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AC Alternating Current The
direction of electrical current reverses, usually many (60) times
per second. Electricity transmission networks use AC because
voltage can be controlled with relative ease.
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. One ampere
is equal to 6.235 x 10 18
electrons per second passing a given point in a circuit.
AMPERE HOUR (amp hr. or AH),
a measure of current over time, 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
between the direct solar beam and the normal (90 degrees) to
the active surface. (degrees)
ARRAY Any number of Photovoltaic
modules connected together electrically to provide a single electrical
output. An array is a mechanically integrated assembly of modules
or panels together with support structure (including foundation
and other components, as required) to form a free-standing field
installed unit that produces DC power.
BALANCE OF SYSTEMS (BOS)
Parts or components of a photovoltaic system other than the photovoltaic
array.
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 CAPACITY The maximum
total electrical charge, expressed in ampere-hours (AH), that
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 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ampere-hours, that can be withdrawn from a cell or battery under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 cutoff voltage.
BATTERY ENERGY CAPACITY The
total energy available, expressed in watt-hours
(kilowatt-hours), that can be withdrawn from a fully-charged
cell or battery. The energy capacity of a given cell varies with
temperature, rate, age, and cutoff voltage. This term is more
common to system designers than it is to the battery industry
where capacity usually refers to
ampere-hours.
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 LIFE The period during
which a cell or battery is capable of operating above a
specified capacity or efficiency performance level. For example,
with lead-acid batteries, end-of-life is generally taken as the
point in time when a fully charged cell can deliver only 80%
of its rated capacity. Beyond this state of aging, deterioration
and loss of capacity begins to accelerate rapidly. Life may be
measured in cycles and/or years, depending on the type of service
for which the cell or battery is intended.
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.
CATHODIC PROTECTION Systems
that protect underground metal from corrosion by running small
electrical currents along the metal. Most often used to protect
well heads, oil, gas, and water pipelines.
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).
CELL (solar) The smallest, basic
Photovoltaic device that generates electricity when exposed to
light.
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.
CHARGE CONTROLLER A component
of photovoltaic system that controls the flow of current to and
from the battery to protect the batteries from over-charge and
over-discharge. The charge controller may also indicate the system
operational status.
CONCENTRATOR A photovoltaic
module which includes optical components, such as lenses, to
direct and concentrate sunlight onto a solar cell of smaller
area. Most concentrator arrays must directly face or track the
sun.
DC Direct Current. A one way
flow of electric current. Typical sources of direct currents
are electric cells, rectified power units and direct current
generators. This is the current flow
produced by a solar system. To be used for typical 120 volt or
220 volt household appliances, it must be converted to AC (alternating
current).
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%.
DIFFUSE INSOLATION The radiant
energy from the sky incident upon unit surface area during a
specified time period (Same units as for direct insolation).
DIRECT INSOLATION The radiant
energy from the sun (and a small area of sky surrounding it,
defined by the acceptance angle of the pyrheliometer) incident
upon unit surface area during a specified time period. (MJ/m2
per hour, day, week, month or year, as the case may be).
EFFICIENCY The ratio of power
output of a Photovoltaic cell to the incident power from the
sun or simulated sun sources under specified standard insolation
conditions.
ELECTROLYTE The fluid used in
batteries as the transport medium for positively and
negatively charged ions.
ELECTRIC CURRENT The rate at
which electricity flows through an electrical conductor, usually
measured in amperes (amps).
ELECTRICITY
Energy resulting from the flow of charge particles, such
as electrons or ions.
ELECTRONS A negatively charged
particle. The movement of electrons in an electrical
conductor constitutes an electric current.
EQUALIZATION The process of
restoring all cells in a battery to an equal state-of-charge.
For lead-acid batteries, this is a charging process designed
to bring all cells to 100% state-of-
charge. Some battery types may require a complete discharge as
a part of the equalization
process.
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.
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.
FULL SUN
The full sun condition is the amount of power density
received at the surface of the earth at noon on a clear day -
about 100 mW/cm2. Lower levels of sunlight are often expressed
as 0.5 sun or 0.1 sun. A figure of 0.5 sun means that the power
density of the sunlight is one-half of that of a full sun.
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.
GRID Transmission line network
used to distribute electric power.
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.
HERMETIC SEAL Being impervious
to external influences. Typically associated with the sealing
of a package so that oxygen, moisture, and other outside environments
cannot enter the
package.
HYBRID SYSTEM A power system
consisting of two or more power generating subsystems
(e.g., the combination of a wind turbine or diesel generator
and a photovoltaic system. The SES
Photogenset® is an example of such a system
INCIDENT LIGHT The incident
light is the amount of light reaching an object.
INSOLATION The amount of sunlight
reaching an area. Usually expressed in milliwatts per
square centimeter, or langleys.
INVERTER A device that converts
electricity from DC to AC.
KILOWATT-HOUR Unit of energy
used to perform work (energy and work are equivalent in units,
energy being the potential value and work the achieved value)
Fuel equivalents:
One barrel of crude contains roughly 1700 kWh
One ton of coal contains roughly 7500 kWh
One gallon of gasoline contains roughly 37 kWh
One cubic foot of natural gas contains 0.3 kWh
One ton of uranium ore contains 164 million kWh 1.34 horsepower-hours.
Thermal unit:
One kWh = 3400 BTU. Can be compared to 860 calories (food value)
Example:
The "average" American home uses about 24 kWh of electricity
per day. Heating 3 gallons of
water from 75 degrees to the boiling point requires 1 kWh. A
200 watt Photovoltaic array of six, 50 watt Solar modules, will
generate 1 kWh in an "average" day (annualized average
equivalent of 5 hours peak sunlight per day).
