Glossary - Solar Photovoltaic (PV) system

Dark-colored objects that soak up heat in thermal solar collectors.

The factor by which photons are absorbed as they travel a unit distance through a material.

A solar water or space-heating system that moves heated air or water using pumps or fans.

Absorbed Glass Mat, a newer type of battery construction that uses saturated absorbent glass mats rather than gelled or liquid electrolyte. Somewhat more expensive than flooded (liquid), but offers very good reliability.

A noncrystalline semiconductor material. Easier and cheaper to make than crystalline, but less efficient and slowly degrades over time. Also called thin film.

The quantity of electrical energy equal to the flow of current of one ampere for one hour. The term is used to quantify the energy stored in a battery.

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. The loss is a function of the cosine, so at 45 degree angle, output drops off by about 30%.

The positive electrode in an electrochemical cell (battery). Also, the earth ground in a cathodic protection system. Also, the positive terminal of a diode.

A thin coating of a material, which reduces the light reflection and increases light transmission, applied to a photovoltaic cell surface.

A collection of electrically connected photovoltaic (PV) modules.

The electrical current produced by a PV array when it is exposed to sunlight.

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.

Angle between the north direction and the projection of the surface normal into the horizontal

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.

The average amount of electric power that a utility must supply in any period.

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.

A diode used to restrict or block reverse current from flowing backward through a module. [UL 1703] 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.

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. [UL 1703] 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.

A polycrystalline thin-film photovoltaic material.

The total number of ampere-hours that can be withdrawn from a fully charged battery at a specified discharge rate and temperature.

The negative electrode in an electrochemical cell. Also, the negative terminal of a diode.

A method of preventing oxidation (rusting) of exposed metal structures, such as bridges and pipelines, by imposing between the structure and the ground a small electrical voltage that opposes the flow of electrons and that is greater than the voltage present during oxidation.

The basic unit of a photovoltaic panel or battery

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.

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.

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.

A method of depositing thin semiconductor films. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

Copper-Indium-Diselelide.

A photovoltaic device or module that provides useful heat energy in addition to electricity.

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.

The ratio of the electric energy produced by a photovoltaic device (under one-sun conditions) to the energy from sunlight incident upon the cell.

A unit that converts a dc voltage to another dc voltage.

A type of PV cell made from a single crystal or polycrystalline slice of silicon.

The voltage levels (activation) at which the charge controller disconnects the array from the battery or the load from the battery.

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).

The percent of the rated battery capacity that has been withdrawn.

Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.

Radiation received from the sun after reflection and scattering by the atmosphere and ground.

Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.

Electric current in which electrons flow in one direction only. Opposite of alternating current.

Sunlight falling directly upon a collector. Opposite of diffuse insolation.

The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

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.

'Depth of Discharge,' from 100-percent state of charge (SOC), in a battery or battery system.

The ratio of active time to total time. Used to describe the operating regime of appliances or loads in PV systems.

digital Volt-meter.

The ratio of output power (or energy) to input power (or energy). Expressed in percent.

Edge definde Film Growth, A method for making sheets of polycrystalline silicon in which molten silicon is drawn upward by capillary action through a mold.

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.

An integrated system of electricity distribution, usually covering a large area.

The medium that provides the ion transport mechanism between the positive and negative electrodes of a battery.

The ratio of energy available from a battery to its volume (Wh/1) or mass (Wh/kg). "watts to weight" ratio.

The time required for any energy producing system or device to produce as much

The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time.

Ethylene-Vinile-Acetate Foil, it will be used by module production for covering the cells.

For an I-V curve, the ratio of the maximum power to the product of the open-circuit voltage and the short-circuit current. Fill factor is a measure of the "squareness" of the I-V curve.

A PV array set in at a fixed angle with respect to horizontal.

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 is the voltage required to counteract the self-discharge of the battery at a certain temperature.

Number of years that a battery can keep its stated capacity when it is kept at float charge (see float charge).

The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

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.

A crystalline, high-efficiency semiconductor/photovoltaic material.

Gas by-products, primarily hydrogen, produced when charging a battery. Also, termed out-gassing.

Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

Term used to describe an electrical utility distribution network.

A PV system in which the PV array acts like a central generating plant, supplying power to the grid.

An undesirable feedback condition caused by two or more circuits sharing a common electrical line, usually a grounded conductor.

An undesirable phenomenon of PV device operation whereby one or more cells within a PV

A PV system that includes other sources of electricity generation, such as wind or diesel generators.

The plot of the current versus voltage characteristics of a photovoltaic cell, module, or array. Three important points on the I-V curve are the open-circuit voltage, short-circuit current, and peak power operating point.

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.

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). [ASTM E 1036]

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 in Dodge City.

A conductor within a module or other means of connection which provides an electrical interconnection between the solar cells. [UL 1703]

(Power Conditioning Unit, PCU, or Power Conditioning System, PCS) In a PV system, an inverter converts dc power from the PV array/battery to ac power compatible with the utility and ac loads.

The solar power incident on a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals Insolation.

The solar radiation incident on an area over time. Equivalent to energy and usually expressed in kilowatt-hours per square meter.

