Renewable Energy · Grade 12 Science

Renewable Energy

Learning goals

• I can describe how solar, wind, hydroelectric, geothermal, and biomass energy generate power.
• I can evaluate the benefits and trade-offs of each renewable energy source.
• I can explain what intermittency means and why energy storage is central to a renewable grid.

Readings for this unit

• Compare the service rather than the label on the technology

Renewable energy draws on replenishing processes such as sunlight, wind, flowing water, geothermal heat, and biomass growth. Replenishment does not imply unlimited local extraction or no environmental impacts. Biomass harvest can exceed regrowth, and water or geothermal use can affect the supporting system. Compare useful output, timing, materials, land, water, and lifecycle effects under the proposed conditions.

Photovoltaic cells convert incident light into electrical output without combustion at the panel. Output varies with irradiance, temperature, orientation, and device conditions; clouds often reduce output rather than making it universally zero. No sunlight at night requires another delivery source or stored energy. Manufacturing, transport, infrastructure, and end-of-life stages contribute to a wider environmental account beyond panel operation.

Wind turbines convert the kinetic energy of moving air into electricity, and thanks to steady improvements they are now among the cheapest sources of new power anywhere. Wind emits nothing as it runs, and the land beneath a turbine can still be farmed or grazed. The trade-offs: wind is intermittent (it blows when it blows, not when you need it), turbines can kill birds and bats, some people object to their look and noise, and the best winds are often offshore or in remote regions far from cities. Hydroelectric power, by contrast, is wonderfully controllable — a dam can release water to make electricity on demand — but building the dam floods large areas of land, blocks fish migration, traps sediment, and permanently reshapes a river.

Geothermal energy taps the natural heat of Earth's interior, using hot water or steam from underground to spin turbines or warm buildings directly. Because that heat never stops flowing, geothermal runs steadily around the clock with a small surface footprint — but it is largely limited to geologically active regions like Iceland or the western United States, where the heat is close enough to the surface to reach affordably. Biomass means burning plant matter or waste — wood, crop residue, or biogas — or refining crops into liquid biofuels like ethanol. Biomass is sometimes called 'carbon-neutral' because the plants absorbed CO2 as they grew, so regrowing them can reabsorb what burning releases. But that only holds if the plants are genuinely replaced, burning still produces air pollution, and growing fuel crops can compete with food and clear forests.

Nameplate capacity and actual generated energy are different quantities. Capacity factor is actual energy over an interval divided by the energy that continuous nameplate output would provide during that interval. It is not simply the fraction of time a source runs at full output and is not conversion efficiency. Dispatchability describes the ability to control delivery under operating constraints; hydrological, fuel, equipment, and other limits still matter.

And planning around intermittency is the central engineering challenge of a renewable future. When the wind dies down at dinnertime or clouds roll over a solar farm, the grid still has to keep the lights on. The answer is energy storage — grid-scale batteries, or clever tricks like pumping water uphill to release later — together with smarter grids that shift and share power across wide regions. Renewables are a powerful technological solution for reducing human impact on the climate and on finite fuel supplies, but realizing that promise depends less on the panels and turbines themselves than on solving the storage-and-timing puzzle around them.

Model check: Renewable describes replenishment, not zero impact or unlimited site availability. 12/(2×24)=.25=25%; it is not conversion efficiency.

Capacity Factor

capacity factor = average actual output / maximum (nameplate) capacity

the result is written as a fraction or percent; steady sources like geothermal have high capacity factors, while intermittent solar and wind have lower ones because they only produce power when the sun shines or wind blows. Rearranged, average output = nameplate capacity x capacity factor.

Worked examples

Avoid these mistakes

• Renewable describes replenishment, not zero impact or unlimited site availability.

Words to know

renewable energy

Energy drawn from sources that nature replenishes on a human timescale — sunlight, wind, flowing water, Earth's internal heat, and plant matter.

photovoltaic (PV) cell

A device, usually made of silicon, that converts sunlight directly into electricity.

intermittency

The tendency of some renewable sources — especially solar and wind — to produce power only when the sun is shining or the wind is blowing, rather than on demand.

capacity factor

The fraction of the time a power source actually delivers its maximum possible output, written as a percent; intermittent sources have lower capacity factors than steady ones.

How much sunlight? Far more than we could ever use

The scale of the solar resource is hard to overstate. The Sun bathes the Earth in roughly 173,000 terawatts of power continuously, while all of human civilization uses on the order of just 18 terawatts. Put another way, enough sunlight strikes the Earth in a single hour to power everything humanity does for an entire year. The challenge of solar energy, then, has never really been quantity — it is capturing that diffuse, on-and-off energy affordably and storing it for when the Sun isn't shining. Every advance in cheaper panels and better batteries chips away at that engineering puzzle rather than at any shortage of sunlight.

The duck curve: when too much solar becomes its own problem

Grid operators in sunny places like California watch a graph nicknamed the duck curve. As rooftop and utility solar flood the grid at midday, the net demand that other power plants must meet sags into the shape of a duck's belly — then, as the sun sets and people arrive home and switch everything on, demand shoots up the duck's 'neck' in just a few hours. Meeting that steep evening ramp without cheap storage forces operators to fire up fast, often gas-burning, 'peaker' plants. The duck curve is a vivid reminder that a renewable grid's hardest problem is timing, not total energy.

Not all 'green' is equal: the trade-offs of dams and biofuels

It is tempting to treat 'renewable' as a synonym for 'harmless,' but the two are not the same. Large hydroelectric dams flood whole valleys and ecosystems, block fish migrations, and — surprisingly — can emit significant methane as drowned vegetation rots in the reservoir. Corn ethanol, a common biofuel, can take nearly as much fossil energy to grow, fertilize, and distill as it ultimately delivers, and growing fuel crops competes with growing food. The lesson is that 'renewable' describes where the energy comes from, while 'sustainable' describes whether we can keep doing it without serious harm — and a source can be one without fully being the other.

Pumped hydro: the world's biggest battery is made of water

When people picture grid storage they think of giant lithium batteries, but by far the most energy stored on the world's grids today sits in pumped-storage hydropower. The idea is elegantly simple: when electricity is cheap and plentiful, surplus power pumps water uphill into a high reservoir; when demand spikes, that water is released back down through turbines to regenerate electricity, at round-trip efficiencies near 80%. New York runs exactly such a plant — the Blenheim-Gilboa pumped-storage project in the Catskills — which acts like a huge rechargeable battery for the regional grid, smoothing out the peaks and valleys of demand.

New York's offshore wind bet

New York has committed by law to getting 70% of its electricity from renewables by 2030 and to building 9,000 megawatts of offshore wind by 2035. That target moved from paper to reality when South Fork Wind, off the eastern tip of Long Island, delivered its first power in late 2023 as one of the first utility-scale offshore wind farms in the United States. Offshore wind is attractive for a densely populated coastal state because ocean winds are strong and steady and the turbines sit far from most neighborhoods — though the projects still must navigate costs, fishing interests, and effects on marine life, the familiar trade-offs of any real energy choice.

In the real world

New York's plan to power the state largely with offshore wind farms off Long Island and big upstate solar arrays is a real-world bet on renewables — and on solving their intermittency with grid-scale batteries.

Exit check

A2 kW turbine produces 12 kWh in 24h. Find capacity factor.