Monday, June 3, 2013

Is It Time for DC Power?

In the 1880s, the War of Currents was raging between the two main factions - direct current (DC) which was heavily promoted by none other than Thomas Edison and alternating current (AC) advocated by George Westinghouse. The DC system was developed first and worked well for lighting which was the primary use of electricity in its early days. The AC system, however, has proven to be more efficient for powering motors and for carrying energy across long distances. In the end, AC won and the rest is a history. Today, our electricity grid is AC based.

Thomas A. Edison
When I look around my house today, I see a number of motor-based appliances including the washer, dryer, refrigerator, heater, air conditioner, etc. Those appliances use AC and that is the more efficient source of power for them. Yet increasingly, more and more of my electrical devices use a power adapter because they run on DC power: computer, printer, iPhone, iPad, PC speakers, cordless phone, Apple TV, TV set, alarm clock, radio, not to mention the many devices that use batteries: camera, keyboard, mouse, flashlight, fire alarm, etc.

Using all the power adapters to generate DC power is a hassle because of the lack of standardization. We practically have a different adapter for each device which is hugely inconvenient. Traveling with a bag full of power supplies is a major pain as I have written about in my post Environment and the Power Charger. In addition, power supplies are only about 70-80% efficient which means that about a quarter of the energy we produce (and pay for) is wasted on the AC to DC conversion.

This situation is particularly absurd for the increasing number of households that use solar panels to augment their power supply, often making them entirely independent from the AC power grid. The power produced by the solar panels is DC power. All the solar systems today require expensive inverters to invert the DC current into AC current. These inverters are expensive, often representing a significant portion of the entire cost for the solar power system. They are also inefficient, with efficiencies ranging from 50-90% - this is where we lose up to 50% of the energy produced by the solar panels!

So we are inverting DC solar power into AC current to power the house while losing up to 50% of the energy. At the same time, our devices increasingly use the DC power which requires an adapter that loses another 25% of the energy. So we are losing a significant percentage of the energy that we pay for. That sounds pretty inefficient, doesn’t it?

LED light bulbs may be the trigger
The story becomes even more interesting with the advent of LED-based light-bulbs. Lighting represents about 20% of household’s energy consumption today and switching to the LED light bulbs offers great opportunity to save on the monthly energy bill while doing something good for the environment at the same time. The LED lights are still pricey but those prices will surely go down, just like they did for the fluorescent light bulbs a decade ago. The problem with the LED light bulbs is LEDs work on DC and so each LED bulb has to contain a power converter which converts the house AC into the DC that the LED lights need. More AC/DC craziness, not to offend any rock fans...

All of this begs the question - is it time to wire our houses for DC power? We have standardized DC power in our cars with a slew of gadgets and appliances using the "cigarette lighter outlets" - from phone chargers and GPS to air pumps and mini-refrigerators. Many airlines provide a DC outlet in every seat to power our laptops and other gadgets. Why not have such DC outlets in every room of the house? Why not have the lights wired on a DC circuit?

USB outlets exist today
Sure, we will still need to transport power across long distances and we’ll need an AC circuit to power the big appliances with motors. But most houses have a separate 220V circuit for big appliances in addition to the standardized 110V wiring. In Europe, most house have 220V (well, 240V really) and they also have a 380V circuit for their washer, dryer, water heater and other big appliances. Why not have a separate DC circuit for all the devices? AC would come to the house like it does today but one single converter would replace all of those individual power adapters. On top of that, DC power is easier to store and a couple of batteries could provide an effective backup power supply.

Re-standardizing something as essential as the power system is a major undertaking. But we live in the times of major undertakings. If Google can take pictures of every street in the world and Tesla can build a network of charging stations throughout the entire country and SpaceX can fly to space, we might be also capable of switching to a more efficient power circuitry. We even have a standard - USB - which may not be meeting all the needs, but could be a starting point. Of course, we would first have to convince Apple to add USB interface to all their devices...

PS: Thank you, Brett, for an inspiring dinner conversation!


  1. Interesting idea. Consolidated Edison provided parts of NYC with DC until as late as 2007:

  2. Interesting. But not as practical as it sounds...

    AC has one advantage... Converting it from 110 (or 230V) to a lower AC voltage was relatively easy with a very simple device - a transformer - magnetic - or now more common - electronic.

    The problem with DC in the home is, is what voltage do you standardize on? 12V (Car)? 5V (USB) Both have the problem that in order to transfer sufficient energy the wires need to be really thick. Secondary issue is if the current is high enough you can weld or start a fire...

    Higher voltage requires converters again, and anything above about 48V has safety risks when exposed to body contact. And DC/DC converters are more expensive, and then there is the problem of groundloops if they don't isolate their outputs...

    Transformers do implicitly isolate the outputs from the inputs.

    On the other side... DC motors are easier to control in speed, and I suspect they are used more than we know in washing machines etc. Whether AC or DC wouldn't matter for electric stoves (although AC is easier to be used for triac/thyristor circuits)

  3. Led’s has revolutionized the whole world with the quality of light and ability to work on DC. moreover the operation and maintenance cost are the key factors, which made led’s more popular.