The United States gets about 40 percent of its electricity from carbon-free sources, including renewables and nuclear, and researchers have a pretty good idea of how to cost-effectively get to about 90 percent.

But that last 10 percent? It gets expensive, and there is little agreement about how to do it.

A new paper in the journal Joule identifies six approaches for achieving that last 10 percent, including a reliance on wind and solar, a build-out of nuclear power, and development of long-term energy storage using hydrogen.

This isn’t a matter of one pathway winning out over the others, said Trieu Mai, the paper’s lead author and senior energy researcher for the National Renewable Energy Laboratory in Golden, Colorado.

The larger point, he said, is that researchers and industry need to be doing the work now to figure out which technologies are the most viable in order to meet the goal, set by the Biden administration, to get to net-zero emissions in the electricity sector by 2035.

Here are the six options from the paper, along with what I see as their pluses and minuses:

• Wind and solar power with short-term energy storage and an expansion of interstate power lines. Pluses: Low costs and the technology is already available. Minuses: To be available around the clock, wind and solar need to work alongside energy storage systems, so this is a package deal. Also, the country will need a major expansion of interstate power lines to deliver wind and solar, a prospect that is costly and politically fraught. And, the growth of wind and solar requires a lot of open land, which has led to conflicts at the local level in communities that don’t want to host the projects.
• Other renewables, including geothermal, hydropower, and biomass. Pluses: The technologies are already available, and they can operate around the clock. Minuses: Relatively high costs. Geothermal has limits in where it can be built. Hydropower is vulnerable to falling water levels, and construction of new hydropower plants could be a danger to ecosystems. Biomass, which includes power plants that burn wood, is controversial because of the loss of trees and a disagreement about whether it should be considered a carbon-free energy source.
• Nuclear, along with fossil fuels and carbon capture. Pluses: Nuclear is a vital part of today’s mix of carbon-free electricity, which shows how the technology could be used in a future grid. Fossil fuel plants, if they could be retrofitted with carbon capture technology, are well-suited to providing peak power. Both can operate around the clock. Minuses: Nuclear is expensive and has a lot of baggage in terms of safety concerns. As for carbon capture, the technology has not been deployed successfully on a large scale, and it has been dismissed by some analysts as too expensive and inefficient. (The NREL report put these two resources in the same category because of some common traits, including 24-7 capability and high upfront costs and medium operational costs.)
• Long-duration energy storage using hydrogen. Pluses: The idea is that energy companies would use renewable energy to produce hydrogen from water, and then the hydrogen could be stored in vast caverns. During times of high electricity demand, the hydrogen could be released and burned to power a gas turbine or a fuel cell. If this could be done cheaply enough and stored in large enough quantities, it’s an opportunity to replicate the role of natural gas power plants in today’s grid. Minuses: Of all of the six, this may be the farthest away from being ready for the market, so talk of its benefits and drawbacks involve a lot of speculation.
• Carbon dioxide removal. Pluses: This covers a variety of approaches to removing carbon from the environment, including direct air capture and tree-planting. Even if this doesn’t end up being a major part of reducing emissions in the electricity sector, it could be an essential in other sectors, like heavy industry, that are more difficult to decarbonize. Minuses: Some carbon removal technologies are expensive relative to other options, and it’s not clear that they would work on the scale needed.
• Reducing electricity demand. Pluses: Low costs. As energy conservation advocates like to say, it is much cheaper to conserve a kilowatt-hour than it is to generate one. Minuses: Energy conservation measures, which include a gamut of programs and technologies, can be complicated to implement and require buy-in from policymakers and consumers.