The US is using more and more sustainable energy, getting about 40% of its electricity from renewables. Further, researchers see a clear path forward on how to get the US to generate 90% of its energy from carbon-free sources in a relatively cost-effective way. But getting to 100% clean energy is much harder to achieve, as there is no perfect way to store renewable energy cheaply and universally. Now, a new study presents six options for getting that last 10%.

The options are as follows: short-term energy storage with more interstate power lines; less-common renewables like geothermal, hydropower, and biomass; nuclear energy paired with carbon capture; long-term energy storage with hydrogen; carbon removal; and reducing energy demand.

Each course of action has its own set of pros and cons. Some, like nuclear and biomass, have safety and environmental concerns. Others, like long-duration energy storage and carbon capture, use unproven, expensive technology.

“A 100% carbon-free power system will require a portfolio of resources,” said Trieu Mai, the paper’s lead author and senior energy researcher for the National Renewable Energy Laboratory in Golden, Colorado. “But humility is needed to accept that we don’t know what the optimal mix to solving the last 10%” will be.

Why This Matters

According to a recent IPCC report, even though wind, solar, and batteries have become cheaper and more effective, fossil fuels still supply 80% of global energy. But even though the global average of renewable use is low, some countries, like Kenya and Uruguay, have almost entirely phased out fossil fuels altogether.

This study provides an overview of the strategies the US can use to join these 100%-renewable nations and the obstacles to achieving them. Even despite these obstacles, renewables are on the rise throughout the US. Wind and solar power are the cheapest energy sources on the market, and even oil states like Texas rely on sustainable energy to meet increased electricity demands.

States Take Renewables To Scale

Though America’s grid needs federal support to function properly, some states are picking up the slack and using electricity more efficiently and sustainably.

In April, Washington became the first state in the US to legally require the installation of heat pumps, a much more sustainable alternative to traditional gas-powered furnaces and water heaters in new construction. California followed suit a few days ago, voting to ban the sale of all natural gas-fired space heaters and water-heating appliances by 2030 to limit ozone levels in the atmosphere. 

California is also phasing out the sale of new gas-powered cars and trucks by 2035, so the state is shoring up its electric car infrastructure. In fact, a recent Stanford study projects that increased EV use over the next decade will change how and when people charge their cars. Rather than plugging in overnight, as the state’s utilities currently incentivize, charging will also need to occur throughout the day to balance demand and not overtax the grid.