Photovoltaic Glossary
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 or (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 - Ah
A measure of current over time, used to measure battery capacity.
Amp Hour Meter
An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).
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The total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state of charge, age, and cutoff voltage.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The total energy available, expressed in watt-hours (kilowatt-hours), that can be withdrawn from a fully-charged cell or battery.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.
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.
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 flowproduced 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).
Efficientcy
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.
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 Connected System
A PV system that is connected to the national grids by an inverter. The inverter will export unused electricity to the grid earning the user money. A Grid Connected PV system usually is made from Photovoltaic Modules and a Grid Connected Inverter.
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.
Hemetic 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 Systems
A power system consisting of two or more power generating systems combined.
Incident Light
The incident light is the amount of light reaching an object.
Insolation
The amount of sunlight reaching an area. Sometimes expressed in milliwatts per
square centimeter, or langleys.
Inverter
A device that converts electricity from DC to AC.
kWh
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 cubic foot of natural gas contains 0.3 kWh
Thermal unit:
One kWh = 3400 BTU. Can be compared to 860 calories (food value)
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.
Off-Grid System
A system which operates independently of the utility lines. An offgird system is made up from, PV Modules, Charge Controller, Battery Bank and an Inverter. It may draw additional power from a generator and is not capable of providing power to the utility.
Open Circuit Volatage - 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 - PV 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 arrangement of solar modules and other equipment generating power for a given application.
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
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.
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
See Off-Grid System.
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.
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.
"The Earth isn't dying, it's being killed. And those who are killing it have names and addresses." Utah Phillips
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