battery.
Low Voltage Disconnect — The voltage at which a charge
controller will disconnect the load from the batteries
to prevent over-discharging.
Low Voltage Disconnect Hysteresis — The voltage
difference between the low voltage disconnect set point
and the voltage at which the load will be reconnected.
Low Voltage Warning — A warning buzzer or light that
indicates the low battery voltage set point has been
reached.
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M
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.
Maximum Power Point Tracker (MPPT) — Means of a power
conditioning unit that automatically operates the
photovoltaic generator at its maximum power point under
all conditions.
Maximum Power Tracking — Operating a photovoltaic array
at the peak power point of the array's I-V curve where
maximum power is obtained. Also called peak power
tracking.
Megawatt (MW) — 1,000 kilowatts, or 1 million watts;
standard measure of electric power plant generating
capacity.
Megawatt-Hour — 1,000 kilowatt-hours or 1 million
watt-hours.
Microgroove — A small groove scribed into the surface of
a solar cell, which is filled with metal for contacts.
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.
Modularity — The use of multiple inverters connected in
parallel to service different loads.
Module — 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.
Monolithic — Fabricated as a single structure.
Movistor — Metal Oxide Varistor. Used to protect
electronic circuits from surge currents such as those
produced by lightning.
Multicrystalline — A semiconductor (photovoltaic)
material composed of variously oriented, small,
individual crystals. Sometimes referred to as
polycrystalline or semicrystalline.
Multijunction Device — A high-efficiency photovoltaic
device containing two or more cell junctions, each of
which is optimized for a particular part of the solar
spectrum.
Multi-Stage Controller — A charging controller unit that
allows different charging currents as the battery nears
full state_of_charge.
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N
National Electrical Code (NEC) — Contains guidelines for
all types of electrical installations. The 1984 and
later editions of the NEC contain Article 690, "Solar
Photovoltaic Systems" which should be followed when
installing a PV system.
National Electrical Manufacturers Association (NEMA) —
This organization sets standards for some non-electronic
products like junction boxes.
NEC — See National Electrical Code.
NEMA — See National Electrical Manufacturers
Association.
Nickel Cadmium Battery — A battery containing nickel and
cadmium plates and an alkaline electrolyte.
Nominal Voltage — A reference voltage used to describe
batteries, modules, or systems (i.e., a 12-volt or
24-volt battery, module, or system).
Normal Operating Cell Temperature (NOCT) — The estimated
temperature of a photovoltaic module when operating
under 800 w/m2 irradiance, 20�C ambient temperature and
wind speed of 1 meter per second. NOCT is used to
estimate the nominal operating temperature of a module
in its working environment.
N-Type — Negative semiconductor material in which there
are more electrons than holes; current is carried
through it by the flow of electrons.
N-Type Semiconductor — A semiconductor produced by
doping an intrinsic semiconductor with an electron-donor
impurity (e.g., phosphorus in silicon).
N-Type Silicon — Silicon material that has been doped
with a material that has more electrons in its atomic
structure than does silicon.
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O
Ohm — 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.
One-Axis Tracking — A system capable of rotating about
one axis.
Open-Circuit Voltage (Voc) — The maximum possible
voltage across a photovoltaic cell; the voltage across
the cell in sunlight when no current is flowing.
Operating Point — The current and voltage that a
photovoltaic module or array produces when connected to
a load. The operating point is dependent on the load or
the batteries connected to the output terminals of the
array.
Orientation — Placement with respect to the cardinal
directions, N, S, E, W; azimuth is the measure of
orientation from north.
Outgas — See gassing.
Overcharge — Forcing current into a fully charged
battery. The battery will be damaged if overcharged for
a long period.
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P
Packing Factor — The ratio of array area to actual land
area or building envelope area for a system; or, the
ratio of total solar cell area to the total module area,
for a module.
Panel — See photovoltaic (PV) panel.
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.
Passivation — A chemical reaction that eliminates the
detrimental effect of electrically reactive atoms on a
solar cell's surface.
Peak Demand/Load — The maximum energy demand or load in
a specified time period.
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 Power Tracking — see maximum power tracking.
Peak Sun Hours — The equivalent number of hours per day
when solar irradiance averages 1,000 w/m2. For example,
six peak sun hours means that the energy received during
total daylight hours equals the energy that would have
been received had the irradiance for six hours been
1,000 w/m2.
