ALTERNATE GRID: Direct current is being newly reconsidered as an alternative to alternating current for electricity transmission.
Image: 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
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26 Comments
Add CommentThere are a number of inaccuracies here. For one, the battery of your laptop gets hot when it is recharged because of the inefficiency of the battery, not the charger. Check the temperature of the wall wart or power module to see how much power is wasted by the AC to DC conversion. Not very warm, is it? These things usually manage about 95% efficiency nowadays.
Reply | Report Abuse | Link to thisAlso, if you are going to go entirely DC, the voltage of the battery or other power source has to be the same as that needed by the device. Otherwise you need to do a DC-DC conversion, which is actually DC-AC-DC, which is what is already happening.
An interesting case of AC/DC would be the reasons electro-diesel locomotives go back and forth this. Both on the generation side and on the motor side. There is likely a lot of engineering and physics available published on this.
Reply | Report Abuse | Link to thisSo, if I want to run my house off solar panels and a small wind turbine, would I have to change my stove or frig or furnace? What would I have to do to make the DC current coming inside my house safe so my computer can use it?
Reply | Report Abuse | Link to thisUm, High voltage DC is the ubiquitous method used to transmit large amounts of electric power over long distances. How could you write an article and not know this?
Reply | Report Abuse | Link to thisThis last post actually has a number of inaccuracies. Not only did the article never mention a laptop's battery, it already stated that the power bricks (wall warts) waste 5-20% of their energy (hence 80-95% efficient). I don't know what TTLG is talking about, but when I touch my laptop's power brick, it's definitely warm. Maybe not enough to cook an egg, but it's definitely enough for me to sense; that's wasted energy, albeit a 5-20% waste. But, with everybody and their mothers on a computer nowadays, that 5-20% adds up.
Reply | Report Abuse | Link to thisIn regards to DC-DC conversion, this can easily be done at the integrated-circuit level and usually has a higher efficiency than an AC-DC converter. Indeed, your cell phone has about a dozen separate DC-DC converters inside of it (different components need different voltages). Last time I checked, they're making cell phones these days smaller and less heat-producing than those clunky power bricks.
Isolating grounds is an important advantage of the AC transformer for both safety and electrical noise reduction. Ground loops can be sneaky and dangerous.
Reply | Report Abuse | Link to thisIf a plug design for DC is standardized one would hope the safety chassis ground is considered from the start. The third prong on an AC plug became an afterthought after more than 50 years and is occasionally easily defeated.
This ClimateWire blurb makes it all sound so simple and imminently feasible! For some reason I'm skeptical.
Reply | Report Abuse | Link to thisAC makes sense in most situations, DC makes more sense in others.
Reply | Report Abuse | Link to thisBoth have been in use since the days of Tesla and Edison. The Author doesn't seem to understand a lot. DC power transmission has been in place large scale for 50 years now. It's still too expensive for most uses, but prices and technology keep changing.
Computers are already DC. The power converter in every computer or the brick for a laptop convert AC to DC for the computer. In fact, nearly every electronic device in your house, Flat screen TVs, DVD players, game systems and anything else with a power converter are DC already. The only issue would be what voltage. If you look on all these devices the DC voltage can range anywhere from 1 to 20 volts, with a computer at around 12 volts, so there would still need to be a converter to get the volts right or a need to standardize on one DC volt standard for all devices. Since the converter still needs to be there it is better than an AC/DC converter but not the complete elimination for the need to do some converting.
Reply | Report Abuse | Link to thisThis isn't really new, it's just a clueless author.
Reply | Report Abuse | Link to thisThere is a great difference in equipment for low power and high power, but, with solid state, power can be converted to different voltages without using a transformer. There's a unit in your computer that does that. It's easy at low power, but only recently has it become economical to do it with high voltages.
This guy should really talk to IEEE. They've been doing this for a long time.
When I suggested this years ago ,I was told that DC overloads are harder to control.
Reply | Report Abuse | Link to thisHVDC transmission is certainly not ubiquitous in the United States. According to a 2008 GAO report there were only five HVDC lines in the country and they covered about 2 per cent of all transmission line distance.
Reply | Report Abuse | Link to thisI'm just an ignorant bystander, but as others have mentioned this article seems somewhat confused/confusing. The enclosed photo caption seems to refer to future plans to install additional HVDC transmission lines. These offer the benefit of lower line losses due to coronal discharges. As I understand, this is especially important for large capacity (rural) generation facilities that are great distances from large capacity (urban) consumers.
