Absorbers

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

Absorption coefficient

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

AC/DC converter (Rectifier)

Device for converting alternating current into direct current.

Accumulator

Instrument designed to absorb or accumulate energy (electric, thermal, mechanical) to be distributed at the right moment and in the measure required.

Accumulator batteries

Devices to accumulate electric energy, usually connected together, which produce voltages or currents, electric powers or capacities higher than the ones produced by their single components.
In the charging phase they are used to store electric energy, then restored in the discharging phase.

Acid lead battery

Type of accumulator formed by plates of pure lead, lead-antimony or lead-calcium immersed in an acid electrolyte (usually sulphuric acid).

Active solar heater

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

Albedo

Component of the solar radiation received by a light-catching surface after being reflected by ground, water, vegetation and surrounding buildings.

Alternating current (AC)

Electric current in which the direction of the flow is reversed at frequent intervals, 100 times per second in Europe (50 cycles per second) and 120 times per second in the USA. Opposite of AC is direct current (DC).

Alternating voltage (Vac)

Voltage between two points of a circuit changing at frequent intervals and having sinusoidal pattern. It’s typical of electrical distribution systems as well as of domestic and industrial uses.

Amorphous silicon

Thin-film PV silicon cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate.

Ampere [A]

Unit of electric current. The rate of flow of electrons in a conductor equal to one coulomb per second.

Ampere-hour [Ah]

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 of Azimuth

Angle between the north direction and the projection of the surface normal into the horizontal
plane; measured clockwise from north. As applied to the PV array, 180 degree azimuth means the array faces due south.

Angle of declination

Angular position of the sun with respect to the equator.

Angle of elevation

Angular distance of the sun from the horizon level.

Angle of incidence

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..

Angle of inclination (Tilt angle)

Angle formed by the light-catching surface with the horizon level; it is equal to 0° with a horizontal surface and +90° with a surface perpendicular to the ground.

Anode

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

Array

See: Photovoltaic Array.

Azimuth

Horizontal angle measured clockwise starting from SOUTH; a 0° Azimuth indicates SOUTH; a +90° Azimuth indicates WEST; a –90° Azimuth indicates EAST.

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 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 cut-off 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 cut-off voltage. This term is more common to system designers than it is to the battery industry where capacity usually refers to ampere-hours.

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 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.

BIPV
(Building Integrated PhotoVoltaics)

A term for the design and integration of PV into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, into shading "eyebrows" over windows; or other building envelope systems.

Blocking diode

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.

BOS
(Balance Of System)

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.

British thermal unit (BTU)

The amount of heat energy required to raise the temperature of one pound of water from 60 degrees F to 61 degrees F at one atmosphere pressure.

Busbar

Principal connection wire, usually of silver and applied on the PV cell surface exposed to the light.

By-pass 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. 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.

Cadmium Telluride (CdTe)

A polycrystalline thin-film photovoltaic material.

Cathode

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

Cathodic protection

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.

Cell

The basic unit of a photovoltaic panel or battery

Cell junction

The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the centre of the cell barrier or depletion zone.

Charge controller

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.

Cogeneration

The production of electricity and another form of useful energy (such as heat or steam) used for industrial, commercial, heating, or cooling purposes.

Concentrator

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.

Concentrator (module, array, or collector)

An arrangement of photovoltaic cells that includes a lens to concentrate sunlight onto small-area cells. Concentrators can increase the power flux of sunlight hundreds of times.

Conversion efficiency (cell or module)

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

Converter with Forced Commutation

Inverter whose output voltage form and frequency is produced by an oscillator electronic circuit, which allows the machine to work as a generator on an insulated grid.

Converter with Natural Commutation

Inverter whose output voltage frequency is set by the electric grid: it can function only if connected to electric grid.

Copper Indium Diselenide [CuInSe2]

Semiconductor polycrystalline compound used to produce PV cells.
It is usually indicated with the conventional acronym CIS.

Cosine

In a AC line, voltage and current are not always in phase; on the contrary, some phase displacement can occur, due to the inductive and capacitative effects of the loads. When the cosine of the phase angle between V and I (cos j) is equal to 1, there is no phase displacement; on the other side, when it is equal to 0, voltage and current are wattless (j = 90°), so that there is no transfer of power. Values between 0 and 1 indicate more or less marked displacements.

Crystalline silicon

Type of silicon having monocrystalline or polycrystalline structure.

Current, I [A]

The flow of electric charge in a conductor between two points with potential difference (voltage).

Current at maximum power (Imp)

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

Cycle life

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).

Czochralski (Process -)

A method of growing large size, high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.

DC/AC converter (Inverter)

Device for converting direct current into alternating current.


DC/DC CONVERTER
Electronic circuit to convert dc voltages (e.g., PV module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker (MPPT).

Deep Of Discharge

See: DOD.

Delivery point

A location or facility designated for the delivery of the electric power produced by the distributor (ENEL or Municipal Concerns).

Diffused irradiance

Component of the solar irradiance received from a light-catching surface after the reflection and dispersion due to the atmosphere.

Diffusion

Process by which the silicon cell is doped with phosphor ions to allow the creation of a superficial n-type layer on its surface; that is necessary to obtain the p-n junction, which is essential to make the PV cell function. The process of diffusion takes place in special ovens, at temperatures not inferior to 870 °C and for about 15÷20 minutes.

Diode

Electronic component that allows current flow in one direction only.

Direct irradiance

Component of the solar irradiance striking a light-catching surface with a sole defined angle of incidence.

Direct current (DC)
Electric current in which electrons flow in one direction only. Opposite of alternating current.

Direct voltage (Vdc)

Voltage between two points of a circuit maintaining the same sign and value with changing of time. It's typical of some stand-alone systems (railways, ships) as well as of battery-fed devices.

Distributed systems

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.

DOD
(Deep Of Discharge)

Discharging a battery to 20-percent or less of its full charge.

Doping

Introduction of impurities (doping elements) in very small amounts (1 out of 1.000.000) inside the semiconductor material, in order to increase its intrinsic electric properties.

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.

Efficiency of conversion [%]

Relationship between the electric energy produced and the incident solar energy on a PV device.

Electric contacts

Conductor elements which establish or cut off the continuity of an electric circuit. In the PV cell contacts are realized with high conductive material (i.e. silver) and with a grid configuration formed by fingers and busbars, so as not to obscure the cell.

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.

Electric load [W]

The instantaneous electric power (the consumed energy) required by any electric user. It is measured in Watt (W).

Electrochemical accumulator

Device to convert chemical energy into electric energy through an oxidation-reduction process (redox).

See also: Accumulator batteries.

Electrolyte

The fluid used in batteries as the transport medium for positively and
negatively charged ions.

Electrolyte density

In a battery, it's the relationship between the weight and the volume of the electrolyte. The electrolyte density shows the battery SOC and it is expressed in gr/cm³. It is measured by a float densimeter.

Electronical grade silicon

Highly pure and very expensive silicon, normally used in semiconductor industry (also called Semiconductor grade silicon).

Electrons
A negatively charged particle. The movement of electrons in an electrical conductor constitutes an electric current.

Energy

Generally measured in J (Joule); electric energy is measured in Wh (Watt-hours) and is equal to the energy provided by a device distributing one Watt of power per hour (1 Wh = 3.600 J, 1 cal = 4,186, 1 Wh = 860 cal).

Energy density

Relationship between the available energy of a battery and its volume (in Wh/cm³), or its weight (in Wh/kg).

Equalization recharge

Periodic recharge of an accumulator in order to equalize (i.e. homogenize) the battery accumulators. It is usually practised once or twice a year with the aim of bringing the battery to the “high voltage” zone (2.6÷2.7 V/el) and letting it gasificate for some hours.

EVA (ETHYLENE VINYL ACETATE)

An encapsulant used between the glass cover and the solar cells in PV modules. It is durable, transparent, resistant to corrosion, and flame retardant.

Fill Factor (FF) [%]

Relationship between the maximum power and the product resulting from the open circuit voltage by the short-circuit current of a PV device. This factor shows how much the pattern of the device I-V curve is near to the ideal rectangular shape.

Finger

Secondary connection, usually of silver, connected to the busbar and applied on the cell side exposed to light.

Fixed tilt array

PV array whose module supporting structure has a fixed tilt.

Frequency [Hz]

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

Fuel cell
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.

Gallium Arsenide [GaAs]

Chemical compound with typical electrical features of semiconductors. It is employed to produce very high efficiency cells, though usually limited to space uses for its considerable costs of production.

Gasification

Process of gas production (usually hydrogen) executed by the electrolyte in an accumulator subject to Equalization recharge (see). The gasification reduces the difference of density (especially in the batteries) developed particularly at a high altitude, therefore solving the problem of Stratification (see).