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.
Photocurrent — An electric current induced by radiant
energy.
Photoelectric Cell — A device for measuring light
intensity that works by converting light falling on, or
reach it, to electricity, and then measuring the
current; 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.
Photon — 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 one-module 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.
Photovoltaic-Thermal (PV/T) System — A photovoltaic
system that, in addition to converting sunlight into
electricity, collects the residual heat energy and
delivers both heat and electricity in usable form. Also
called a total energy system.
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.
P-I-N — 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.
Plates — A metal plate, usually lead or lead compound,
immersed in the electrolyte in a battery.
P/N — A semiconductor photovoltaic device structure in
which the junction is formed between a p-type layer and
an n-type layer.
Pocket Plate — A plate for a battery in which active
materials are held in a perforated metal pocket.
Point-Contact Cell — A high efficiency silicon
photovoltaic concentrator cell that employs light
trapping techniques and point-diffused contacts on the
rear surface for current collection.
Polycrystalline — See Multicrystalline.
Polycrystalline Silicon — A material used to make
photovoltaic cells, which consist of many crystals
unlike single-crystal silicon.
Power Conditioning — The process of modifying the
characteristics of electrical power (for e.g., inverting
direct current to alternating current).
Power Conditioning Equipment — Electrical equipment, or
power electronics, used to convert power from a
photovoltaic array into a form suitable for subsequent
use. A collective term for inverter, converter, battery
charge regulator, and blocking diode.
Power Conversion Efficiency — The ratio of output power
to input power of the inverter.
Power Density — The ratio of the power available from a
battery to its mass (W/kg) or volume (W/l).
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.
Projected Area — The net south-facing glazing area
projected on a vertical plane.
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).
Pulse-Width-Modulated (PWM) Wave Inverter — A type of
power inverter that produce a high quality (nearly
sinusoidal) voltage, at minimum current harmonics.
PV — See photovoltaic(s).
Pyranometer — An instrument used for measuring global
solar irradiance.
Pyrheliometer — An instrument used for measuring direct
beam solar irradiance. Uses an aperture of 5.7° to
transcribe the solar disc.
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Q
Quad — One quadrillion Btu (1,000,000,000,000,000 Btu).
Qualification Test — A procedure applied to a selected
set of photovoltaic modules involving the application of
defined electrical, mechanical, or thermal stress in a
prescribed manner and amount. Test results are subject
to a list of defined requirements.
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R
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.
Rated Module Current (A) — The current output of a
photovoltaic module measured at standard test conditions
of 1,000 w/m2 and 25�C cell temperature.
Rated Power — Rated power of the inverter. However, some
units can not produce rated power continuously. See duty
rating.
Reactive Power — The sine of the phase angle between the
current and voltage waveforms in an alternating current
system. See power factor.
Recombination — The action of a free electron falling
back into a hole. Recombination processes are either
radiative, where the energy of recombination results in
the emission of a photon, or nonradiative, where the
energy of recombination is given to a second electron
which then relaxes back to its original energy by
emitting phonons. Recombination can take place in the
bulk of the semiconductor, at the surfaces, in the
junction region, at defects, or between interfaces.
Rectifier — A device that converts alternating current
to direct current. See inverter.
Regulator — Prevents overcharging of batteries by
controlling charge cycle-usually adjustable to conform
to specific battery needs.
Remote Systems — See stand-alone systems.
Reserve Capacity — The amount of generating capacity a
central power system must maintain to meet peak loads.
Resistance (R) — The property of a conductor, which
opposes the flow of an electric current resulting in the
generation of heat in the conducting material. The
measure of the resistance of a given conductor is the
electromotive force needed for a unit current flow. The
unit of resistance is ohms.
Resistive Voltage Drop — The voltage developed across a
cell by the current flow through the resistance of the
cell.
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.
RMS — See root mean square.
Root Mean Square (RMS) — The square root of the average
square of the instantaneous values of an ac output. For
a sine wave the RMS value is 0.707 times the peak value.
The equivalent value of alternating current, I, that
will produce the same heating in a conductor with
resistance, R, as a dc current of value I.
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S
Sacrificial Anode — A piece of metal buried near a
structure that is to be protected from corrosion. The
metal of the sacrificial anode is intended to corrode
and reduce the corrosion of the protected structure.