Reply | Report Abuse | Link to thisHowever, the article seems entirely focused on a possible conversion of customer premise equipment to DC. I don't understand how anyone building a new facility could reduce capital expenditures by installing local DC distribution equipment, since available electrical equipment, appliances, etc., are all configured for AC local distribution facilities. It would seem that additional, perhaps custom, conversion equipment would be required. But I'm just guessing...
I don't see too much problem.
Reply | Report Abuse | Link to thisIt is easy to rectify AC, especially the most common three phase AC to DC, not so easy to convert (invert) from DC to AC. Even Automobiles which run on DC control & battery power, nowadays, use AC alternators, which they rectify to DC.
Reply | Report Abuse | Link to thisAnd Aircraft have their own independent power supply, so they chose to use 400 hz AC, due to lighter weight motors, generators and transformers.
You should realize that Industry runs on Big Motors and they are almost all Three Phase AC, which supplies a rotating magnetic field causing the torque that motors produce. And easy to change voltages over site power distribution, to minimize cost, mainly the high cost of copper. Double the voltage, 1/2 the current, 1/2 the copper.
A more sensible article would be to kick America's butt to reform their STUPID 480v, 3 phase medium voltage system, to Canada's much more sensible 600v system. And gradually push both of them up to 750 v, the code limit for medium voltage. Because of their stupid 480v system, Americans have to use HUMUNGOUS motors & cable & switchgear, with mega-tons of copper wasted stupidly.
And another thing, kick these morons like Honda in the butt, who seem to only make generators for off-grid households & cottages as single phase AC, when the vast majority of off-grid (at least if they have any sense), use batteries & inverter for home power, with the generator only used full-out at max efficiency, to charge the batteries, once a day or so. So, Honda - you numbskulls, how about selling generators optimized for battery charging, like everyone wants? And maybe make some diesel units with efficiencies of more like 35%, which you are quite capable of doing, fuel energy to DC battery charge power, instead of the lackluster 25% efficiency, like you are selling. Toyota can make a gasoline Atkinson generator for the Prius that is 38% efficient, fuel energy to DC battery/motor power. So how about selling something like that for off-grid home & cottage owners. And set up for CHP (combined heat&power) usage. You know you can do it.
And Honda makes a CHP NG fueled furnace replacement, that generates all a households power plus hot water & heat, but the crooks only sell them in the Northwest USA and only available installed through a licensed way overpriced contractor/installer. Why haven't these been available for years now? A bit of influence peddling going on I suspect. Wouldn't want homeowners in Northern locations to generate all their household power, hot water & heat much cheaper than the Utility charges would we. Crooks.
Agree about the Honda statement. The small motors for scooters can use 1970s 2 valve cams. The low cost of small motors means they are placed on the backburner for innovation. 5 valve titanium block is ignored to make inexpensive wasters. Corporate headquarters make these decisions. Yamaha has modern tech in their scooters. Ignorance at Honda.So wasteful. I think DC comes into its own in many small generator scenarios. Wind turbines, Neighborhood blook sized solar farms ect.
Reply | Report Abuse | Link to thisWhat I got from the article was that DC distribution within a home or building would be more efficient. It would also be easier to back up using batteries. The telecommunications industry has been doing this for as long as the industry has been in business. They use a -48V distribution system that can be backed up by multiple sets of 12V lead acid batteries. This is why the telephone still works even when the AC power goes out.
Reply | Report Abuse | Link to thisInternal voltages used by the individual equipment relies on DC-DC convertors. As a manufacturing engineer in the telecom market, I have had access to -48V DC distributed power throughout manufacturing for direct hookup. We still need AC for our common test equipment though.
I for one would be happy to have a DC power distribution scheme in my home IF I could find appliances and other consumer products that would run off that power. Unfortunately all current consumer products run off of AC.
For this idea to take off it would require a major restructuring of the consumer product industries. To start off, it would require either an additional DC Power input on all consumer equipment to give the option to the homeowner (similar to having multiple format video cables on TV's) or a whole new series of product developed to be DC only. You would also need a standard to define what the distributed voltages would need to be so manufacturers can build equipment to run on that voltage. I do not think the -48V used by the telecom industry would be the best voltage for distibution in a home but that would have to be looked at by the standard setting authority.
In summary, I think that DC distribution at the building level would be more efficient especially if there were plans to incorporate renewable energy systems into the design of the building. But it would only work if consumer products were available to use that power. I don't see this happening anytime soon.
Going DC will not solve much of the problems. It is not reasonable to use low voltage DC, such as 12V because the currents will be very high. For example, a 1 kilowatt microwave oven will need 83A current from 12V DC grid. And the current of the whole house will be very high meaning the wires have to be very thick. So DC voltage in the grid must be high, in the same range as in present AC grid.
