| Aberration | The apparent displacement of a celestial object’s position due to the finite speed of light combined with earth’s orbital motion. The sun appears slightly displaced from its true geometric position, requiring a correction of approximately 20 arcseconds in longitude. |
| Absorption Coefficient | A material property quantifying the rate at which light intensity decreases per unit path length due to absorption (units: m⁻¹). Appears in optical transmittance equations. |
| Air Mass | The ratio of the path length of sunlight through earth’s atmosphere to the path length at zenith (sun directly overhead). Air mass equals 1 at zenith and increases with solar zenith angle, reaching ~38 at the horizon. Higher air mass means more atmospheric absorption and scattering. |
| Albedo | Ground reflectance, expressed as a dimensionless ratio from 0 to 1, representing the fraction of incident solar radiation reflected by the surface. Fresh snow has high albedo (~0.8–0.9), while dark soil has low albedo (~0.1–0.2). Albedo affects the ground-reflected component of plane-of-array irradiance and is particularly important for bifacial module modeling. |
| Along-Axis Slope | The terrain slope component parallel to the tracker row orientation (or parallel to fixed-tilt rows). Along-axis slope creates elevation differences between bays within the same row, affecting which neighboring bays can cast shadows on a receiver bay. Used in slope-aware shading calculations. See also: Cross-Axis Slope. |
| Angle of Incidence | The angle between the sun’s rays and the perpendicular (normal) to a surface. When the sun is directly facing a surface, the angle of incidence is 0° and maximum irradiance is received. As the angle increases, the irradiance on the surface decreases proportionally to cos(θ). For tracking systems, the goal is to minimize angle of incidence to maximize energy capture. |
| Anisotropic | Varying with direction. In transposition models, anisotropic diffuse irradiance refers to sky diffuse that has directional components—such as circumsolar brightening near the sun and horizon brightening near the horizon—rather than being uniformly distributed across the sky dome. The Perez model is an example of an anisotropic transposition model. |
| Anisotropy Index | A dimensionless ratio of direct normal irradiance (DNI) to extraterrestrial DNI, used in the Hay-Davies transposition model to quantify what fraction of diffuse irradiance exhibits directional (circumsolar) characteristics. Values range from 0 (overcast sky, purely isotropic diffuse) to approximately 1 (clear sky, high circumsolar fraction). |
| Anti-Reflective Coating (ARC) | A thin optical layer applied to the front glass of PV modules to reduce reflection losses. The coating has an intermediate refractive index (typically n ≈ 1.22–1.30) between air (n = 1.0) and glass (n ≈ 1.52), which minimizes the refractive index mismatch at each interface. The Physical IAM model explicitly accounts for the ARC layer in its optical calculations. |
| Aerosol Optical Depth | A measure of atmospheric turbidity caused by suspended aerosol particles (dust, smoke, pollution). Higher values indicate hazier conditions with more scattering and absorption of sunlight. Aerosol optical depth affects the ratio of direct to diffuse irradiance and is used in clear-sky irradiance models. |
| Astronomical Unit (AU) | The mean distance from earth to the sun, approximately 149.6 million kilometers (93 million miles). Used as a standard unit for measuring distances within the solar system. Earth’s distance from the sun varies from ~0.983 AU at perihelion to ~1.017 AU at aphelion due to its elliptical orbit. |
| Atmospheric Attenuation | The reduction in solar radiation intensity as it passes through earth’s atmosphere, caused by absorption (by gases like ozone, water vapor, and CO₂) and scattering (by molecules and aerosols). Attenuation increases with air mass—at higher zenith angles, sunlight travels through more atmosphere and experiences greater losses. |
| Azimuth | The horizontal angle measured clockwise from north (0° = North, 90° = East, 180° = South, 270° = West). For the sun (solar azimuth), this is the compass direction to the sun’s position projected onto the horizon. For a surface (surface azimuth), this is the compass direction the surface faces—the horizontal projection of the outward normal vector. |
| Backtracking | A tracker control strategy that rotates modules away from the optimal sun-tracking angle to prevent inter-row shading during low sun angles (morning and evening). When trackers would otherwise cast shadows on adjacent rows, backtracking reduces the rotation angle to eliminate shading, accepting a slightly higher angle of incidence in exchange for avoiding shading losses. Standard backtracking assumes uniform terrain; terrain-aware backtracking accounts for elevation differences between rows. |
