REP Solar Energy Systems 5 — Questions and Answers
Question 1: Which factor most significantly limits the efficiency of a single-junction silicon photovoltaic cell according to the Shockley-Queisser limit?
- Reflection losses from the glass surface
- Photons with energy below the bandgap are not absorbed and photons above it lose excess energy as heat (Correct answer)
- Resistive losses in the metal contacts
- Soiling on the panel surface
Correct answer: Photons with energy below the bandgap are not absorbed and photons above it lose excess energy as heat
The Shockley-Queisser limit (~33%) arises because sub-bandgap photons are unused and above-bandgap photons lose excess energy as thermalization heat.
Question 2: In a solar PV feasibility study, what does the 'levelized cost of energy' (LCOE) represent?
- The purchase price of the PV modules per watt
- The average cost per kWh over the system's lifetime, accounting for all costs and production (Correct answer)
- The utility's retail electricity rate at project commissioning
- The cost per kW of inverter capacity
Correct answer: The average cost per kWh over the system's lifetime, accounting for all costs and production
LCOE divides the total lifetime costs (capital, O&M, financing) by the total lifetime energy produced to yield a comparable $/kWh metric.
Question 3: What condition must be met before a solar installation crew performs work on energized PV circuits above 50 V DC?
- Only one technician needs to be present
- Workers must follow arc flash and shock hazard NFPA 70E requirements with appropriate PPE (Correct answer)
- The inverter must be producing full rated power
- The utility disconnect must be closed
Correct answer: Workers must follow arc flash and shock hazard NFPA 70E requirements with appropriate PPE
NFPA 70E requires an arc flash hazard analysis and appropriate PPE for work on energized electrical equipment above 50 V DC to protect against shock and arc flash injuries.
Question 4: A solar PV array has a DC-to-AC ratio (inverter loading ratio) of 1.25. What does this indicate?
- The array is 25% undersized for the inverter
- The DC array capacity is 25% greater than the inverter's AC rated output (Correct answer)
- The system loses 25% of power through wiring
- The inverter efficiency is 125%
Correct answer: The DC array capacity is 25% greater than the inverter's AC rated output
A DC/AC ratio of 1.25 means the STC-rated array power is 25% larger than inverter capacity, which is common practice to account for real-world losses and maximize inverter utilization.
Question 5: In bifacial PV modules, what is the 'bifaciality factor'?
- The ratio of front-side to rear-side cell efficiency
- The ratio of the rear-side power output to the front-side power output under the same irradiance (Correct answer)
- The angle at which rear-side irradiance is maximized
- The percentage of sunlight reflected by the ground onto the rear of the panel
Correct answer: The ratio of the rear-side power output to the front-side power output under the same irradiance
Bifaciality factor (typically 0.65–0.80) quantifies how efficiently the rear side converts light relative to the front side under identical test conditions.
Question 6: Which interconnection queue process reform, often called the 'first ready, first served' model, was proposed by FERC Order 2023 to address PV project delays?
- Cluster study process replacing serial project-by-project studies (Correct answer)
- Direct interconnection without utility studies
- Automatic approval for systems under 10 MW
- Priority queuing based on project financing status
Correct answer: Cluster study process replacing serial project-by-project studies
FERC Order 2023 (2023) mandates cluster study processes where groups of projects are studied together, replacing the slow serial study process to reduce interconnection queue backlogs.
Question 7: What is the primary purpose of an 'IV curve tracer' during solar PV commissioning or troubleshooting?
- Measure AC output voltage of the inverter
- Plot the current-voltage characteristic of a module or string to identify performance degradation or faults (Correct answer)
- Check the insulation resistance of DC wiring
- Verify the anti-islanding protection of the inverter
Correct answer: Plot the current-voltage characteristic of a module or string to identify performance degradation or faults
An IV curve tracer sweeps the operating point of a PV module or string and plots current vs. voltage, allowing comparison with the manufacturer's flash test data to detect problems like shading, bypass diode failures, or cell damage.
REP Solar Energy Systems 5
Which factor most significantly limits the efficiency of a single-junction silicon photovoltaic cell according to the Shockley-Queisser limit?