Glossary of Terms: Solar Energy Systems
air mass: The optical path through the earth's atmosphere traversed by light passing from a celestial body to a point on the earth's surface. Ignoring clouds and surface elevation, the air mass for a given ray of light is roughly the inverse of the cosine of the angle between the ray's celestial source and the zenith.
aperture: Used as a dimension, the area of the projection of the opening of a solar collector onto a plane perpendicular to the direction of sunlight. Throughout the day and seasons, the aperture of a stationary solar panel varies in proportion to the cosine of the angle between the direction of sunlight and the normal to the panel's face.
aperture loss: Light that falls within the aperture of a stage such as the system, panel, or module, but then fails to be transmitted to the next stage for reasons other than lack of optical efficiency. Typically such losses result from light being blocked or 'falling between the cracks', as is seen especially in HCPV systems.
array: Referring to a solar energy system, the arrangement of generally identical elements -- such as solar panels, CPV elements, or CPV modules in a one- or two-dimensional grid, where the elements may be fixed or individually mounted to track the sun.
bandgap energy: The characteristic energy of the photodiode, corresponding to a specific wavelength of light.
BTU (British Thermal Unit): Unit of energy equal to about 1060 joules, and approximating the energy needed to heat one pound of water one degree Fahrenheit.
bulk silicon: Form of silicon used in photovoltaics typically manufactured by growing ingots of silicon, sawing it into thin wafers and doping the wafers' two sides. Contrasted with thin film methods of photovoltaic manufacture.
concentrator: In solar energy generation, a device which captures sunlight and focuses it onto one or more receivers whose aperture is much smaller than that of the concentrator, using concentrating optics.
CPV (Concentrating PhotoVoltaic): The use of optics to concentrate and direct sunlight to photovoltaic cells, reducing the amount of photovoltaic material required to convert the solar energy passing into a solar collector.
CPV panel: A CPV device that functions in a fixed-position installation like a plate-type solar panel. A practical MCPV or HCPV panel encloses an array of tiltably-mounted CPV elements within a relatively thin slab.
efficiency: As applied to solar panels, the ratio of the energy -- electrical and/or thermal -- captured and converted by the panels to the available electromagnetic energy falling on the panels' aperture, under standard illumination conditions. See kilowatt peak.
heliostat: A system that maintains alignment between an instrument, such as a CPV module, and the direction of sunlight as the sun moves across the sky.
HCPV (High-Concentration PhotoVoltaic): Concentrating photovoltaic systems that use high sunlight concentration ratios, typically greater then 100. HCPV systems generally require the use of two-axis tracking of optics to keep sunlight convergent on small spots.
incidence angle: In reference to light falling on a PV cell, the departure of the direction of a ray of light from the cell's normal direction. Conversion efficiency of a PV cell decreases with increasing incidence angle, with steep declines beyond incidence angles of about 45 degrees.
line-focusing optics: Concentrating optics that focus light parallel to an optical plane into narrow bands proximal to that plane, and thereby requires only single-axis tracking to maintain focusing alignment with the sun.
MCPV (Medium-Concentration PhotoVoltaic): Concentrating photovoltaic systems that use a moderate ratio of sunlight concentration, typically between 10 and 100. MCPV systems generally employ single-axis tracking of elongate optics to keep sunlight convergent on narrow bands.
normal direction: or normal; The direction perpendicular to a surface, pointing away from it.
optical efficiency: The fraction of light power that is transmitted from the aperture of a concentrator to the receiver surface. Imperfections in the surfaces of lenses and mirrors, lack of clarity of lenses, and lack of reflectance in mirrors are all factors reducing optical efficiency.
photon: A particle of light, being quantum packet of electromagnetic radiation having a characteristic energy, or frequency, which is the invese of its wavelength.
point-focusing optics: Concentrating optics that focus light parallel to an optical axis into small spots proximal to that axis, and thereby requires multi-axis tracking to maintain focusing alignment with the sun.
PTC (PVUSA Test Condition): Also called 'Performance Test Condition', a set of test conditions developed by the Photovoltaics for Utility Scale Applications project, which more closely approximates real-world conditions than the STC.
PV cell: A photodiode that converts energy in the form of light into electricity.
renewable energy: Energy generated from sources that either grow or are inexhaustible, such as sunlight and wind.
semiconductor: A material whose electrical conductivity can be directional and influenced by light, heat, and electrical or magnetic fields.
solar module: Component of a solar energy system that captures and converts solar energy, and is manufactured, rated, and shipped as a unit. In reference to PV panels, module is generally synonymous with panel. In reference to CPV systems, module refers to a rigid CPV unit that functions when light is aligned with its optical axis or axes. Not to be confused with CPV-elements-as-modules populating a CPV panel.
solar energy system: A system that harvests energy from sunlight by capturing the sun's radiant energy falling within the system's aperture and converting it to a desired form of energy such as electricity or heat carried by a fluid.
solar panel: A device that harvests electrical and/or thermal energy from sunlight, has an easily measured normal aperture -- typically that of its front rectangular face -- and does not depend on an external heliostat to function.
stage of energy capture: One in a series of passages by solar energy moving through a solar energy system en route to its receiver(s) that can be measured and assigned an efficiency coefficient. We define stages of system, panel, module, and PV cell.
triple-junction cell: Photovoltaic cells composed of a sandwich of three different photodiodes, each tuned to a different bandgap energy. The most efficient PV cells available use triple junctions consisting of group III-V semiconductors. Originally designed for powering spacecraft, most such cells are produced for terrestrial use in CPV applications. Cells are optimized for particular concentration ratios, and their efficiency peaks at high concentrations -- typically between 100 and 1000 suns.
watt: Standard unit of electrical power.
zenith: The direction of up, directly opposite the center of the earth.