Photovoltaics

Overview

Photovoltaics (PV) is the technology of converting light energy directly into electrical energy using semiconductor materials. At the heart of PV system modeling is the ability to accurately predict the electrical performance of solar cells and modules under varying environmental conditions. This capability is essential for system design, performance analysis, bankability studies, and operational optimization.

All photovoltaic modeling tools rely on pvlib-python, the industry-standard open-source library for simulating the performance of photovoltaic energy systems. Built on NumPy and pandas, pvlib provides validated implementations of established models and algorithms used throughout the PV industry.

Solar Position and Irradiance: Before modeling electrical performance, we must determine the sun’s position and the irradiance on the panel surface. The SOLARPOSITION tool calculates solar azimuth, elevation, and zenith angles using validated astronomical algorithms. The IRRADIANCE tool then translates global horizontal irradiance into plane-of-array irradiance components (direct, diffuse, and ground-reflected) accounting for the panel’s tilt and orientation.

Electrical Performance Modeling: The fundamental challenge in PV modeling is understanding how the current-voltage (IV) characteristics of a solar cell change with irradiance and temperature. The single diode model (equivalent circuit model) is the industry-standard approach, representing the cell as a current source in parallel with a diode, along with series and shunt resistances. The CALCPARAMS_CEC tool implements the California Energy Commission (CEC) parameter calculation method, translating manufacturer datasheet values into the five parameters needed for the single diode equation at any operating condition. Once parameters are known, I_FROM_V solves the implicit single diode equation to find current at any given voltage—essential for detailed IV curve analysis.

Simplified System Modeling: For system-level performance estimation where detailed IV curves aren’t needed, the PVWATTS_DC tool provides a simpler empirical model that directly estimates DC power output from irradiance and temperature. This approach is widely used for feasibility studies and preliminary system sizing (Figure 1).

Figure 1: Photovoltaic performance characteristics: (A) IV curves shift vertically with solar irradiance, maintaining similar voltage characteristics. (B) Temperature significantly affects open-circuit voltage, with IV curves shifting horizontally as temperature increases.

Tools

Tool	Description
CALCPARAMS_CEC	Calculate five CEC model parameters for the single diode equation at given irradiance and cell temperature.
I_FROM_V	Calculate the device current at a given device voltage for a PV cell/module using the single diode model.
IRRADIANCE	Calculate the plane of array irradiance components on a tilted surface using PVLib.
PVWATTS_DC	Calculate the DC power output of a PV module using the PVWatts DC model.
SOLARPOSITION	Calculate solar azimuth, elevation, and apparent zenith for given times and location.