Satellite Power System (SPS) — Concept for providing
large amounts of electricity for use on the Earth from
one or more satellites in geosynchronous Earth orbit. A
very large array of solar cells on each satellite would
provide electricity, which would be converted to
microwave energy and beamed to a receiving antenna on
the ground. There, it would be reconverted into
electricity and distributed the same as any other
centrally generated power, through a grid.
Schottky Barrier — A cell barrier established as the
interface between a semiconductor, such as silicon, and
a sheet of metal.
Scribing — The cutting of a grid pattern of grooves in a
semiconductor material, generally for the purpose of
making interconnections.
Sealed Battery — A battery with a captive electrolyte
and a resealing vent cap, also called a valve-regulated
battery. Electrolyte cannot be added.
Seasonal Depth of Discharge — An adjustment factor used
in some system sizing procedures which "allows" the
battery to be gradually discharged over a 30-90 day
period of poor solar insolation. This factor results in
a slightly smaller photovoltaic array.
Secondary Battery — A battery that can be recharged.
Self-Discharge — The rate at which a battery, without a
load, will lose its charge.
Semiconductor — Any material that has a limited capacity
for conducting an electric current. Certain
semiconductors, including silicon, gallium arsenide,
copper indium diselenide, and cadmium telluride, are
uniquely suited to the photovoltaic conversion process.
Semicrystalline — See Multicrystalline.
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 Regulator — Type of battery charge regulator
where the charging current is controlled by a switch
connected in series with the photovoltaic module or
array.
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.
Shallow-Cycle Battery — A battery with small plates that
cannot withstand many discharges to a low
state-of-charge.
Shelf Life of Batteries — The length of time, under
specified conditions, that a battery can be stored so
that it keeps its guaranteed capacity.
Short-Circuit Current (Isc) — The current flowing freely
through an external circuit that has no load or
resistance; the maximum current possible.
Shunt Controller — A charge controller that redirects or
shunts the charging current away from the battery. The
controller requires a large heat sink to dissipate the
current from the short-circuited photovoltaic array.
Most shunt controllers are for smaller systems producing
30 amperes or less.
Shunt Regulator — Type of a battery charge regulator
where the charging current is controlled by a switch
connected in parallel with the photovoltaic (PV)
generator. Shorting the PV generator prevents
overcharging of the battery.
Siemens Process — A commercial method of making purified
silicon.
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.
Single-Crystal Material — A material that is composed of
a single crystal or a few large crystals.
Single-Crystal Silicon — Material with a single
crystalline formation. Many photovoltaic cells are made
from single-crystal silicon.
Single-Stage Controller — A charge controller that
redirects all charging current as the battery nears full
state-of-charge.
Solar Cell — see photovoltaic (PV) cell.
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 Cooling — The use of solar thermal energy or solar
electricity to power a cooling appliance. Photovoltaic
systems can power evaporative coolers ("swamp" coolers),
heat-pumps, and air conditioners.
Solar Energy — Electromagnetic energy transmitted from
the sun (solar radiation). The amount 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 Insolation — See insolation.
Solar Irradiance — See irradiance.
Solar Noon — The time of the day, at a specific
location, when the sun reaches its highest, apparent
point in the sky; equal to true or due, geographic
south.
Solar Panel — See photovoltaic (PV) panel.
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). About 99 percent of solar
radiation is contained in a wavelength region from 300
nm (ultraviolet) to 3,000 nm (near-infrared). The
combined radiation in the wavelength region from 280 nm
to 4,000 nm is called the broadband, or total, solar
radiation.
Solar Thermal Electric Systems — Solar energy conversion
technologies that convert solar energy to electricity,
by heating a working fluid to power a turbine that
drives a generator. Examples of these systems include
central receiver systems, parabolic dish, and solar
trough.
Space Charge — See cell barrier.
Specific Gravity — The ratio of the weight of the
solution to the weight of an equal volume of water at a
specified temperature. Used as an indicator of battery
state-of-charge.
Spinning Reserve — Electric power plant or utility
capacity on-line and running at low power in excess of
actual load.
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.
Sputtering — 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.
Square Wave — A waveform that has only two states,
(i.e., positive or negative). A square wave contains a
large number of harmonics.
