Renewable Energy Basics: Solar, Wind, Hydro and How the Grid Copes
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Generating electricity is one of the largest sources of greenhouse gas emissions in the world, because so much of it still comes from burning coal and natural gas. Renewable energy offers a way to produce power with far lower emissions. Over the past decade or so, solar and wind in particular have gone from niche to mainstream. This explainer covers the main types of renewable energy, why they have grown so quickly, and the real challenges of running a grid on them.
Key points
- Renewable energy comes from sources that are naturally replenished, such as sunlight, wind, flowing water and the Earth's internal heat.
- Solar and wind produce no emissions while operating, and their life-cycle emissions are far lower than those of coal or gas power.
- The cost of solar panels, wind turbines and batteries has fallen dramatically since 2010.
- Solar and wind are variable, so grids balance them with storage, flexible demand, transmission lines and other sources.
- Electricity is only part of the picture; transport, heating and industry also need to shift to clean energy.
What is renewable energy?
The US Energy Information Administration defines renewable energy as energy from sources that are naturally replenishing but flow-limited: they are virtually inexhaustible in duration, but limited in how much energy is available per unit of time. Fossil fuels, by contrast, formed over millions of years and are being used far faster than they form.
"Renewable" and "low-carbon" overlap but are not the same. Nuclear power is low-carbon but not renewable. Some forms of bioenergy are renewable but can have significant emissions depending on how the fuel is grown and used.
The main types of renewable energy
Solar power
Solar photovoltaic (PV) panels turn sunlight directly into electricity. When light hits a solar cell, usually made of silicon, it knocks electrons loose and creates an electric current. Panels can go on rooftops, over parking lots, or in large solar farms. A different technology, concentrating solar power, uses mirrors to focus sunlight and heat a fluid that drives a turbine. The US Department of Energy has a clear introduction to how solar works.
Wind power
Wind turbines use large blades to capture the energy of moving air, spinning a generator. Onshore wind farms are common in windy plains and hills; offshore wind farms take advantage of stronger, steadier winds at sea. Modern turbines are very large, because bigger rotors capture much more energy. See the Department of Energy's explainer on how wind turbines work.
Hydropower
Hydropower uses flowing or falling water to turn turbines. It is the oldest large-scale renewable source and still one of the biggest globally. Reservoir hydro can be turned up or down quickly, which makes it valuable for balancing the grid. Its downsides include effects on river ecosystems and fish, displacement of communities by large dams, and vulnerability to drought.
Geothermal energy
Geothermal plants tap heat from deep underground, usually in volcanically active regions, to make steam and drive turbines. Geothermal power runs steadily day and night. Separately, geothermal heat pumps use the stable temperature of shallow ground to heat and cool buildings efficiently almost anywhere.
Bioenergy
Bioenergy comes from plant and animal material, such as wood, crop waste, or biogas from landfills and manure. Its climate benefits vary widely. Using waste can be helpful, while clearing forests or farmland to grow fuel can cause large emissions; see Deforestation and climate change.
Why renewables grew so fast
The biggest story in energy over the past decade has been cost. According to the IPCC's Sixth Assessment Report on mitigation, from 2010 to 2019 the unit costs of solar energy fell by about 85 percent, wind energy by about 55 percent, and lithium-ion batteries by about 85 percent. In many places, new solar and wind are now among the cheapest sources of new electricity generation.
Costs fell because of mass manufacturing, better technology, larger turbines, cheaper financing, and experience: the more of something the world builds, the better and cheaper it tends to get. Government policies, from research funding to tax credits and auctions, helped kick-start that cycle.
How clean are renewables really?
Solar panels and wind turbines produce no emissions while generating power. Making them does involve mining, manufacturing and transport, which produce emissions. Life-cycle studies, which add up emissions from manufacturing through to disposal, consistently find that solar, wind, hydro and geothermal have much lower emissions per unit of electricity than coal or natural gas, typically by a large margin. Mining for materials such as lithium, copper and rare earths does raise real environmental and social concerns, which is why recycling and responsible sourcing are growing priorities.
The variability challenge
The sun sets and the wind does not always blow. Electricity grids must match supply and demand second by second, so variable sources require planning. Grid operators use a mix of tools:
| Tool | What it does |
|---|---|
| Battery storage | Stores cheap midday solar power and releases it in the evening; batteries are now being installed at large scale in several countries. |
| Pumped hydro storage | Pumps water uphill when power is plentiful and releases it through turbines when needed. |
| Transmission lines | Move power across wide areas, since it is usually windy or sunny somewhere. |
| Mixing sources | Solar and wind often peak at different times of day or year, so combining them smooths output. |
| Flexible demand | Shifts some electricity use, such as electric vehicle charging or water heating, to times when power is abundant. |
| Firm low-carbon power | Hydro, geothermal, nuclear and, for now, some gas plants that can run when needed. |
Grids with high shares of wind and solar already operate reliably in several regions, but each step higher requires more investment in these tools. Long periods of low wind and sun, sometimes lasting days, remain a planning challenge that researchers and grid operators are actively working on.
Beyond electricity
Electricity is only part of total energy use. Cars, trucks, home heating and industry still largely run on fossil fuels directly. A common strategy is to "electrify everything" where practical, using electric vehicles and heat pumps, while cleaning up the grid at the same time. Some sectors, such as aviation, shipping and steelmaking, will need other solutions too. To understand the electricity system more broadly, the EIA has an introduction to how electricity is generated and delivered.
What you can do
Depending on where you live and your budget, options may include rooftop solar, joining a community solar project, choosing a green electricity tariff, switching to a heat pump or electric vehicle when replacing old equipment, and using energy efficiently. Supporting local planning decisions for clean energy projects and transmission lines matters too. See What can one person do? for more.
The bottom line
Renewable energy captures natural flows of sunlight, wind, water and heat to make power with far lower emissions than fossil fuels. Sharp cost declines have made solar and wind mainstream. The main challenge now is integrating large amounts of variable power through storage, transmission and flexible demand, while extending clean energy to transport, heating and industry. Since burning fossil fuels is the main driver of rising CO2, as explained in What is the greenhouse effect?, the shift to clean energy is central to slowing climate change.