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Glossary of Related Solar Power Terms
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-A- Top
absorbers--Dark-colored objects that soak up heat in thermal solar power
collectors used to produce heat energy.
active solar heater--A solar water or space-heating system that moves
heated air or water using pumps or fans.
AGM -- Absorbed Glass Mat, a newer type of battery construction that
uses saturated absorbant glass mats rather than gelled or liquid electrolyte.
Somewhat more expensive than flooded (liquid), but offers very good reliability.
Alternating current--Electric current in which the direction of flow
is reversed at frequent intervals--usually 100 or 120 times per second
(50 or 60 cycles per second or 50//60 Hz).
amorphous semiconductor--A non-crystalline semiconductor material. Easier
and cheaper to make than crystalline, but less efficient and slowly degrades
over time. Also called thin film.
ampere (A) or amp--The unit for the electric current; the flow of electrons.
One amp is 1 coulomb passing in one second. One amp is produced by an electric
force of 1 volt acting across a resistance of 1 ohm.
ampere-hour (Ah)--Quantity of electricity or measure of charge. How
many amps of flow or which can be provided over a one hour period. Deep
cycle batteries used in solar power systems are rated in Ah (amp-hour).
angle of incidence--Angle between the normal to a surface and the direction
of incident radiation; applies to the aperture plane of a solar collector.
Most modern solar panels have only minor reductions in power output within
plus/minus 15 degrees.
antireflection coating--A thin coating of a material, which reduces
the light reflection and increases light transmission, applied to a photovoltaic
cell surface.
array--Any number of photovoltaic modules connected together to provide
a single electrical output. Arrays are often designed to produce significant
amounts of electricity.
autonomous system--A stand-alone PV system that has no back-up generating
source. May or may not include storage batteries. Most battery systems
are designed for a certain minimum "days of autonomy" - which means that
the batteries can supply sufficient power with no sunlight to charge the
batteries. This varies from 3-5 days in the sunbelt, to 5 to 10 days elsewhere.
AWG -- American Wire Gauge, a standard system for designating the size
of electrical wire. The higher the number, the smaller the wire. Most house
wiring is #12 or 14. In most other countries, wire is specified by the
size in millimeters.
azimuth--Angle between the north direction and the projection of the
surface normal into the horizontal plane; measured clockwise from north.
As applied to the PV array, 180 degree azimuth means the array faces due
south.
-B- Top
balance of system (BOS)--Represents all components and costs other than
the PV modules. It includes design costs, land, site preparation, system
installation, support structures, power conditioning, operation and maintenance
costs, batteries, indirect storage, and related costs.
baseline performance value--Initial values of Isc, Voc, Pmp, Imp measured
by the accredited laboratory and corrected to Standard Test Conditions,
used to validate the manufacturer's performance measurements provided with
the qualification modules per IEEE 1262.
blocking diode--A diode used to restrict or block reverse current from
flowing backward through a module. [UL 1703] Alternatively, diode connected
in series to a PV string; it protects its modules from a reverse power
flow and, thus, against the risk of thermal destruction of solar cells.
British thermal unit (BTU)--The amount of heat energy required to raise
the temperature of one pound of water from 60 degrees F to 61 degrees F
at one atmosphere pressure.
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. [UL 1703] Alternatively, diode connected anti-parallel
across a part of the solar cells of a PV module. It protects these solar
cells from thermal destruction in case of total or partial shading, broken
cells, or cell string failures of individual solar cells while other cells
are exposed to full light.
-C- Top
cathodic protection--A method of preventing oxidation (rusting) of exposed
metal structures, such as bridges and pipelines, by imposing a small electrical
voltage that opposes the flow of electrons between the structure and the
ground that is greater than the voltage present during oxidation.
cell--The basic unit of a photovoltaic panel or battery.
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, cell junction, 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--An electronic device which regulates the voltage
applied to the battery system from the PV array. Essential for ensuring
that batteries obtain maximum state of charge and longest life.
combined collector--A photovoltaic device or module that provides useful
heat energy in addition to electricity.
concentrator--A PV module that uses optical elements to increase the
amount of sunlight incident on a PV cell. Concentrating arrays must track
the sun and use only the direct sunlight because the diffuse portion cannot
be focused onto the PV cells. Efficiency is increased, but lifespan is
usually decreased due to the high heat.
concentrator (module, array, or collector)--An arrangement of photovoltaic
cells that includes a lens to concentrate sunlight onto small-area cells.
