Glossary of Terms
|
|
Glossary
|
Absorption
Coefficient
The factor by which photons are absorbed as they travel a unit distance
through a material.
Air mass
A measure of how far light travels through the Earth's atmosphere. One
air mass, or AM1, is the thickness of the Earth's atmosphere. Air mass
zero (AM0) describes solar irradiance in space, where it is unaffected
by the atmosphere. The power density of AM1 light is about 100 mW/cm²;
the power density of AM0 light about 136 mW/cm².
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).
Ampere
A unit of electrical
current. A potential of one volt
across a resistance of one ohm causes a current of one ampere (6.25 X
1018 electrons per second) to flow.
Ampere-hour
A unit of energy,
typically referring to battery capacity. One ampere of current flowing
for one hour.
Array
A number of photovoltaic
modules electrically connected to produce a single electrical output.
Angle of incidence
The angle
between a ray of sunlight
striking a surface and a line perpendicular to that surface. Rays
perpendicular to a surface have a zero angle of incidence.
Azimuth
The angular measure
between due south and the point on the horizon directly below the sun.
Balance-of-Systems
(BOS)
components
All the components in a power system other than the
photovoltaic array.
Battery
Two or more
electrochemical cells connected to provide energy storage. Commonly
used to designate one cell also.
Blocking diode
A diode which
prevents reverse current flow
in a circuit, commonly used to prevent a battery from discharging
through the array at night.
Charge controller
The PV system component which controls the battery's state of charge.
It may also provides other system control functions.
Charge rate
The current applied to
a battery to restore its
energy capacity. The rate is typically normalized with respect to the
battery's full capacity and a designated time period. Thus, the current
necessary to nominally charge a 100-ampere-hour battery from zero to
full charge in 5 hours (20 amperes) is referred to as the battery's C/5
rate. The term is also applied to discharge rate.
Concentrator array
A PV array
which uses concentrating devices (reflectors, lenses) to increase the
intensity of the sunlight striking the array.
Depth of discharge
A measure of
how much energy has been
withdrawn from a battery, expressed as a percentage of full capacity. A
100 Ah battery from which 30 Ah has been withdrawn has undergone a 30%
depth of discharge (DOD). This term is the inverse of state of charge
(SOC); the example battery would be at 70% SOC.
Diffuse radiation
The sunlight
received indirectly, as a
result of scattering due to clouds, fog, dust, moisture vapor or other
substances in the atmosphere.
Direct
current (DC)
The type of current provided by a battery or solar cell, which flows in
one direction.
Direct radiation
Sunlight received
directly, which has traveled in a straight path from the sun, also
referred to as beam radiation.
Efficiency
With
respect to solar cells, the
percentage of light energy that is converted to electricity by the
cell. Depending on cell technology and production technique, this
ranges from as low as 5% to as high as 30%.
Elevation (solar)
The sun's angle
above the horizon.
Equalizing charge
A controlled
overcharge of a battery bank for the purpose of restoring equality of
charge in all cells. |
|
|
Finishing
charge
That part of the charging process which restores the
final segment of a battery's charge, roughly between 90% and 100% SOC.
Flat-plate array
A PV array which
does not use concentration.
Frequency
The rate at which a
periodic event occurs. In
electricity, the rate at which current reverses direction in an
alternating current system. In the US, alternating current systems use
a frequency of 60 cycles per second (60 Hz); in Europe, the standard is
50 Hz.
Global radiation
Total solar radiant energy impinging on a surface, equal to the sum of
direct and diffuse radiation.
Grid-connected
A power system
interconnected with the grid (or mains) of the local electric utility.
Also referred to as utility-interactive, or grid-intertied.
Hole
A vacancy
in a crystalline structure, which would be filled by an electron if the
structure were electrically perfect.
Hybrid system
A power system
consisting of two or more energy sources (e.g., a PV array and a wind
generator.)
Insolation
The
solar energy received
at a place over a given period. May be expressed as sun-hours per day,
langleys per hour, watts per square meter per hour, or any number of
other units.
Isolation diode
A diode which
prevents one segment of a
PV array from interacting with another array segment. Usually used to
prevent array energy from flowing backwards through a sub-voltage
series string. May also serve the function of blocking diode.
Inverter
A device which converts
DC electricity to AC.
