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The Rise of Direct Current
Just about every discussion of alternating current (AC) and direct current (DC) starts with the great debate between Thomas Edison (advocating for DC), and Nikola Tesla, who was ultimately successful in showing that AC was the better choice for large-scale generation and distribution.* While this was certainly the correct outcome in the early 19th century, leading to the rapid build-out of an immense and reliable electrical grid, it may be time to revisit the use of DC in our homes and in most businesses. New technologies and the information economy have changed the demand for power and the way we use it.
Nearly all of the devices we rely upon in our houses and offices, from cell phones to computers to TVs natively use DC power (typically 3-5V). Our 120V AC power must be stepped down within the device or through the ubiquitous “brick.” The reason these bricks are warm is because they’re bleeding off energy as they make this transformation.
Changing Technologies: Lights, Gadgets, and Motors
Breakthroughs in manufacturing have led to a dramatic rise in the use of LED lighting in everything from flashlights to general interior lighting. LED lights produce better light quality than fluorescents, are generally more efficient, and last much longer. Best of all, the cost per lumen continues to fall and is expected to reach parity with standard fluorescent lighting in the next three years. When reduced maintenance is factored in, they are already more life-cycle cost-effective than fluorescent and high-intensity discharge (HID) fixtures for many applications.
LED lights require low-voltage DC power, so most LED fixtures have a driver which among other things converts AC power to DC. To reduce the cost and energy loss associated with this conversion, some manufacturers are now offering DC-based LED lighting systems. These systems allow quick installation and may be reconfigured easily by the building occupants since the voltage is so low an electrician is not required.
Small AC motors generally have poor efficiencies, usually converting less than 50% of the electricity they draw into useful mechanical power. Electronically Commutated Motors, or ECM motors (yes, the “motor” is redundant here, but that’s what they’re called) operate in the 60-80% efficiency range. This technology relies on internal electronics to create a rotating field so it can run on direct current. HVAC applications in fans and small refrigeration compressors, both of which are used in newer split-ductless air conditioners and VRF systems, increasingly apply ECM motors.
Photovoltaic panels produce DC power. Batteries are direct current devices. The cost for both continues to fall, and it’s estimated that PV-based power will be cost competitive with conventional utility power by 2020 or sooner. As distributed generation matures, there will be less of a reason to transform power from renewable systems to higher voltage AC only to convert it back down to DC at the point of use.
In developing areas without access to large electric grids, we’re likely to see the deployment of DC-based power systems based on solar PV panels with battery back-up. Houses and small public buildings such as clinics and schools can be lit, heated and cooled, and have their phones, computers, and other electronic equipment powered by native DC systems. This will prove a boon to the more than one billion people in the world who currently lack any sort of access to electricity.
In North America and Europe, DC-based microgrids are currently being developed that integrate a variety of DC generation sources such as PV, wind, fuel cells, and battery storage into a limited distribution network such as a campus or base. Many data centers are now running DC power to their servers to simplify their system architecture and improve efficiency.
The Near Future
The large AC distribution grid that arose in the first half of the 20th century will surely be with us for many decades to come, but there will be increasing pressures and opportunities in the coming years to change the business of how we consume energy.
*The story of this conflict is itself fascinating and is the subject of many books. In his book More Information Than You Require, author John Hodgman describes the epic lighting battle between Edison and Tesla in which the great debate was finally decided. Alternating current became the law of the land, but Nikola Tesla never recovered.
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