Concentrators can increase the power flux of sunlight hundreds of times.
conversion efficiency (cell or module)--The ratio of the electric energy
produced by a photovoltaic device (under one-sun conditions) to the energy
from sunlight incident upon the cell.
current at maximum power (Imp)--The current at which maximum power is
available from a module. [UL 1703]
cycle life--Number of discharge-charge cycles that a battery can tolerate
under specified conditions before it fails to meet specified criteria as
to performance (e.g., capacity decreases to 80-percent of the nominal capacity).
-D- Top
dc to dc converter--Electronic circuit to convert dc voltages (e.g.,
PV module voltage) into other levels (e.g., load voltage). Can be part
of a maximum power point tracker (MPPT). deep discharge--Discharging a battery to 20-percent or less of its full
charge.
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.
direct current (dc)--Electric current in which electrons flow in one
direction only. Opposite of alternating current.
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.
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.
DOD--'Depth of Discharge,' from 100-percent state of charge (SOC), in
a battery or battery system.
-E- Top
electric circuit--Path followed by electrons from a power source (generator
or battery) through an external line (including devices that use the electricity)
and returning through another line to the source.
electric current--A flow of electrons; electricity, amps.
electrical grid--An integrated system of electricity distribution, usually
covering a large area. As in "off the grid".
electrolyte--A liquid conductor of electricity. In batteries, usualy
H2SO4, sulfuric acid, but may be any number of things. Seawater is the
mostcommon electrolyte in the world - and by suspending a zinc and a steel
sheet in it, you can get a little electricity.
energy--The ability to do work. Stored energy becomes working energy
when we use it.
energy density--The ratio of energy available from a battery to its
volume (Wh/1) or mass (Wh/kg). "watts to weight" ratio.
energy payback time--The time required for any energy producing system
or device to produce as much energy as was required in its manufacture.
For solar electric panels, this is about 16-20 months.
EVA--(ETHYLENE VINYL ACETATE) An encapsulant used between the glass
cover and the solar cells in PV modules. It is durable, transparent,resistant
to corrosion, and flame retardant.
-F- Top
flat-plate PV--Refers to a PV array or module that consists of non-concentrating
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--Float charge is the voltage required to counteract the
self-discharge of the battery at a certain temperature.
float life--Number of years that a battery can keep its stated capacity
when it is kept at float charge (see float charge).
fuel cell--A device that converts the energy of a fuel directly to electricity
and heat, without combustion. Because there is no combustion, fuel cells
give off few emissions; because there are no moving parts, fuel cells are
quiet.
-G- Top
gel-type battery--Lead-acid battery in which the electrolyte is composed
of a silica gel matrix.
grid-connected (PV system)--A PV system in which the PV array acts like
a central generating plant, supplying power to the grid.
ground loop--An undesirable feedback condition caused by two or more
circuits sharing a common electrical line, usually a grounded conductor.
grid tie-a PV system used exclusively for powering production
without storage. Effectively uses the battery as storage.
-H- Top
hot spot--An undesirable phenomenon of PV device operation whereby one
or more cells within a PV module or array act as a resistive load, resulting
in local overheating or melting of the cell(s).
hybrid system--A PV system that includes other sources of electricity
generation, such as wind or fossil fuel generators.
-I- Top
incident light--Light that shines onto the face of a solar cell or module.
insolation--Sunlight, direct or diffuse; from 'incident solar radiation.'
Not to be confused with 'insulation.' Equal to about 1000 watts per square
meter at high no
on. interconnect--A conductor within a module or other means of connection
which provides an electrical interconnection between the solar cells. [UL
1703]
inverters--Devices that convert dc electricity into ac electricity (single
or multiphase), either for stand-alone systems (not connected to the grid)
or for utility-interactive systems
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 characterized cell performance.
I-V data--The relationship between current and voltage of a photovoltaic
device in the power-producing quadrant, as a set of ordered pairs of current
and voltage readings in a table, or as a curve plotted in a suitable coordinate
system (i.e., Cartesian). [ASTM E 1036]
-J- Top
junction box--A 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.
-K- Top
kilowatt (kW)--1000 watts. kilowatt-hour (kWh)--One thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.
-L- Top
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.
ine-commutated inverter--An inverter that is tied to a power grid or
line. The commutation of power (conversion from dc to ac) is controlled
by the power line, so that, if there is a failure in the power grid, the
PV system cannot feed power into the line.
load--Anything in an electrical circuit that, when the circuit is turned
on, draws power from that circuit.
