Offshore windmill. Photo courtesy obrien26382 via Flickr The great hope for powering a sustainable world is renewable energy. The great barrier to powering a sustainable world is the cost and complexity of building a new national transmission grid that will transmit the carbon-free electricity generated by remote wind farms and solar power plants to population centers.

In 2008, the U.S. Department of Energy released a report that concluded the United States could obtain 20 percent of its electricity from wind power by 2030. This week the Energy Department’s National Renewable Energy Laboratory issued a study that shows how the eastern half of the U.S. could obtain as much as 30 percent of its electricity from wind by 2024. The study focused on what transmission geeks call the Eastern Interconnection, six linked regional power grids that run from the Great Plains to the Eastern Seaboard and from the Canadian border to the tip of Florida.

“Although significant costs, challenges, and impacts are associated with a 20 percent wind scenario, substantial benefits can be shown to overcome the costs,” the report’s authors wrote. “Such a scenario is unlikely to be realized with a business-as-usual approach, and that a major national commitment to clean, domestic energy sources with desirable environmental attributes would be required.”

Essentially, all we need to do is come up with at least $93 billion for new power lines and infrastructure and get myriad transmission operators and local agencies to cooperate on the design of a new high-voltage grid.

Sounds daunting. But let’s put the numbers in context. The $93 billion is roughly what the U.S. spends in eight months on the Iraq and Afghanistan wars. Or to use another metric, a little more than what Joe Taxpayer forked over to bail out AIG.

The payback to build a transmission grid, however, is likely to be far more productive. Such an expansion of transmission capacity (and the resulting increase ) in wind power — and one assumes, other renewable energy — would displace coal-fired power plants, according to the NREL.

It would also solve one of the biggest problems with wind — its intermittency, which plays havoc with keeping electricity flowing smoothly. To greatly simplify, the wind is usually blowing somewhere. By multiplying the number of wind farms that feed into a single transmission grid from a broad swath of the country, transmission operators can rely less on fossil fuel power plants — read coal — as a backup when the wind dies, say, in South Dakota.

That will lower not only the price of renewable energy but utilities’ capital costs, which are, of course, passed on to their customers.

In California, for instance, utilities like PG&E spend billions on natural gas-fired power plants in order to provide emergency power for those few days each year when the grid is overloaded — hot summer afternoons when everyone cranks up their air conditioners at the same time.

“About 10 percent of our generation capacity sits idle for all but 50 hours a year,” Andrew Tang, a senior director at PG&E, tells me. “This industry is predicated on the premise that you always prepare for the worst day. It’s hugely expensive.”

Left unsaid in the NREL report is that the massive expansion of wind power needed to supply 20 to 30 percent of the nation’s electricity would be a green jobs machine.

At the beginning of 2009, according to the report, the U.S. had 25,000 megawatts of wind capacity installed and added another 4,500 megawatts during the first half of that year, despite the recession. To reach the 20 percent target, NREL estimates that 225,000 megawatts of new wind capacity must come online. Add another 105,000 megawatts to hit 30 percent.

The researchers offered different scenarios on how to achieve those goals, relying on varying mixes of wind from the Great Plains, the East and offshore. The 30 percent target relies heavily on developing wind farms off the East Coast, a capital-intensive undertaking that so far has run into huge political problems.

The NREL report, which was prepared by the consulting firm EnerNex, offers a highly technical discussion on how to reconfigure the grid to accommodate all that wind. But the bottom line is that between 8,352 and 11,102 miles of 800-volt direct current transmission lines, as well as thousands of miles of lower-voltage power lines, must be built. All in all, as many as 22,697 miles of new transmission lines would need to be installed, along with all the supporting infrastructure.

But the key takeaway is that NREL has concluded that there are no overriding technological hurdles or insurmountable financial obstacles to be overcome on the way to achieving the 20 percent target. It is basically a political problem — just imagine the NIMBY nightmare all those power lines would create.

Well, a political solution has been offered up by, of all people, Republican pollster Frank Luntz, who previously advised President George W. Bush on how to neutralize demands that the government take action on global warming.

Luntz has apparently undergone a climate change conversion. In a poll he released on Thursday, Luntz found that there is broad support among both Republicans and Democrats for climate change legislation when the issue is couched in terms of national security and energy independence.

“National security tops every other reason to support cap-and-trade,” Luntz concluded. “It’s about freeing the U.S. from foreign oil — and opening the door to greater security and prosperity.”

And as green energy advocates press to translate the NREL report into action, it’s worth remembering how a 20th century president managed to persuade Congress to fund a similarly ambitious infrastructure project, the interstate highway system.

Eisenhower did not argue that we needed to spend billions of dollars on a vast road system so we could develop the suburbs or drive coast-to-coast with ease. In the fearful fifties, he said building such a transportation network was all about creating the ability to move troops around the country in a national emergency. In other words, a national security argument secured what would become the driver of American prosperity in the coming decades.