III-V cell — A high-efficiency solar cell made from materials including Group III and Group V elements from the periodic table .
AC — See alternating current.
acceptor — 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.
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.
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).
alternating current (AC) — 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.
amorphous semiconductor — A non-crystalline semiconductor material that has no long-range order.
amorphous silicon — A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.
amperage interrupt capability (AIC) — direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current.
ampere hour meter — An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).
ancillary services — Services that assist the grid operator in maintaining system balance. These include regulation and the contingency reserves: spinning, non-spinning, and in some regions, supplemental operating reserve.
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.
antireflection coating — A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.
array — See photovoltaic (PV) array.
autonomous system — See stand-alone system.
balance of system — Represents all components and costs other than the photovoltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.
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.
base load — The average amount of electric power that a utility must supply in any period.
base load generating plants — Typically coal or nuclear generating units that are committed and dispatched at constant or near-constant levels with minimum cycling. They are often the sources of lowest-cost of energy when run at very high capacity factors.
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 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 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 — See building integrated photovoltaics.
blocking diode — 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.
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.
building integrated photovoltaics — A term for the design and integration of photovoltaic (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.
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.
cadmium (Cd) — A chemical element used in making certain types of solar cells and batteries.
capacity (C) — See battery capacity.
captive electrolyte battery — A battery having an immobilized electrolyte (gelled or absorbed in a material).
Cd — See cadmium.
CdTe — See cadmium telluride.
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). See also photovoltaic (PV) cell.
cell barrier — 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 or space charge.
charge — The process of adding electrical energy to a battery.
charge controller — A component of a photovoltaic 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.
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 photovoltaic 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) photovoltaic 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.
concentrating photovoltaics (CPV) — A solar technology that uses lenses or mirrors to concentrate sunlight onto high-efficiency solar cells.
concentrating solar power (CSP) — A solar technology that use mirrors to reflect and concentrate sunlight onto receivers that convert solar energy to heat. This thermal energy is then used to produce electricity with a steam turbine or heat engine driving a generator.
concentrator — A photovoltaic 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.
contact resistance — The resistance between metallic contacts and the semiconductor.
contingency reserves — Reserve services that are sufficient to cover the unplanned trip (disconnect) of a large generator or transmission line and maintain system balance. Contingency reserves are generally split between spinning and non-spinning reserves, and are often based on the largest single hazard (generator or transmission capacity).
conversion efficiency — See photovoltaic (conversion) efficiency.
converter — A unit that converts a direct current (dc) voltage to another dc voltage.
copper zinc tin sulfide/selenide (CZTS) — A polycrystalline thin-film photovoltaic material.
current — See electric current.
current-voltage (I-V) curve — See I-V curve
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.
dangling bonds — A chemical bond associated with an atom on the surface layer of a crystal. The bond does not join with another atom of the crystal, but extends in the direction of exterior of the surface.
DC — See direct current.
deep-cycle battery — A battery with large plates that can withstand many discharges to a low state-of-charge.
deep discharge — Discharging a battery to 20% or less of its full charge capacity.
defect — See light-induced defects
demand response — The process of using voluntary load reductions during peak hours.
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%.
dendrite — A slender threadlike spike of pure crystalline material, such as silicon.
dendritic web technique — A method for making sheets of polycrystalline silicon in which silicon dendrites are slowly withdrawn from a melt of silicon whereupon a web of silicon forms between the dendrites and solidifies as it rises from the melt and cools.
diffuse insolation — Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.
diffuse radiation — Radiation received from the sun after reflection and scattering by the atmosphere and ground.
diffusion furnace — Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material.
direct current (DC) — A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.
direct insolation — Sunlight falling directly upon a collector. Opposite of diffuse insolation.
discharge — The withdrawal of electrical energy from a battery.
discharge factor — A number equivalent to the time in hours during which a battery is discharged at constant current usually expressed as a percentage of the total battery capacity, i.e., C/5 indicates a discharge factor of 5 hours. Related to discharge rate.
disconnect — Switch gear used to connect or disconnect components in a photovoltaic system.
dispatching (economic dispatch) — A method by which system operators decide how much output should be scheduled from plants.
distributed energy resources (DER) — A variety of small, modular power-generating technologies that can be combined with energy management and storage systems and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid.
distributed generation — A popular term for localized or on-site power generation.
distributed power — Generic term for any power supply located near the point where the power is used. Opposite of central power. See also stand-alone systems.
