Environmental Science Renewable Energy Systems 1 — Questions and Answers
Question 1: Which of the following best describes how a wind turbine generates electricity?
- Wind compresses a refrigerant that drives a heat engine connected to a generator
- Moving air turns rotor blades, spinning a shaft connected to a generator through a gearbox or direct drive (Correct answer)
- Wind pressure pushes pistons in a reciprocating engine connected to an alternator
- Wind forces air into a compressed tank; the compressed air then powers a turbine
Correct answer: Moving air turns rotor blades, spinning a shaft connected to a generator through a gearbox or direct drive
Wind turns aerodynamic rotor blades, which rotate a low-speed shaft; a gearbox (or direct-drive system) increases rotational speed to drive a generator producing electricity.
Modern wind turbines use horizontal-axis, three-blade upwind designs. Wind exerts lift and drag on the aerodynamic blades, causing rotation. The low-speed shaft (typically 5-20 RPM) connects via a gearbox to a high-speed shaft driving an induction generator, or uses a direct-drive permanent-magnet generator without a gearbox. Power output is proportional to the cube of wind speed. The Betz limit (~59.3%) sets the theoretical maximum fraction of wind energy extractable; commercial turbines achieve ~35-45%.
Question 2: The intermittency problem with solar and wind power refers to:
- The high upfront capital cost that makes projects financially uncertain
- The fact that output varies with weather and time of day, creating challenges for grid balancing (Correct answer)
- The environmental impact of manufacturing solar panels and wind turbine blades
- The limited number of suitable locations for utility-scale renewable installations
Correct answer: The fact that output varies with weather and time of day, creating challenges for grid balancing
Unlike dispatchable power plants (coal, gas, nuclear) that generate on demand, solar and wind output fluctuates with conditions, requiring storage, backup generation, or grid flexibility to maintain reliable supply.
Intermittency (or variability) means that solar and wind cannot be controlled to match demand exactly. Solutions include: energy storage (batteries, pumped hydro); demand response (shifting loads to periods of high generation); long-distance transmission to smooth regional variability; flexible backup generation; and overbuilding capacity. As renewables penetrate higher grid shares, managing variability becomes increasingly important and expensive.
Question 3: Levelized Cost of Energy (LCOE) is calculated as:
- The fuel cost per kilowatt-hour over a power plant's operating life
- The total lifecycle cost of building and operating a power plant divided by the total energy it produces (Correct answer)
- The upfront capital cost of a renewable project divided by its rated capacity
- The annual revenue required to make a power plant financially viable
Correct answer: The total lifecycle cost of building and operating a power plant divided by the total energy it produces
LCOE = (total lifetime costs including capital, O&M, fuel, decommissioning) divided by (total lifetime energy output), expressed as dollars per MWh, enabling comparison across different technologies.
Levelized Cost of Energy provides a common metric to compare generation technologies with different cost structures. LCOE discounts future costs and revenues to present value using a discount rate. Utility-scale solar PV LCOE has fallen ~90% since 2010 to ~$30-50/MWh in many markets, now below new coal ($65-150/MWh) and often below new gas combined-cycle (~$45-75/MWh). Critics note LCOE does not capture the value of dispatchability or integration costs.
Question 4: Tidal energy harvests power from:
- Temperature differences between deep and surface ocean water
- The kinetic energy of ocean waves generated by wind at the surface
- The predictable rise and fall of sea level caused by gravitational forces of the Moon and Sun (Correct answer)
- Differences in salinity between freshwater river mouths and seawater
Correct answer: The predictable rise and fall of sea level caused by gravitational forces of the Moon and Sun
Tidal power captures energy from the twice-daily tidal cycles driven by lunar and solar gravitational forces - via tidal barrages (potential energy of tidal range) or tidal stream generators (kinetic energy of tidal currents).
Tidal energy is generated by two main technologies: (1) Tidal barrages - dam-like structures across estuaries that capture potential energy in the tidal range; La Rance (France, 240 MW, 1966) is the largest. (2) Tidal stream generators - turbines placed in fast-moving tidal currents. Tidal energy is predictable years in advance, highly location-specific, and constrained by tidal range and current speed.
Question 5: The primary environmental concern associated with large hydroelectric dams is:
- Emission of carbon dioxide from turbine combustion gases
- Disruption of river ecosystems, blocking fish migration, and flooding of upstream habitats and communities (Correct answer)
- Radioactive contamination of downstream water supplies
- Depletion of groundwater aquifers that feed the river system
Correct answer: Disruption of river ecosystems, blocking fish migration, and flooding of upstream habitats and communities
Large dams block fish passage (salmon, shad, sturgeon), flood unique terrestrial and riparian ecosystems, displace communities, alter downstream sediment and flow regimes, and can emit methane from decomposing organic matter in reservoirs.
Large hydroelectric projects cause major ecological and social impacts: fish passage barriers (many salmon/steelhead populations in the Pacific Northwest declined after dam construction); reservoir flooding of forests, wetlands, agricultural land, cultural sites, and communities (Three Gorges Dam displaced ~1.3 million people); altered thermal and sediment regimes downstream; greenhouse gas emissions from reservoirs in tropical areas where decomposing biomass releases CO2 and methane; and seismic risk from reservoir impoundment.
Question 6: Second-generation biofuels differ from first-generation biofuels in that they:
- Are produced from food crops like corn and sugarcane rather than waste biomass
- Are derived from non-food feedstocks (cellulosic biomass, agricultural residues, algae) that don't compete with food production (Correct answer)
- Use the same fermentation processes as first-generation but with more efficient yeast strains
- Are liquid fuels burned in conventional engines without any engine modification
Correct answer: Are derived from non-food feedstocks (cellulosic biomass, agricultural residues, algae) that don't compete with food production
Second-generation (advanced) biofuels use lignocellulosic feedstocks (switchgrass, wood chips, corn stover, municipal solid waste) or algae, avoiding the food vs. fuel conflict of corn ethanol and sugarcane ethanol.
First-generation biofuels use food crops: corn starch or sugarcane (ethanol), soybean or palm oil (biodiesel). These raise ethical concerns about food price impacts and land use change. Second-generation (cellulosic) biofuels break down the lignocellulose in agricultural residues, energy crops (switchgrass, Miscanthus), forestry residues, or municipal solid waste. Challenges include the high cost of cellulase enzymes and economic viability of biorefineries. Third-generation biofuels (algae) can be grown on non-arable land using saline water.
Environmental Science Renewable Energy Systems 1
Which of the following best describes how a wind turbine generates electricity?