Solar Engineering
71 terms in the solar engineering category.
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Albedo
Albedo is the fraction of incident solar radiation reflected by the ground surface beneath PV modules. Albedo values range from 0.05 (dark asphalt) to 0.85 (snow), critically affecting bifacial gain and rear-side energy.
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Arc Fault
An arc fault in a solar PV DC circuit is an unintended electric arc between conductors caused by loose connections, damaged insulation, or component failure. NEC 690.11 requires Arc-Fault Circuit Interrupter (AFCI) protection on DC circuits operating above 80 V.
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Azimuth
Azimuth is the compass orientation of a PV array, measured from true north (0°) clockwise to 360°. South-facing (180°) is optimal for annual energy in the northern hemisphere; deviations cost yield but may align with tariff incentives.
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Backtracking
Backtracking is the control algorithm in single-axis solar trackers that rotates modules against the sun's direction during early morning and late afternoon, preventing one row from shading the adjacent row at low solar angles. Recovers 3–5% annual energy.
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Bifacial Gain
Bifacial gain is the additional annual energy produced by bifacial PV modules from rear-side irradiance — typically 5–25% above monofacial output, driven by ground albedo, ground clearance, GCR, and tracker geometry.
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Bypass Diode Activation
Bypass diode activation is the conduction of a PV module bypass diode when a cell group is partially shaded. Current routes around the shaded cells, preventing hot-spot damage but creating a step in the I-V curve and reducing module Pmax.
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Cell Microcrack
Cell microcracks are sub-visible fractures in PV silicon cells caused by transport, installation, or thermal stress. Initially invisible but worsen over thermal cycling, leading to power loss, mismatch, and hot spots.
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Combiner Box
A combiner box is an electrical enclosure that consolidates multiple PV strings into a single DC output for transmission to the inverter. Each string input includes an OCPD (typically a fuse), and the combiner output may include a disconnect, surge protection device (SPD), and monitoring.
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Conductor Ampacity
Conductor ampacity is the maximum current (in amperes) a conductor can carry continuously under specified ambient and bundling conditions without exceeding its temperature rating. NEC Table 310.16 provides base ampacities for 60°C, 75°C, and 90°C insulations.
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Conduit Fill
Conduit fill is the NEC-defined limit on the percentage of conduit cross-sectional area occupied by conductors. Maximum fill is 40% for ≥3 conductors. More than 3 current-carrying conductors triggers ampacity derating per NEC Table 310.15(B)(3).
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DC Optimizer
A DC optimizer is a small electronic device attached to each PV module that provides per-module MPPT and rapid shutdown compliance. Modules are wired in series to a central string inverter, while the optimizer manages individual module performance and partial shading mitigation.
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DHI (Diffuse Horizontal Irradiance)
Diffuse Horizontal Irradiance (DHI) is the diffuse component of solar irradiance on a horizontal surface, in W/m². Sum of DHI plus direct beam (projected onto horizontal) = GHI.
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Diffuse Irradiance
Diffuse irradiance is the portion of solar radiation that has been scattered by atmospheric particles, clouds, and aerosols, reaching the surface without a defined direction. Important in cloudy climates where diffuse can exceed 50% of GHI.
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DNI (Direct Normal Irradiance)
Direct Normal Irradiance (DNI) is the solar radiation received per unit area perpendicular to the sun's direct beam, excluding diffuse and reflected components. Critical for solar tracker and concentrated solar power (CSP) yield modeling.
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Dual-Axis Tracker
A dual-axis solar tracker rotates modules on two axes — east-west daily and north-south seasonally — to maintain perpendicular orientation to the sun throughout the year. Yields +25–30% over fixed tilt but costs roughly 2× a single-axis tracker.
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East-West Layout
An east-west solar layout mounts modules in alternating east-facing and west-facing rows or sub-arrays, producing a flatter daily energy profile than south-facing arrays. Common on carports, low-tilt commercial roofs, and NEM 3.0 residential design.
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EGC (Equipment Grounding Conductor)
Equipment Grounding Conductor (EGC) is the dedicated conductor that bonds all metallic electrical equipment to the grounding electrode system, providing a low-impedance fault current path. Sized per NEC 250.122 Table based on OCPD rating.