| Bandgap Energy | The minimum energy required to excite an electron from the valence band to the conduction band in a semiconductor. The bandgap determines which wavelengths of light a solar cell can absorb and convert to electricity. For crystalline silicon, the bandgap is approximately 1.12 eV, corresponding to a cutoff wavelength of ~1100 nm. Wider-bandgap materials (e.g., CdTe at 1.5 eV) absorb less of the solar spectrum but produce higher voltage per cell. |
| Bay | A section of a single-axis tracker, typically corresponding to one torque tube segment, where all mounted modules share the same tilt angle and follow the same rotation angle. Modules within a bay receive uniform irradiance and have the same angle of incidence. A single tracker is comprised of multiple bays; in articulated trackers, these bays may have different tilts to follow terrain contours. |
| Beam Irradiance | The direct component of solar radiation traveling in a straight line from the sun, as opposed to diffuse irradiance which has been scattered by the atmosphere. Beam irradiance is the dominant component under clear skies and is maximized when the module surface is perpendicular to the sun’s rays. Also called direct irradiance. See also: DNI. |
| Bifacial | A type of PV module that can absorb light from both its front and rear surfaces, generating additional energy from ground-reflected and rear-incident irradiance. Bifacial modules are characterized by their bifaciality factor, which quantifies the rear surface’s relative efficiency compared to the front. See also: Bifaciality Factor. |
| Bifaciality Factor | The ratio of a bifacial module’s rear-side efficiency to its front-side efficiency, expressed as a dimensionless value typically ranging from 0.65 to 0.90. A bifaciality factor of 0.70 means the rear side converts light to electricity at 70% of the front side’s efficiency. Used to weight rear-side irradiance when calculating effective POA irradiance for bifacial modules. |
| Bypass Diode | A diode connected in parallel with a group of solar cells within a PV module. When cells become shaded or damaged, the bypass diode provides an alternative current path, preventing the shaded cells from limiting the entire string’s output and avoiding localized heating (hot spots). Typical crystalline silicon modules have 3 bypass diodes, each protecting approximately one-third of the module’s cells. |
| Built-in Voltage | The contact potential (Vbi) that arises across a semiconductor junction at thermal equilibrium, equal to the difference in chemical potential between the two sides of the junction in isolation. Vbi is an equilibrium property of the junction that characterizes the difference in carrier concentrations between the two sides of the junction. |
| Clearness Index | The ratio of measured global horizontal irradiance (GHI) to extraterrestrial horizontal irradiance, quantifying atmospheric transmittance. Values range from 0 (complete overcast) to ~0.8 (very clear sky). Used in diffuse-direct decomposition models. |
| Circumsolar | Diffuse irradiance concentrated in a bright ring around the solar disk, caused by forward scattering of sunlight by atmospheric particles. Under clear skies, a significant fraction of diffuse irradiance comes from the circumsolar region. Transposition models like Hay-Davies and Perez treat circumsolar irradiance separately from isotropic diffuse because it behaves geometrically similar to direct beam irradiance. |
| Column Depth | The total amount of an atmospheric constituent (such as ozone or water vapor) contained in a vertical column of atmosphere, expressed as the equivalent thickness the substance would have if compressed to standard conditions. For ozone, column depth is typically ~0.3 cm; for water vapor (called precipitable water), typical values range from 0.5 to 5 cm. Column depth is multiplied by air mass to obtain the slant column along the sun’s path. |
| Clipping | The condition where DC power available from the PV array exceeds the inverter’s rated AC output capacity. The inverter moves its operating point away from the array’s maximum power point to limit input power, curtailing output to its rated capacity. Clipping occurs most often around solar noon on clear days in systems with high DC-to-AC ratios. |
| Closure Equation | The fundamental relationship between horizontal irradiance components: GHI = DNI × cos(θz) + DHI, where θz is the solar zenith angle. This equation states that global horizontal irradiance equals the horizontal projection of direct normal irradiance plus diffuse horizontal irradiance. Used for quality control validation and to calculate missing irradiance components. |
| Cross-Axis Slope | The terrain slope component perpendicular to the tracker row orientation (or perpendicular to fixed-tilt rows). Cross-axis slope affects row-to-row shading by changing the relative elevation between adjacent rows. Positive cross-axis slope means rows further from the front are higher. Used in slope-aware shading and terrain-aware backtracking calculations. See also: Along-Axis Slope. |
| Day Angle | The angular position of earth in its orbit, measured in radians from the start of the year. Ranges from 0 to 2π over the course of a year and is used in calculating extraterrestrial irradiance and the equation of time. |
