The U.S. and European examples above -- now including Spain's 52-percent-renewable electricity supply for all March and 54 percent for all April (when the mix included 25 percent hydro, 22 percent wind, 5 percent solar) -- were achieved not by bulk storage but by deftly choreographing the operation of utilities' existing assets, using wind and sun to back out fossil fuels. In this way, San Diego Gas & Electric expects its fossil-fueled generation on sunny afternoons (when electricity is most scarce and valuable) to drop to zero within the next few years as solar power burgeons.
But we can go further. For example, my colleagues' hourly simulations show that after efficiency gains that the National Academies consider highly profitable, the isolated Texas grid (ERCOT) can meet demand reliably for every hour of the year 2050 with 100 percent renewables by using just options (a) through (e) -- using no fossil-fueled or nuclear power, no bulk electrical storage, and with only 5 percent of renewable generation left over. Reinventing Fire's 80-percent-renewable, 50-percent-distributed, highly resilient 2050 "Transform" scenario would need bulk storage only 5 percent as big as its renewable capacity and would cost essentially the same as business-as-usual, but would make cascading blackouts impossible and mitigate the grid's six other big risks. The National Renewable Energy Laboratory's equally rigorous REFS study showed how an 80 to 90 percent centralized-renewable U.S. grid could run reliably, with reasonable economics, and again with relatively little storage. Indeed, high-renewables futures may need less storage and backup than utilities have already bought (or would need to buy) to manage the intermittence of their big thermal power stations, which fail far less gracefully than a sensible renewable portfolio.
It also doesn't matter that there's currently "no way for power from places like the southwest, with plenty of sun, to flow to places like the northeast, which lack sun," because that's unnecessary and uneconomic. The cloudiest parts of the U.S. have at least half as much annual sun as the sunniest. New Jersey is now the Number 2 solar-powered state, after California, with ample sunshine to make a good business case. But the sun also isn't the only renewable source: There's wind, small hydro, geothermal, waste and biogas, tidal, wave, and more. Each region has its own mix. NREL's REFS study, Reinventing Fire, and others confirm that each region of the United States has an abundant and cost-effective range of renewable power choices. But nearer is often better: for example, Great Lakes wind rivals the Dakotas for powering Chicago, while offshore windpower could power the Northeast (though one would actually use a diversified portfolio).
The emerging regional strategy looks cheaper, easier, and more secure than shipping renewable power cross-country. Indeed, new transmission will increasingly have to compete with efficient use and distributed generation, and often won't be able to. And distributed renewable power plus "microgrid" architecture is the best if not the only way to make the grid highly resilient, enhancing our national security at essentially no extra cost.