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EL Imaging (Electroluminescence)
Electroluminescence (EL) imaging captures the infrared emission from PV cells under applied current, revealing microcracks, soldering defects, PID, and dead cells. Standard quality control during manufacturing and field commissioning.
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Equipotential Bonding
Equipotential bonding is the connection of all metallic components — racking, frames, conduits, equipment chassis — to a common ground, ensuring they remain at the same potential. Prevents shock during faults and reduces induced lightning damage.
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Fill Factor
Fill Factor (FF) is the ratio of Pmax to the product of Voc × Isc, measuring how 'square' the I-V curve is. Modern c-Si modules: 0.75–0.85. Higher FF indicates better cell quality and lower series resistance.
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Fixed Tilt
Fixed-tilt mounting holds PV modules at a constant tilt angle and azimuth, typically aligned for maximum annual energy at the site's latitude. Simpler, more reliable, and ~15–20% lower yield than horizontal single-axis trackers.
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GCR (Ground Coverage Ratio)
Ground Coverage Ratio (GCR) is the ratio of total PV module area to total ground area of a solar installation. Higher GCR fits more modules per acre but increases inter-row self-shading and reduces bifacial gain.
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GHI (Global Horizontal Irradiance)
Global Horizontal Irradiance (GHI) is the total solar radiation received on a horizontal surface per unit area, in W/m². It is the sum of direct beam (DNI projected) and diffuse horizontal irradiance (DHI). GHI is the foundation metric for solar resource assessment.
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Ground Fault
A ground fault in a solar PV system is an unintended current path between an energized conductor and ground or grounded equipment. NEC 690.41(B) requires ground-fault protection that detects, indicates, and interrupts dangerous ground-fault currents.
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Horizon Profile
Horizon profile is the azimuthal elevation angle of distant obstructions (mountains, ridgelines, distant buildings) around a solar site. Input to PVsyst's far-shading analysis as opposed to near-shading from immediate obstructions.
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Hot Spot
A hot spot is localized overheating in a PV cell, occurring when a partially shaded or damaged cell reverse-biases under the full string current. Bypass diodes mitigate; cracked cells without functioning bypass can cause fire.
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I-V Curve
The I-V curve is the graphical relationship between current (I) and voltage (V) of a PV module under specified irradiance and temperature. Four characteristic points: Isc (V=0), Vmp/Imp (max power), Pmax, and Voc (I=0).
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Imp (Maximum Power Current)
Imp (Maximum Power Current) is the current at which a PV module produces its maximum power, at STC. Typically 0.92 × Isc. Used to verify the inverter per-MPPT current limit accommodates the parallel string count.
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IR Thermography
IR thermography is the use of thermal infrared cameras to identify temperature anomalies in operating PV modules and electrical connections. Detects hot spots, failed bypass diodes, loose MC4 connectors, and module-level issues.
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Isc (Short-Circuit Current)
Short-Circuit Current (Isc) is the maximum current a PV module produces when its output terminals are shorted, measured at Standard Test Conditions. NEC 690.8 conductor and OCPD sizing is based on 1.25 × Isc.
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LeTID
LeTID (Light and elevated Temperature Induced Degradation) is a slower form of PV module degradation in PERC cells occurring over months at elevated cell temperatures. Total loss typically 2–4% over module lifetime; reduced in modern PERC and absent in TOPCon/HJT.
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LID (Light Induced Degradation)
Light Induced Degradation (LID) is the initial power loss in PV modules during the first weeks of sunlight exposure. PERC: 1.5–2.5% LID. TOPCon: <0.5%. HJT: <0.3%. Accounted for in 25-year warranty calculations.
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Lightning Protection System (LPS)
A Lightning Protection System (LPS) is the integrated set of air terminals, down conductors, and earth electrodes that captures direct lightning strikes and safely dissipates the current to ground, protecting solar PV equipment per IEC 62305 (international) and IS 16942 (India).
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MLPE (Module-Level Power Electronics)
Module-Level Power Electronics (MLPE) refers to electronic devices installed at each PV module — typically microinverters or DC optimizers — that enable per-module MPPT, rapid shutdown compliance per NEC 690.12, and granular performance monitoring.
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Module Efficiency
Module efficiency is the ratio of electrical power output to incident solar power per unit area, measured at STC. Modern crystalline silicon modules: 20–24%. Determines kW per area for given site.
