DC Versus AC
The Second War of Currents Has Already Begun
This spring I published an article in MIT’s Technology Review magazine, “Edison’s Revenge: The Rise of DC Power,” predicting that dc might once again flow through the power distribution grids that deliver electricity to power consumers. It is an improbable scenario in the view of many power engineers, so I half expected some blowback. The most colorful response I received, however, was a note from a Canadian engineer who had reason to question the seemingly unimpeachable presumption that dc had long since disappeared from the distribution scene.
Surely ac’s victory in Edison and Westinghouse’s turn-of-the-century “war of currents” had relegated dc power distribution to museums of science and technology, right? Not completely, wrote David Cousins of Victoria, B.C., who told me he’d run across something surprising while conducting building energy audits in San Francisco: a dc power grid distributing dc power to the city’s fleet of ancient elevators. Remnants of dc power distribution quietly endured, it seemed, more than a century after ac usurped it to become the default currency of power grids worldwide.
I dropped through a Pacific Gas & Electric (PG&E) manhole south of Market Street this summer to document dc’s survival for IEEE Spectrum. What I found was a rectifier feeding dc power to the surrounding blocks on some of the original dc cables installed in the city during dc’s heyday—one of 97 dc power islands that PG&E operates to service seven to ten buildings each. These dc minigrids sustain the legacy of variable-speed dc-motored elevators locked into the bowels of countless historic buildings. It’s a pretty small niche as power grid technology goes, but one of immense value to San Francisco building owners.
It is, in fact, an exception that proves a rule. For while elevators preserved dc’s toehold in power distribution, dc power entrepreneurs have been steadily winning additional odd jobs at other levels of the modern power grid. Over the past century, innovators have secured additional niche applications for dc in power generation, transmission, and regulation, to name but a few. And this trend is accelerating with the proliferation of semiconductor electronics. They improve the power-electronics employed in many dc grid devices and, since all electronics run on dc, are also fueling exponential demand for dc power.
Could dc have finally secured enough turf in the early days of the 21st century to make this technological also-ran a contender, once again, for the soul of the power grid?
San Francisco Treat
DC applications are diverse, but nearly all promise reduced power consumption and eased integration of renewable energy. This green dividend and its significance are easy to spot in a glance around San Francisco. Exhibit A is the two-year-old Trans Bay cable, a 53-mi high-voltage direct current (HVdc) cable that transfers energy from Pittsburg, California, to San Francisco.
Trans Bay could be laid under the San Francisco Bay, easing its siting in this densely populated area, because dc cables’ steady current generates far weaker electromagnetic fields than an ac cable (and therefore experience fewer losses underwater). Thanks to its voltage source converter (VSC)-based terminals, a technology commercialized in the early 1990s, the system guarantees the availability of 400 mW of electricity to San Francisco while enhancing grid stability, an impressive feat in the heart of a meshed ac network.
The bottom line impact for the city is more than additional power: Trans Bay enabled the shut down of the Potrero power plant, whose 1960s-era primary unit was the city’s largest single source of air pollution.
Exhibit B delivers less power but puts it right where it’s needed: more than 2,600 solar panel arrays erected across the city’s skyline. Solid-state inverters convert the photovoltaic’s (PV’s) more than 18-MW of dc generation into ac to serve the electrical demand of their owners; surplus is fed to the grid to meet neighboring loads. By generating power at the point of consumption during the work day, when power demand usually peaks, the panels minimize strain on congested distribution lines.
Surprisingly, there’s room for plenty more solar power in famously foggy San Francisco. Studies by the California Energy Commission indicate a theoretical potential of approximately 500 MW of solar capacity, enough to cover roughly a third of the city’s residential power consumption.
Even Alcatraz, which is off grid, is benefiting from a major dc-enabled power upgrade thanks to integration of PV generation and a battery energy-storage system. The Rock’s 307- kW PV array feeds two 2,000-Ah battery strings, enabling 24-h operation to minimize reliance on the diesel generators that have supplied electricity on the infamous island for three-quarters of a century.
Overall, the U.S. National Park Service expects Alcatraz’s system will reduce generator runtime by 60%, slashing carbon dioxide emissions by 337,000 kg a year.
All that is just the present. Developments just getting underway promise to multiply dc’s domain. For example, a little ways down the Bay from San Francisco, dc is butting its way into one of the power grid’s newest roles: fueling up electrical vehicles (EVs).
In July Volkswagen’s U.S. subsidiary and electric charging firm ECOtality opened the first public dc charger in northern California. By connecting directly to an EV’s dc batteries, the station’s 90-kw power pump can deliver a fill in 1/2 hour that would take 3–8 h with a conventional ac charger.
That kind of performance is not just about convenience. It has the potential to eliminate the “range anxiety” that is a major impediment to the broad adoption of electric cars. The ability to boost charge in minutes rather than hours means commuters to the South Bay no longer need to worry that they’ll find themselves stranded by an empty battery.
Of course, San Francisco is but a microcosm, and these highlighted dc developments are but a taste of dc’s proliferation. As the articles in this issue show, the dc revolution is a broad and international phenomenon:
- VSC-based HVdc systems akin to the Trans Bay cable increasingly evacuate generation from Europe’s offshore wind farms.
