Environmental Science Renewable Energy Systems 2 — Questions and Answers
Question 1: Concentrated Solar Power (CSP) differs from photovoltaic (PV) solar in that CSP:
- Converts sunlight directly into electricity using semiconductor materials
- Uses mirrors or lenses to concentrate sunlight as heat to drive a conventional steam turbine (Correct answer)
- Requires direct contact with sunlight and cannot use diffuse radiation
- Generates electricity only during brief periods of peak solar irradiance
Correct answer: Uses mirrors or lenses to concentrate sunlight as heat to drive a conventional steam turbine
CSP concentrates sunlight to produce high-temperature heat used in a conventional thermodynamic power cycle (steam or Stirling engine), and can incorporate thermal storage to dispatch power after sunset.
CSP technologies include parabolic troughs (curved mirrors focus sun on a heat-transfer fluid pipe), solar power towers (heliostats focus on a central receiver), linear Fresnel reflectors, and dish-Stirling systems. CSP generates heat (400-1,000 degrees C) to run steam turbines. A key advantage over PV is thermal energy storage (TES): molten salt can store heat for hours, allowing CSP to generate electricity after sunset. CSP is most economical in high direct normal irradiance areas - deserts at 30-40 degrees latitude.
Question 2: An offshore wind farm generally produces more electricity per unit of installed capacity than an onshore wind farm because:
- Salt water conducts electricity better, reducing transmission losses
- Offshore wind speeds are higher and more consistent due to reduced surface friction and fewer obstructions (Correct answer)
- Offshore turbines use larger blades that are prohibited in many onshore locations
- Ocean temperatures moderate the air density, increasing turbine efficiency
Correct answer: Offshore wind speeds are higher and more consistent due to reduced surface friction and fewer obstructions
Oceans have low surface roughness, fewer turbulence-causing obstacles, and higher average wind speeds - improving capacity factors from ~25-40% onshore to ~40-55% offshore.
Wind speed increases with height and decreases with surface roughness (trees, buildings, terrain). Offshore, surface roughness is minimal, mean wind speeds are higher, and winds are less turbulent and more consistent. This translates to higher capacity factors. Offshore also allows siting near population centers and permits larger turbines that cannot be transported to onshore sites. Trade-offs: offshore installation and maintenance costs are ~2-3x higher than onshore; corrosive marine environment increases maintenance needs.
Question 3: Lithium-ion batteries are currently the dominant technology for grid-scale and electric vehicle energy storage primarily because:
- Lithium is the most abundant mineral on Earth, ensuring low material costs
- They offer high energy density, long cycle life, and rapidly declining costs driven by manufacturing scale (Correct answer)
- Lithium-ion batteries have no safety risks and require no thermal management
- They can be fully charged and discharged infinitely without capacity degradation
Correct answer: They offer high energy density, long cycle life, and rapidly declining costs driven by manufacturing scale
Li-ion batteries have high gravimetric/volumetric energy density, high round-trip efficiency (~90-95%), long cycle life, and costs that have fallen ~97% since 1991 due to manufacturing scale from the consumer electronics and EV industries.
Lithium-ion batteries use lithium-ion intercalation in layered cathode (LiFePO4, NMC, NCA) and anode (graphite) materials. Key advantages: high energy density (150-250 Wh/kg); high round-trip efficiency (90-95%); scalability; and dramatic cost reductions (from $1,200/kWh in 1991 to ~$100/kWh in 2023). Challenges include: lithium and cobalt supply chain concerns; thermal runaway risks; capacity fade over cycles; and end-of-life recycling. Alternatives include sodium-ion, solid-state, flow batteries, and iron-air batteries for long-duration storage.
Question 4: Green hydrogen is produced through:
- Reforming natural gas with carbon capture and storage (CCS)
- Electrolysis of water powered by renewable electricity, producing zero lifecycle emissions (Correct answer)
- Fermentation of biomass to yield hydrogen as a byproduct
- Thermal decomposition of ammonia using solar concentrator heat
Correct answer: Electrolysis of water powered by renewable electricity, producing zero lifecycle emissions
Green hydrogen uses renewable electricity to power an electrolyzer that splits water (H2O) into hydrogen and oxygen, with no greenhouse gas emissions when the electricity comes from solar/wind.
Hydrogen production pathways are color-coded by emissions intensity: Gray (steam methane reforming without CCS, ~10 kg CO2/kg H2); Blue (SMR + CCS, ~2-4 kg CO2/kg H2); Green (water electrolysis powered by renewables, ~0 kg CO2/kg H2). Electrolysis technologies include PEM (proton exchange membrane), alkaline (lower cost, mature technology), and SOEC (solid oxide, high temperature, highest efficiency). Green hydrogen is seen as essential for decarbonizing hard-to-electrify sectors: steelmaking, ammonia production, shipping, and aviation.
Question 5: The primary disadvantage of biomass combustion as a renewable energy source compared to solar or wind is:
- Biomass plants cannot generate electricity - only heat for buildings
- Combustion emits CO2 and air pollutants, and large-scale biomass demand can drive deforestation and land use change (Correct answer)
- Biomass energy is too expensive to compete with any other electricity source
- Biomass cannot be used in combined heat and power systems
Correct answer: Combustion emits CO2 and air pollutants, and large-scale biomass demand can drive deforestation and land use change
While biomass is considered carbon-neutral in theory (re-growing plants reabsorbs CO2), combustion still emits particulates and NOx, and demand for biomass at scale risks deforestation, habitat loss, and food security impacts.
Biomass combustion is nominally carbon-neutral if the carbon released was recently fixed by photosynthesis and plants are replanted. However, full lifecycle analysis reveals complexities: (1) Carbon debt: cutting and burning a forest releases carbon immediately, while regrowth repays it over decades to centuries; (2) Land use change: converting forests or grasslands to energy crops releases soil carbon; (3) Air pollution: biomass combustion emits PM2.5, VOCs, and NOx; (4) High water and fertilizer requirements for dedicated energy crops.
Question 6: The net metering policy allows solar PV owners to:
- Avoid property taxes on the increased home value from solar panel installation
- Export excess electricity to the grid and receive credit against their electricity bill at the retail rate (Correct answer)
- Sell electricity directly to neighbors without going through a utility company
- Install solar panels without permits if system size is below a certain threshold
Correct answer: Export excess electricity to the grid and receive credit against their electricity bill at the retail rate
Net metering lets customers with rooftop solar offset their consumption: excess generation exported to the grid spins the meter backward (or is credited), with the utility billing only the net consumption.
Net metering (NEM) is a billing arrangement where a residential or commercial solar customer is credited for electricity fed back to the grid at (usually) the retail electricity price. The customer pays only the net of consumption minus generation over a billing period. NEM has been a major driver of distributed solar adoption in the U.S. and other countries. Utilities argue NEM customers don't pay their full share of grid infrastructure costs. Variants include NEM 2.0/3.0 (lower export credits), virtual net metering (for shared solar), and feed-in tariffs (fixed export price).
Environmental Science Renewable Energy Systems 2
Concentrated Solar Power (CSP) differs from photovoltaic (PV) solar in that CSP: