Models & Algorithms
Summary
This section provides technical documentation of the physical and empirical models that form PlantPredict’s prediction engine. Each page describes a specific model or algorithm used to simulate solar PV system performance, from solar resource assessment through grid-delivered power including optional battery energy storage.Purpose and Scope
The Models & Algorithms section documents the calculation methods used in PlantPredict Version 12 to predict energy production from utility-scale solar photovoltaic systems. These models translate solar physics and electrical engineering principles into quantitative predictions of system performance under varying environmental and operational conditions. The prediction engine is organized into five main stages that correspond to the physical energy conversion process:Stage 1: Irradiance Calculation
From solar resource characterization (sun position, horizontal irradiance components, and system geometry) to effective plane-of-array irradiance:- Sun Position & Extraterrestrial Irradiance: Solar position algorithm, sunrise/sunset times, extraterrestrial irradiance, and correction
- Horizontal Irradiance Data Processing: Weather data quality control and, if needed, diffuse-direct
- System Geometry: Fixed-tilt and single-axis tracker orientations, including tracking algorithm (true tracking, backtracking, terrain-aware backtracking, or user-defined tracking angles)
- Models: Converting horizontal irradiance components to plane-of-array irradiance, including 3D transposition
- Shading Models: Horizon shading, sky diffuse shading, ground-reflected shading, direct beam shading (geometric, legacy 3D, or detailed 3D scene), and electrical shading effects
- Plane-of-Array Irradiance: Soiling, incidence angle modifiers, spectral corrections, and bifacial irradiance gain
Stage 2: Photovoltaic Conversion
From plane-of-array irradiance to module-level DC electrical power:- Cell Temperature: Thermal models for cell temperature estimation (Heat Balance, Sandia, NOCT-SAM, or measured surface temperature)
- DC System Losses: Mismatch, module quality, LID, and DC health losses applied to effective irradiance
- DC Performance Models: Single diode model, recombination models, and temperature coefficients
Stage 3: DC Aggregation and DC–AC Conversion
From module-level DC power to inverter-level AC power:- DC Field Aggregation: Combining multiple DC fields at a common operating voltage
- Degradation (DC Applied): Time-dependent power reduction due to module aging, applied to aggregated DC power before inverter operating region determination
- Inverter Models: Temperature derating, operating regions, and efficiency
Stage 4: AC Collection and Interconnection
From inverter-level AC power to grid-delivered energy:- Degradation (AC Applied): Time-dependent degradation applied at the AC level (Linear AC, Stepped AC, LeTID)
- Transformer Loss Model: Shared quadratic loss model for MV and HV step-up transformers
- Array-Level Aggregation and AC Losses: Inverter aggregation, auxiliary loads, MV transformer, AC collection system, and block aggregation
- Plant-Level Aggregation and AC Losses: Block-to-plant aggregation, HV transformers, transmission lines, availability, and LGIA limit
Stage 5: Energy Storage
Optional AC-coupled battery energy storage system integrated with PV. The full PV pipeline (Stages 1–4) runs once; the ESS calculation then uses the retained PV results, and only the plant-level power flow (HV equipment, availability, LGIA limit) is re-run with the combined PV+ESS output:- Charge & Discharge Limits: Maximum charge/discharge power from system topology and constraints
- Dispatch Algorithms: LGIA excess, energy available, and custom dispatch modes
- Battery Model: Degradation, state of charge, DC and AC power derivation
- Power Flow to Grid: Storage MV transformer losses, PV adjustment, and combined output
Documentation Structure
Each model page follows a consistent structure:- Title: Clear, descriptive model name
- Summary: Concise description of the model purpose and outputs
- Inputs: Table listing all input parameters with symbols, units, and descriptions
- Outputs: Table listing all output parameters with symbols, units, and descriptions
- Detailed Description: Complete mathematical description of the model, including equations and algorithmic steps
- References: Academic and technical references for the model
Version 12 Focus
This documentation describes models as implemented in PlantPredict Version 12. Where Version 12 differs from previous versions, those differences are explicitly noted. New features in Version 12 include:- Site-Level 3D Scene Shading: Polygon clipping algorithm for fast, accurate shade calculations across entire site
- Terrain-Aware Backtracking (TABT): Optimization of tracker angles accounting for local terrain slopes
- 3D Transposition: Bay-to-bay (tracking systems) or rack-to-rack (fixed-tilt systems) irradiance variations in 3D scene calculations
- Enhanced Bifacial Modeling: Improved rear-surface irradiance calculations with view factors
- Quadratic AC Collection Losses: Load-dependent AC collection system losses
- Battery Energy Storage: Dispatch algorithms, degradation modeling, and PV+storage integration
Conventions
All models in this documentation follow consistent conventions for units, angles, and coordinate systems. Key conventions include:- Azimuth angles: Measured clockwise from north (0° = North, 90° = East, 180° = South, 270° = West)
- Tilt angles: Measured from horizontal (0° = horizontal, 90° = vertical)
- Zenith angles: Measured from vertical (0° = overhead, 90° = horizon)
- Irradiance units: W/m² unless otherwise specified
- Temperature: Celsius unless otherwise specified
- Pressure: hPa (hectopascals) unless otherwise specified