Square Wave Inverter — A type of inverter that produces
square wave output. It consists of a direct current
source, four switches, and the load. The switches are
power semiconductors that can carry a large current and
withstand a high voltage rating. The switches are turned
on and off at a correct sequence, at a certain
frequency.
Staebler-Wronski Effect — The tendency of the sunlight
to electricity conversion efficiency of amorphous
silicon photovoltaic devices to degrade (drop) upon
initial exposure to light.
Stand-Alone System — An autonomous or hybrid
photovoltaic system not connected to a grid. May or may
not have storage, but most stand-alone systems require
batteries or some other form of storage.
Stand-Off Mounting — Technique for mounting a
photovoltaic array on a sloped roof, which involves
mounting the modules a short distance above the pitched
roof and tilting them to the optimum angle.
Standard Reporting Conditions (SRC) — A fixed set of
conditions (including meteorological) to which the
electrical performance data of a photovoltaic module are
translated from the set of actual test conditions.
Standard Test Conditions (STC) — Conditions under which
a module is typically tested in a laboratory.
Standby Current — This is the amount of current (power)
used by the inverter when no load is active (lost
power). The efficiency of the inverter is lowest when
the load demand is low.
Starved Electrolyte Cell — A battery containing little
or no free fluid electrolyte.
State-of-Charge (SOC) — The available capacity remaining
in the battery, expressed as a percentage of the rated
capacity.
Storage Battery — A device capable of transforming
energy from electric to chemical form and vice versa.
The reactions are almost completely reversible. During
discharge, chemical energy is converted to electric
energy and is consumed in an external circuit or
apparatus.
Stratification — A condition that occurs when the acid
concentration varies from top to bottom in the battery
electrolyte. Periodic, controlled charging at voltages
that produce gassing will mix the electrolyte. See
equalization.
String — A number of photovoltaic modules or panels
interconnected electrically in series to produce the
operating voltage required by the load.
Substrate — The physical material upon which a
photovoltaic cell is applied.
Subsystem — Any one of several components in a
photovoltaic system (i.e., array, controller, batteries,
inverter, load).
Sulfation — A condition that afflicts unused and
discharged batteries; large crystals of lead sulfate
grow on the plate, instead of the usual tiny crystals,
making the battery extremely difficult to recharge.
Superconducting Magnetic Energy Storage (SMES) — SMES
technology uses the superconducting characteristics of
low-temperature materials to produce intense magnetic
fields to store energy. It has been proposed as a
storage option to support large-scale use of
photovoltaics as a means to smooth out fluctuations in
power generation.
Superconductivity — The abrupt and large increase in
electrical conductivity exhibited by some metals as the
temperature approaches absolute zero.
Superstrate — The covering on the sunny side of a
photovoltaic (PV) module, providing protection for the
PV materials from impact and environmental degradation
while allowing maximum transmission of the appropriate
wavelengths of the solar spectrum.
Surge Capacity — The maximum power, usually 3-5 times
the rated power, that can be provided over a short time.
System Availability — The percentage of time (usually
expressed in hours per year) when a photovoltaic system
will be able to fully meet the load demand.
System Operating Voltage — The photovoltaic array output
voltage under load. The system operating voltage is
dependent on the load or batteries connected to the
output terminals.
System Storage — See battery capacity.
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Tare Loss — Loss caused by a charge controller. One
minus tare loss, expressed as a percentage, is equal to
the controller efficiency.
Temperature Compensation — A circuit that adjusts the
charge controller activation points depending on battery
temperature. This feature is recommended if the battery
temperature is expected to vary more than ±5°C from
ambient temperature.
Temperature Factors — It is common for three elements in
photovoltaic system sizing to have distinct temperature
corrections: a factor used to decrease battery capacity
at cold temperatures; a factor used to decrease PV
module voltage at high temperatures; and a factor used
to decrease the current carrying capability of wire at
high temperatures.
Thermophotovoltaic Cell (TPV) — A device where sunlight
concentrated onto a absorber heats it to a high
temperature, and the thermal radiation emitted by the
absorber is used as the energy source for a photovoltaic
cell that is designed to maximize conversion efficiency
at the wavelength of the thermal radiation.
Thick-Crystalline Materials — Semiconductor material,
typically measuring from 200-400 microns thick, that is
cut from ingots or ribbons.
