Renewable energy is generally limited by the weather. Wind, solar, and hydroelectric are all sensitive to the local conditions. Given that the US doesn't have a national electric grid, that means they're very intermittent; if the sun's not shining in California, then the golden state doesn't get much photovoltaic power.
Expanding the source of power over much larger regions can overcome the weather dependence; it's essentially unheard of for the entire US to be experiencing low-wind conditions. But this runs up against the structural limits of the US grid, where shifting power over large distances is either impossible or highly inefficient.
A new study released Monday looks into what would happen if that limitation were eliminated. It envisions a massive web of high-voltage, direct-current transmission lines, hooked up to 32 nodes spread across the US. This allows a massive spread of renewable power that could be dispatched anywhere in the nation. The result is a grid with dramatically lower carbon emissions and the bonus of lower costs to consumers.