Glossary of Terms: Solar Energy Systems

aggregate efficiency: Applied to solar panels, the electrical efficiency, thermal efficiency, or sum of the above; depending on whether the panel is photovoltaic thermal, or hybrid.

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.

aperture efficiency: The complement of aperture loss.

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.

calibration: An adjustment required for the accurate functioning of an instrument. A CPV element that has mechanical linkages to other elements or uses open-loop tracking likely requires calibration.

closed loop tracking: A method of heliostatic tracking in which orienting movements are based on measurements of sunlight falling on the solar module or CPV element.

concentrating optics: A system of non-imaging optics that captures and focuses directional light, where the focus is generally either point-like or line-like.

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.

concentration ratio: The ratio of a concentrator's aperture to the area of its receivers.

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 element: A rigid assembly having concentrating optics and one or more PV cells at the optics' focus or foci, designed to operate in an array.

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.

crystalline silicon: Form of bulk silicon -- of either mono- or poly-crystalline type -- used to manufacture the PV cells found in the vast majority of PV panels.

efficiency: As applied to solar panels, the ratio of output power -- electrical and/or thermal -- to the luminous solar power falling in the panels' aperture, under standard illumination conditions. See kilowatt peak.

efficiency coefficient: Property of energy capture stage representing the fraction of received energy that is transmitted to the next stage.

electrical efficiency: For a photovoltaic device, the ratio of output electrical power to input electromagnetic radiation power.

gigawatt (gW): One billion watts.

group III-V semiconductor: Semiconductor based on elements from groups III and V of the Periodic Table, such as germanium, gallium, indium, and arsenic.

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.

hybrid: Solar energy system that uses both photovoltaic and thermal energy harvesting means.

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.

kilowatt (kW): One thousand watts.

kilowatt hour (kWh): Unit of energy equal to the work done by the power of a kilowatt operating for one hour.

kilowatt peak (kWp): Unit of measure for the output of a PV module under standard test conditions, where the module is irradiated with a flux of 1,000 W/m² at a temperature of 25ºC.

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.

maximum power point: The point on the current-voltage curve of a PV module under illumination where the power (voltage times current) is maximum.

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.

MEMS (MicroElectroMechanical System): A technology that embeds mechanical devices -- such as actuators, mirrors, sensors, and valves -- within semiconductor chips.

multi-axis tracking: Heliostatic tracking in which the sun is kept on a line defined by the instrument through pivoting about multiple axes, such as two nested axes.

normal aperture: The aperture of a solar energy device when the sun is in its normal direction.

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.

photovoltaic (PV): Using the photoelectric effect, as does the familiar PV panel, to convert sunlight directly to electricity.

plate-type panel: The familiar type of solar panel, in which the energy is captured throughout the panel's surface, over a wide range of incidence angles.

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.

PV cell: A photodiode that converts energy in the form of light into electricity.

PV module: A solar module that uses PV cells.

PV panel: A solar panel that uses PV cells.

receiver: Device lying at the focus of an optical concentrator, typically a PV cell and/or thermal transfer apparatus.

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.

silicon: Element immediately below carbon on the Periodic Table, semiconductor that is the basis of most PV panels.

silicon dioxide: Main constituent of the feedstock used to make Silicon-based semiconductors, the basis for the most widely used form of photovoltaic panels.

single-axis tracking: Heliostatic tracking in which the sun is kept in a plane defined by the instrument through tilt about a single axis on that plane.

solar cell: An individual photosensitive device that converts sunlight to electricity, usually assembled into larger units such as solar panels. Generally synonymous with PV cell.

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.

solar panel value equation: A framework for computing the value of a solar panel based on its efficiency, purchase cost, and maintenance costs.

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.

STC (Standard Test Conditions): A set of reference measurement conditions consisting of an irradiance of 1000 W/m², a reference air mass of 1.5, and a cell temperature of 25ºC.

system: Referring to a stage of energy capture in a solar energy system as a whole, including the space occupied by panels, tracking mounts, and other equipment.

thermal: In a solar energy system, the usable heat energy held in a working fluid due to its elevated temperature.

thin-film technology: Method of manufacturing photovoltaics in which thin films of material are deposited on a base material. Contrasted with photovoltaics based on bulk silicon fabrication methods.

tracking mount: The structure and mechanism that supports a solar module and changes its tilt, on one or more axes, as required for heliostasis.

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.