NABCEP Module Technology 2 — Questions and Answers
Question 1: What is the primary cause of potential-induced degradation (PID) in crystalline silicon PV modules?
- Excessive UV exposure breaking encapsulant bonds
- High voltage potential between cells and grounded frame causing ion migration (Correct answer)
- Thermal cycling cracking solder joints between cells
- Delamination of the backsheet from moisture ingress
Correct answer: High voltage potential between cells and grounded frame causing ion migration
PID occurs when a high voltage difference between the cell circuit and the grounded module frame drives sodium ions through the glass, degrading the anti-reflective coating and cell junction.
Question 2: Which cell technology is most commonly used in bifacial PV modules to maximize rear-side light absorption?
- Standard aluminum back-surface field (Al-BSF) cells
- PERC (Passivated Emitter and Rear Cell) cells (Correct answer)
- Amorphous silicon thin-film cells
- Cadmium telluride (CdTe) cells
Correct answer: PERC (Passivated Emitter and Rear Cell) cells
PERC cells use a dielectric passivation layer on the rear that reflects unabsorbed light back into the cell and allows rear-side bifacial generation, making them the dominant bifacial module technology.
Question 3: On a PV module datasheet, what does the temperature coefficient of Pmax (γ) indicate?
- The rate at which open-circuit voltage decreases per degree Celsius above STC
- The percentage change in maximum power output per degree Celsius change in cell temperature (Correct answer)
- The thermal resistance between the cell and ambient environment
- The rate at which short-circuit current increases with temperature
Correct answer: The percentage change in maximum power output per degree Celsius change in cell temperature
γ (gamma) is the temperature coefficient of Pmax, expressing how much the maximum power output changes (typically negative, e.g., -0.35%/°C) for each degree of cell temperature deviation from STC.
Question 4: What is 'light-induced degradation' (LID) and which cell type is most susceptible?
- UV discoloration of EVA encapsulant, worst in thin-film modules
- Initial power loss in the first hours of sunlight exposure, most common in boron-doped p-type crystalline silicon (Correct answer)
- Gradual efficiency loss from thermal cycling, worst in multi-junction cells
- Arc damage from partial shading, most common in string inverter systems
Correct answer: Initial power loss in the first hours of sunlight exposure, most common in boron-doped p-type crystalline silicon
LID is a reversible initial power loss (typically 1–3%) in boron-doped p-type c-Si cells caused by the formation of boron-oxygen defect complexes when first exposed to light.
Question 5: A module's bypass diode is rated for a forward current of 15 A. What is its primary purpose?
- To prevent reverse current flow from the battery bank into the module
- To provide a current path around shaded or damaged cells, limiting hot-spot heating (Correct answer)
- To clamp overvoltage transients caused by lightning on the string circuit
- To equalize current between parallel module strings
Correct answer: To provide a current path around shaded or damaged cells, limiting hot-spot heating
Bypass diodes conduct when cells in their protected group are reverse-biased due to shading or damage, preventing those cells from dissipating power as heat and becoming hot spots.
Question 6: What is the standard irradiance level used for Standard Test Conditions (STC) ratings on PV modules?
- 800 W/m²
- 900 W/m²
- 1000 W/m² (Correct answer)
- 1200 W/m²
Correct answer: 1000 W/m²
STC is defined as 1000 W/m² irradiance, AM1.5 spectrum, and 25°C cell temperature — the universal baseline for comparing PV module performance ratings.
Question 7: Which property of a PV module's I-V curve is most directly used to determine the maximum power point (MPP)?
- The point where the I-V curve crosses the current axis (Isc)
- The knee of the I-V curve where the product of voltage and current is maximized (Correct answer)
- The point where the I-V curve crosses the voltage axis (Voc)
- The linear region of the I-V curve at low voltage
Correct answer: The knee of the I-V curve where the product of voltage and current is maximized
The MPP is found at the knee of the I-V curve where the P-V curve peaks, i.e., where V × I is at its maximum value, defining Vmpp and Impp.
NABCEP Module Technology 2
What is the primary cause of potential-induced degradation (PID) in crystalline silicon PV modules?