DC is back

Power sub distribution board by Tom Raftery (cc) (from Flickr)
Power sub distribution board by Tom Raftery (cc) (from Flickr)

Read an article in this month’s IEEE Spectrum on providing direct current (DC) power distribution to consumers.  Apparently various groups around the world are preparing standards to provide 24-V DC and 380-V DC power distribution to home and office.

Why DC is good

It turns out that most things electronic today (with the possible exception of electro-magnetic motors) run off DC power.  In fact, the LED desklamp I just purchased has a converter card in the plug adapter that converts 120-V alternating current (AC ) to 24-V DC to power the lamp.

If you look at any PC, server, data storage, etc., you will find power supplies that convert AC to DC for internal electronic use.  Most data centers take in 480-V AC which is converted to DC to charge up uninterruptible power supply batteries that discharge DC power that is  converted back to AC which is then converted internally back to DC for server electronics.  I count 3 conversions there: AC to DC, DC to AC and AC to DC.

But the problem with all this AC-DC conversion going on, is that it takes energy.

The War of Currents or why we have AC power today

Edison was a major proponent of DC power distribution early in the history of electronic power distribution. But the issues with DC or even AC for that matter is that voltage is lost over any serious line distances which required that DC generation stations of the time had to be located within a mile of consumers.

In contrast, Tesla and Westinghouse first proposed distributing AC power because of the ability to convert high voltage AC to low voltage using transformers.  To see why this made a difference read on, …

It turns out the major problem with the amount of line loss depends on the current being transmitted.  But current is only one factor in the equation that determines electrical power, the other factor being voltage.  You see any electrical power level can be represented by high current-low voltage or low current-high voltage.

Because AC at the time could easily be converted from high to low voltage or vice versa, high voltage-low current AC power lines could easily be converted (locally) to low voltage-high current power lines.   A high voltage-low current line lost less power and as AC voltage could be converted more easily, AC won the War of Currents.

Move ahead a century or so, and electronics have advanced to the point that converting DC voltage is almost as easy as AC today.  But more to the point, with today’s AC distribution, changing from lot’s of small, individual AC to DC and DC to AC converters in each appliance, server, UPS, etc., can be better served by a few, larger AC to DC converters at the building or household level, improving energy efficiency.

Where DC does better today

Batteries, solar panels, solid state electronics (pretty much any electronic chip, anywhere today), and LED lighting all operate on DC power alone and in most cases convert AC to DC to use today’s AC power distribution.  But by having 24-V or 380-V DC power in the home or office, it would allow these devices to operate without converters and be more efficient.  The Spectrum article states that LED lighting infrastructure can save up to 15% of the energy required if it was just powered by DC rather than having to convert AC to DC.

However, with the industry standards coming out of Emerge and European Telecommunications Standards Institute we may have one other significant benefit.  We could have one worldwide plug/receptacle standard for DC power.  Whether this happens is anyone’s guess, and given today’s nationalism may not be feasible.  But we can always hope for sanity to prevail…