Clark W. Gellings
Vice President - Innovation
Electric Power Research Institute

 

Thomas Edison's nineteenth-century electric distribution system relied on direct current (dc) power generation, delivery, and use. This pioneering system, however, turned out to be impractical and uneconomical, largely because in the 19th century, dc power generation was limited to a relatively low voltage 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 subsequently stepped down for delivery to appliances and equipment in the home or factory. 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. Despite a vigorous campaign against the adoption of alternating current, Edison could not overcome the shortcomings of his dc system. AC won out, and today utilities generate, transmit, and deliver electricity in the form of alternating current.

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, no one is advocating a wholesale change of the infrastructure from ac to dc – as this would be wildly impractical. Nonetheless, a new debate is arising over ac versus dc: should dc power 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 centers, campus-like groups of buildings, or building sub-systems may find a compelling value proposition in using dc power.

Several converging factors have spurred the recent interest in dc power delivery. One of the most important is that an increasing number of microprocessor-based electronic devices use dc power internally, converted inside the device from standard ac supply. Another factor is that new distributed resources 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?

Advocates point to greater efficiency and reliability from a dc power delivery system. Eliminating the need for multiple conversions could potentially prevent energy losses of up to 35%. Less waste heat and a less complicated conversion system could also potentially translate into lower maintenance requirements, longer-lived system components, and lower operating costs. In a larger context, deployment of dc power delivery systems as part of ac-dc "hybrid" buildings – or as a dc power micro-grid "island" that can operate independently of the bulk power grid – could enhance the reliability and security of the electric power system.

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