AP Environmental Science Unit 6 Renewable Energy: Concepts, Mechanisms, and Trade-offs

Biomass energy

Energy captured from recently living organic material (plants, algae, animal waste, food waste, some paper/wood waste); its impacts depend on sourcing and processing.

Stored solar energy (biomass concept)

Idea that plant tissues store chemical energy originally captured from sunlight via photosynthesis, later released by burning or conversion to fuels.

Carbon neutrality (biomass claim)

The argument that CO2 released when burning biomass is offset by CO2 absorbed during regrowth; may involve long time lags and requires sustainable harvesting.

Direct combustion (biomass)

Burning biomass (wood, pellets, residues) to produce heat and/or electricity (often by boiling water to make steam that drives a turbine).

Anaerobic decomposition

Breakdown of organic matter without oxygen, producing methane-rich biogas.

Biogas

Methane-rich gas produced by anaerobic decomposition; can be captured and burned for energy.

Landfill gas

Biogas generated as trash decomposes in a landfill; capturing it reduces methane emissions and provides usable fuel.

Anaerobic digester

A controlled tank/system that decomposes manure or food waste anaerobically to produce methane for energy and a remaining digestate.

Biofuels

Liquid fuels derived from biomass that can replace or blend with gasoline/diesel (commonly ethanol and biodiesel).

Ethanol

A biofuel typically made by fermenting sugars/starches (e.g., corn or sugarcane) and distilling the alcohol; still emits CO2 when burned.

Biodiesel

A fuel made from plant oils or animal fats used in diesel engines (often as a blend); still emits CO2 when burned.

Life-cycle emissions

Total environmental impact of a fuel across production and use (e.g., farming inputs, fertilizer, processing energy, land-use change, combustion).

Photovoltaic (PV) solar

Solar technology that converts light directly into electricity using semiconductor cells.

Photovoltaic cell

A semiconductor device (commonly silicon) where photons excite electrons, generating direct current (DC) when a circuit is connected.

Inverter (solar)

Device that converts DC electricity from PV panels into AC electricity used by homes and the grid.

Solar thermal

Solar technology that captures sunlight as heat to warm fluids for hot water, space heating, or electricity generation.

Concentrated solar power (CSP)

Solar thermal system using mirrors to focus sunlight to heat a fluid, make steam, and spin a turbine (often can include heat storage).

Intermittency (solar/wind)

Variability of power output due to changing natural conditions (time of day, weather, seasons, wind speeds), requiring storage or grid flexibility.

Hydroelectric power

Electricity generated from moving water, commonly by releasing stored water through turbines at a dam.

Dam-and-reservoir hydropower

Hydropower system that stores water behind a dam (gravitational potential energy) and releases it through turbines to generate electricity.

Run-of-river hydropower

Hydropower that uses river flow with minimal reservoir storage; can reduce (but not eliminate) flooding impacts and still affects river ecology.

Pumped-storage hydropower

Grid-scale energy storage where electricity pumps water uphill during low demand and releases it downhill through turbines during high demand; storage, not a primary energy source.

Geothermal energy

Energy derived from Earth’s internal heat; can provide reliable baseload electricity and direct heating in suitable locations.

Hydrogen fuel cell

Device that produces electricity electrochemically by combining hydrogen and oxygen to form water (no tailpipe CO2 with pure hydrogen), with impacts depending on hydrogen production.

Wind turbine

Machine that converts wind’s kinetic energy into electricity as airfoil-shaped blades spin a rotor connected to a generator; output varies with wind speed (cut-in and cut-out speeds).