PV systems fundamentals and applications
What the exam tests
PV systems fundamentals and applications covers the photovoltaic effect, how cells are packaged into modules and arrays, and how irradiance, temperature, and shading change output. Expect definitions, IV-curve reading, and simple calculations rather than product memorization.
Core rules
- A PV cell is a large-area semiconductor diode; photons free carriers and the built-in field separates them, producing current.
- Irradiance (W/m²) drives current; cell temperature mainly drives voltage. Higher irradiance raises Isc and power; higher temperature lowers Voc and power.
- Standard Test Conditions (STC): 1000 W/m², 25°C cell temperature, AM 1.5 spectrum. Nameplate ratings use STC.
- Key IV points: Isc (short-circuit current), Voc (open-circuit voltage), Imp/Vmp at the maximum power point. Fill factor = (Vmp × Imp) / (Voc × Isc).
- Cells in series add voltage; parallel strings add current. A bypass diode limits hotspot damage from shading; a blocking diode prevents reverse current.
- Orientation and tilt set incident irradiance; performance ratio compares actual yield with the irradiance-based ideal.
Worked example
A module has Voc = 40.0 V at STC and a Voc temperature coefficient of −0.30%/°C. At a record-low cell temperature of −10°C, ΔT = −35°C. Voltage change = 40.0 × (−0.003/°C) × (−35°C) = +4.2 V, so cold Voc ≈ 44.2 V. Use this value for maximum system voltage and string sizing, not the 40.0 V nameplate value.
Traps
- Using ambient temperature instead of cell temperature; modules run hotter than air.
- Assuming irradiance changes Voc proportionally; it changes current far more.
- Confusing bypass diodes (across groups of cells) with blocking diodes (in series with strings).
- Reporting STC power when the question asks about actual operating conditions.