Renewable Energy Resources
Renewable energy resources are energy sources that are replenished by natural processes on a human timescale, such as sunlight, wind, flowing water, geothermal heat, tides and biomass, and they produce electricity or heat with low or no net carbon dioxide emissions during operation. In brief: solar panels convert light to electricity, wind turbines extract kinetic energy from air (power ∝ v3), and hydroelectric stations use the gravitational potential energy of water (P = ηρgQh). Many renewables are variable, so storage and grid management are important.
Renewable sources supply a growing share of the world’s electricity, over 30 percent in recent years according to the International Energy Agency. This guide describes the main renewable resources and how they work, discusses advantages and limitations, compares them in a table, works through power calculations for solar, wind and hydro and corrects common mistakes.
How does solar energy work?
Sunlight can be used directly. Photovoltaic (PV) cells are made of semiconductors such as silicon, in which photons release electrons to give a direct current. Solar thermal collectors heat water or a fluid for buildings, and concentrated solar power uses mirrors to produce steam for a turbine. At the top of the atmosphere, the solar power is about 1,361 W/m2, and on a clear day at ground level it is about 1,000 W/m2 on a surface facing the Sun. Output varies with the time of day, weather and season.
How does wind energy work?
Moving air has kinetic energy. A turbine’s blades turn a generator. The power in the wind passing through a swept area A is P = ½ρAv3, where ρ is the air density (1.2 kg/m3) and v the wind speed. Because of the cube, doubling the wind speed increases the power eight times. A turbine cannot extract all of this: the theoretical maximum is 59 percent (the Betz limit), and real turbines reach about 35 to 45 percent. Wind farms are placed on land (onshore) or at sea (offshore), where the wind is stronger and steadier.
How do hydroelectric, tidal and wave energy work?
- Hydroelectric: water stored behind a dam falls through pipes and turns turbines. Power P = ηρgQh, with Q the volume flow rate and h the height of the fall. It is reliable and can be switched on quickly, and pumped storage can store energy by pumping water uphill. However, dams change ecosystems and displace people.
- Tidal: the rise and fall of tides drives turbines in a barrage or in the stream. Tides are predictable, but suitable sites are few.
- Wave: the motion of waves moves floats or air columns. The technology is less developed.
What are geothermal energy and biomass?
- Geothermal: heat from within the Earth is used to make steam or to heat buildings. It is available constantly, and is most effective in volcanic regions such as Iceland, where it provides a large share of heating.
- Biomass: wood, crops, waste and biofuels (ethanol, biodiesel) are burned or converted. It is renewable if plants are regrown, as the carbon dioxide released was taken up by the plants, but land use, emissions from growing and processing, and competition with food must be considered.
Renewable resources compared
| Resource | Main advantage | Main limitation |
|---|---|---|
| Solar PV | No fuel, low maintenance | Only in daylight, weather dependent |
| Wind | No fuel, low emissions | Variable, noise and visual impact |
| Hydroelectric | Reliable, storage possible | Environmental impact of dams |
| Geothermal | Constant output | Limited to suitable regions |
| Tidal | Predictable | Few sites, high cost |
| Biomass | Can use waste, dispatchable | Land use, local air pollution |
Worked example: power from sun, wind and water
- Solar panel. A panel of area 1.6 m2, efficiency 20 percent, in sunlight of 1,000 W/m2: input power = 1.6 × 1,000 = 1,600 W; output = 0.20 × 1,600 = 320 W. If the day has the equivalent of 5 hours of full sun, the energy is 0.32 kW × 5 h = 1.6 kWh.
- Wind turbine. Blades of radius 40 m, so A = πr2 = π × 402 = 5,027 m2. In a wind of 10 m/s: P = ½ × 1.2 × 5,027 × 103 = 3.0 × 106 W = 3.0 MW in the wind. With 40 percent conversion: 1.2 MW. In a wind of 5 m/s the power in the wind is one-eighth: 0.38 MW.
- Hydroelectric station. A flow of 100 m3/s falls 50 m, and the efficiency is 90 percent: P = 0.90 × 1,000 × 9.8 × 100 × 50 = 4.4 × 107 W = 44 MW.
How has renewable energy developed?
Windmills and water wheels have been used for over a thousand years. Edmond Becquerel discovered the photovoltaic effect in 1839, and Bell Labs made the first practical silicon solar cell in 1954. Large hydroelectric dams, such as the Hoover Dam (1936), expanded hydropower in the twentieth century. The prices of solar panels and wind turbines fell sharply in the 2010s, and in many places they are now the cheapest sources of new electricity.
Which mistakes are common?
Mistake 1: Calling renewable energy “free of emissions”
Operation produces little or no carbon dioxide, but manufacturing, transport and construction cause emissions.
Mistake 2: Expecting a turbine to produce its rated power all the time
The average output (capacity factor) is lower, typically 25 to 45 percent for wind and 10 to 25 percent for solar.
Mistake 3: Linear thinking about wind power
The power is proportional to the cube of the wind speed.
Mistake 4: Assuming that biomass is always carbon neutral
It depends on how quickly the plants regrow and on the emissions from production.