| Declination | The angular distance of a celestial object north or south of the celestial equator, analogous to latitude on earth. Solar declination ranges from +23.45° (summer solstice) to −23.45° (winter solstice) and determines the sun’s path across the sky at different times of year. |
| Decomposition | The process of separating global horizontal irradiance (GHI) into its direct (DNI) and diffuse (DHI) components using empirical models. Common decomposition models include Erbs, Reindl, and DIRINT. These models use the clearness index and other parameters to estimate the fraction of irradiance that is direct vs. diffuse based on atmospheric conditions. |
| Dewpoint Temperature | The temperature at which air becomes saturated with water vapor (relative humidity reaches 100%). At the dewpoint, the air can no longer hold all its moisture and condensation begins. Dewpoint is a direct measure of atmospheric moisture content—higher dewpoint indicates more water vapor in the air. |
| DHI (Diffuse Horizontal Irradiance) | Solar radiation received on a horizontal surface from the sky dome after scattering by atmospheric constituents (molecules, aerosols, clouds), excluding the direct beam component from the sun’s disk. DHI represents the “soft” light that arrives from all directions of the sky. Typical clear-sky values range from 50–150 W/m², increasing under partly cloudy conditions. |
| Diffuse Irradiance | Solar radiation that has been scattered by atmospheric constituents (molecules, aerosols, clouds) and arrives at a surface from all directions of the sky dome rather than directly from the sun. Diffuse irradiance increases under cloudy or hazy conditions and provides the “soft” ambient light even when the sun is obscured. See also: DHI. |
| Diode Ideality Factor | A dimensionless parameter (typically between 1 and 2) that characterizes how closely a PV cell follows the ideal diode equation. In the single-diode model, the ideality factor appears in the exponential term and affects the shape of the I-V curve, particularly the fill factor. |
| DNI (Direct Normal Irradiance) | Solar radiation received on a surface held perpendicular (normal) to the sun’s rays, representing only the direct beam component from the sun’s disk. DNI is the highest-intensity component under clear skies (typically 800–1000 W/m²) and is the primary driver of energy production for tracking systems. DNI drops to zero under overcast conditions. |
| Ecliptic | The plane of earth’s orbit around the sun, or equivalently, the apparent annual path of the sun across the celestial sphere. The ecliptic is inclined at approximately 23.44° to the celestial equator (earth’s equatorial plane projected onto the sky). This tilt causes the seasons. |
| Ecliptic Coordinates | A celestial coordinate system based on the ecliptic plane as the fundamental reference. Positions are specified by ecliptic longitude (measured eastward from the vernal equinox along the ecliptic, 0°–360°) and ecliptic latitude (angular distance north or south of the ecliptic, ±90°). Used in planetary and solar position calculations. |
| Ecliptic Reference Frame | A coordinate system with the ecliptic (earth’s orbital plane) as its fundamental plane. The x-axis points toward the vernal equinox, and the z-axis is perpendicular to the ecliptic. Used for describing planetary positions and as an intermediate step in solar position calculations. See also: Ecliptic Coordinates. |
| Electrical Mismatch | Power loss that occurs when series-connected solar cells or modules operate at different current levels due to partial shading, soiling, manufacturing variations, or temperature differences. Because cells in a string must carry the same current, the lowest-performing cell limits the entire string’s output. At the module level, mismatch between modules in the same string similarly constrains overall power. Electrical mismatch losses can significantly exceed the geometric shading fraction, particularly for crystalline silicon modules. |
| Elevation (Solar) | The angular height of the sun above the horizon, measured in degrees. Also called solar altitude. Ranges from 0° (horizon) to 90° (directly overhead). Related to zenith angle by: elevation = 90° − zenith. |
| Energization | The date on which a PV system block is first connected to the grid and begins producing power. The energization date serves as the reference point for degradation and LeTID calculations; before this date, system output is zero. |
| Ephemeris Time | A historical uniform time scale based on the motion of celestial bodies, independent of earth’s variable rotation. Ephemeris Time was replaced by Terrestrial Time (TT) in 1984, though the two are continuous and equivalent for practical purposes. Used in astronomical calculations where precise orbital mechanics are needed. Differs from Universal Time by ΔT. |
| Equatorial Coordinates | A celestial coordinate system based on earth’s equatorial plane projected onto the celestial sphere. Positions are specified by right ascension (measured eastward from the vernal equinox along the celestial equator, 0h–24h or 0°–360°) and declination (angular distance north or south of the equator, ±90°). Used for specifying positions as seen from earth. |
