Solar Energy Glossary I-R
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.
insolation — The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour. See also diffuse insolation and direct insolation.
internal quantum efficiency (internal QE or IQE) — A type of quantum efficiency. Refers to the efficiency with which light not transmitted through or reflected away from the cell can generate charge carriers that can generate current.
intrinsic semiconductor — An undoped semiconductor.
inverted metamorphic multijunction (IMM) cell — A photovoltaic cell that is a multijunction device whose layers of semiconductors are grown upside down. This special manufacturing process yields an ultra-light and flexible cell that also converts solar energy with high efficiency.
ion — An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.
ISPRA guidelines — Guidelines for the assessment of photovoltaic power plants, published by the Joint Research Centre of the Commission of the European Communities, Ispra, Italy.
i-type semiconductor — Semiconductor material that is left intrinsic, or undoped so that the concentration of charge carriers is characteristic of the material itself rather than of added impurities.
I-V curve — 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 characterizes cell performance.
junction — A region of transition between semiconductor layers, such as a p/n junction, which goes from a region that has a high concentration of acceptors (p-type) to one that has a high concentration of donors (n-type).
junction box — A photovoltaic (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.
kerf — The width of a cut used to create wafers from silicon ingots, often resulting in the loss of semiconductor material.
langley (L) — Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2.
lead-acid battery — A general category that includes batteries with plates made of pure lead, lead-antimony, or lead-calcium immersed in an acid electrolyte.
levelized cost of energy (LCOE) — The cost of energy of a solar system that is based on the system's installed price, its total lifetime cost, and its lifetime electricity production.
life — The period during which a system is capable of operating above a specified performance level.
life-cycle cost — The estimated cost of owning and operating a photovoltaic system for the period of its useful life.
light trapping — 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.
line-commutated inverter — An inverter that is tied to a power grid or line. The commutation of power (conversion from direct current to alternating current) is controlled by the power line, so that, if there is a failure in the power grid, the photovoltaic system cannot feed power into the line.
liquid electrolyte battery — A battery containing a liquid solution of acid and water. Distilled water may be added to these batteries to replenish the electrolyte as necessary. Also called a flooded battery because the plates are covered with the electrolyte.
load circuit — The wire, switches, fuses, etc. that connect the load to the power source.
load current (A) — The current required by the electrical device.
load forecast — Predictions of future demand. For normal operations, daily and weekly forecasts of the hour-by-hour demand are used to help develop generation schedules to ensure that sufficient quantities and types of generation are available when needed.
load resistance — The resistance presented by the load. See also resistance.
locational marginal price (LMP) — The price of a unit of energy at a particular electrical location at a given time. LMPs are influenced by the nearby generation, load level, and transmission constraints and losses.
low voltage warning — A warning buzzer or light that indicates the low battery voltage set point has been reached.
maximum power point (MPP) — 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 volts.
maximum power point tracker (MPPT) — Means of a power conditioning unit that automatically operates the photovoltaic generator at its maximum power point under all conditions.
measurement and characterization — A field of research that involves assessing the characteristics of photovoltaic materials and devices.
metrology — The science of measurement.
microgroove — A small groove scribed into the surface of a solar cell, which is filled with metal for contacts.
micrometer (micron) — One millionth of a meter.
minority carrier — A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.
modularity — The use of multiple inverters connected in parallel to service different loads.
module — See photovoltaic (PV) module.
monolithic — Fabricated as a single structure.
movistor — Short for metal oxide varistor. Used to protect electronic circuits from surge currents such as those produced by lightning.
multicrystalline — A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.
nanometer — One billionth of a meter.
National Electrical Code (NEC) — Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, "Solar Photovoltaic Systems" which should be followed when installing a PV system.
NEC — See National Electrical Code.