LOAD Refers
to equipment that is powered by electricity. Usually expressed
in terms of amperes or watts. In an electrical circuit, any devise
or appliance that uses power (such as a light bulb or water pump).
MAXIMUM POWER The power at the
point on the current-voltage characteristic where the
product of current and voltage is a maximum (measured in watts).
MODULE The smallest non divisible,
self-contained and environmentally protected physical
structure housing interconnected Photovoltaic cells and providing
a single DC electrical output.
OPEN CIRCUIT VOLTAGE (VOC) Voltage
produced by a Photovoltaic cell with no load applied when the
cell is exposed to standard insolation conditions, measured with
a voltmeter.
PANEL A collection of one or
more modules fastened together into a single unit, often factory
pre- assembled and wired, forming a field-installable unit.
PARALLEL CONNECTION A wiring
configuration used to increase current (amperage). Parallel wiring
is positive to positive (+ to +) and negative to negative (-
to -). Opposite of a series connection.
PEAK POWER POINT
Operating point of the I-V (current-voltage) curve for
a Photovoltaic cell or module where the product of the current
value times the voltage value is a maximum.
PEAK WATTS
The measurement of electricity produced by a solar generator
at noon on a sunny day, under predetermined standard conditions.
PHOTON The actual (physical)
particle unit of light, as the electron is of electric charge
and the atom and molecule are of matter. Light has both wave
properties and particle properties. Violet light has relatively
short wavelength and higher energy in its photons; red light
has longer
wavelength, lower-energy photons. The wavelength and/or energy
spectrum of the sun extends
in both directions beyond the visible range of light, of course,
and the silicon module solar cell
can capture some energy in both of these invisible zones. Photons
not captured by the cell are
either reflected or converted to heat in the solar array.
PHOTOVOLTAIC CELL A device composed
of specially prepared semiconductor material or material combinations
exhibiting the ability to convert incident solar energy directly
into electrical energy.
PHOTOVOLTAIC 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 SYSTEM An installed
aggregate of solar arrays generating power for a given application.
A system may include the following sub-systems:
Support Foundation
Power conditioning and control equipment
Storage
Active Thermal control
Land security systems and buildings
Conduit/wiring
Instrumentation
POWER CONDITIONER The electrical equipment used
to convert power from a photovoltaic array into a form suitable
for subsequent use. Loosely, a collective term for inverter,
transformer, voltage regulator and other power controls.
POWER FACTOR The ratio of real
power (watts) to apparent power (volt-amps) in an AC circuit.
Displacement power factor is the ratio of fundamental watts to
fundamental RMS volts times
RMS amps, excluding the effects
of all harmonic exponents; it could be called fundamental power
factor.
REGULATOR Prevents overcharging
of batteries by controlling charge cycle-usually adjustable to
conform to specific battery needs.
RENEWABLE ENERGY Flows of energy
that are regenerative or virtually inexhaustible. Most commonly
includes solar (electric and thermal), biomass, geothermal, wind,
tidal, wave, and hydro power sources.
SEMICONDUCTOR A material such
as silicon, which has a crystalline structure that will allow
current to flow under certain conditions. Semiconductors are
usually less conductive than metals but not an insulator like
rubber.
SERIES CONNECTION
A wiring configuration used to increase voltage. Series
wiring is positive to negative (+ to -) or negative to positive
(- to +). Opposite of parallel connection.
SILICON A non-metallic element,
that when specially treated, is sensitive to light and capable
of transforming light into electricity. Silicon is the basic
material of most beach sand, and is the raw material used to
manufacture most photovoltaic cells.
SINGLE-CRYSTAL STRUCTURE A material
having a crystalline structure such that a
repeatable or periodic molecular pattern exists in all three
dimensions.
SOLAR CELL The basic photovoltaic
device which generates electricity when exposed to sunlight.
SOLAR PANEL A collection of
solar modules connected in series, in parallel, or in series-
parallel combination to provide greater voltage, current, or
power than can be furnished by a
single solar module. Solar panels can be provided to furnish
any desired voltage, current, or
power. They are made up as a complete assembly. Larger collections
of solar panels are usually called solar arrays.
STAND-ALONE SYSTEM (SA) A system
which operates independently of the utility lines. It may draw
supplementary power from the utility but is not capable of providing
power to the utility.
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.
SULFATION The formation of lead-sulfate
crystals on the plates of a lead-acid battery.
Commonly used to indicate the large crystals which form in partially
discharged cells as the
result of temperature cycling. These large crystals are more
difficult to reduce by the charging
current than are the smaller crystals that result from normal
and self-discharge reactions.
Sulfating can be caused by leaving the battery in a discharged
state for long periods of time.
TELEMETRY DEVICE Devices used
to transmit or receive data in a digital form.
TILT ANGLE A fixed angle measured
from the horizontal to which a solar array is tilted. The tilt
angle is chosen to maximize the array output. Depending upon
latitude, season and time of day this angle will vary.
TRACKING ARRAY
An array that is mounted on a movable structure that attempts
to follow the path of the sun. Some tracking arrays are single
axis while others are dual.
VOLTAGE A measure of the force
or "push" given the electrons in an electrical circuit;
a
measure of electrical potential. One volt produces one amp of
current when acting against a
resistance of one ohm.
WATT Unit of power. Power is
the rate of using energy to do work.
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Glossary of Photovoltaic Terms