Conversion inside PV modules

Overview

The single diode model serves as a cornerstone for PV system simulation and optimization by enabling precise and detailed analysis of the electrical behavior of solar cells under varying environmental and operational scenarios. This foundational model provides accurate current-voltage (IV) characteristics that are critical for understanding how cells perform in real-world conditions.

Aggregating IV curves to the plant level is also a critical step that ensures an accurate representation of the system's overall electrical behavior. This numerical combination accounts for the electrical layout and wiring configurations to generate the input IV curve for the inverter. This aggregated curve is essential for modeling how the entire PV system will perform.

Complementing the single diode approach, the temperature model plays a vital role in calculating cell temperature, a key factor that directly influences the electrical performance of the PV system. By incorporating variables such as ambient temperature, irradiance, and module-specific parameters, the temperature model ensures accurate predictions of how thermal effects impact efficiency and power output.

To maintain the long-term reliability and efficiency of PV systems, module degradation models are also essential. These models account for the gradual reduction in module performance due to environmental exposure and material aging, enabling operators to anticipate performance declines and plan for maintenance or module replacements.

Single diode model

The Single Diode model is a fundamental approach used to generate the IV curve of a solar cell based on its Global Tilted Irradiance (GTI) and temperature. It calculates the current-voltage (IV) characteristics of all cells in a photovoltaic system, providing the foundation for understanding and simulating system performance.

The Single diode model requires five key parameters to describe the current-voltage (IV) curves of PV cells, typically acquired at Standard Test Conditions (STC):

• Modified ideality factor

• Diode saturation current

• Light current (photocurrent)

• Series resistance

• Parallel resistance

These parameters, along with the Global tilted irradiance (GTI) and cell temperature, are used to generate the IV curves for each PV cell.

Calculation process

• GTI and cell temperature: The GTI is the output of the optical simulation, while the cell temperature is determined using the model proposed by Duffie and Beckman. Solargis has modified this model to account for module efficiency dependency on module temperature.
  

• IV curve generation: The five model parameters, GTI, and cell temperature are used to generate the IV curves for each PV cell. The Lambert W function by Darko Veberic is utilized to simulate the diode part of the cell model.

• Aggregation of IV curves: The IV curves of individual cells are summed up into submodule IV curves. These are then combined with the characteristics of bypass diodes (modeled as ideal P-N junction diodes) to form a single PV module. The resulting IV curves of the PV modules are summed to string characteristics and connected to DC cabling.

Cell temperature model

Cell temperature is a critical parameter in determining the IV curves of PV systems. It is calculated based on air temperature, the cell’s Global Tilted Irradiance (GTI), and a set of model-specific parameters that account for environmental and material factors. These parameters include:

• Nominal Operating Cell Temperature (NOCT): A standard characteristic of the module that defines its temperature under specific reference conditions.

• Thermal Model Correction Coefficient: A user-defined simulation input that adjusts the model for site-specific conditions.

• Wind Speed Empirical Constant: A factor that accounts for cooling effects of wind on the module, derived from empirical data.

• Module Efficiency at 25°C: The efficiency of the module under standard testing conditions.

• Module Type Factor: A correction factor that distinguishes between module technologies, such as crystalline silicon (CSI) or cadmium telluride (CdTe).

• Emissivity Coefficient Approximation: An estimate of the module’s ability to emit energy as thermal radiation.

Module degradation

The lifespan of a PV module typically ranges between 25 to 30 years, during which the cumulative effects of annual degradation gradually reduce the module's power output. Understanding degradation parameters and rates is essential for accurately predicting the long-term performance of PV installations. This knowledge also informs decisions about potential module replacements, ensuring consistent and optimal energy production throughout the system's lifecycle.

PV module degradation is generally modeled as having two distinct phases: a higher degradation rate in the first year, followed by a steadier, lower rate in subsequent years. Specifically:

1. First-Year Degradation: Modules often experience an efficiency loss of approximately 1% during the first year of operation. This initial reduction is primarily due to factors like light-induced degradation (LID) and the stabilization of module materials.

2. Subsequent Years’ Degradation: After the first year, the annual degradation rate typically stabilizes at around 0.5%. This steady decline results from prolonged exposure to environmental stressors such as ultraviolet (UV) radiation, temperature fluctuations, and mechanical stress over time.

Further reading

• Improvement and validation of a model for photovoltaic array performance. W. De Soto et al. Solar Energy 2006.