Solar Energy Glossary S-Z
sacrificial anode — A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.
satellite power system (SPS) — Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous Earth orbit. A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.
scheduling — The general practice of ensuring that a generator is committed and available when needed. It also can refer to scheduling of imports or exports of energy into or out of a balancing area.
sealed battery — A battery with a captive electrolyte and a resealing vent cap, also called a valve-regulated battery. Electrolyte cannot be added.
seasonal depth of discharge — An adjustment factor used in some system sizing procedures which "allows" the battery to be gradually discharged over a 30-90 day period of poor solar insolation. This factor results in a slightly smaller photovoltaic array.
secondary battery — A battery that can be recharged.
semiconductor — Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.
semicrystalline — See multicrystalline.
shallow-cycle battery — A battery with small plates that cannot withstand many discharges to a low state-of-charge.
shelf life of batteries — The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.
shunt controller — A charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less.
shunt regulator — Type of a battery charge regulator where the charging current is controlled by a switch connected in parallel with the photovoltaic (PV) generator. Shorting the PV generator prevents overcharging of the battery.
Siemens process — A commercial method of making purified silicon.
silicon (Si) — A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It's commonly found in sand and quartz (as the oxide).
sine wave — A waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle.
sine wave inverter — An inverter that produces utility-quality, sine wave power forms.
single-crystal silicon — Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon.
smart grid — An intelligent electric power system that regulates the two-way flow of electricity and information between power plants and consumers to control grid activity.
soft costs — Non-hardware costs related to PV systems, such as financing, permitting, installation, interconnection, and inspection.
solar cell — See photovoltaic (PV) cell.
solar constant — The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 watts per square meter or 492 Btu per square foot.
solar cooling — The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners.
solar energy — Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.
solar insolation — See insolation.
solar irradiance — See irradiance.
solar noon — The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky.
solar panel — See photovoltaic (PV) panel.
solar ready main service panel – A term used to describe different types of service panels that can accommodate more PV output than a typical main service panel.
solar spectrum — The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation.
solar thermal electric systems — Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.
space charge — See cell barrier.
specific gravity — The ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. Used as an indicator of battery state-of-charge.
spinning reserve — Electric power plant or utility capacity on-line and running at low power in excess of actual load.
splice bar – A piece of hardware that connects two pieces of PV racking rails together.
split-spectrum cell — 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 also mulitjunction device.
sputtering — A process used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate.
square wave — A waveform that has only two states, (i.e., positive or negative). A square wave contains a large number of harmonics.
square wave inverter — A type of inverter that produces square wave output. It consists of a direct current 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.
stand-off – A term used for the part of the PV module racking that provides distance between the roof surface and the PV modules.
standard reporting conditions (SRC) — A fixed set of conditions (including meteorological) to which the electrical performance data of a photovoltaic module are translated from the set of actual test conditions.
standard test conditions (STC) — Conditions under which a module is typically tested in a laboratory.
state-of-charge (SOC) — The available capacity remaining in the battery, expressed as a percentage of the rated capacity.
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.
stratification — A condition that occurs when the acid concentration varies from top to bottom in the battery electrolyte. Periodic, controlled charging at voltages that produce gassing will mix the electrolyte. See also equalization.
sub-hourly energy markets — Electricity markets that operate on time steps of 5 minutes. Approximately 60% of all electricity in the United States is currently traded in sub-hourly markets, running at 5-minute intervals so that maximum flexibility can be obtained from the generation fleet.
substrate — The physical material upon which a photovoltaic cell is applied.
sulfation — 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.
superconducting magnetic energy storage (SMES) — SMES technology uses the superconducting characteristics of low-temperature materials to produce intense magnetic fields to store energy. It has been proposed as a storage option to support large-scale use of photovoltaics as a means to smooth out fluctuations in power generation.
superconductivity — The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.
superstrate — The covering on the sunny side of a photovoltaic (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.
supply-side interconnection – A type of grid interconnection where a PV system is connected on the supply-side of a facility’s main service breaker. This type of interconnection is typically made between the main service breaker and the utility meter.
surge capacity — The maximum power, usually 3-5 times the rated power, that can be provided over a short time.
system storage — See battery capacity.
temperature compensation — A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than ±5°C from ambient temperature.
temperature factors — It is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures.
thermophotovoltaic cell (TPV) — A device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.
thick-crystalline materials — Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.
tile hook – A type of roof attachment for PV racking that is designed for tile roofs.
tilt angle — The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.
time of use (TOU) electric rate – An electric rate where the utility company charges the customer more money per kilowatt hour for energy used during the “peak” time which are typically the late afternoon and evening on weekdays.
total harmonic distortion — The measure of closeness in shape between a waveform and it's fundamental component.
total internal reflection — The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.
tracking array — A photovoltaic (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.
transformerless inverter – A PV inverter that uses a computerized multi-step process instead of a large transformer to convert the DC voltage from PV modules to the desired AC voltage.
transparent conducting oxide (TCO) — A doped metal oxide used to coat and improve the performance of optoelectronic devices such as photovoltaics and flat panel displays. Most TCO films are fabricated with polycrystalline or amorphous microstructures and are deposited on glass. The current industry-standard TCO is indium tin oxide. Indium is relatively rare and expensive, so research is ongoing to develop improved TCOs based on alternative materials.
tray cable (TC) - may be used for interconnecting balance-of-systems.
trickle charge — A charge at a low rate, balancing through self-discharge losses, to maintain a battery that is fully charged.
trunk cable – Multiconductor cable with proprietary connectors used to connect microinverters in a branch circuit.
tunneling — Quantum mechanical concept whereby an electron is found on the opposite side of an insulating barrier without having passed through or around the barrier.
two-axis tracking — A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).
ultraviolet — Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.
underground feeder (UF) — May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures.
underground service entrance (USE) — May be used within battery enclosures and for interconnecting balance-of-systems.
ungrounded system – A term used to refer to a PV system where the DC negative conductor is not bonded to ground.
uninterruptible power supply (UPS) — The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.
utility-interactive inverter — 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 photovoltaic system's output is fully synchronized with the utility power.
vacuum zero — The energy of an electron at rest in empty space; used as a reference level in energy band diagrams.
valence level energy/valence state — Energy content of an electron in orbit about an atomic nucleus. Also called bound state.
varistor — A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.
vented cell — A battery designed with a vent mechanism to expel gases generated during charging.
vertical multijunction (VMJ) cell — A compound cell made of different semiconductor materials in layers, one above the other. Sunlight entering the top passes through successive cell barriers, each of which converts a separate portion of the spectrum into electricity, thus achieving greater total conversion efficiency of the incident light. Also called a multiple junction cell. See also multijunction device and split-spectrum cell.
voltage — The amount of electromotive force, measured in volts, that exists between two points.
voltage drop – The amount of voltage loss due to impedance in a circuit. In PV systems, excessive voltage drop can occur in long conductor runs between components.
wafer — A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.
watt — The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).
window — A wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.
wire types — See Article 300 of National Electric Code for more information.