Thin Film — A layer of semiconductor material, such as
copper indium diselenide or gallium arsenide, a few
microns or less in thickness, used to make photovoltaic
cells.
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.
Total AC Load Demand — The sum of the alternating
current loads. This value is important when selecting an
inverter.
Total Harmonic Distortion — The measure of closeness in
shape between a waveform and it's fundamental component.
Total Internal Reflection — The trapping of light by
refraction and reflection at critical angles inside a
semiconductor device so that it cannot escape the device
and must be eventually absorbed by the semiconductor.
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.
Transformer — An electromagnetic device that changes the
voltage of alternating current electricity.
Tray Cable (TC) - may be used for interconnecting
balance-of-systems.
Trickle Charge — A charge at a low rate, balancing
through self-discharge losses, to maintain a cell or
battery in a fully charged condition.
Two-Axis Tracking — A photovoltaic array tracking system
capable of rotating independently about two axes (e.g.,
vertical and horizontal).
Tunneling — Quantum mechanical concept whereby an
electron is found on the opposite side of an insulating
barrier without having passed through or around the
barrier.
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U
Ultraviolet — Electromagnetic radiation in the
wavelength range of 4 to 400 nanometers.
Underground Feeder (UF) — May be used for photovoltaic
array wiring if sunlight resistant coating is specified;
can be used for interconnecting balance-of-system
components but not recommended for use within battery
enclosures.
Underground Service Entrance (USE) — May be used within
battery enclosures and for interconnecting
balance-of-systems.
Uninterruptible Power Supply (UPS) — The designation of
a power supply providing continuous uninterruptible
service. The UPS will contain batteries.
Utility-Interactive Inverter — An inverter that can
function only when tied to the utility grid, and uses
the prevailing line-voltage frequency on the utility
line as a control parameter to ensure that the
photovoltaic system's output is fully synchronized with
the utility power.
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V
Vacuum Evaporation - The deposition of thin films of
semiconductor material by the evaporation of elemental
sources in a vacuum.
Vacuum Zero — The energy of an electron at rest in empty
space; used as a reference level in energy band
diagrams.
Valence Band — The highest energy band in a
semiconductor that can be filled with electrons.
Valence Level Energy/Valence State — Energy content of
an electron in orbit about an atomic nucleus. Also
called bound state.
Varistor — A voltage-dependent variable resistor.
Normally used to protect sensitive equipment from power
spikes or lightning strikes by shunting the energy to
ground.
Vented Cell — A battery designed with a vent mechanism
to expel gases generated during charging.
Vertical Multijunction (VMJ) Cell — A compound cell made
of different semiconductor materials in layers, one
above the other. Sunlight entering the top passes
through successive cell barriers, each of which converts
a separate portion of the spectrum into electricity,
thus achieving greater total conversion efficiency of
the incident light. Also called a multiple junction
cell. See multijunction device and split-spectrum cell.
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.
Voltage — The amount of electromotive force, measured in
volts, that exists between two points.
Voltage at Maximum Power (Vmp) — The voltage at which
maximum power is available from a photovoltaic module.
Voltage Protection — Many inverters have sensing
circuits that will disconnect the unit from the battery
if input voltage limits are exceeded.
Voltage Regulation — This indicates the variability in
the output voltage. Some loads will not tolerate voltage
variations greater than a few percent.
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W
Wafer — A thin sheet of semiconductor (photovoltaic
material) made by cutting it from a single crystal or
ingot.
Watt — The rate of energy transfer equivalent to one
ampere under an electrical pressure of one volt. One
watt equals 1/746 horsepower, or one joule per second.
It is the product of voltage and current (amperage).
Waveform — The shape of the phase power at a certain
frequency and amplitude.
Wet Shelf Life — The period of time that a charged
battery, when filled with electrolyte, can remain unused
before dropping below a specified level of performance.
Window — A wide band gap material chosen for its
transparency to light. Generally used as the top layer
of a photovoltaic device, the window allows almost all
of the light to reach the semiconductor layers beneath.
Wire Types — See Article 300 of National Electric Code
for more information.
Work Function — The energy difference between the Fermi
level and vacuum zero. The minimum amount of energy it
takes to remove an electron from a substance into the
vacuum.
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Z
Zenith Angle — the angle between the direction of
interest (of the sun, for example) and the zenith
(directly overhead).
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