Renewable Energy Systems: Solar, Wind, and Hydro

Renewable energy systems produce power from natural flows that are constantly replenished: solar energy from sunlight, wind energy from moving air, hydroelectric power from flowing or falling water, and also geothermal heat from inside Earth and biomass from organic matter. In brief: solar panels convert sunlight directly into electricity (photovoltaics) or use its heat; wind turbines convert the kinetic energy of the wind, with power proportional to the cube of wind speed; hydroelectric plants use the potential energy of water stored behind a dam, with power proportional to the flow and the height of the fall. Renewables emit little or no carbon dioxide in operation, but they vary with weather and place, and need land, materials and storage or grid integration.

Renewable energy is growing rapidly as costs fall and countries aim to cut emissions. This guide describes each source and how it works, its advantages and limits, compares them in a table, works through calculations of wind, hydro and solar output, outlines the history of renewable energy and corrects common mistakes.

How does solar energy work?

The Sun delivers on average about 1,000 W/m² to a surface facing it on a clear day at noon (the solar constant at the top of the atmosphere is about 1,361 W/m²). Photovoltaic (PV) cells, usually made of silicon, convert light directly into direct current by the photovoltaic effect; commercial panels have efficiencies of about 15–23 percent. Solar thermal systems use the heat: solar water heaters on roofs, and concentrating solar power (CSP) plants, which use mirrors to focus sunlight and produce steam for turbines. The output depends on sunshine, latitude, the angle of the panel, clouds and temperature, so it follows the day and the seasons, and it is zero at night. The capacity factor of PV (the actual energy as a percentage of the maximum if run at rated power all the time) is typically 10 to 25 percent. Solar is best in sunny, low-latitude regions, such as deserts, but works at moderate latitudes too. Storage in batteries or other means is needed to supply energy after sunset.

How does wind energy work?

Wind is caused by differences in air pressure resulting from uneven heating of Earth’s surface. A wind turbine’s blades capture the kinetic energy of the wind and turn a generator. The power in the wind passing through a rotor of area A is P = ½ρAv³, where ρ is the air density (about 1.225 kg/m³ at sea level) and v is the wind speed. A turbine can extract only part of it: the theoretical maximum is 59.3 percent (the Betz limit), and real turbines reach about 35–45 percent. Because of the cube, a small increase in wind speed gives a large increase in power, which is why turbines are put on hills, coasts and offshore, and on tall towers. Turbines operate between a cut-in speed (about 3–4 m/s) and a cut-out speed (about 25 m/s). Onshore capacity factors are about 25–40 percent, and offshore are higher. Issues: variability, visual impact, noise, and effects on birds and bats.

How does hydroelectric power work?

Water stored behind a dam falls through pipes (penstocks) and turns turbines connected to generators. The power is P = ηρgQH, where η is the efficiency (typically 85–95 percent), ρ is the density of water (1,000 kg/m³), g is 9.81 m/s², Q is the flow in m³/s and H is the head (the height of the fall) in meters. Types: large dams with reservoirs, run-of-river plants (with little storage), and pumped storage, which pumps water uphill when there is surplus electricity and releases it when needed, acting as a large battery. Hydropower is the largest renewable source of electricity, providing about 15 percent of the world’s electricity (a rough, recent figure), and it is reliable and can be controlled, but dams flood land, displace people, change ecosystems and sediment flow, and may emit methane. Tidal and wave power are smaller related sources.

What are geothermal energy and biomass?

Geothermal energy uses heat from within Earth, from hot rock and water near volcanic or tectonic zones (Iceland, New Zealand, Kenya, the Philippines, western United States). Steam or hot water drives turbines or heats buildings directly. It provides constant power day and night. Biomass is energy from organic material: wood, crop residues, dung, biogas from waste and biofuels such as ethanol and biodiesel. It is renewable if the plants are regrown, and the carbon dioxide released was recently absorbed, but growing fuel crops may compete with food and land, and burning wood and dung indoors causes serious health problems.

Renewable sources compared

Worked example: power from wind, water and sun

1. Wind power: a rotor with a diameter of 100 m has an area of π × 50² = 7,854 m². In a 12 m/s wind, the power in the wind is ½ × 1.225 × 7,854 × 12³ = 8.31 million W = 8.31 MW. With a power coefficient of 0.40, the turbine extracts 3.33 MW, consistent with a 3 MW rated turbine.
2. Effect of the cube: at 6 m/s, half the speed, the power is (½)³ = 1/8 as large: 0.42 MW for the same turbine (0.40 × 1.04 MW).
3. Annual wind energy: a 3 MW turbine with a capacity factor of 35 percent produces 3 × 8,760 × 0.35 = 9,198 MWh = 9.2 GWh a year. At 4,000 kWh a household per year, this supplies 9,198,000 ÷ 4,000 = about 2,300 homes.
4. Hydropower: with a flow of 50 m³/s, a head of 80 m and an efficiency of 90 percent: P = 0.90 × 1,000 × 9.81 × 50 × 80 = 35.3 million W = 35.3 MW.
5. Solar PV: a 1 kW (peak) system with a capacity factor of 18 percent produces 1 × 8,760 × 0.18 = 1,577 kWh a year.
6. Comparison: to produce the same 9,198 MWh as the wind turbine, a solar farm with a capacity factor of 18 percent needs 9,198 ÷ (8,760 × 0.18) = 5.8 MW of installed capacity, nearly twice the rated power of the turbine.

How have renewables developed?

Windmills and watermills have been used for over a thousand years. The first modern photovoltaic cell with a practical efficiency was developed at Bell Laboratories in 1954, and the first large hydroelectric plants appeared in the 1880s–1890s; Niagara Falls started generating power in 1895. The oil crises of the 1970s stimulated wind and solar research. Denmark pioneered modern wind turbines in the 1980s. Since 2010, the cost of solar PV has fallen by about 90 percent (a widely cited estimate), and wind and solar are now often the cheapest source of new electricity in many places. Renewables, including hydro, now supply roughly 30 percent of the world’s electricity.

Which mistakes are common?

Mistake 1: Confusing power and energy

Power (watts) is the rate. Energy (watt-hours) is the power multiplied by time. A 3 MW turbine does not produce 3 MWh every hour of the year.

Mistake 2: Ignoring the capacity factor

The rated power is the maximum. The actual output depends on sunshine and wind, so the capacity factor must be applied.

Mistake 3: Assuming that wind power rises in proportion to wind speed

It rises with the cube of the speed. Doubling the speed gives eight times the power.

Mistake 4: Assuming that renewable means no environmental impact

All energy systems use land and materials, and large dams can have major ecological and social effects.