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DC Power Production, Delivery and Utilization

Advocates point to greater efficiency and reliability from a DC power delivery system. Eliminating the need for multiple con- versions 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.

Benefits and drivers of DC power delivery systems

Several potential benefits are driving newfound interest in DC power delivery systems:

Increasingly, equipment operates on DC, requiring conversion from AC sources. All microprocessors re- quire direct current and many devices operate inter- nally on DC power since it can be precisely regulated for sensitive components. Building electrical systems are fed with AC that is converted to DC at every fluores- cent ballast, computer system power supply, and other electronic device. As one specialty electronics manu- facturer put it, “DC is the blood of electronics.”1

AC-DC conversions within these devices waste power. The power supplies that convert high-voltage AC power into the low-voltage DC power needed by the electronic equipment used in commercial buildings and data cen- ters typically operate at roughly 65% to 75% efficiency, meaning that 25 to 35% of all the energy consumed is wasted. About half the losses are from AC to DC con- versions, the rest from stepping down DC voltage in DC to DC conversions. Simply getting rid of the losses from AC to DC conversion could reduce energy losses by about 10 to 20%. Likewise, an increasing number of portable gadgets such as cell phones and personal digi- tal assistants (PDAs) require an AC-DC adapter, which also results in power losses during conversion. Consid- ered in aggregate, the millions of AC to DC conversions

June 2006

An EPRI White Paper

necessitated for the operation of electronics extract a huge energy loss penalty.

Distributed generation systems produce DC power. Many distributed generation sources such as photovol- taic cells and fuel cells—and advanced energy storage systems (batteries, flywheels, and ultra capacitors) pro- duce energy in the form of DC power. Other devices can also be suited to DC output, such as microturbines and wind turbines. Even hybrid vehicles such as the Toyota Prius could serve as DC generators in emergencies with the right equipment to connect them to the electrical system.

The energy losses entailed in converting DC to AC power for distribution could be eliminated with DC power delivery, enhancing efficiency and reliability and system cost-effectiveness. For instance, EPRI So- lutions estimated that the total lifecycle cost of PV en- ergy for certain DC applications could be reduced by more than 25% compared to using a conventional DC to AC approach—assuming that the specific end-use applications are carefully selected.2 The costs of new distributed generation such as PV arrays are still high, so optimization of designs with DC power delivery may help spur adoption and efficient operation.

Storage devices such as batteries, flywheels and ca- pacitors store and deliver DC power. This again helps avoid unnecessary conversions between AC and DC.

DC power could help power hybrid automobiles, tran-

sit buses, and commercial fleets, Plug-in hybrid vehi- cles can go greater distances on electricity than today’s hybrids since they have larger batteries. These batteries store DC power, so charging them with electricity from solar photovoltaic arrays and other distributed sources could reduce reliance on gasoline, enhancing security and emergency preparedness.

DC power delivery could potentially enhance energy efficiency in data centers, a pressing need. One of the most promising potential applications of DC power delivery is in data centers, which have densely packed racks of servers that use DC power. In such centers, AC

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