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Module Frame Bonding
Module frame bonding is the electrical connection of all PV module aluminum frames to the equipment grounding conductor (EGC), providing a safety ground path. Required by NEC 690.43; commonly accomplished via UL 2703 listed racking systems.
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Module Mismatch
Module mismatch is the energy loss caused by variations in individual PV module electrical characteristics (Pmax, Imp, Vmp) within a series string. The string operates at the lowest common current, costing typically 1–3% annual energy.
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Module Power Tolerance
Module power tolerance is the manufacturer-guaranteed range around the STC Pmax nameplate. Premium modules: 0 to +5 W (no negative). Standard: ±3%. Tighter tolerance reduces module mismatch loss in strings.
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MPPT (Maximum Power Point Tracking)
MPPT (Maximum Power Point Tracking) is a control algorithm inside a solar inverter or charge controller that continuously adjusts the operating voltage and current of a PV array to extract the maximum available power. Because the array's optimal operating point shifts with irradiance and temperature, MPPT recalculates it many times per second.
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NOCT (Nominal Operating Cell Temperature)
NOCT (Nominal Operating Cell Temperature) is a PV module rating measured at 800 W/m² irradiance, 20°C ambient temperature, 1 m/s wind speed, open-rack mounting. Typical NOCT 42–48°C. Used to predict realistic operating cell temperatures.
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OCPD (Overcurrent Protection Device)
Overcurrent Protection Device (OCPD) is a fuse or circuit breaker that interrupts current flow when it exceeds a safe threshold. NEC 690.9 sizes OCPD at 1.25 × Isc for PV source circuits.
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Performance Ratio (PR)
Performance Ratio (PR) is the ratio of a solar plant's actual energy output to its theoretical maximum output under measured irradiance, normalized for temperature. PR captures all real-world losses in a single dimensionless number, typically 0.75–0.88 for utility-scale plants.
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PID (Potential Induced Degradation)
PID (Potential Induced Degradation) is a leakage-current-driven loss of PV module performance caused by high-voltage stress between cells and grounded frame. Manifests as 20–50% power loss within months in unmanaged systems.
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POA Irradiance (Plane of Array)
Plane of Array (POA) irradiance is the total solar radiation received per unit area on a tilted PV module surface, computed as a transposition of GHI plus direct and diffuse contributions. The direct upstream input to PV energy yield.
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PTC (PVUSA Test Conditions)
PVUSA Test Conditions (PTC) is a module rating method developed by NREL/PG&E: 1000 W/m² irradiance, 20°C ambient temperature, 1 m/s wind, AM 1.5 spectrum. CEC publishes PTC values; used in California Solar Initiative.
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PV Wire vs USE-2
PV Wire and USE-2 are both common conductor types for solar PV source circuits. PV Wire is single-conductor, sunlight-resistant, suitable for exposed outdoor installation. USE-2 is direct-burial-rated but not approved for prolonged sun exposure above ground.
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Rapid Shutdown
Rapid Shutdown is the NEC 690.12 requirement that within 30 seconds of activation, voltage on PV conductors outside the array boundary drops below 80 V (and below 30 V inside the array, unless UL 3741 hazard-control compliant). It exists to protect firefighters and first responders.
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Recombiner Box
A recombiner box is a second-stage DC combiner used in utility-scale solar plants, consolidating the outputs of multiple first-stage combiners before feeding a central inverter. Includes fuses, disconnect, monitoring.
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Reverse Current
Reverse current is current flowing backward (anti-parallel) through a PV module, typically occurring when a shorted module in parallel string draws current from other strings. Module's bypass diodes and series fuse mitigate damage.
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Series Fuse Rating
Series Fuse Rating (also Max Series Fuse) on a PV module datasheet is the maximum OCPD rating permitted in series with the module to prevent reverse-current damage. Typically 15–25 A for standard modules; 20–30 A for high-current bifacial.
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Shading Analysis
Shading analysis is the engineering process of quantifying how obstructions (trees, buildings, RTUs, neighboring rows) reduce solar PV array energy yield. It uses sun-path geometry, 3D site modeling, and time-step simulation to compute annual shading loss.
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Single-Axis Tracker
A single-axis tracker is a solar tracker that rotates about one axis to follow the sun's daily east-west motion. Horizontal single-axis trackers (HSAT) on a north-south axis dominate utility-scale solar.