| Equatorial Reference Frame | A coordinate system with earth’s equatorial plane as its fundamental plane. The x-axis points toward the vernal equinox, and the z-axis points toward the celestial north pole. Used for describing celestial positions as seen from earth and for calculating hour angles. See also: Equatorial Coordinates. |
| Equinox | The moment when the sun crosses the celestial equator, making day and night approximately equal length worldwide. The vernal (spring) equinox occurs around March 20 when the sun moves northward; the autumnal equinox occurs around September 22 when it moves southward. The equinox points also serve as reference points for celestial coordinate systems. |
| Equation of Time | The difference between apparent solar time (based on the actual sun’s position) and mean solar time (based on a hypothetical sun moving at constant rate). The equation of time varies from approximately −14 to +16 minutes throughout the year due to two effects: (1) earth’s elliptical orbit causes varying orbital speed per Kepler’s second law, and (2) earth’s axial tilt causes the sun’s east-west motion projected onto the celestial equator to vary seasonally. PlantPredict uses a second-order Fourier series approximation following Spencer (1971). |
| Fractional Year Angle | The angular position in the annual cycle, ranging from 0 to 2π radians over the course of a year. Computed as γ = (2π/365.24)(n − 1 + t/24), where n is the day of year and t is the hour of day. Used in Fourier series approximations for seasonal solar variations such as the equation of time and extraterrestrial irradiance. Also called the day angle. |
| GAST (Greenwich Apparent Sidereal Time) | The hour angle of the true vernal equinox at the Greenwich meridian. GAST measures earth’s rotational position relative to the stars, accounting for nutation. It is computed from Greenwich Mean Sidereal Time (GMST) plus the equation of the equinoxes (Δψ cos ε). Used to calculate the local hour angle of celestial objects. |
| GCR (Ground Coverage Ratio) | The ratio of collector width (the dimension of the PV module array perpendicular to the tracker axis) to row pitch (the horizontal distance between adjacent tracker rotation axes). GCR is a dimensionless value typically ranging from 0.2 to 0.5 for single-axis trackers. Higher GCR values indicate denser arrays with more ground coverage but increased inter-row shading; lower values reduce shading but require more land area. GCR is a key parameter in backtracking calculations. |
| Geocentric | Measured from earth’s center, treating the planet as a point mass. Geocentric coordinates do not account for the observer’s position on earth’s surface, making them suitable for astronomical calculations where parallax effects are negligible. |
| GHI (Global Horizontal Irradiance) | Total solar radiation received on a horizontal surface from the entire sky hemisphere, including direct beam, diffuse sky radiation, and ground-reflected radiation. GHI is the most commonly measured irradiance component and equals DNI × cos(θz) + DHI, where θz is the solar zenith angle. Typical peak values are 900–1100 W/m² at solar noon under clear skies. |
| Heliocentric | Measured from the center of the sun. Heliocentric coordinates describe earth’s position in its orbit around the sun, used as an intermediate step in calculating the sun’s apparent position as seen from earth. |
| Horizon Brightening | Enhanced diffuse irradiance near the horizon band, caused by atmospheric scattering at low elevation angles where the optical path through the atmosphere is longest. The Perez transposition model includes a horizon brightening component that can be positive (bright horizon band) or negative (dark horizon relative to zenith). Horizon brightening is most pronounced under partly cloudy conditions. |
| IAM (Incidence Angle Modifier) | A correction factor that accounts for optical losses when light strikes a module surface at non-perpendicular angles. As the angle of incidence increases, more light is reflected away rather than transmitted through the glass cover. IAM equals 1 at normal incidence and decreases toward 0 at grazing angles. |
| IEC 61215 | An international standard that defines design qualification and type approval requirements for crystalline silicon terrestrial PV modules. It specifies environmental and performance tests—including thermal cycling, damp heat, and mechanical load—that modules must pass to demonstrate long-term reliability. The standard also defines the procedure for measuring Nominal Operating Cell Temperature (NOCT) under standardized open-rack conditions (800 W/m² irradiance, 20 °C ambient, 1 m/s wind speed). |
| I-V Curve | The current-voltage characteristic of a PV cell or module, describing all possible operating points from short-circuit current (Isc) to open-circuit voltage (Voc). The shape of the modeled I-V curve is determined by the single-diode model parameters and shifts with irradiance and temperature. The maximum power point (MPP) is the point on the curve where the product of current and voltage is greatest. |