I-V curve
A current/voltage curve,
which expresses the possible combinations of current and voltage output
of a photovoltaic device.
Junction diode
A
semiconductor device with a junction and a built-in potential that
passes current better in one direction than the other. All solar cells
are junction diodes.
Kilowatt-hour (kWh)
One thousand watts acting over a period of 1 hour. The kWh is a unit of
energy. 1 kWh=3600 kJ.
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.
Maximum
power
Also referred to as peak
power. The point on a device's I-V curve where the product of I and V
(Pmax, measured in watts) is maximized. The points on the I and V
scales which describe this curve point are named Imp (current @ max
power) and Vmp (voltage @ max power.)
Module
A number of solar cells
electrically connected,
protected from environmental stresses, self-contained and not
subdividable, providing a single electrical output.
NOCT
Nominal
Operating Cell Temperature, the temperature at which cells in a module
operate under
Standard Operating Conditions (SOC), which are: irradiance of 0.8
kW/m2, 20ºC ambient temperature, and average windspeed of 1 m./s, with
the wind oriented parallel to the plane of the array, and all sides of
the array fully exposed to the wind.
Open-circuit voltage
Abbreviated Voc, refers to a PV device's voltage potential when it is
providing no current.
Parallel connection
Electrical
connection where the positive terminals of a number of devices are
connected together, as are their negative terminals. The output voltage
of the paralleled devices is equal to the average of the devices, and
the total current is the sum of the current of all the devices.
p-n junction
The junction at the
interface between two
differently doped layers of semiconductor material, one layer doped
with a positive-type dopant, the other layer with a negative-type
dopant. An electrical field is established at the p-n junction which
gives direction to the flow of light-stimulated electrons. |
|
|
Reserve
capacity
The amount of generating capacity a central power system
must maintain to meet peak loads.
Self-discharge
Batteries lose charge even when not in use. For many batteries,
self-discharge rate increases as the battery ages.
Series connection
Electrical
connection where the positive
terminal of one device is attached to the negative terminal of the next
in a series string; in this connection, the string voltage is the sum
of the device voltages and the string current is limited to the current
of the least productive device in the string.
Short-circuit current
Abbreviated
Isc, refers to a PV device's current output when short-circuited.
Solar panel
A group of
photovoltaic modules mechanically mounted on a single frame.
Solar spectrum
The total
distribution of
electromagnetic radiant energy over the band of wavelengths present in
solar radiation. The total energy received on a given surface, and how
that energy is distributed among various wavelengths, depends on how
much of the Earth's atmosphere light has traversed.
Standalone
system
A power system not connected to the utility grid
(mains.) Sometimes referred to as an autonomous system or an off-grid
system.
Standard operating conditions
Abbreviated SOC, a set
of reference PV device measurement conditions consisting of irradiance
of 0.8 kW/m2, 20ºC ambient temperature, and average windspeed of 1
m./s, with the wind oriented parallel to the plane of the array, and
all sides of the array fully exposed to the wind.
Standard test conditions
Abbreviated STC, a set of
reference PV device measurement conditions consisting of irradiance of
1 kW/m2, AM 1.5, and 25ºC cell temperature.
Standoff mount
A mounting system
which supports a PV array above a roof surface.
State of charge
Abbreviated SOC,
the percentage of energy in a battery referenced to its nominal full
capacity.
Sulfation
The formation of lead
sulfate crystals on the
plates of a lead-acid battery. Normally used to refer to large sulfate
crystals, rather than small crystals formed in normal battery
operation, formed as a result of temperature cycling while the battery
is in a partially charged state.
Thin film cell
A
PV cell formed by
depositing thin layers of conductive and semiconductive materials,
usually using a chemical vapor deposition (CVD) process. Also referred
to as amorphous cells because they have no crystalline structure, such
cells use less material than cells sawn from crystalline ingots.
Two-axis tracking
A tracking
system which follows the sun's azimuth and elevation.
Utility-interactive
A power system which
interacts with the utility grid (mains), taking power from the grid to
satisfy its loads as necessary, and returning power to the grid when
not required by the loads.
Voltage
Measured
in volts (V), the
electrical potential between two points. One volt of potential causes
one ampere of current to flow through a resistance of one ohm. The
open-circuit voltage of a silicon solar cell is about half a volt; the
operating voltage of a lead-acid cell is about two volts.
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.
|
|
|
|
