Do you have a solar system at the home? An electric car in your drive way? Smart phones, tablets, game consoles and other electronics proliferating in your home?
If you answered yes to any of those questions, it's time to take a minute to learn about the difference between alternating current (AC) and direct current (DC) electricity and what it means for your household energy use.
As Doug Houseman explains at IEEE Smart Grid, "According to the Energy Information Agency (EIA), the fastest growing portion of residential electricity use is consumer electronics and small appliances. These devices primarily run on DC power."
Mr. Houseman is talking about any of your electronics that plug in to the wall through one of those AC rectifiers or adapters, (known as "power bricks," "wall warts," etc.) including your smart phone, computer, gaming consoles, many flatscreen TVs, LED lights, and a growing range of other electronic devices.
These digital devices now consume anywhere between 15 and 30 percent of a typical household's electricity, up from a negligible share just a couple decades ago.
“Within the next 20 years we could definitely see as much as 50 percent of our total [electricity consumption] be made up of DC consumption,” according to Greg Reed, director of the Power & Energy Initiative at the University of Pittsburgh. “It’s accelerating even more than we’d expected,” Reed told MIT Technology Review in April 2012.
So what's the big deal? Well, converting from AC to DC power involves wasted energy -- a lot of it.
"Even with improvements in power supplies," Mr. Houseman explains, "many of these devices have a conversion efficiency of no better than 80 percent and some low-end devices have efficiencies as low as 65 percent in converting power."
According to Houseman's calculations
, on the order of 5 percent of all electricity used in the typical U.S. home is lost to conversion of AC to DC power to run DC devices.
Worse yet, if you have a solar photovoltaic (PV) system on your home, you may lose close to a quarter
of its output at the inverter, the device that converts the DC power generated by the PV panels to the AC power used in your home's circuits and the broader power grid. The same goes for the output of any batteries you might use to store your solar output or provide backup power in the case of an outage; chemical batteries also output DC power, as do fuel cells.
Finally, if you've got an electric car parked in the driveway, charging that puppy up also bleeds energy converting from the AC power coming out of your socket to the DC power stored in the vehicle's on-board batteries. How much energy? Again, Houseman estimates that you lose about 24 percent of all power used to charge your electric car. That's enough to cost you close to $100 per year at average household electricity costs if you drive your EV about 35 miles a day, about the distance of the average American's round-trip daily commute.
So if you have a solar system at your home producing DC power, why are you converting it to AC and then back to DC to run your electronics and charge your electric car? Should you be wiring up a DC circuit in your house as well to power all your DC electronics?
For now, this sounds like a nice job for a DIY type with a bit of electrical skills -- although it might not be as easy or useful
as you might hope.
In the future, more and more
electricity experts are talking about whether or not new homes should be built with PV systems integrated into the roof and both DC and AC circuits in the home. If we want to both maximize the output of our solar systems and minimize wasted energy, this may become increasingly common.
Even today, some large consumers of DC power are already wiring up DC microgrids
to more efficiently fuel their needs. The large data centers that run the Internet, cloud computing services, and telecommunications networks are a logical candidate. These "server farms" now consume more than 1.3 percent of all electricity worldwide, according to Tech Review
, and that figure is surging.
Since all those servers run internally on DC power, the incoming AC power needs to be converted. "Instead of having power converters on each computer," Tech Review explains, "some companies are installing large centralized converters and distributing 380-volt DC power across their server farms. Japanese telecom giant NTT has four data centers in the Tokyo region operating on DC; last year it completed a DC-based server center in Atsugi City, southwest of Tokyo, that is its first to serve external clients."
An added bonus: reducing AC-DC conversion losses also reduces the waste heat generated by all those power converters, and that further saves on the large amounts of energy used to cool hot-running data centers.
The U.S. military
and large campuses such as universities
and hospitals are also experimenting with DC systems, particularly to run critical systems in the case of a failure of the AC power grid.
For more reading:
Jesse Jenkins is a graduate student and researcher at the Massachusetts Institute of Technology, where he is a candidate for a Masters of Science in Technology & Policy. At MIT, Jesse works as a researcher with the "Utility of the Future" project and is an MIT Energy Initiative Energy Fellow and a National Science Foundation Graduate Research Fellow.
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