EMP Renewable Energy Systems 3 — Questions and Answers
Question 1: How does a pumped hydro energy storage (PHES) facility store electrical energy?
- Converts electricity to hydrogen via electrolysis stored in tanks
- Uses surplus electricity to pump water to an upper reservoir for later release through turbines (Correct answer)
- Charges large lithium-ion battery banks fed by run-of-river flow
- Compresses air into underground caverns using hydroelectric generators
Correct answer: Uses surplus electricity to pump water to an upper reservoir for later release through turbines
PHES stores energy as gravitational potential energy by pumping water uphill during low-demand periods and recovers it by releasing water through turbines when demand is high.
Question 2: What effect does increasing a PV module's cell temperature have on its power output?
- Power output increases linearly with temperature
- Power output decreases because open-circuit voltage drops (Correct answer)
- Power output is unaffected; only current changes
- Short-circuit current drops while voltage rises, maintaining power
Correct answer: Power output decreases because open-circuit voltage drops
Rising cell temperature reduces open-circuit voltage (Voc) at roughly -0.3% to -0.5%/°C, which decreases overall module power output despite a slight increase in short-circuit current.
Question 3: An offshore wind project has a hub height of 120 m and measures wind speed of 9 m/s at that height. Using the wind power equation P = ½ρAv³, if wind speed increases to 10 m/s, how does power output change approximately?
- Increases by about 11%
- Increases by about 23%
- Increases by about 37% (Correct answer)
- Doubles
Correct answer: Increases by about 37%
Power scales with the cube of wind speed: (10/9)³ ≈ 1.372, so power increases approximately 37% for an 11% increase in wind speed.
Question 4: In the context of solar resource assessment, what does GHI represent?
- Grid Harmonic Index — a power quality measurement
- Global Horizontal Irradiance — total solar radiation on a horizontal surface (Correct answer)
- Generator Heat Input — fuel energy consumed per MWh
- Ground Humidity Index — soil moisture affecting PV cooling
Correct answer: Global Horizontal Irradiance — total solar radiation on a horizontal surface
Global Horizontal Irradiance (GHI) is the total shortwave radiation received on a horizontal surface and is the primary metric used in solar resource evaluation and energy yield modeling.
Question 5: What is 'curtailment' in the context of renewable energy systems?
- Intentional reduction of a renewable generator's output below its available capacity (Correct answer)
- Seasonal maintenance shutdown of wind or solar farms
- Financial penalty applied when a project misses production targets
- Emergency shutdown triggered by grid frequency deviations
Correct answer: Intentional reduction of a renewable generator's output below its available capacity
Curtailment occurs when grid operators instruct renewable generators to reduce output—often due to transmission constraints or oversupply—resulting in foregone clean energy production.
Question 6: Which type of solar collector is best suited for generating high-temperature industrial process heat above 200°C?
- Flat-plate collector
- Evacuated tube collector
- Concentrating parabolic trough collector (Correct answer)
- Unglazed polymer pool heater
Correct answer: Concentrating parabolic trough collector
Concentrating solar collectors like parabolic troughs use mirrors to focus sunlight onto a receiver, achieving fluid temperatures of 300–550°C suitable for industrial processes and power generation.
Question 7: What does a negative Levelized Cost of Energy (LCOE) imply for a renewable project with high production tax credits?
- The project is thermodynamically impossible and data must be incorrect
- Subsidies and tax credits exceed total lifetime costs, making each MWh economically net-positive for the developer (Correct answer)
- The grid operator must pay the developer to accept power
- LCOE cannot be negative; a floor of zero always applies
Correct answer: Subsidies and tax credits exceed total lifetime costs, making each MWh economically net-positive for the developer
When production tax credits and other incentives exceed the sum of capital, O&M, and financing costs, LCOE can technically go negative, indicating the developer earns more from incentives than it costs to produce each MWh.
EMP Renewable Energy Systems 3
How does a pumped hydro energy storage (PHES) facility store electrical energy?