| Inverter | A power electronics device that converts direct current (DC) from PV modules into alternating current (AC) for grid injection. The inverter determines its operating point on the DC field’s I-V curve and has a rated AC capacity that limits maximum power delivery. When DC input exceeds this capacity, the inverter clips. |
| Isotropic | Uniform in all directions. In transposition models, isotropic diffuse irradiance refers to sky diffuse that is evenly distributed across the entire sky dome, without directional concentration. Simple transposition models assume all diffuse irradiance is isotropic, while advanced models like Hay-Davies and Perez separate isotropic from anisotropic (circumsolar, horizon) components. |
| Julian Calendar | A calendar system introduced by Julius Caesar in 46 BC, using a 365.25-day year with leap years every 4 years. In astronomy, “Julian” refers to the Julian Day Number—a continuous count of days since January 1, 4713 BC—which provides a uniform time reference independent of calendar systems. Julian Day 2451545.0 corresponds to January 1, 2000, 12:00 TT (the J2000.0 epoch). |
| Local Hour Angle | The angular distance of a celestial object (typically the sun) from the observer’s local meridian, measured westward in degrees or hours. An hour angle of 0° means the object is on the meridian (local noon for the sun); positive values indicate the object has passed the meridian (afternoon). Computed as H = GAST + observer’s longitude − right ascension. |
| Local Solar Time | A time scale based on the sun’s actual position relative to the observer’s meridian. Solar noon (12:00 local solar time) occurs when the sun crosses the local meridian. Local solar time differs from clock time due to the observer’s position within their time zone and the equation of time (which accounts for earth’s elliptical orbit and axial tilt). |
| Local Standard Time | Clock time for a given time zone, defined as a fixed offset from Coordinated Universal Time (UTC). For example, US Eastern Standard Time is UTC−5. Local standard time differs from local solar time due to the observer’s longitude within the time zone and the equation of time. To convert from local solar time to local standard time, apply the equation of time correction and adjust for the difference between the observer’s longitude and the time zone’s central meridian. |
| LID (Light-Induced Degradation) | Initial power loss experienced by PV modules upon first exposure to light, primarily caused by boron-oxygen defect complexes in p-type crystalline silicon cells. LID typically results in a 1–3% reduction in module power during the first hours to days of operation. |
| LeTID (Light and Elevated Temperature Induced Degradation) | A degradation mechanism in silicon solar cells where carrier-induced defects form under illumination at elevated operating temperatures, reducing module efficiency over the first few years of operation. Unlike conventional degradation, LeTID is partially reversible, leading to some level of performance recovery. |
| Maximum Power Point (MPP) | The operating point on a PV module’s current-voltage (I-V) curve where the product of voltage and current yields the highest power output. The MPP shifts with irradiance and temperature — higher irradiance increases current (and thus power), while higher temperature reduces voltage (and thus power). |
| Metastability | A transient performance phenomenon observed in certain PV module technologies (notably CdTe and some HJT designs) where module efficiency varies reversibly depending on recent thermal and irradiance history. Metastable effects arise from changes in material defect states that shift the I-V curve. |
| NOCT (Nominal Operating Cell Temperature) | The cell temperature of a PV module measured under standard open-rack conditions: 800 W/m² irradiance, 20 °C ambient temperature, and 1 m/s wind speed. NOCT provides a standardized thermal reference point for a specific module design and is used in temperature models to predict cell temperature under varying environmental conditions by scaling from this reference. |
| Normalized Path Length | The ratio of the actual path length that sunlight travels through the atmosphere to the path length at zenith (sun directly overhead). This is equivalent to air mass. A normalized path length of 1 means the sun is overhead; values increase with zenith angle, reaching ~38 at the horizon. Used in transmittance calculations where the amount of absorbing or scattering material encountered is proportional to path length. |
| Nutation | Short-period oscillations in earth’s rotational axis caused by gravitational torques from the sun and moon. Nutation causes periodic variations of up to ±17 arcseconds in longitude and ±9 arcseconds in obliquity, with the primary period being 18.6 years. |
| Obliquity | The angle between earth’s rotational axis and the perpendicular to its orbital plane (the ecliptic), currently approximately 23.44°. Obliquity determines the severity of seasons and varies slowly over time due to gravitational perturbations. |
