NABCEP Solar PV Fundamentals 3 — Questions and Answers
Question 1: What is the purpose of a blocking diode in a PV string connected to a battery system?
- To increase the voltage of each series-connected module
- To prevent battery current from flowing backward through the PV string at night (Correct answer)
- To bypass shaded cells and prevent hot spots
- To regulate the current from the PV array to the load
Correct answer: To prevent battery current from flowing backward through the PV string at night
Blocking diodes prevent reverse current flow from the battery back through the PV modules when the array voltage is lower than battery voltage.
Question 2: An array has 4 strings of 10 modules each wired in parallel. If each module is rated at 8A Isc, what is the total array Isc?
- 8 A
- 32 A (Correct answer)
- 80 A
- 320 A
Correct answer: 32 A
In parallel strings, currents add: 4 strings × 8 A per string = 32 A total array Isc.
Question 3: What effect does partial shading of a single module in a series string have on overall string output without bypass diodes?
- Only that module's output is reduced proportionally
- The entire string current is limited to the shaded module's reduced current (Correct answer)
- String voltage decreases but current remains unaffected
- Output increases as other modules compensate
Correct answer: The entire string current is limited to the shaded module's reduced current
In a series string, all modules must carry the same current, so a shaded module limits the current for the entire string.
Question 4: Which of the following best describes the AM1.5 spectrum used in STC module ratings?
- Sunlight filtered through 1.5 inches of atmospheric water vapor
- Sunlight passing through 1.5 times the atmosphere's thickness at a 48.2° angle (Correct answer)
- The solar spectrum measured at 1500 m altitude
- Sunlight at 1.5 kW/m² irradiance at sea level
Correct answer: Sunlight passing through 1.5 times the atmosphere's thickness at a 48.2° angle
AM1.5 represents the solar spectrum when sunlight travels through 1.5 air masses (a path length 1.5× the zenith path), corresponding to a solar angle of about 48.2°.
Question 5: A PV module has a Voc of 40V at 25°C and a temperature coefficient of -0.3%/°C. What is the approximate Voc at 60°C?
- 44.2 V
- 40.0 V
- 35.8 V (Correct answer)
- 33.2 V
Correct answer: 35.8 V
ΔT = 35°C; voltage change = 40V × 0.003 × 35 = 4.2V decrease; Voc ≈ 40 – 4.2 = 35.8V.
Question 6: What is the primary advantage of monocrystalline silicon modules compared to polycrystalline silicon modules?
- Lower manufacturing cost per watt
- Higher conversion efficiency due to uniform crystal structure (Correct answer)
- Better performance in high-temperature environments
- Greater resistance to potential-induced degradation (PID)
Correct answer: Higher conversion efficiency due to uniform crystal structure
Monocrystalline cells have a single uniform crystal lattice with fewer grain boundaries, yielding higher efficiency than polycrystalline cells.
Question 7: In thin-film PV technology, what property allows the active semiconductor layer to be much thinner than crystalline silicon cells?
- Higher silicon purity in thin-film materials
- Direct bandgap enabling strong light absorption with less material (Correct answer)
- Thin-film cells operate at higher temperatures
- Thin-film modules use concentrating optics to increase light intensity
Correct answer: Direct bandgap enabling strong light absorption with less material
Direct-bandgap semiconductors like CdTe and CIGS absorb photons very efficiently, requiring only a few microns of material versus ~180 microns for silicon.
NABCEP Solar PV Fundamentals 3
What is the purpose of a blocking diode in a PV string connected to a battery system?