Lesson 21: Renewable Energy

Warm-Up

The sun that warms your face delivers more energy to Earth in an hour than humanity uses in a year. The wind, the tides, the heat of the planet's interior, the growth of plants — renewable energy is all around us, constantly replenished. The challenge was never availability; it's capturing it affordably and reliably, especially when the sun sets and the wind stops. This lesson tours the renewable toolkit — solar, wind, hydro, geothermal, biomass, tidal, and hydrogen — with the honest trade-offs the exam expects. No source is perfect; each has a niche, a cost, and an environmental footprint of its own.

Core Concept

Solar energy

• Photovoltaic (PV) cells convert sunlight directly into electricity using semiconductors — no steam or turbine.
• Solar thermal (concentrated solar power) uses mirrors to focus sunlight, heating a fluid to produce steam that drives a turbine.
• Passive solar design uses building orientation, windows, and materials to capture heat without mechanical systems (Lesson 22).

Benefits: no emissions during operation, renewable, low operating cost, scalable (rooftop to utility), no water for PV. Drawbacks: intermittent (no sun at night/clouds), needs storage or backup, manufacturing uses energy and some toxic materials, land use for large arrays, upfront cost.

Wind energy

Wind turbines convert kinetic energy of moving air into electricity (blades spin a generator). Best in consistently windy areas (plains, coasts, offshore).

Benefits: no emissions in operation, renewable, low operating cost, land beneath turbines still usable for farming. Drawbacks: intermittent (variable wind), bird/bat mortality, noise and aesthetic objections, requires suitable windy sites and transmission lines.

Hydroelectric energy

Hydroelectric dams use flowing/falling water to spin turbines. Currently the largest renewable electricity source globally.

[DIAGRAM: Hydroelectric dam cross-section — reservoir behind dam, water flows through intake down a penstock, spins a turbine connected to a generator, then exits downstream. Label reservoir, dam, penstock, turbine, generator.]

Benefits: no combustion emissions, reliable, can adjust output quickly, long-lived, provides water storage/flood control. Drawbacks: dams flood large areas (habitat/community loss), block fish migration (e.g., salmon), alter downstream flow/sediment/temperature, reservoirs can emit methane from decomposing vegetation. Example: Three Gorges Dam (China) — huge output but massive displacement and ecological change.

Geothermal energy

Geothermal taps Earth's internal heat: hot water/steam from underground drives turbines (for electricity) or heats buildings directly. Geothermal heat pumps use stable shallow-ground temperatures to heat/cool buildings efficiently.

Benefits: reliable baseload (not intermittent), low emissions, small land footprint. Drawbacks: location-limited (best near tectonic/volcanic activity — Iceland, western US), high upfront drilling cost, can release some gases (H₂S) and cause minor seismicity.

Biomass energy

Biomass burns organic material (wood, crop waste, animal waste) or converts it to biofuels (ethanol from corn/sugarcane, biodiesel) or biogas (methane from decomposition).

Benefits: renewable, uses waste, can replace fossil fuels for transport (biofuels). Drawbacks: emits CO₂ and air pollutants when burned (not carbon-free), can compete with food crops/land (ethanol from corn), deforestation risk, lower energy density.

Tidal, wave, and hydrogen

• Tidal/wave energy — captures ocean movement; reliable/predictable (tides) but geographically limited and can affect marine habitat; early-stage.
• Hydrogen fuel cells — combine hydrogen and oxygen to produce electricity with water as the only byproduct. Clean at point of use, but hydrogen must be produced (often from natural gas or via electrolysis), so its cleanliness depends on the energy source; storage/infrastructure challenges remain.

Intermittency and the grid

The central challenge for solar and wind is intermittency — output varies with weather and time of day, unlike baseload sources (nuclear, geothermal, hydro, fossil) that run steadily. Solutions: energy storage (batteries, pumped hydro), grid interconnection over wide areas, demand management, and pairing intermittent sources with dispatchable backup.

Why this matters

The exam expects you to match each renewable to how it works and its specific trade-offs, and to reason about intermittency and grid integration. "Propose a renewable solution and justify it" is a common FRQ; the justification must name the source's benefit and acknowledge its drawback.

Worked Examples

Example 1 (easy): Match source to mechanism

How does a photovoltaic (PV) cell differ from a coal plant in producing electricity?

Solution: A PV cell converts sunlight directly into electricity via semiconductors — no combustion, no steam, no turbine. A coal plant burns fuel to boil water and spin a turbine. PV skips the entire heat-and-steam cycle.

Interpretation: PV is direct light-to-electricity; most other plants boil water to spin turbines.

Example 2 (medium): Hydro trade-offs

Give two benefits and two drawbacks of a large hydroelectric dam.

Solution: Benefits: reliable, adjustable, emission-free electricity; water storage/flood control. Drawbacks: floods large areas (habitat/community loss) and blocks fish migration; alters downstream flow, sediment, and temperature (reservoirs may emit methane).

Interpretation: Hydro is clean and reliable but ecologically disruptive to rivers and migratory fish.

Example 3 (AP-style): Intermittency

Explain why solar and wind require energy storage or backup, and name one storage method.

Solution: Solar and wind are intermittent — solar produces nothing at night/under clouds, wind varies — so supply doesn't always match demand. To provide reliable power, excess energy must be stored (e.g., batteries or pumped-hydro storage) for use when generation is low, or paired with dispatchable backup.

Interpretation: Intermittent sources need storage or backup to act like baseload power.

Example 4 (AP-style): Choosing a source by location

A volcanic island nation wants reliable, low-carbon baseload power. Which renewable is best suited, and why?

Solution: Geothermal. Its volcanic geology provides abundant underground heat, and geothermal is reliable baseload (not intermittent like solar/wind), delivering steady low-carbon electricity. (This is why Iceland relies heavily on geothermal.)

Interpretation: Match the source to the site — geothermal suits volcanic regions and provides baseload.

Common Mistakes

• Calling biomass carbon-free. Burning biomass emits CO₂ and air pollutants; it's renewable, not clean-burning.
• Forgetting hydro's ecological costs. "Clean" hydro floods land, blocks fish migration, and alters rivers.
• Ignoring intermittency. Solar and wind can't provide steady baseload without storage/backup — a key exam point.
• Saying PV uses steam/turbines. PV converts light directly to electricity; solar thermal uses steam, but PV does not.
• Treating hydrogen as a primary energy source. Hydrogen is an energy carrier; it must be produced, so its footprint depends on how.

Practice Problems

11. (FRQ-style) A sunny, windy coastal region wants to shift from coal to renewables. Recommend two renewable sources, explain how each works, and identify one drawback of each and how to address it.

12. (Data) A region gets 40% solar, 30% wind, 30% hydro. (a) Which two sources are intermittent? (b) Explain one strategy to keep the grid reliable given that mix.

FRQ Practice — Analyze an Environmental Problem and Propose a Solution (10 pts)

A country relies on coal and wants to transition to renewables. It has strong sun, steady coastal wind, major rivers, and no volcanic activity.

(a) Recommend two renewable sources well-suited to this country and explain how each generates energy. (4 pts) (b) Explain why geothermal is a poor choice here. (1 pt) (c) Explain the intermittency problem for solar and wind and propose one solution. (2 pts) (d) Identify one environmental drawback of large-scale hydro in this country and one way to mitigate it. (3 pts)