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A — In a photo voltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).

AC — see alternating current.

Activated Shelf Life — The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.

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.

— A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

— A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.

AIC — See amperage interrupt capability.

Air mass (sometimes called air mass ratio) — Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).

— The temperature of the surrounding area.

Amorphous Semiconductor — A non-crystalline semiconductor material that has no long-range order.

— A thin-film, silicon photo voltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.

— direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current.

— 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.

— A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

Ampere Hour Meter — An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).

Angle of Incidence — The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.

Annual Solar Savings — The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.

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

Anti reflection Coating — A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.

Array — see photo voltaic (PV) array.

Array Current — The electrical current produced by a photo voltaic array when it is exposed to sunlight.

Array Operating Voltage — The voltage produced by a photo voltaic array when exposed to sunlight and connected to a load.

Autonomous System — See stand-alone system.

Availability — The quality or condition of a photo voltaic system being available to provide power to a load. Usually measured in hours per year. One minus availability equals downtime.

— The angle between true south and the point on the horizon directly below the sun. 

B — Represents all components and costs other than the photo voltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.

— In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.

Band Gap Energy (Eg) — The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.

Barrier Energy — The energy given up by an electron in penetrating the cell barrier; a measure of the electrostatic potential of the barrier.

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

— 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.

— The maximum total electrical charge, expressed in ampere-hours, which 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 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 Energy Capacity — The total energy available, expressed in watt-hours (kilowatt-hours), which 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 Energy Storage — Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

Battery Life — The period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. 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 photo voltaic (PV) technology 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, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.

— A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.

— The chemical element commonly used as the dopant in photo voltaic device or cell material.

Boule — A sausage-shaped, synthetic single-crystal mass grown in a special furnace, pulled and turned at a rate necessary to maintain the single-crystal structure during growth.

— The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

— A diode connected across one or more solar cells in a photo voltaic 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 — A chemical element used in making certain types of solar cells and batteries.

— A polycrystalline thin-film photo voltaic material.

— See battery capacity.

Capacity Factor — The ratio of the average load on (or power output of) an electricity generating unit or system to the capacity rating of the unit or system over a specified period of time.

Captive Electrolyte Battery — A battery having an immobilized electrolyte (gelled or absorbed in a material).

— The negative pole or electrode of an electrolytic cell, vacuum tube, etc., where electrons enter (current leaves) the system; the opposite of an anode.

— A method of preventing oxidation of the exposed metal in structures by imposing a small electrical voltage between the structure and the ground.

Cd — see cadmium.

CdTe — see cadmium telluride.

(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). Also see photo voltaic (PV) cell.

— A very thin region of static electric charge along the interface of the positive and negative layers in a photo voltaic 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 or space charge.

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

Charge — The process of adding electrical energy to a battery.

— A free and mobile conduction electron or hole in a semiconductor.

— A component of a photo voltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status.

Charge Factor — A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to 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.

Chemical Vapor Deposition (CVD) — A method of depositing thin semiconductor films used to make certain types of photo voltaic devices. 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.

Cleavage of Lateral Epitaxial Films for Transfer (CLEFT) — A process for making inexpensive Gallium Arsenide (GaAs) photo voltaic cells in which a thin film of GaAs is grown atop a thick, single-crystal GaAs (or other suitable material) substrate and then is cleaved from the substrate and incorporated into a cell, allowing the substrate to be reused to grow more thin-film GaAs.

Cloud Enhancement — The increase in solar intensity caused by reflected irradiance from nearby clouds.

Combined Collector — A photo voltaic device or module that provides useful heat energy in addition to electricity.

— A photo voltaic module, which includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.

(or conduction level) — An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge.

— The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.

Contact Resistance — The resistance between metallic contacts and the semiconductor.

Conversion Efficiency — See photo voltaic conversion efficiency.

Converter — A unit that converts a direct current (dc) voltage to another dc voltage.

— A polycrystalline thin-film photo voltaic material (sometimes incorporating gallium (CIGS) and/or sulfur).

Crystalline Silicon — A type of photo voltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.

Current — See electric current.

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 photo voltaic array from the battery or the load from the battery.

Cycle — The discharge and subsequent charge of a battery.

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. 

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