-M- Top
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. [UL 1703] For a typical silicon cell panel, this is about 17
volts for a 36 cell configuration.
maximum power point tracker (MPPT)--A power conditioning unit that automatically
operates the PV-generator at its MPP under all conditions. An MPPT will
typically increase power delivered to the system by 10% to 40%, depending
on climate conditions and battery state of charge.
microgroove--A small groove scribed into the surface of a cell which
is filled with metal for contacts.
module--A number of PV cells connected together,sealed with an encapsulant,
and having a standard size and output power; the smallest building block
of the power generating part of a PV array. Also called panel.
monolithic--Fabricated as a single structure.
multicrystalline--Material that is solidified at such as rate that many
small crystals (crystallites) form. The atoms within a single crystallite
are symmetrically arranged, whereas crystallites are jumbled together.
These numerous grain boundaries reduce the device efficiency. A material
composed of variously oriented, small individual crystals. (Sometimes referred
to as polycrystalline or semicrystalline).
multijunction device--A photovoltaic device containing two or more cell
junctions, each of which is optimized for a particular part of the solar
spectrum, to achieve greater overall efficiency.
-N- Top
n-type semiconductor--A semiconductor produced by doping an intrinsic
semiconductor with an electron-donor impurity (e.g., phosphorous in silicon).
NOCT--Nominal Operating Cell Temperature. The solar cell temperature
at a reference environment defined as 800 W/m2 irradiance, 20°C ambient
air temperature, and 1 m/s wind speed with the cell or module in an electrically
open circuit state.
-O- Top
ohm--The unit of resistance to the flow of an electric current.
one-axis tracking--A system capable of rotating about one axis.
open-circuit voltage (Voc)--The maximum possible voltage across a photovoltaic
cell or module; the voltage across the cell in sunlight when no current
is flowing.
-P- Top
parallel connection--A way of joining two or more electricity-producing
devices (i.e., PV cells or modules) by connecting positive leads together
and negative leads together; such a configuration increases the current.
passive solar home--A house that uses a room or another part of the
building as a solar collector, as opposed to active solar, such as PV.
peak load; peak demand--The maximum load, or usage, of electrical power
occurring in a given period of time, typically a day.
peak watts (Wp)--See 'Photovoltaic peak watt.'
photon--A particle of light that acts as an individual unit of energy.
photovoltaic (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 (dc voltage and current).
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) efficiency--The ratio of electric power produced by
a cell at any instant to the power of the sunlight striking the cell. This
is typically about 9% to 14% for commercially available cells.
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
dc power under un-concentrated sunlight. The structural (load carrying)
member of a module can either be the top layer (superstrate) or the back
layer (substrate). [UL 1703]
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) peak watt--Maximum "rated" output of a cell, module,
or system. Typical rating conditions are 0.645 watts per square inch (1000
watts per square meter) of sunlight, 68 degrees F (20 degrees C) ambient
air temperature and 6.2 x 10-3 mi/s (1 m/s) wind speed.
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.
polycrystalline--See 'Multicrystalline.'
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 factor--The ratio of the average power and the apparent volt-amperes.
pulse-width-modulated (pwm) wave inverter (PWM)--PWM inverters are the
most expensive, but produce a high quality of output signal at minimum
current harmonics. The output voltage is very close to sinusoidal. PV--Abbreviation for photovoltaic(s).
pyronometer--An instrument for measuring total hemispherical solar irradiance
on a flat surface, or "global" irradiance; thermopile sensors have been
generally identified as pyranometers,however, silicon sensors are also
referred to as pyranometers.
-Q- Top
qualification test (PV)--A procedure applied to a selected set of PV 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.
-R- Top
rectifier--A device that converts ac to dc, as in a battery charger
or converter. See 'inverter.'
remote systems--Systems off of the utility grid.
resistive voltage drop--The voltage developed across a cell by the current
flow through the resistance of the cell.
reverse bias--Condition where the current producing capability of a
PV cell is significantly less than that of other cells in its series string.
This can occur when a cell is shaded, cracked, or otherwise degraded or
when it is electrically poorly matched with other cells in its string.
-S- Top
Schottky barrier--A cell barrier established as the interface between
a semiconductor, such as silicon, and a sheet of metal.