Reply | Report Abuse | Link to thisAlso no modern electronics device is using 12V inside. First chips were working at 5V voltage, technological advances made each new generation to work from the lower voltage and modern processor works from voltage as low as 0.6V consuming somewhere 150A of current! DC/DC converters are unavoidable in all electronics.
All present electronics devices have input rectifier and DC/DC converter. Both of them have loses, but rectifier is responsible for a small part of them. Going DC will eliminate this rectifier but it is a very small advantage. Disadvantages are bigger: all equipment currently designed to work on AC, will not work on DC, DC/AC converters have to be used. This includes all transformers and AC motors.
Generally, converting AC to DC is easier than vice-versa.
There are some cases where DC grid is reasonable. A big number of identical consumers may be the case. Also solar power is DC by nature and in some cases it may be used directly instead of AC.
If you thought that going digital with Television was expensive...wait until industry computes to cost of switching *everything* to D.C.
Reply | Report Abuse | Link to thisLook out GDP of China!!!
Reply | Report Abuse | Link to thisDC as the author writes is a very viable power grid especially local with RE. I'm building a house now that has 12vdc and 120/240AC because many DC units are more eff than AC and batteries easily run it for days.
Next feeding in RE in DC is far less expensive, 1/3 as much. DC and AC 3000 wt inverter not hooked to the grid costs under $800 vs $2000 grid tie inverter plus the monthly bill.
PV panel are now $1/wt so an independent DC/AC system with PV and/or wind is an excelent way to go especially with a used veg oil diesel gen for backup and heat.
Every rectification almost to DC has a diode drop of 1vdc which at 12vdc is 9% loss.
DC/DC does not have to go AC but pulsed DC instead vs 3+ x's losses with DC/AC/DC way.
DC doesn't make so much inductive losses and can be 2.7? x's less voltage than AC for the same power/cable thus a cable can carry over 2x's as much with DC.
It's the new high powered DC/DC converters that can handle AC level voltages that is making DC viable now.
High Voltage DC transmission lines are not all that common but many countries are designing and installing these megavolt lines, especially in Europe and Asian countries. These types of lines are more efficient and with the advent of high voltage semiconductors, are more cost-effective than traditional high voltage AC transmission lines. Transmission lines is the key term here, distribution is another matter.
Reply | Report Abuse | Link to thisThe author mentions server-farms as strong candidates for a high voltage DC connection which does make sense, but for commercial and household use, AC distribution is probably a better choice; especially considering all the legacy 'stuff' that would still need to be supported.
The most advanced vehicles that utilize hybrid-electric drive technology utilize DC distribution within the chassis (fully isolated from the chassis with ground fault monitoring) with voltages up to 1000 VDC. A number of companies manufacture HVDC power modules that can provide any 'flavor' of electricity required, at power levels easily exceeding 100kW.
By the way; all motors and generators are AC machines.
When I supply 12-volts DC to my car starter motor, I am supplying DC power to a DC motor. My 1964 Comet had a 12-V DC generator (not an alternator). There are indeed DC motors, which any kid with a battery-operated motor toy knows, and DC generators. I would think commenters to SciAm would be less ill-informed.
Reply | Report Abuse | Link to thisOn what planet do they use high voltage DC? How do they deal with the enormous power loss and how do they deal with the deadly sparking that starts fires? Working for an electrical utility, I can assure you that every utility in North America and Europe do long distance transmission via AC. As far as I'm aware the rest of the world uses AC as well. Most countries use 220V instead of the American standard 110V but it is still AC.
Reply | Report Abuse | Link to thisI really don't know anything about it, but according to: http://en.wikipedia.org/wiki/HVDC
Reply | Report Abuse | Link to this"The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km."
"The disadvantages of HVDC are in conversion, switching, control, availability and maintenance."
An inventory of >200 global HVDC projects includes dozens in the U.S., along with map links:
http://en.wikipedia.org/wiki/List_of_HVDC_projects
Many installations in Europe at least are used for long distance underwater power transmission...
It does seem that the use of DC for customer premise equipment is another story, since most electrical appliances are AC, except for those intended to use batteries as their primary power source.
Reply | Report Abuse | Link to thisI don't see any inaccuracies. The article doesn't say the battery heats up, but the "power brick." In a desktop this is internal. For a laptop it is the DC conversion pack (its the bulky object either at the wall or about halfway up the power cord, also on game consoles and other devices). That is where the conversion takes place, and these devices do heat up significantly, at least in the case of laptops, game consoles, even speaker or sound systems.
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