Cells, Modules and Arrays
Cells, Modules and Arrays
This domain tests the series-parallel build-up from PV cell to module to array, including voltage/current relationships, bypass diodes and mismatch.
Core rules
- A crystalline silicon cell produces about 0.5–0.6 V; current depends on cell area and irradiance.
- Cells in series add voltage; current is limited by the weakest cell. Cells in parallel add current; voltage must match.
- A 60-cell module typically has Voc around 36–38 V and Vmp around 30 V; a 72-cell module has higher voltage.
- Bypass diodes provide an alternate path around shaded cells to reduce hot-spot heating; blocking diodes prevent reverse current in parallel strings.
- Array voltage must stay within inverter/charge-controller limits at the lowest expected temperature, and Vmp must remain high enough at the highest temperature.
Worked example
A module has Vmp = 18 V and Imp = 8 A. Two modules in series give 36 V at 8 A = 288 W. Four such series strings in parallel give 36 V at 32 A = 1152 W. If one module in a series string is fully shaded, its current limit can reduce the whole string unless the bypass diode conducts.
Traps that cost marks
Adding both voltage and current when modules are in series; connecting strings of different voltages in parallel; ignoring Voc rise in cold weather; confusing bypass and blocking diodes; sizing cable for Imp only while ignoring Isc and safety factors.