Alaska Village Electric Cooperative (AVEC), based in Anchorage, serves 58 villages stretching across more than a thousand miles of rugged, wild country. Only one—Minto—is accessible by road. The others must be reached by plane or boat. Diesel generators provide the bulk of power, requiring around 9 million gallons of fuel to be shipped in annually.
A grid connecting those communities would provide important benefits, saving fuel, transportation, and other costs while increasing overall resiliency. But the distances between villages, which largely operate as islanded micro-grids, can be huge. Transmission would have to reach across the state’s vast landscape and withstand its harsh weather.
These challenges have led
AVEC to consider direct current (DC) transmission as a way to tie its far-flung communities together and even to connect certain facilities within those villages. In doing so, the co-op is part of a movement in the electric utility industry to give DC power another look.
“AC went out and took over more than a 100 years ago,” says Meera Kohler, AVEC’s president and CEO. “Now it’s time for DC to reassert its presence.”
The battle between DC and alternating current (AC) power has a long history. When Thomas Edison opened his first generating plant in New York in 1882, it provided DC power. Edison’s company would help spark a revolution that lit the world. But the DC power he endorsed had a problem: There was no way at the time to raise or lower DC voltages, which meant DC generators created power at a lower voltage, safe for household use, but only able to travel short distances.
Edison envisioned a world of small, local grids powered by DC current. In the next four years, his company would install 58 such microgrids and about 500 individual lighting plants in the United States and around the world.
But a former Edison employee, Nikola Tesla, believed AC was a better solution. Alternating current could be stepped up or down in voltage through contemporary power transformers, meaning electricity could be generated and transmitted at higher voltages, traveling farther and allowing for fewer centralized power plants.
Direct and alternating current dueled for supremacy for nearly two decades in what was known as “The War of the Currents.” But in 1896, when Buffalo, New York, successfully lit the city with AC carried along 26 miles of transmission lines from a hydroelectric plant at Niagara Falls, the battle swung decisively toward AC.
‘Why convert it?’
Today, resurgent interest in DC is being driven by several factors: First, technological advances have reduced the challenge in changing DC voltage. Second, the lower line losses that come with DC transmission have made it more cost effective for certain long-distance lines. Finally, the growth of distributed renewable generation has increased the amount of DC power available to the grid. Solar, whether utility scale or rooftop, starts out as DC before being converted to AC by an inverter as it’s fed into the grid or a household.
“We’re seeing DC at two levels,” says Craig Miller, NRECA chief scientist. “One level is high-voltage DC transmission in particular places. We’re seeing a lot of it in transmission under water bodies, for example. It works better in those circumstances than AC does. The second place it’s showing up is in buildings. Solar, for example, is DC, and so are batteries. So if you’ve got solar panels and any kind of storage, why convert it into AC?”
DC also avoids the frequency and harmonics issues that come with AC, which in the U.S. reverses direction 60 times a second. “You don’t have to worry about frequency synchronization,” he says. “There’s no frequency in DC.”
Around the world, several long-distance power connections are taking advantage of DC. The United Kingdom and Norway, for example, are working together to lay the longest underwater power cables in the world: 447-mile DC connections between the grids of the two nations crossing the North Sea. The interconnectors will carry 515 kV and will allow Great Britain to tap into hydroelectric power from Kvilldal, Norway, that nation’s largest hydro-electric generation plant.
By using DC power, the line will have only a 1% loss and will avoid problems of frequency modulation that come with long AC spans, according to reports. Conversion plants on both ends of the line will shift from AC to DC and back.
The idea of tying together AVEC’s villages with DC lines is only in the conceptual phase but could provide similar advantages.
“It’s a bit more robust and able to carry more power with lower losses,” Kohler says. “So, technically, it makes a lot of sense.”
Theory becoming reality
Building traditional AC transmission lines to connect villages in Alaska comes with several challenges, says William Thomson, AVEC’s technical and engineering advisor to the CEO. First, the small size of these communities means very high construction costs per customer.
“And then you’ve got a continuous maintenance headache—some of the worst weather in the world and difficult logistics getting to those lines,” he says. “Sometimes you just can’t get out.”
But steel-armored submarine DC cable, the kind used in seabeds, could be laid across the frozen tundra in the winter much more affordably because the cost of building transmission towers and hanging line is avoided, he explains. In the warmer weather, as the tundra thaws, the lines would sink beneath the surface.
The project would still require significant investment and inverters to handle the medium-voltage DC-to-AC conversion. When AVEC first began looking at DC a few years ago, inverters weren’t being built in the size that would be needed. The co-op worked with Princeton Power, a company specializing in microgrid technology, on a medium-voltage design, Kohler says.
The results weren’t robust enough to be tested in the field, but Thomson says the technology has advanced, and an affordable product could be available within the next couple of years.
“It does look feasible at this point,” he says. “The next step is to look at villages where it makes sense to tie them together.”
DC is also gaining traction in smaller settings tied to solar’s growth and the proliferation of DC-powered devices, such as electric vehicles, computers, cellphones, and many modern household appliances. Photovoltaic cells provide DC power, which is converted to AC through inverters for distribution systems and building wiring and then converted back to DC to power in an increasing number of products.
“The amount of inverted-based distributed generation and load have increased exponentially over the last decade or so,” says Venkat Banunarayanan, NRECA’s senior director for the integrated grid. “You have solar and battery, and both are DC and inverter based, and inverter accounts for a significant amount of the cost in those cases. Any incentive to take away all those inverters would reduce cost. From the load side, you have all the iPhones and laptops and iPads, all of them powered by direct current. They all require converters too.”
Companies have begun installing smaller-scale DC projects. In September, Sandia National Laboratory and Emera Technologies cut the ribbon on a DC microgrid in Albuquerque, New Mexico, a pilot project that will test DC microgrid technology by powering a community center and housing units at Kirkland Air Force Base.
A Canadian company, ARDA Power, has developed a building-scale DC project at a manufacturing facility in Burlington, Ontario, that uses photovoltaic cells and a battery system to power electric vehicle charging stations, LED lights, and other systems.
Some analysts believe the growth of DC consumer products and generation mean DC will eventually come to rival AC. Greg Reed, director of the Power & Energy Initiative at the University of Pittsburgh, told MIT Technology Review he expects that 50 percent of load will be DC by the early 2030s, which will lead to expanding DC transmission and distribution.
But Banunarayanan says there are reasons to be skeptical: Circuit breaker technology for DC remains more complicated and expensive, and there are also questions about control and optimization in a grid with multiple sources of DC generation and load.
Finally, he adds, “you’ve got to define the business case.” An investment in DC has to come with a clear economic advantage over AC. The cases where that exists are still limited, he says.
NRECA’s Miller says the biggest hurdle is the existing electrical grid.
“We have a legacy system,” he says. “Every part of it is AC. If we were starting from a clean sheet, there would be a lot more DC, but we aren’t.”
He expects DC will be adopted more quickly in underdeveloped countries with limited electrical infrastructure. In the United States, Miller believes DC transmission will grow, and we will see more DC microgrids on campuses, military bases, and similar sites.
“A college puts in a microgrid, they may put in a DC substation,” Miller says. “They build a new building, they put some solar on top, they put in DC. It makes sense, and eventually it becomes more common.”