ADVERTISEMENT
latest stories:

Edison's Revenge: Will Direct Current Make a Comeback in the U.S.?

The rise of direct current using devices and direct current generation have some rethinking the use of alternating current in the grid
thomas edison, george westinghouse, AC, DC, direct current, alternating current, grid, electricity, renewables



Wikimedia Commons/Biho

A new front in an old feud is being opened in the push for greater energy efficiency.

In the late 19th century, two competing electricity systems jostled for dominance in electric power distribution in the United States and much of the industrialized world. Alternating current (AC) and direct current (DC) were both used to power devices like motors and light bulbs, but they were not interchangeable.

A battle for the grid emerged from the Apple and Microsoft of the Gilded Age. Thomas Edison, who invented many devices that used DC power, developed the first power transmission systems using this standard. Meanwhile, AC was pushed by George Westinghouse and several European companies that used Nikola Tesla's inventions to step up current to higher voltages, making it easier to transmit power over long distances using thinner and cheaper wires.

The rivalry was fraught with acrimony and publicity stunts -- like Edison electrocuting an elephant to show AC was dangerous -- but AC eventually won out as the standard for transmission, reigning for more than a century.

Now comes the EMerge Alliance, a consortium of agencies and industry groups that thinks DC will make a comeback. With so many portable electronic devices and large electricity users like data centers running on DC, the technology can fill a growing niche while cutting energy consumption.

In addition, as more renewable electricity generators like photovoltaics and wind turbines producing DC come online, DC power systems can ease their integration into the grid. "We were asking, as a group, of ourselves, 'If we're generating DC power and we're using DC power, why are we converting it to AC to move it a few hundred feet, or even a few feet?'" said Brian Patterson, chairman of EMerge.

Correcting a wasteful 'mismatch'
The group is developing standards for DC power on small scales for individual buildings and specific applications like lighting. Patterson explained that the alliance is correcting "a fundamental mismatch between the power-based utility system and the user base, which is predominantly DC."

Though we now have the technology to transmit DC over the grid, most generators produce AC. This is then pushed to a higher voltage to overcome resistance in transmission lines. The current swings back and forth from a positive to a negative voltage generally 50 to 60 times per second, depending on the country.

When the power gets to the user, it is stepped down to more usable levels. Since computers, televisions and cellphones run on DC, the power has to be rectified from AC so the undulating current becomes flat and "direct."

This conversion isn't always efficient, wasting between 5 and 20 percent of the energy as heat. That's why your computer's power brick warms up when you charge it. "Your laptop is kind of its own nano-DC grid. If you can imagine that scaling out to our whole power system, you can kind of see an evolution similar to how the Internet formed," said Patterson.

Data centers form the foundations of the online world and may help DC gain traction. They house anywhere from a few dozen to thousands of servers, each with their own processors, hard drives and memory. These facilities form the back end for major enterprises -- not just in the tech sector, but for firms in consulting, the financial industry and research. However, they are notoriously heavy energy users.

For example, the Lakeside Technology Center in Chicago is one of the largest data centers in the world. Fed by more than 100 megawatts, it's the region's second-largest energy consumer behind O'Hare International Airport. Nationwide, these centers use 14.6 terawatt-hours of electricity annually, according to a report from Lawrence Berkeley National Laboratory (LBNL).

Economics could make DC rise again
These data centers have some quirks that make them inviting targets for DC microgrids. Since server uptime means money to businesses, many of these facilities are supported by uninterruptible power supplies (UPS), battery backup systems that ensure websites like Google stay online during power outages.

Brian Fortenbery, a program manager in the efficiency group of the Electric Power Research Institute (EPRI), noted that there are some glaring inefficiencies in such setups. "The thing that got us interested was that in the data center space, when they go through those conversions, the UPS they like to use converts from AC to DC to AC," he said, adding that the AC power from the backup is then converted back to DC inside servers, since that's how they run internally.

The conversions produce heat, so server rooms need very energy-intensive cooling systems, requiring upward of twice as much power to run air conditioning as is needed to run the servers themselves. "It actually looked kind of silly," he said.

EPRI and LBNL launched a pilot study on DC data centers in 2008. In their installation, they found that the DC systems were 6 to 8 percent more efficient and were 5 to 7 percent more efficient in energy consumption compared to AC data centers.

Fortenbery said these improvements are not the only upside to DC-powered server rooms. There are other variables that will draw companies to the paradigm. "The elements that are probably going to drive the market are to sell those DC systems for less capital expense, smaller footprints and efficiency gains. The biggest player, the biggest driver, is going to be the reliability in the system," he said, explaining that DC power is generally more stable and that eliminating conversion losses makes batteries last longer.

Renewable energy will create the 'Prius' of buildings
Another driver is renewable energy proliferation. Solar panels produce DC power, which has to be inverted to AC before it is fed into a home, an office or the grid. For companies striving for a net-zero-energy building -- one that produces as much energy as it consumes -- or trying to shrink the payback period for their solar investments, bypassing AC can help squeeze more energy from the sun, according to Brinda Thomas, a doctoral student in engineering and public policy at Carnegie Mellon University.

She said installing a DC power system will become cheaper over time, and she envisions buildings with both AC and DC power outlets.

EMerge's Patterson agreed that hybrid electrical systems are the future and said his group is developing standards for "creating the 'Prius' of buildings," writing rules for how current is routed, how plugs should be designed and how to make the systems safe.

These are important considerations because DC's voltage stays fairly constant when a device is run, unlike AC, in which the voltage goes to zero dozens of times per second. This means that if you unplug a DC device while it's running, the electricity could arc through the air. This can also cause corrosion and pitting in the metal components.

"We're doing the work that needs to be done to make the code and regulatory side of things a level playing field" for DC, he explained.

But away from transmission lines, DC is also gaining ground as an alternative in the developing world, according to Karina Garbesi, a professor and visiting researcher at LBNL. Getting power to remote areas from an AC grid is very costly and doesn't make much sense, since some of these regions can construct wind turbines and solar farms.

"Once you start going to this whole scenario, this direct DC becomes more and more attractive," she said. "The biggest issue is going to be the transition: How are you going to go from an AC-centric world to a DC-centric world?"

Patterson said that as buildings are retrofitted for DC delivery, the technology will spread in a manner analogous to the Internet, driven by larger firms before it spreads to homes, where rooftop solar panels charge electric cars.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

Share this Article:

Comments

You must sign in or register as a ScientificAmerican.com member to submit a comment.
Scientific American Holiday Sale

Give a Gift &
Get a Gift - Free!

Give a 1 year subscription as low as $14.99

Subscribe Now! >

X

Email this Article

X