NEMA — See National Electrical Manufacturers Association.
nickel cadmium battery — A battery containing nickel and cadmium plates and an alkaline electrolyte.
normal operating cell temperature (NOCT) — The estimated temperature of a photovoltaic module when operating under 800 w/m2 irradiance, 20°C ambient temperature and wind speed of 1 meter per second. NOCT is used to estimate the nominal operating temperature of a module in its working environment.
one-axis tracking — A system capable of rotating about one axis.
open-circuit voltage (Voc) — The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.
operating point — The current and voltage that a photovoltaic 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.
orientation — Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.
outgas — See gassing.
panel — See photovoltaic (PV) panel.
parallel connection — A way of joining solar cells or photovoltaic modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage.
passivation — A chemical reaction that eliminates the detrimental effect of electrically reactive atoms on a solar cell's surface.
peak power tracking — See maximum power tracking.
peak sun hours — 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.
peak watt — A unit used to rate the performance of solar cells, modules, or arrays; the maximum nominal output of a photovoltaic device, in watts (Wp) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified.
photocurrent — An electric current induced by radiant energy.
photoelectric cell — A device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers.
photoelectrochemical cell — A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use.
photovoltaic (PV) array — 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.
photovoltaic (PV) cell — The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.
photovoltaic (PV) device — A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device. Solar photovoltaic devices are made of various semiconductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multicrystalline, or amorphous forms.
photovoltaic (PV) effect — The phenomenon that occurs when photons, the "particles" in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available.
photovoltaic (PV) module — 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 direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).
photovoltaic (PV) panel — 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).
photovoltaic-thermal (PV/T) system — A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system or solar thermal system.
physical vapor deposition — A method of depositing thin semiconductor photovoltaic films. With this method, physical processes, such as thermal evaporation or bombardment of ions, are used to deposit elemental semiconductor material on a substrate.
P-I-N — A semiconductor photovoltaic (PV) device structure that layers an intrinsic semiconductor between a p-type semiconductor and an n-type semiconductor; this structure is most often used with amorphous silicon PV devices.
plug-and-play PV system — A commercial, off-the-shelf photovoltaic system that is fully inclusive with little need for individual customization. The system can be installed without special training and using few tools. The homeowner plugs the system into a PV-ready circuit and an automatic PV discovery process initiates communication between the system and the utility. The system and grid are automatically configured for optimal operation.
pocket plate — A plate for a battery in which active materials are held in a perforated metal pocket.
polycrystalline — See multicrystalline.
polycrystalline thin film — A thin film made of multicrystalline material.
power — The amount of electrical energy available for doing work, measured in horsepower, Watts, or Btu per hour.
power conditioning equipment — 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.
power conversion efficiency — The ratio of output power to input power of the inverter.
power density — The ratio of the power available from a battery to its mass (W/kg) or volume (W/l).
power factor (PF) — The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.
projected area — The net south-facing glazing area projected on a vertical plane.
pulse-width-modulated (PWM) wave inverter — A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.
PV — See photovoltaic(s).
pyranometer — An instrument used for measuring global solar irradiance.
quad — One quadrillion Btu (1,000,000,000,000,000 Btu).
qualification test — A procedure applied to a selected set of photovoltaic modules involving the application of defined electrical, mechanical, or thermal stress in a prescribed manner and amount. Test results are subject to a list of defined requirements.
quantum efficiency (QE) — The ratio of the number of charge carriers collected by a photovoltaic cell to the number of photons of a given energy shining on the cell. Quantum efficiency relates to the response of a solar cell to the different wavelengths in the spectrum of light shining on the cell. QE is given as a function of either wavelength or energy. Optimally, a solar cell should generate considerable electrical current for wavelengths that are most abundant in sunlight.
ramp — A change in generation output.
ramp rate — The ability of a generating unit to change its output over some unit of time, often measured in MW/min.
Rankine cycle — A thermodynamic cycle used in steam turbines to convert heat energy into work. Concentrating solar power plants often rely on the Rankine cycle. In CSP systems, mirrors focus sunlight on a heat-transfer fluid. This is used to creates steam, which spins a turbine to generate electricity.
rated battery capacity — The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature. See also battery capacity.
recombination — The action of a free electron falling back into a hole. Recombination processes are either radiative, where the energy of recombination results in the emission of a photon, or nonradiative, where the energy of recombination is given to a second electron which then relaxes back to its original energy by emitting phonons. Recombination can take place in the bulk of the semiconductor, at the surfaces, in the junction region, at defects, or between interfaces.
regulator — Prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.
remote systems — See stand-alone systems.
reserve capacity — The amount of generating capacity a central power system must maintain to meet peak loads.
resistance (R) — The property of a conductor, which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.
ribbon (photovoltaic) cells — A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.
RMS — See root mean square.
root mean square (RMS) — The square root of the average square of the instantaneous values of an ac output. For a sine wave the RMS value is 0.707 times the peak value. The equivalent value of alternating current, I, that will produce the same heating in a conductor with resistance, R, as a dc current of value I.