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SLD (Single Line Diagram)
A Single Line Diagram (SLD) is the simplified one-line electrical schematic of a solar PV system showing modules, strings, inverters, disconnects, protection devices, conductors, and the point of interconnection. It is the single most reviewed document in any solar permit submittal.
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Soiling Loss
Soiling loss is the energy yield reduction caused by accumulation of dust, dirt, pollen, leaves, and bird droppings on PV module surfaces. Annual losses range from 1–2% in clean climates to 5–15% in arid regions without regular cleaning.
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Solar Pathfinder
Solar Pathfinder is a handheld optical shading measurement instrument using a polished dome and sun-path chart to record obstructions visible from a proposed module location. Was the industry standard before LIDAR + software.
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Solar Tracker
A solar tracker is a mechanical system that rotates PV modules through the day to maintain optimal orientation toward the sun. Single-axis trackers boost annual yield 15–25% over fixed-tilt; dual-axis adds another 3–8% at significantly higher cost.
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Solar Window
The solar window is the daily and seasonal time range during which solar irradiance is sufficient for productive PV generation at a site. Reduced by morning/evening obstructions and seasonal sun-path variation.
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SPD (Surge Protection Device)
A Surge Protection Device (SPD) clamps voltage surges induced by lightning or grid disturbances to protect downstream electronics. Type 1 at service entrance, Type 2 at distribution, Type 3 at sensitive equipment.
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Specific Yield
Specific yield is the annual energy production per unit of installed DC nameplate capacity, expressed in kWh/kWp/year. It captures the combined effect of site irradiance, system efficiency, and orientation in a single normalized number.
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STC (Standard Test Conditions)
Standard Test Conditions (STC) are the industry-standard laboratory conditions for rating PV modules: 1000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum. STC values are what appears on module datasheets and labels.
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String Current
String current is the current flowing through a series-connected PV string. In series wiring, current is constant across all modules, equal to the module-level current (Imp at MPP, Isc at short circuit).
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String Sizing
String sizing is the calculation of the number of modules to connect in series within a PV string. It is bounded by NEC 690.7 maximum system voltage at the coldest expected temperature and the inverter MPPT lower bound at the hottest expected operating temperature.
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String Voltage
String voltage is the sum of voltages across series-connected PV modules. Cold-day Voc must stay below NEC 690.7 limits and inverter MPPT max. Hot-day Vmp must stay above inverter MPPT min.
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Stringing
Stringing is the process of wiring PV modules in series to form strings of suitable voltage and current for the inverter MPPT. Encompasses layout planning, MC4 connector terminations, conductor routing, and per-MPPT current balancing.
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Temperature Coefficient
Temperature coefficients are the percentage change per °C of a PV module's electrical parameters — Voc, Vmp, Isc, Imp, and Pmax. They quantify how module performance varies between cold and hot operating conditions.
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Three-Line Diagram
A three-line diagram (3LD) is the electrical schematic that explicitly shows all three phases plus neutral and ground conductors separately, used for commercial-scale solar PV protection coordination and utility interconnection studies.
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Tilt Angle
Tilt angle is the inclination of a PV module's surface from horizontal, measured in degrees. Optimal annual tilt approximates the site's latitude; lower tilts favor summer production; higher tilts favor winter.
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Vmp (Maximum Power Voltage)
Vmp (Maximum Power Voltage) is the voltage at which a PV module produces its maximum power, measured at STC. Typically 0.78–0.85 × Voc. Critical for ensuring string voltage stays above the inverter MPPT minimum at hot operating temperature.
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Voc (Open-Circuit Voltage)
Open-Circuit Voltage (Voc) is the maximum voltage a PV module produces when no load is connected, measured at Standard Test Conditions. It is the controlling parameter for string sizing against NEC 690.7 maximum system voltage.
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Voltage Drop
Voltage drop (Vd) is the loss of voltage along a conductor due to its resistance and the current flowing through it. NEC recommends ≤ 3% for branch circuits; solar industry targets ≤ 2% DC and ≤ 1% AC for efficiency and MPPT margin.
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Wire Sizing
Wire sizing for solar PV systems determines conductor type, AWG/kcmil size, and length to safely carry the maximum circuit current under NEC 690.8, with acceptable voltage drop (typically ≤ 2–3%) and temperature/conduit-fill derating.
