Glossary of Terms
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Glossary
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A
Absorber
In a photovoltaic device, the material that readily absorbs photons to generate charge carriers
(free electrons or holes).
Absorption Coefficient
The factor by which photons are absorbed as they travel a unit distance through a material.
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.
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.
B
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.
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.
C
Charge Controller
The PV system component which controls the battery's state of charge. It may also provide 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.
Conductor
The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.
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.
D
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.
E
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.
F
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.
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G
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.
H
High Voltage Disconnect
The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging.
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.)
I
Incident Light
Light that shines onto the face of a solar cell or module.
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.
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.
I-V Curve
A current/voltage curve, which expresses the possible combinations of current and voltage output of a photovoltaic device.
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 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)
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)
One thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.
L
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.
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.
M
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).
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.
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.
Module
A number of solar cells electrically connected, protected from environmental stresses, self-contained and not subdividable,
providing a single electrical output.
N
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.
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.
Open-Circuit Voltage
Abbreviated Voc, refers to a PV device's voltage potential when it is providing no current.
Overvoltage
When the voltage in a circuit or part of a circuit is raised above its upper design limit, this is known as overvoltage. The condition
may be hazardous. Depending on its duration, the overvoltage event can be permanent or transient (a oltage spike).
P
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.
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.
p-n Junction
The junction at the interface between two differently doped layers of semiconductor material, one layer doped with 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.
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Q
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.
R
Reserve Capacity
The amount of generating capacity a central power system must maintain to meet peak loads.
S
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.
T
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.
U
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.
V
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.
W
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.
Z
Zenith Angle
The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).
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