REP Renewable Energy Technologies & Applications 3 — Questions and Answers
Question 1: Which ocean energy technology converts the temperature difference between warm surface water and cold deep water into electricity?
- Tidal stream turbine
- Oscillating water column
- Ocean thermal energy conversion (OTEC) (Correct answer)
- Salinity gradient osmotic power
Correct answer: Ocean thermal energy conversion (OTEC)
OTEC exploits the thermal gradient between warm surface seawater (~25°C) and cold deep seawater (~5°C) to drive a thermodynamic cycle and generate electricity.
Question 2: In a grid-tied PV system, what happens to power generation when the utility grid goes down (without a battery backup)?
- The system continues supplying power to the home only
- The inverter shuts down to prevent islanding (Correct answer)
- Power is stored in the DC bus capacitors
- Output is automatically reduced to 50%
Correct answer: The inverter shuts down to prevent islanding
Grid-tied inverters are required to detect grid loss and shut down immediately (anti-islanding protection) to prevent energizing lines that utility workers might contact.
Question 3: What is the Betz limit and what value does it represent?
- Maximum theoretical efficiency of a wind turbine, approximately 59.3% (Correct answer)
- Minimum cut-in wind speed required for turbine operation, 3 m/s
- Maximum allowable tower height-to-rotor-diameter ratio, 2:1
- Efficiency ceiling for solar PV cells, 33.7%
Correct answer: Maximum theoretical efficiency of a wind turbine, approximately 59.3%
The Betz limit, derived by Albert Betz in 1919, states that no wind turbine can capture more than 16/27 ≈ 59.3% of the kinetic energy in wind.
Question 4: Which solar cell technology currently holds the highest confirmed single-junction efficiency record for terrestrial flat-plate cells?
- Multicrystalline silicon
- Organic photovoltaic (OPV)
- Gallium arsenide (GaAs) single crystal (Correct answer)
- Amorphous silicon thin-film
Correct answer: Gallium arsenide (GaAs) single crystal
Single-crystal GaAs cells hold the highest confirmed efficiency for single-junction terrestrial flat-plate cells, exceeding 29%, due to their near-ideal bandgap and material quality.
Question 5: What is 'levelized cost of energy' (LCOE) used to measure?
- The peak power output per dollar of capital investment
- The average cost per unit of electricity over a plant's lifetime (Correct answer)
- The ratio of renewable to fossil fuel generation in a grid
- The cost of grid interconnection per kilometer
Correct answer: The average cost per unit of electricity over a plant's lifetime
LCOE represents the net present value of all costs (capital, O&M, fuel) divided by lifetime energy production, enabling comparison across different generation technologies.
Question 6: Which type of electrolyzer technology is most suitable for pairing with intermittent renewable energy sources due to its fast response time and ability to operate at partial load?
- Alkaline (AWE) electrolyzer
- Proton exchange membrane (PEM) electrolyzer (Correct answer)
- Solid oxide electrolyzer (SOEC)
- Molten carbonate electrolyzer
Correct answer: Proton exchange membrane (PEM) electrolyzer
PEM electrolyzers can ramp rapidly and operate efficiently across a wide load range (5–100%), making them well-suited for coupling with variable solar and wind generation.
Question 7: A biomass power plant using a combined heat and power (CHP) configuration achieves what primary benefit over electricity-only generation?
- Lower NOx emissions per unit of biomass consumed
- Higher overall fuel utilization efficiency (Correct answer)
- Reduced need for feedstock preprocessing
- Elimination of flue gas treatment requirements
Correct answer: Higher overall fuel utilization efficiency
CHP (cogeneration) captures waste heat for useful purposes such as space heating or industrial processes, raising overall fuel-to-useful-energy efficiency from ~25–35% to 70–85%.
REP Renewable Energy Technologies & Applications 3
Which ocean energy technology converts the temperature difference between warm surface water and cold deep water into electricity?