A
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).
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°.
B
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 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The total energy available, expressed in watt-hours (kilowatt-hours), which can be withdrawn from a fully charged cell or battery.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.
BIPV (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.
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.
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.
C
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). Also see 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.
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.
D
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%.
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.
E
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.
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.
I
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.
K
K-value
The k-value, or heat transfer coefficient, is the measured value of the heat flow which is transferred through an area of 1 m² at a temperature difference of 1 K. The units of measure are watts per square meter per temperature difference (W/m²K). K-value = energy / (area x temperature difference x time).
This value is used to measure the Heat Insulation Efficiency of Buildings, and in the case of Passive Buildings, all components must have a k-value of at least 0.16 W/m2K to be considered energy efficient.
Kilowatt-Peak (kWh)
Abbreviation for kilowatt-peak, a measure of the peak output of a photovoltaic system, for example.
The electric characteristics of Solar Cells, and therefore of the entire Generator, vary with respect to various general conditions, especially the radiation intensity. In Photovoltaics, the maximum possible output of a solar generator operating under standard conditions is defined as its peak output, which is measured in watts or kilowatts and stated as either Wp (watt, peak) or kWp, respectively. An optimal Solar Radiation of 1000 W/m2 is defined as the standard condition, and it can be reached early afternoon on a sunny summer day (however, the mean output over the period of a year is only about one thenth of the peak output due to night-time and less than optimal day-time sun conditions). The peak output is so based on measurements under optimal conditions, and, specifically, the peak output (some Manufacturers also designate this as the nominal or rated value: rated power, rated output, nominal power, nominal output) results from the product of the nominal voltage and the nominal current. More enlightening information over the properties of solar cells or generators can be found on the current/voltage curve at the right. When planning a photovoltaic system, its Performance Ratio (conversion Efficiency) is important as it describes which part of the radiation energy is converted into useable electric current.
N
Normal Operating Cell Temperature (NOCT)
Operating temperature at which the cells in a solar module under standard operating conditions (SOC). These conditions are: the radiation of 0.8 kW/m2, 20º C ambient and average wind speed of 1 m/s, with the cell or module in a state of open circuit temperature, wind oriented parallel to the plane of the array, and all parts of the matrix is completely exposed to the wind.
P
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).
R
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 non radiative, 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.
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.
S
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.
SOC
It is a set of reference conditions for the measurement of a photovoltaic device to the irradiation of 0.8 kW/m 2, 20° C ambient temperature and average speed of 1 m/s, with the wind oriented parallel to the plane of the matrix, and all parts of the matrix exposed to wind completely. See also the normal operating temperature of the cell (NOCT).
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.
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 multitjunction device.
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.
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.
T
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.
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.
V
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 multijunction device and split-spectrum cell.