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.
donor — In a photovoltaic device, an n-type dopant, such as phosphorus, that puts an additional electron into an energy level very near the conduction band; this electron is easily exited into the conduction band where it increases the electrical conductivity over than of an undoped semiconductor.
dopant — A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).
downtime — Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.
duty cycle — The ratio of active time to total time. Used to describe the operating regime of appliances or loads in photovoltaic systems.
duty rating — The amount of time an inverter (power conditioning unit) can produce at full rated power.
electric circuit — The path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.
electrochemical cell — A device containing two conducting electrodes, one positive and the other negative, made of dissimilar materials (usually metals) that are immersed in a chemical solution (electrolyte) that transmits positive ions from the negative to the positive electrode and thus forms an electrical charge. One or more cells constitute a battery.
electrodeposition — Electrolytic process in which a metal is deposited at the cathode from a solution of its ions.
electron — An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom. The movement of electrons in an electrical conductor constitutes an electric current.
electron hole pair — The result of light of sufficient energy dislodging an electron from its bond in a crystal, which creates a hole. The free electron (negative charge) and the hole (positive charge) are a pair. These pairs are the constituents of electricity.
energy — The capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.
energy audit — A survey that shows how much energy used in a home, which helps find ways to use less energy.
energy density — The ratio of available energy per pound; usually used to compare storage batteries.
energy imbalance service — A market service that provides for the management of unscheduled deviations in individual generator output or load consumption.
energy levels — The energy represented by an electron in the band model of a substance.
epitaxial growth — The growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal.
equalization charge — The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time.
equinox — The two times of the year when the sun crosses the equator and night and day are of equal length; occurring around March 20 or 21 (spring equinox) and September 22 or 23 (fall equinox).
external quantum efficiency (external QE or EQE) — Quantum efficiency that includes the effect of optical losses, such as transmission through the cell and reflection of light away from the cell.
Fermi level — Energy level at which the probability of finding an electron is one-half. In a metal, the Fermi level is very near the top of the filled levels in the partially filled valence band. In a semiconductor, the Fermi level is in the band gap.
fixed tilt array — A photovoltaic array set in at a fixed angle with respect to horizontal.
flat-plate module — An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.
flat-plate photovoltaics (PV) — 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 life — The number of years that a battery can keep its stated capacity when it is kept at float charge.
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.
float-zone process — In reference to solar photovoltaic cell manufacture, a method of growing a large-size, high-quality crystal whereby coils heat a polycrystalline ingot placed atop a single-crystal seed. As the coils are slowly raised the molten interface beneath the coils becomes single crystal.
full sun — The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).
Ga — See gallium.
GaAs — See gallium arsenide.
gallium (Ga) — A chemical element, metallic in nature, used in making certain kinds of solar cells and semiconductor devices.
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.
gassing current — The portion of charge current that goes into electrolytical production of hydrogen and oxygen from the electrolytic liquid. This current increases with increasing voltage and temperature.
gigawatt (GW) — A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.
grid — See electrical grid.
grid-interactive system — Same as grid-connected system.
harmonic content — The number of frequencies in the output waveform in addition to the primary frequency (50 or 60 Hz.). Energy in these harmonic frequencies is lost and may cause excessive heating of the load.
homojunction — The region between an n-layer and a p-layer in a single material, photovoltaic cell.
hybrid system — A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.
hydrogenated amorphous silicon — Amorphous silicon with a small amount of incorporated hydrogen. The hydrogen neutralizes dangling bonds in the amorphous silicon, allowing charge carriers to flow more freely.
independent system operator (ISO) — The entity responsible for maintaining system balance, reliability, and electricity market operation.
indium oxide — A wide band gap semiconductor that can be heavily doped with tin to make a highly conductive, transparent thin film. Often used as a front contact or one component of a heterojunction solar cell.
infrared radiation — Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.
ingot — A casting of material, usually crystalline silicon, from which slices or wafers can be cut for use in a solar cell.
input voltage — This is determined by the total power required by the alternating current loads and the voltage of any direct current loads. Generally, the larger the load, the higher the inverter input voltage. This keeps the current at levels where switches and other components are readily available.
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