Gel-type battery

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

Gettering

Process which moves away from the cell the residual elements (iron, chrome, etc.) resulting from the processes of silicon growth and cut of wafers from the ingot.

Global insolation

Total irradiance received from a solar device depending on the sun, the atmosphere clearness and the general environment conditions.

Global irradiance

Sum of the three components of solar irradiance: direct, diffused and reflected.

Grid
Network of transmission lines, substations, distribution lines and transformers used by central power systems

Grid-connected (PV system)
A PV system in which the PV array acts like a central generating plant, supplying power to the grid.

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

Harmonic distortion

When the pattern of voltage or alternating current moves away from the sinusoidal one, there is a harmonic distortion. In fact, a non-sinusoidal pattern is composed by a basic sinusoid (of 50 Hz in the European electric grid), to which other sinusoids with multiple frequency overlap. According to the multiple of the basic one taken into consideration (of frequency f), we have second harmonic (2f), third harmonic (3f) and so on. Constructors of inverters usually supply the TDH (Total Harmonic Distortion) of the voltage or the output current; in other words, the per cent value of the relationship between the sum of all the harmonic components and the basic one.

Heat absorption

Dark metallic sheet called also picking up plate, on which tubes have been welded; some convector liquid flows in these tubes.

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.

HIT
(Heterojunction with Intrinsic Thin-layer)

Bifacial PV cells made of a very thin layer of amorphous silicon placed on a high efficiency sublayer of monocrystalline silicon.

Hot spot

An undesirable phenomenon of PV device operation whereby one or more cells within a PV module or array act as a resistive load, resulting in local overheating or melting of the cell(s).

Hour angle

Angular distance of the sun from the straight line joining the sun and the earth at midday: this angle changes of 15° every hour and it is measured in daomega = 15 x (12 - ts) where ts is the unit of solar hour.

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

Idling absorption

When functioning, stand-alone inverters absorb power from the batteries and the PV generator, even with no load in output. This power, called idling absorption, depends basically on the inverter size and it can vary from few to some dozens of Watts. Also grid-connected inverters in series present a certain idling absorption (from the grid); this happens when the power supplied by the PV generator drops under the minimum value, necessary to trigger the power transfer between the sections DC and AC of the inverter. Anyway, these power levels are extremely modest.

Incident light

Light that shines onto the face of a solar cell or module.

Insolation

The amount of energy in sunlight reaching an area. Usually expressed in watts per square meter (W/m^2), but also expressed on a daily basis as watts per square meter per day (W/m^2/day).

Inverter

Device that converts DC electricity into AC electricity (single or multiphase), either for stand-alone systems (not connected to the grid) or for utility-interactive systems.

Ion accelerator

Device similar to a particle accelerator which bombards the silicon surface of the PV cell with high-speed phosphor ions. This procedure (called sputtering) allows to examine carefully the depth of ion penetration on the silicon surface.

I-V characteristic

Graphic of the current characteristic as a function of the voltage of a PV device.

I-V curve

Graphic showing the features of current as a function of voltage in a PV device.

Joule (J)
Unit of energy equal to 1/3600 kilowatt-hours.

Junction

The border between the semiconductor areas with opposed polarities. It is called “homojunction” if the semiconductors are equals; “heterojunction” if they are different.

Junction box

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.

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 [kW]

One thousand watts. A unit of power.

KiloWatt-hour [kWh]

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

Langley [L]

Unit of solar radiation. It corresponds to one gram calorie per square centimeter. 1 L corresponds then to 85.93 kWh/m².

Latitude

The angular distance to a point on a celestial object, measured north or south from the celestial equator; it is defined as NORTH or SOUTH latitude according to the hemisphere in which the point is.

LCC
(Life-Cycle Cost)

Method to determine the installation and maintenance costs of a system of electric generation during its total lifetime.

Life cycle or Duration of a battery

Conventional value showing the number of charging and discharging cycles executed by the battery before shutting down. It is usually featured with mode limitations of charging and discharging cycles.

Limit voltage

In ON-OFF regulators it is referred to the amount of voltage beyond which the PV generator automatically disconnects from the accumulator batteries.

Load

Anything in an electrical circuit that, when the circuit is turned on, draws power from that circuit.

Longitude

The arc or portion of the equator intersected between the meridian of a given place and the meridian of Greenwich, passing through the north and south poles at right angles to the equator.

Maximum Power Point Tracker (MPPT)

A power conditioning unit that automatically opera