High-voltage direct-current transmission lines hold the key to slashing greenhouse gases.
everal miles south of Rawlins, Wyoming, on a cattle ranch east of the Continental Divide, construction crews have begun laying down roads and pads that could eventually underpin up to 1,000 wind turbines. Once complete, the Chokecherry and Sierra Madre project could generate around 12 million megawatt-hours of electricity annually, making it the nation’s largest wind farm.
But how do you get that much wind power to where it’s actually needed?
The Denver-based company behind the project hopes to erect a series of steel transmission towers that would stretch a high-voltage direct-current transmission line 730 miles across the American West. It could carry as much as 3,000 megawatts of Wyoming wind power to the electricity markets of California, Nevada, and Arizona. With the right deals in place, the transmission line could deliver solar-generated electricity back as well, balancing Wyoming’s powerful late-afternoon winds with California’s bright daytime sun.
The $3 billion TransWest Express Transmission Project is among a handful of proposed direct-current transmission lines in the United States, and one of the furthest along in the planning process. It underscores the huge promise of these high-capacity lines to unlock the full potential of renewable energy.
Transmission isn’t sexy (see our story art above). It’s basic infrastructure. Long wires and tall towers (see “How Blockchain Could Give Us a Smarter Energy Grid”).
But a growing body of studies conclude that building out a nationwide network of DC transmission lines could help enable renewable sources to supplant the majority of U.S. energy generation, offering perhaps the fastest, cheapest, and most efficient way of slashing greenhouse-gas emissions.
Developing these transmission lines, however, is incredibly time-consuming and expensive. The TransWest project was first proposed in 2005, but the developers will be lucky to secure their final permits and begin moving dirt at the end of next year.
There’s no single agency in charge of overseeing or ushering along such projects, leaving companies to navigate a thicket of overlapping federal, state, county, and city jurisdictions—every one of which must sign off for a project to begin. As a result, few such transmission lines ever get built.
A macro grid
Direct current, in which electric charges constantly flow in a single direction, is an old technology. It and alternating current were the subject of one of the world’s first technology standards battles, pitting Thomas Edison against his former protégé Nikola Tesla in the “War of the Currents” starting in the 1880s (see “Edison’s Revenge: The Rise of DC Power”).
AC won this early war, mainly because, thanks to the development of transformers, its voltage could be cranked up for long-distance transmission and stepped down for homes and businesses.
But a series of technological improvements have substantially increased the functionality of DC, opening up new ways of designing and interconnecting the electricity grid.
Starting in the 1950s, some companies and countries began to deploy next-generation high-voltage DC transmission lines. These systems could carry more power much farther than AC lines, which suffer far more losses over greater distances. Crucially, direct-current lines can also be used to transmit power between “asynchronous” alternating-current systems like the nation’s three major regional grids, which otherwise can’t share power.
For the past two years, James McCalley, an engineering professor at Iowa State University, has been studying the best way to tie together those massive grid systems as part of the Department of Energy’s $220 million Grid Modernization Initiative.
One way to solve the problem is to expand existing “back-to-back” conversion stations to provide more east-to-west transmission capacity. These systems allow transmission between two grids, by converting the power to DC and then back to AC again at the point where they “cross the seam.”
Another approach adds three point-to-point transmission lines, running east to west, connecting the heart of each grid to that of the other. Yet another solution is a so-called “macro grid” of long DC transmission lines covering much of the country. It runs up the Florida panhandle, across the South, north to Seattle, east to Minneapolis, and back down to Louisiana, with several additional lines crisscrossing the West.
McCalley and his team developed models to simulate each of these scenarios over a 15-year period. They found that all three demonstrated a strong economic payoff, providing a benefit of at least $2.50 in savings for every $1 invested in the transmission system.
With direct-current lines, grid operators have more options for energy sources throughout the day, allowing them to tap into, say, cheap wind two states away during times of peak demand instead of turning to nearby but more expensive natural-gas plants for a few hours. The fact that regions can depend on energy from distant states for their peak demand also means they don’t have to build as much high-cost generation locally.
The point-to-point DC transmission scenario demonstrated the highest immediate economic return in the study, which will be published in the months ahead. But the macro-grid approach offers far greater redundancy and resilience, ensuring that the grid keeps operating if any one line goes down. It also makes it possible to build out far more renewable energy generation.
“The macro grid gives you a highway to all those loads and ties all those markets together,” says Dale Osborn, transmission planning technical director at the Midcontinent Independent System Operator (MISO), which first designed the system. “You get the most efficient, lowest-cost energy possible.”
A national direct-current grid could also help lower emissions to as much as 80 percent below 1990 levels within 15 years, all with commercially available technology and without increasing the costs of electricity, according to an earlier study in Nature Climate Change.
The researchers produced an idealized transmission network that connected 32 nodes across the nation, linking hydroelectric power in the Pacific Northwest, solar in California, wind energy in the Southwest, and nuclear energy on the East Coast, among other sources.