The maximum number of modules per string is determined based on module characteristics, the maximum permissible DC system voltage and the ASHRAE Extreme Annual Mean Minimum Design Dry Bulb temperature obtained from the nearest ASHRAE weather station. The maximum permissible DC voltage is determined from the inverter maximum DC voltage. Therefore, it is possible to model systems with different maximum DC voltages (and therefore different string sizes) within the same optimization run if inverters with different maximum DC voltages are specified by the user.
The string sizing calculation is performed in accordance with NEC Article 690. The number of modules per string used within the final system design may be the maximum number as described above or may be less. This is heavily dependent on the racking configuration specified by the user since different string sizes may “tile” better than others. The optimal number of modules per string is determined automatically by View during runtime as it tests different string sizes up to the maximum number of modules per string.
Because the optimum number of modules per string and the racking configuration are heavily interrelated and can significantly affect the layout, it is worth defining the constraints that HST imposes on string wiring:
- Strings may not be broken across different rows.
- Strings may be broken across fixed-tilt racks within the same row.
- Strings may not be broken across tracking racks within the same row.
- All racks must be completely filled with modules. Partially filled racks are not allowed.