Glossary of
Related Solar Power Terms
-A-
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-
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.
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. 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-
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.
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-
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-
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, usually H2SO4, sulfuric acid, but may be any
number
of things. Seawater is the most common 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-
Flat-plate PV--Refers to 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--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.
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-
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.
-H-
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-
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 noon.
Interconnect--A conductor within a module or
other means of connection which provides an electrical
interconnection
between the solar cells.
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).
-J-
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-
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-
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 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-
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 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-
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-
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-
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 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) 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-
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-
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-
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-
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-
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-
VAC--Volts ac
VDC--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.
-W-
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.
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