This post is part of an ongoing “What Is” series from Energy Innovation that answers some of today’s most pressing climate policy questions. The first in this series answered the question–What is Net-Zero?, and the second answered the question What is The Inflation Reduction Act?.

What is Clean Energy?

Fighting climate change is the challenge of our time—cutting emissions at the speed and scale science deems necessary will determine the future of nearly all living things on the planet for generations. It’s an overarching problem that impacts every corner of our society and economy, and it requires.

However, as complicated as the problem seems, the solution to cleaning up climate pollution is straightforward: Use clean energy to make electricity and use that clean electricity to power equipment like vehicles, factories, and appliances that currently burn fossil fuels.

But what counts as clean energy?

The answer to that question varies depending on who is responding, muddying the landscape, diverting resources from the best solutions, and costing precious time in the effort to cut climate pollution. But the bottom line is clear – energy isn’t clean if it generates greenhouse gas emissions or air pollution.

To set the record straight, here’s a guide that walks through the clearly clean energy technologies and those requiring more nuance.

The obvious candidates: Renewable energy

 Renewables—wind, solar, geothermal, and hydropower—are universally recognized as clean energy. To generate electricity, they harness natural processes like wind, the sun’s rays, the earth’s heat, and the flow of water. And because they don’t burn any feedstock like coal or natural gas, they emit no air or climate pollution, qualifying them as clean energy. These sources currently generate about 20 percent of U.S. electricity.

Renewables are a bedrock climate solution, they’re the cheapest form of power today and will keep getting cheaper over time, and we know how to reliably manage an electric grid with a high share of renewable energy.

Clean but not pollution-free: Nuclear power 

Nuclear power currently supplies 20 percent of the United States’s electricity, making it one of our largest sources of power free from climate pollution. Nuclear plants can provide around-the-clock zero-carbon power, meaning they’re an important part of a clean grid. While constructing new nuclear plants is a timely and expensive process, maintaining the operating nuclear fleet is critical to meeting climate goals, because if they aren’t online, fossil fuel power could fill that required generation capacity.

We consider nuclear power to be “clean” on the basis that it does not emit pollution, although it must be acknowledged nuclear plants do create nuclear waste, which has complicated storage requirements and can be hazardous. The lack of greenhouse gas emissions is the key distinction in this analysis.

Hydrogen: It’s complicated

 Hydrogen has long been discussed as a potential climate solution, and it has certain use cases where it will be needed to provide energy free from climate pollution in a way electrified processes cannot like steel production, aviation, and long-haul maritime shipping.

However, even though burning hydrogen only emits water vapor, it’s complicated from a climate perspective. Hydrogen on its own scarcely exists in nature—generally it must be separated from other molecules using energy-intensive equipment called electrolyzers. If electrolyzers are powered by fossil fuels, the hydrogen they produce isn’t considered clean, since pollution occurs during the creation of that hydrogen, even if it only water vapor is emitted when the hydrogen itself is subsequently burned.

To be considered clean, hydrogen must be produced by electrolyzers powered by another clean energy source, ideally wind or solar. This is called green hydrogen.

Carbon capture and storage (CCS): Theoretically clean, absent in the real world

 CCS entails burning fossil fuels at a facility like a coal power plant or steel mill, and then sequestering the resulting climate pollution in an underground geologic formation or substance like limestone. While theoretically possible, this process is not and has not been used anywhere in the world at scale. Failures and cost overruns have plagued CCS demonstration projects. And while CCS captures carbon dioxide, it doesn’t capture traditional air pollution like soot, NOx, or Sox, all of which harms human health.

Natural gas: A bridge to nowhere

 Many industry groups have worked for years to brand natural gas as clean with the narrative of gas as a “bridge fuel” to a clean energy future often pushed by fossil fuel interest groups. But this is resoundingly false. Although natural gas emits half the carbon dioxide of coal, it still generates substantial amounts of climate pollution when burned.

And natural gas wells, pipelines, and appliances often leak methane, a much more potent greenhouse gas in terms of trapping heat in the atmosphere than carbon dioxide and is responsible for an estimated 20-30 percent of global warming since the Industrial Revolution.

There is no credible scenario in which natural gas can be considered clean from a climate perspective.

How policy can bring more clean energy online, faster

Because of clean energy’s superior economics, state and federal policy, and corporate climate goals, nearly all new additions to the U.S. electric grid are now clean.

In 2024, 96 percent of the new capacity added in the U.S. will consist of wind, solar, storage, and nuclear, all free from climate pollution. While this is a hopeful development, we’re still not adding clean energy fast enough.

Headwinds such as high interest rates, local siting challenges, supply chain constraints, and long wait times to connect to the grid are among the factors adding sand to the gears of the clean transition. Instituting smart policy can help ease the friction, including:

• Improving regional and interregional transmission planning.
• Accelerating interconnection by reducing the requirements on interconnection studies to only those necessary to connect the project to the grid.
• Upgrading transmission lines using affordable technology to get big capacity increases in short timelines.
• Implementing state and national clean electricity standards to mandate the transition to clean energy.
• Improving regional sharing of electricity to improve reliability and resiliency.
• And enabling demand-side solutions to meet peak loads quickly and affordably.

Clean energy is foundational to the fight against climate change—other solutions like electrification of transportation, buildings, and industry will only reach their fullest potential if clean energy supplies the electricity those technologies run on, rather than coal and natural gas.

Therefore, it’s crucial that policymakers, regulators, advocates, and businesses understand what energy sources are truly clean and which are imposters. Otherwise, they risk offering incentives, investments, and support to the wrong technologies that might only make our climate progress worse.