What Renewable Energy Is and How It Works

What “renewable” means

A renewable energy source is one that is naturally replenished on human timescales, so using it does not deplete it in the way mining coal does. Sunlight keeps arriving, wind keeps blowing, and rivers keep flowing as long as rain falls. Most of these can be traced back to the Sun: it drives the weather and the water cycle and powers the growth of plants. Geothermal heat and tidal energy have different origins.

“Renewable” is not the same as “clean” or “free of impact.” Building turbines, dams and panels takes materials and energy, and each source affects land, wildlife or water in some way. The practical question is usually how the impacts compare, not whether they exist.

Solar

Sunlight can be used in two main ways. Photovoltaic solar panels convert light directly into electricity, as described in the article on how they work. Solar thermal systems use mirrors to concentrate sunlight onto a fluid, heat it and run a turbine, or simply heat water for buildings.

Solar works almost anywhere there is daylight, scales from a calculator to huge fields of panels, and has no moving parts in the photovoltaic case. Its output follows the day and seasons and drops under clouds, and nothing is produced at night.

Wind

Moving air pushes against the blades of a turbine, which spin a shaft connected to a generator. The power available rises steeply with wind speed, which is why turbines are placed in windy locations, often on tall towers, and offshore. Wind is a product of uneven heating of the Earth by the Sun, one of the main drivers of weather systems.

Wind farms can share land with farming. Their output varies from hour to hour, and concerns include noise, visual effects and risks to birds and bats, which developers try to reduce through careful siting.

Hydropower

Falling or flowing water spins a turbine. Many hydroelectric plants use a dam to hold back a reservoir, so the water can be released when electricity is needed. This gives them a useful ability to adjust output quickly. Some plants run on the natural flow of a river with little or no storage.

Hydropower has been used for centuries, first to turn mills, and is a long-established source of electricity in many regions. Dams can change river ecosystems, affect fish migration and sometimes displace communities, and output depends on rainfall and drought. Pumped storage, which moves water uphill when power is cheap and releases it later, works as a large kind of battery.

Geothermal

Heat from inside the Earth can be tapped where hot rock or water lies close to the surface. Steam or hot water from the ground can drive turbines or be used directly for heating. Because it does not depend on weather, geothermal can supply power steadily. It is practical mainly in places with suitable geology, often near volcanic or tectonically active regions, and drilling is costly. Smaller systems called ground-source heat pumps use the stable temperature of the shallow ground for heating and cooling buildings.

Biomass

Biomass is energy from plant and animal material, such as wood, crop residues or waste. The plants captured sunlight through plant photosynthesis, and burning or processing the material releases that energy. It can also be turned into liquid fuels or biogas. Biomass is more complicated environmentally than other options. Whether it counts as low-carbon depends on how the material is grown and harvested, how fast it regrows and what else it would have been used for, so experts disagree on particular cases.

Ocean sources

Tides, waves and ocean currents hold large amounts of energy. Tidal power uses the predictable rise and fall caused by ocean tides, for example by letting water flow through turbines in a barrage or free-standing underwater turbines. Wave and current devices exist but are less widespread, in part because the sea is a harsh environment for equipment.

Common challenges

• Variability. Wind and solar output change with the weather and time of day, so grids need ways to cope: battery storage, other sources that can ramp up and down, wider transmission networks, or shifting when people use power.
• Location. The best resources are not always near where people live, so power lines must be built.
• Space and materials. Some sources need large areas or particular minerals, which have their own mining and recycling questions.
• Cost. Costs have changed a lot over time and vary by place. Comparing sources fairly means looking at the whole system, not only the price of one device.

A rough comparison

Most energy planners expect a mix of sources rather than a single answer, because each fits different conditions. Reducing the use of fossil fuels is often linked to concerns about the greenhouse effect, but exactly how fast and by what mix is a matter of policy and economics as well as science.