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An EPRI White Paper

DC Power Production, Delivery and Utilization

Edison Redux:The New AC/DC Debate

Thomas Edison’s nineteenth-century electric distribution sys- tem relied on direct current (DC) power generation, delivery, and use. This pioneering system, however, turned out to be im- practical and uneconomical, largely because in the 19th cen- tury, DC power generation was limited to a relatively low volt- age potential and DC power could not be transmitted beyond a mile. Edison’s power plants had to be local affairs, sited near the load, or the load had to be brought close to the generator.

Alternating current (AC) distribution was far superior for the needs of a robust electrical infrastructure. Unlike DC power, the voltage of AC could be stepped up with relatively simple transformer devices for distance transmission and subse- quently stepped down for delivery to appliances and equip- ment in the home or factory. And Nikola Tesla’s invention of a relatively simple AC induction motor meant end users needed AC, which could be generated at large central plants for high- voltage bulk delivery over long distances. (See the section AC versus DC: An Historical Perspective for more on the attri- butes of AC and DC power, and why AC originally prevailed.)

Several converging factors have spurred the recent interest in DC power delivery. One of the most important is that an in- creasing number of microprocessor-based electronic devices use DC power internally, converted inside the device from standard AC supply. Another factor is that new distributed re- sources such as solar photovoltaic (PV) arrays and fuel cells produce DC power; and batteries and other technologies store it. So why not a DC power distribution system as well? Why not eliminate the equipment that converts DC power to AC for distribution, then back again to DC at the appliance?

Contents

Edison Redux:The New AC/DC Debate.....................................1 Benefits and drivers of DC power delivery systems .......5

Despite a vigorous campaign against the adoption of alternat- ing current, Edison could not overcome the shortcomings of his DC system. AC won out, and today utilities generate, trans- mit, and deliver electricity in the form of alternating current.

Powering Equipment and Appliances with DC...........................9 Equipment compatibility.........................................................9

Example Application: Data Centers and IT Loads ...................12

Seeking relief from skyrocketing power density— and costs..................................................................................12

Although high-voltage direct current (HVDC) is now a viable means of long-distance power transmission and is used in nearly a 100 applications worldwide (see sidebar, High-voltage direct current transmission, p. 6), no one is advocating a whole- sale change of the infrastructure from AC to DC, as this would be wildly impractical.

Potential savings and benefits of DC power delivery in data centers.............................................................................15

Example Application: PV Powered “Hybrid” Building..............17

DC power delivery to optimize PV system economics................................................................................18

Example Application:Your Future Neighborhood....................23

But a new debate is arising over AC versus DC: should DC pow- er delivery systems displace or augment the AC distribution system in buildings or other small, distributed applications? Edison’s original vision for a system that has DC generation, power delivery, and end-use loads may come to fruition—at least for some types of installations. Facilities such as data cen- ters, campus-like groups of buildings, or building sub-systems may find a compelling value proposition in using DC power.

Potential Future Work and Research..........................................26

AC vs. DC Power:An Historical Perspective............................27 Transformers transform the power delivery system.....28 Centralization dictates AC instead of DC........................29

June 2006

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