- Conventional HVdc cables are tying together China’s immense grid, taking advantage of the lower resistance of dc cables and thus lower power losses when pushing power thousands of kilometers. Like the HVdc lines that deliver 3.1-GW of Northwestern hydropower to California, China’s state-of-the-art 750 and 800 kV cables feed gigawatts of remote hydro and wind power to the bustling cities of its Eastern seaboard.
- Low-voltage dc lighting systems employ centralized high-efficiency rectifiers to minimize the energy losses that occur when electricity is converted from ac to dc and vice versa.
- Standards bodies in Japan, Europe, and North America have blessed 380-V circuits for distributing dc power within commercial buildings. These promise to reduce losses in buildings with large and sensitive electronic loads, such as data farms, by centralizing rectification and also eliminating the wasteful cycle of inversion and rectification that is otherwise needed to charge their battery backup power supplies.
There is plenty more en route. For example, engineers are busy designing a meshed HVdc power grid to better interconnect Europe’s national grids while simultaneously distributing power from the North Sea’s offshore wind farms. The European Commission-backed project has given focus to development efforts by major power engineering firms such as Siemens and ABB to deliver dc circuit breakers and protection devices. If they succeed, the end product will be a meshed dc network with the power flow control of HVdc lines, one that could be less vulnerable to blackouts than interconnected ac grids.
DC developments look set to get really interesting as innovators combine them in hybrid systems. Consider an office building with 380-V circuits linking solar generation on the roof, internal dc loads such as servers and lighting, and a rapid EV charging station in the garage.
Dragan Maksimovic, an expert in power electronics at the University of Colorado in Boulder, is cooking up plans for such dc microgrids. The key component in his scheme is a bi directional ac/dc converter that matches the building’s dc generation and loads and manages its interface with the ac power grid to unload surplus generation or pull in additional electricity when its solar power is insufficient.
When the sun shines, dc generation sent directly to dc loads eliminates the cycle of inversion and rectification that wastes about 10% of a PV system’s output. By modulating dc charging, meanwhile, the converter can provide power conditioning and voltage regulation services to the grid. And the installed equipment is less likely to sit idle for long periods of each day, as would a stand-alone converter for a solar system or dc charging station.
I foresee a consumer market for residential-scale microgrids akin to Maksimovic’s offering city dwellers a taste of the antiestablishment chic associated with off-grid living. Think of it as the next step for early adopters of plug-in EV technology, whose petroleum-free statement is going mainstream with every new EV and plug-in hybrid offered by automakers.
Never mind that the required components—rooftop solar panels, dc-charging equipment, advanced converters, perhaps even batteries to minimize grid reliance—make the home microgrid an economic loser (in the context of today’s grid power and gasoline prices). Like hybrid vehicles and EVs, whose purchase price premium usually exceeds savings from avoided petroleum fuel, home microgrids will be embraced as a rejection of the status quo. By maximizing self-reliance, their champions will seek to distance themselves from notorious mainstream energy sources such as Canada’s energy-intensive “tar sands” developments, risky nuclear reactors, and polluting coal-fired power.
To the Ramparts!
DC devices are disruptive technologies. DC chargers such as ECOtality’s could backstop consumer confidence in EVs and thus accelerate their assault on the combustion engine’s century-long dominance of the automobile market. Entrepreneurial transmission players have exploited the flow control provided by VSC-based HVdc to build and operate cables such as Trans Bay as merchant lines, thus challenging the hegemony of the utilities that control regional ac grids.
DC’s ultimate disruptive potential goes back to the technology’s origins: dc equipment is gearing up, once again, to deliver power all the way from high-voltage sources clear through to consumers. The opportunity comes out of the simultaneous expansion of HVdc transmission and the emerging 380-Vdc building circuits. DC distribution closes the gap between the two, promising another layer of efficiency.
Some of the technology required exists. DC distribution believers such as the University of Pittsburgh’s Greg Reed, guest editor for this issue, look to power electronics to provide a key link whose absence doomed dc distribution a century ago: dc transformers.
Solid-state dc-to-dc buck-boost converters employ power electronics switching routines, rather than copper coils and magnets, to step dc voltage up and down. Power transformation from high-voltage to consumer-level 120 V should be on the order of 5% more efficient, Reed estimates. That is largely because the dc transformers do not consume reactive power—a power-sapping phenomenon in which ac equipment such as transformers shift the current and voltage waves in an ac signal out of sync, thus reducing the electricity’s apparent voltage.
Reed has predicted that the first new dc distribution circuits in a century could be operating within five years. What is required, says Reed, is to scale the dc transformers devices up for use at higher voltages and currents. Silicon carbide semiconductor switches will help, because they can handle higher-power operation. Durable and affordable silicon carbide switches for distribution-voltage dc components—expected by mid-decade—are expected to be 96–97% efficient and will operate at 200°C or higher. (In contrast, the silicon insulated-gate bipolar transistors used in today’s HVdc converters are just 92–94% efficient and require active cooling to stay below 100–110°C.)
Of course, novel technologies must overachieve to dislodge their well-entrenched predecessors, and dc distribution will be no different. To make it happen, dc equipment must not only improve, its advances must outpace those occurring in ac technology, some of which exploit the same materials and device advances that Reed and other dc believers are counting on. Silicon carbide switches, for example, will improve the flexible ac transmission system (or FACTS) devices that boost stability and provide partial flow control on ac grids.
In other words, another war of currents has begun. This round is likely to play out over several decades as dc technology leaps forward, and ac parries. That’s just fine by this journalist, who’s looking forward to calling the game.