String Sizing in Photovoltaic Systems

Jul 4, 2024

String Sizing in Photovoltaic Systems

Introduction

  • Focus: Combine photovoltaic (PV) module and array data to find the appropriate inverter.
  • Example: 7400W (7.4 kW) system.

System Requirements and Example

  • System Requirements: 7400W needed.
  • Modules Used: 300W PV modules.
  • Calculation:
    • 7400W / 300W per module = 24.7 modules.
    • Round down to 24 modules.
  • Module Specifications:
    • Open Circuit Voltage (VOC) = 38.2V
    • Maximum Power Voltage (Vmp) = 36V
    • Short Circuit Current (Isc) = 8.6A
    • Maximum Power Current (Imp) = 8.3A

Voltage and Current Calculations

Series Configuration

  • Voltage Calculation:
    • 24 modules * 38.2V VOC = 917V
    • Inverter Max Input Voltage = 600V
    • Conclusion: Series wiring not suitable (too high voltage).

Parallel Configuration

  • Voltage Calculation:
    • 12 modules * 38.2V VOC = 458.4V
    • 12 modules * 36V Vmp = 432V
    • Conclusion: Voltages within inverter specs (<600V, <480V operating).
  • Current Calculation:
    • 2 series strings * 8.6A Isc = 17.2A
    • Inverter Max Input Current = 18A
    • Conclusion: Current within inverter specs.
  • Design: 2 strings of 12 modules
    • Total Power = 7200W (STC), meets inverter input power.

Alternative Wiring Configurations

  • Three Strings of Eight:
    • Voltage: 8 modules * 38.2V VOC = 305.6V, Vmp = 208V
    • Within inverter specs.
    • Current: 3 strings * 8.6A Isc = 25.8A
    • Conclusion: Exceeds max input current (18A); not suitable.
  • Other Possibilities: 24 modules can be wired in diverse configurations but 2 strings of 12 is optimal here.

General Concepts

  • High voltage is preferred over high current due to less resistance loss.
  • Avoid low voltage series with high numbers of parallel strings to reduce losses.

Summary

  1. Define the size or power of the PV array.
  2. Choose a module and inverter.
  3. Balance output of PV string with inverter's input.
  4. Iterate several configurations to optimize efficiency.
  5. Ensure compliance with voltage and current specs of the inverter.

Learning Outcomes

  • Calculate solar gain based on location.
  • Use online resources to determine solar gain.
  • Calculate losses due to shading, tilt, and azimuth.
  • Explain circuitry basics and electrical output for modules and arrays.
  • Apply PV specifications to determine power and energy output.
  • Size PV system strings using module and inverter spec sheets.

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