| Open-Circuit Voltage | The voltage across a PV cell or module at open circuit, when no current flows (I=0). Open-circuit voltage (Voc) is determined by the balance between photocurrent and diode recombination current, and decreases with increasing temperature (typically −0.3 to −0.4 %/°C for crystalline silicon). It is one of the key nameplate parameters reported on module datasheets at STC. |
| Orbital Plane | The flat, two-dimensional surface in which a celestial body travels around another. Earth’s orbital plane around the sun defines the ecliptic. Other planets have orbital planes inclined at various angles to earth’s, causing gravitational perturbations that affect earth’s position. |
| Parallax | The apparent displacement of a celestial object’s position due to the observer’s location on earth’s surface rather than at earth’s center. Solar parallax causes the sun’s position to shift by up to ~8.8 arcseconds depending on the observer’s latitude, longitude, and altitude. PlantPredict applies a parallax correction to convert geocentric coordinates to topocentric coordinates. |
| Plane-of-Array Irradiance (POA) | Solar irradiance incident on the tilted surface of a PV module, measured in W/m². POA irradiance is calculated by transposing horizontal irradiance components (GHI, DNI, DHI) to the module plane, accounting for surface tilt, azimuth, and solar position. Total POA irradiance consists of beam (direct), sky diffuse, and ground-reflected components. This is the irradiance that actually reaches the module surface and drives power generation. |
| Photocurrent | The current generated by light absorption in a PV cell, representing the rate at which photon-generated electron-hole pairs are collected. Photocurrent (Iph) is slightly larger than the measured short-circuit current (Isc) because a small fraction of the light-generated current is lost through the diode junction and shunt resistance even at V=0. |
| Polygon Clipping | A computational geometry technique for computing the intersection of two polygons. In shading calculations, polygon clipping determines the overlapping area between a projected shadow polygon and a receiver surface. The resulting intersection polygon represents the shaded portion of the receiver. Used in 3D shading engines and slope-aware linear shading algorithms. |
| Precipitable Water | The total amount of water vapor contained in a vertical column of atmosphere, expressed as the equivalent depth of liquid water if condensed (typically 0.5–5 cm). Precipitable water affects atmospheric absorption of solar radiation, particularly in the infrared spectrum, and is used in clear-sky irradiance models and spectral calculations. |
| Radius Vector | The distance from the center of the sun to the center of earth, measured in astronomical units (AU). Varies from ~0.983 AU (perihelion, early January) to ~1.017 AU (aphelion, early July) due to earth’s elliptical orbit. |
| Rayleigh Scattering | Elastic scattering of light by particles much smaller than the wavelength (such as air molecules N₂ and O₂). The molecule’s electron cloud oscillates in response to the electromagnetic wave and re-radiates at the same frequency. Rayleigh scattering is symmetric—equal amounts scatter forward and backward—and is wavelength-dependent (∝ λ⁻⁴), scattering blue light more than red. This causes the blue color of the sky and contributes to diffuse irradiance in clear-sky models. |
| Recombination | The process by which photo-generated electron-hole pairs recombine and are lost before being collected as electrical current. Recombination reduces both the current and voltage output of a solar cell. In the single-diode model, recombination is captured by the diode term; the 7-parameter extension adds a voltage-dependent recombination current term for improved accuracy at low irradiance. |
| Saturation Current | The reverse-bias leakage current of the diode in the single-diode model, representing the recombination current of charge carriers within the solar cell. It is typically very small (on the order of 10−10 to 10−12 A for a single crystalline-silicon cell at STC) but governs the voltage characteristics of the I-V curve through the diode equation. |
| Saturation Vapor Pressure | The maximum partial pressure of water vapor that air can hold at a given temperature, expressed in hPa or kPa. Saturation vapor pressure increases exponentially with temperature—warm air can hold substantially more moisture than cold air. The August-Roche-Magnus equation provides an empirical approximation for calculating saturation vapor pressure from temperature. The ratio of actual vapor pressure to saturation vapor pressure defines relative humidity. |
| Refraction | The bending of light as it passes through earth’s atmosphere, caused by the decrease in air density with altitude. Atmospheric refraction makes the sun appear higher than its true geometric position, with the effect increasing at lower elevations. At the horizon, refraction displaces the sun by approximately 0.57°. PlantPredict applies a refraction correction to the calculated zenith angle based on temperature and pressure. |