Unit of energy equal to 1/3600 kilowatt-hours.

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.

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.

One thousand watts. A unit of power.

One thousand watt-hours. A unit of energy. Power multiplied by time equals energy.

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.

Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

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.

The amount of electric power used by any electrical unit or appliance at any given time.

The current required by the electrical device.

The voltage level at which a controller will disconnect the load from the battery.

A sealed battery to which water cannot be added to maintain electrolyte level.

The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. [UL 1703] For a typical silicon cell panel, this is about 17 volts for a 36 cell configuration.

A power conditioning unit that automatically operates the PV-generator at its maximum power point 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. You usually get more gain in winter and in colder weather due to the higher panel output.Most MPPT controllers are down converters - from a higher voltage to a lower one.

A small groove scribed into the surface of a cell which is filled with metal for contacts.

Maximum Power Point; 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, this is at about 0.45 V.

Maximum Power point Tracker; Means of a power conditioning unit that automatically operates the PV-generator at its MPP under all conditions.

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).

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.

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.

A system capable of rotating about one axis, usually following the sun from East to West.

The maximum voltage produced by an illuminated photovoltaic cell, module, or array with no load connected. This value will increase as the temperature of the PV material decreases.

The current and voltage that a module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.

A designation for a number of PV modules assembled in a single mechanical frame.

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.

A house that uses a room or another part of the building as a solar collector, as opposed to active solar, such as PV.

The maximum load demand on a system.

The maximum load, or usage, of electrical power occurring in a given period of time, typically a day.

The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 W/m2.

The amount of power a photovoltaic module will produce at standard test conditions (normally 1000 W/m2 and 25° cell temperature).

Pertaining to the direct conversion of light into electricity.

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.

The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (dc voltage and current).

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.

The total of all PV strings of a PV power supply system, which are electrically interconnected.

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). [UL 1703]

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).

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.

A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.

A material used to make PV cells which consist of many crystals as contrasted with single crystal silicon.

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.

The cosine of the phase angle between the voltage and the current waveforms in an ac circuit. Used as a designator for inverter performance. A power factor of 1 indicates current and voltage are in phase and power is equal to the product of volt-amperes. (no reactive power).

A battery whose initial capacity cannot be restored by charging.

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.

Abbreviation for photovoltaic(s).

An instrument used for measuring global solar irradiance.

An instrument used for measuring direct beam solar irradiance. Uses an aperture of 5.7° to transcribe the solar disc.

The current output of a PV module measured at standard test conditions of 1,000 w/m2 and 25°C cell temperature.

The sine of the phase angle between the current and voltage waveforms in an AC system.

A device that converts ac to dc, as in a battery charger or converter. See inverter and diode.

A site not serviced by an electrical utility grid.

Systems off of the utility grid. Out in the boondocks.

The voltage developed across a cell by the current flow through the resistance of the cell.

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.

The rate at which a battery, without a load, will lose its charge. This can vary considerably depending on the type of battery and age. It can be as low as 3% a month for a new AGM battery, and as high as 10% a week for an older Lead-Antimony (industrial) battery.

The rate at which a battery, without a load, will lose its charge.

See 'Multicrystalline.'

A way of joining photovoltaic cells or batteries by connecting positive leads to negative leads; such a configuration increases the voltage.

The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

The current produced by an illuminated PV cell, module, or array when its output terminals are shorted.

The current flowing freely from a photovoltaic cell through an external circuit that has no load or resistance; the maximum current possible.

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. Most newer controllers have gone to series regulation.

An inverter that produces utility-quality, sine wave power forms.

Material with a single crystalline formation. Many PV cells are made from single crystal silicon.

A material that is composed of a single crystal or a few large crystals.

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. It increases at higher altitudes.

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.

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.

The total distribution of electromagnetic radiation emanating from the sun.

Method of producing electricity from solar energy by using focused sunlight to heat a working fluid, which in turn drives a turbogenerator.

Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.

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.'

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.

An autonomous or hybrid photovoltaic 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.

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.

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]

The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

A number of modules or panels interconnected electrically in series to produce the operating voltage required by the load.

The physical material upon which a photovoltaic cell is made.

Any one of several components in a PV system (i.e., array, controller, batteries, inverter, load).

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.

Graphical representation of the Sun's height and azimuth

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.

The array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.

Electric energy derived from heat energy, usually by heating a working fluid, which drives a turbogenerator. See 'solar thermal electric.'

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.

Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.

A PV module constructed with sequential layers of thin film semiconductor materials.

The angle of inclination of a solar collector measured from the horizontal.

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.

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.

A charge at a low rate, balancing through self-discharge losses, to maintain a cell or battery in a fully charged condition.

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.

The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

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.

Volts ac

A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

A battery designed with a vent mechanism to expel gases generated during charging.

Voltage at maximum power

Open-circuit voltage

The voltage at which maximum power is available from a module.

The shape of the curve graphically representing the change in the ac signal voltage and current amplitude, with respect to time.

The angle between directly overhead and the line intersecting the sun. (90°- zenith) is the elevation angle of the sun above the horizon.