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. Generally falls between a metal and an insulator in
conductivity. 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 or batteries
by connecting positive leads to negative leads; such a configuration increases
the voltage.
series regulator--Type of battery charge regulator where the charging
current is controlled by a switch, transistor, or FET connected in series
with the PV 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.
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 from a photovoltaic
cell through an external circuit that has no load or resistance; the maximum
current possible.
shunt regulator--Type of a battery charge regulator where the charging
current is controlled by a switch or transistor connected in parallel with
the PV panel. Overcharging of the battery is prevented by shorting the
PV output. Shunt regulators are common in PV systems as they are relatively
cheap to build and simple to design. Series regulators usually have better
control and charge characteristics.
silicon (Si)--A chemical element, atomic number 14, semimetallic in
nature, dark gray, an excellent semiconductor material. A common constituent
of sand and quartz (as the oxide). Crystallizes in face-centered cubic
lattice like a diamond. The most common semiconductor material used in
making photovoltaic devices.
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.
solar cell--See 'Photovoltaic cell.'
solar constant--The strength of sunlight; 1353 watts per square meter
in space and about 1000 watts per square meter at sea level at the equator
at solar noon.
solar energy--Energy from the sun. The heat that builds up in your car
when it is parked in the sun is an example of solar energy.
solar-grade silicon--Intermediate-grade silicon used in the manufacture
of solar cells. Less expensive than electronic-grade silicon.
solar noon--That moment of the day that divides the daylight hours for
that day exactly in half. To determine solar noon, calculate the length
of the day from the time of sunset and sunrise and divide by two. Solar
noon may be quite a bit different from 'clock' noon.
solar spectrum--The total distribution of electromagnetic radiation
emanating from the sun.
solar thermal electric--Method of producing electricity from solar energy
by using focused sunlight to heat a working fluid, which in turn drives
a turbogenerator.
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.'
square wave inverter--The inverter consists of a dc 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. The square wave
inverter is the simplest and the least expensive to purchase, but it produces
the lowest quality of power.
stand-alone system--An autonomous or hybrid energy 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 PV 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 test conditions (STC)--Conditions under which a module is typically
tested in a laboratory: (1) Irradiance intensity of 1000 W/square meter
(0.645 watts per square inch), AM1.5 solar reference spectrum, and (3)
a cell (module) temperature of 25 degrees C, plus or minus 2 degrees C
(77 degrees F, plus or minus 3.6 degrees F). [IEC 1215]
state of charge (SOC)--The available capacity remaining in the battery,
expressed as a percentage of the rated capacity.
substrate--The physical material upon which a photovoltaic cell is made.
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.
superstrate--The covering on the sun side of a 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.
-T- Top
thermal electric--Electric energy derived from heat energy, usually
by heating a working fluid, which drives a turbogenerator. See 'solar thermal
electric.'
thermophotovoltaic (TPV) device--A device that converts secondary thermal
radiation, re-emitted by an absorber or heat source, into electricity;
The device is designed for maximum efficiency at the wavelength of the
secondary 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, cadmium telluride, gallium arsenide, or amorphous silicon,
a few microns or less in thickness, used to make photovoltaic cells. Commonly
called amorphous.
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 eventually be absorbed by the semiconductor
tracking array--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. Typically, a single axis tracker
will give you 15% to 25% more power per day, and dual axis tracking will
add about 5% to that. Depends somewhat on latitude and
season.
transformer--Steps AC voltage up or down, depending on the application.
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 system capable of rotating independently about
two axes (e.g., vertical and horizontal) and following the sun for maximum
efficiency of the solar array.
-U- Top
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 PV system's output is fully synchronized with the utility power.
-V- Top
VAC--Volts ac
V
DC--Volts dc
Vmp--Voltage at maximum power
Voc--Open-circuit voltage
volt (V)--A unit of measure of the force, or 'push,' given the electrons
in an electric circuit. One volt produces one ampere of current when acting
a resistance of one ohm.
voltage at maximum power (Vmp)--The voltage at which maximum power is
available from a module. [UL 1703]
-W- Top
wafer--A thin sheet of semiconductor material made by mechanically sawing
it from a single-crystal or multicrystal ingot or casting.
watt (W)--The unit of electric power, or amount of work (J), done in
a unit of time. One ampere of current flowing at a potential of one volt
produces one watt of power.
watt-hour (Wh)--See 'Kilowatt-hour.'
waveform--The shape of the curve graphically representing the change
in the ac signal voltage and current amplitude, with respect to time.
-X-Y-Z- Top
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