| Refractive Index | A dimensionless ratio of the speed of light in vacuum to the speed of light in a material, determining how much light bends and reflects when passing between media. Air has n ≈ 1.0, anti-reflective coatings typically n ≈ 1.22–1.30, and solar glass n ≈ 1.52. The refractive index determines both the refraction angle and the Fresnel reflection at each interface. |
| Relative Humidity | The ratio of the actual water vapor pressure in the air to the saturation vapor pressure at the same temperature, expressed as a percentage (0–100%). At 100% relative humidity, the air is saturated and cannot hold more water vapor without condensation. |
| Right Ascension | The angular distance of a celestial object measured eastward along the celestial equator from the vernal equinox, analogous to longitude on earth. Typically expressed in hours, minutes, and seconds (0h to 24h) or degrees (0° to 360°). |
| Series Resistance | The total ohmic resistance in the current path of a PV module, including cell metallization, solder bonds, cell interconnect ribbons, junction box connections, and DC wiring. Series resistance causes I²R power losses that increase with current. |
| Sidereal Time | Time measured relative to the fixed stars rather than the sun. A sidereal day is ~4 minutes shorter than a solar day because earth advances in its orbit. Local sidereal time is used to calculate the local hour angle of celestial objects. |
| Short-Circuit Current | The maximum current a PV cell or module produces when its terminals are directly connected (V=0). Short-circuit current scales approximately linearly with irradiance and has a small positive temperature coefficient (typically 0.03–0.06 %/K). At STC, it is one of the key nameplate parameters reported on module datasheets. In the single-diode model, Isc is slightly less than the photocurrent Iph because a small fraction of the light-generated current flows through the diode and shunt resistance even at zero voltage. |
| Shunt Resistance | A parasitic resistance across the PV cell junction, representing leakage current paths caused by manufacturing defects, crystal grain boundaries, or edge effects. Ideal cells have infinite shunt resistance; real cells have finite values that allow a small current to bypass the junction, reducing output power — particularly at low irradiance when the photocurrent is small relative to the leakage current. |
| Single-Diode Model | An equivalent circuit that represents a PV cell as a light-generated current source in parallel with a diode and shunt resistance, all in series with a series resistance. The diode models solar cell’s nonlinear voltage-current relationship. Common variants include the 5-parameter model (one diode) and a 7-parameter extension that adds a recombination current term for improved accuracy at low irradiance. |
| Sky Dome | The hemispherical view of the sky from a point on the earth’s surface, representing all possible directions from which diffuse irradiance can arrive. The sky dome is the source of diffuse irradiance, with brightness varying across its extent (brighter near the sun and horizon under clear skies). View factors quantify what fraction of the sky dome is visible from a tilted surface. |
| Shading Factor | A dimensionless value from 0 to 1 representing the fraction of irradiance that reaches the module surface after accounting for shadows. A shading factor of 1 means no shading (full irradiance); 0 means complete shading. PlantPredict calculates separate shading factors for beam, sky diffuse, and ground-reflected irradiance components. |
| Soiling | The accumulation of dust, dirt, pollen, bird droppings, and other debris on the surface of PV modules, reducing light transmission and energy production. Soiling losses vary by location, season, and weather patterns, and can range from 1–2% in rainy climates to 10–25% in dusty or arid regions without regular cleaning. PlantPredict applies a soiling factor (0–1) to all POA irradiance components. |
| Solid Angle | A three-dimensional angle that measures the apparent size of an object as seen from a given point, expressed in steradians (sr). |
| Slant Column | The total amount of an atmospheric constituent (such as ozone or water vapor) along the actual slant path from the sun to the observer, as opposed to the vertical column. Calculated as the product of column depth and air mass (normalized path length): slant column = column depth × AM. For example, with ozone column depth of 0.3 cm and air mass of 2, the ozone slant column is 0.6 cm. |
| Spectral Effects | Changes in the solar spectrum as sunlight passes through the atmosphere, affecting PV module performance. At higher air mass (lower sun angles), the atmosphere absorbs more blue light and transmits relatively more red/infrared light. Different PV cell technologies respond differently to these spectral changes—modules with narrow spectral response (e.g., CdTe) are more affected than those with broad response (e.g., crystalline silicon). PlantPredict uses spectral shift models to account for these effects. |
| Spectral Mismatch | The difference between the actual solar spectrum incident on a PV module and the reference spectrum (typically AM1.5G) used to rate module performance. Spectral mismatch causes modules to perform slightly better or worse than their rated efficiency depending on atmospheric conditions, air mass, precipitable water, and the module’s spectral response. Different cell technologies (c-Si, CdTe, CIGS, etc.) have different spectral responses, making some more sensitive to spectral shifts than others. The spectral mismatch modifier adjusts the effective irradiance to account for this deviation from standard test conditions. See also: Spectral Effects. |
| STC (Standard Test Conditions) | The reference conditions under which PV module performance is rated: 1000 W/m² irradiance, 25 °C cell temperature, and AM1.5G solar spectrum. Module nameplate power (Pmax), short-circuit current (Isc), and open-circuit voltage (Voc) are specified at STC. |
| Terrestrial Time (TT) | A uniform time scale based on celestial mechanics, independent of earth’s variable rotation. TT replaced Ephemeris Time in 1984 and is used for astronomical ephemerides and orbital calculations. TT runs at a constant rate defined by atomic clocks, differing from Universal Time by ΔT (67 seconds in PlantPredict). In the SPA algorithm, TT is used to compute planetary positions while UT is used for earth rotation. |
| Topocentric | Measured from a specific point on earth’s surface, as opposed to geocentric (from earth’s center). Topocentric coordinates account for the observer’s actual position including latitude, longitude, and elevation. |
| Transposition | The process of converting horizontal irradiance components (GHI, DNI, DHI) to plane-of-array (POA) irradiance on a tilted surface. Transposition models calculate beam, sky diffuse, and ground-reflected irradiance contributions based on surface orientation, solar position, and view factors. Common transposition models include Hay-Davies and Perez. |
| Transmittance | The fraction of solar radiation that passes through the atmosphere to reach the earth’s surface. In clear-sky models, transmittance decreases with air mass as sunlight travels through more atmosphere. The DISC model decomposes transmittance into clear-sky transmittance (Knc, the theoretical maximum under clear conditions) minus a transmittance reduction (ΔKn, accounting for clouds, aerosols, and turbidity), yielding actual transmittance (Kn). DNI is then calculated as transmittance times extraterrestrial irradiance. |
| Turbidity | A measure of the cloudiness or haziness of the atmosphere caused by aerosols, water vapor, dust, and other suspended particles that scatter and absorb sunlight. Higher turbidity reduces direct normal irradiance and increases diffuse irradiance. In decomposition models, turbidity effects are captured indirectly through the clearness index and transmittance reduction factors. Unlike clouds, which cause rapid fluctuations in irradiance, turbidity typically produces steady attenuation—a distinction exploited by the DIRINT model’s temporal stability parameter. |
| Universal Time (UT) | A time standard based on earth’s rotation, closely approximating mean solar time at the Prime Meridian. UT1 is determined by astronomical observations and varies slightly due to irregularities in earth’s rotation. UTC (Coordinated Universal Time) is kept within 0.9 seconds of UT1 through leap seconds. |
| Vernal Equinox | The point on the celestial sphere where the sun crosses the celestial equator moving from south to north, occurring around March 20. Also called the “First Point of Aries,” it serves as the zero point for measuring right ascension and ecliptic longitude. The position of the vernal equinox slowly shifts due to precession (~50 arcseconds/year) and oscillates due to nutation. |
| View Factor | The geometric factor describing what fraction of radiation leaving one surface reaches another surface. In transposition models, view factors quantify how much of the sky dome and ground are visible from the tilted module surface. For a surface tilted at angle β, the view factor to the sky dome is (1 + cos β)/2, and the view factor to the ground is (1 − cos β)/2. A horizontal surface (β = 0°) sees the entire sky dome (view factor = 1) and no ground (view factor = 0); a vertical surface (β = 90°) sees half sky and half ground (view factor = 0.5 each). |
| VSOP87 | Variations Séculaires des Orbites Planétaires (Secular Variations of Planetary Orbits), a high-precision planetary theory developed by Bretagnon and Francou (1988). VSOP87 provides polynomial and periodic series for computing the heliocentric positions of planets. The NREL Solar Position Algorithm uses VSOP87 to calculate earth’s heliocentric longitude, latitude, and radius vector. |
| Zenith Angle | The angle between the local vertical (zenith) and the line of sight to the sun, measured in degrees. A zenith angle of 0° means the sun is directly overhead; 90° means it is at the horizon. Related to solar elevation by: zenith angle = 90° − elevation. |
