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An EPRI White Paper

DC Power Production, Delivery and Utilization

Another IT application: Power over the Ethernet

Another information technology (IT) application that may lend it- self to DC power delivery is supply for power over the Ethernet (PoE). According to the Institute of Electrical and Electronics Engi- neers (IEEE):

The voltage for PoE is 48 V, with about 13 to 15 W of available power at the device. A DC-DC converter transforms the 48V to lower voltages needed for electronics.

Power over Ethernet technology allows IP [Internet protocol] telephones, wireless LAN [local area network] access points and other appliances to receive power as well as data over LAN cabling without needing to modify the existing Ethernet infrastructure.

The LAN cables carry both data and power, just as traditional tele- phone lines carry both voice and power on the same lines. PoE can supply power to computer phones (also known as voice over In- ternet protocol orVOIP) as well as other devices such as web cam- eras, electronic badge readers, and even electric guitars or other musical instruments.

The power infrastructure must be able to support the increased power requirements of PoE. In its own white paper on the top- ic, Cisco Systems states that the overall power budget for both the switch and powered devices is 5,000 to 6,000W. The ultimate source of the power may be AC, but as with the increased power requirements of data centers, facility managers must consider how best to deliver this energy, taking into consideration component costs, efficiencies, and cooling considerations. Cisco notes that its technology can support DC power delivery for the network that is suitable “for installations where highly available power delivery is critical and where an investment in DC power infrastructure is considered to be a business benefit.”13

Example Application: PV Powered “Hybrid” Building

Ongoing and mounting issues related to energy security, re- liability, and emissions reduction make photovoltaic (PV) distributed generation an appealing resource for increased deployment and development. However, costs are still high, despite government rebates such as the “Million Solar Roofs” program in California and federal tax incentives.

For building applications, PV systems typically are supple- ments to grid power, providing some portion of the load. These systems feature cell arrays that produce DC power, which is converted to AC 60 Hz power for distribution to end-use ap- pliances and equipment.

Inverter efficiency. Rated inverter efficiencies are between 90% and 95%, representing 5–10% losses, but actual field ef- ficiencies can be even lower.

Reconversion losses. On top of the losses of inverting, addi- tional losses are incurred by converting back to DC in the elec- tronic devices like fluorescent ballasts, computers, and more.

Anti-islanding. For the protection of utility line workers, invert- ers are required to shut down in the event of grid failure. This means that, for most solar systems, electricity delivery stops during a power failure (when it is likely to be most needed).

But the inverter model for PV power system designs has several efficiency penalties and grid connection issues:

Net metering. Power sent back into the grid is not always re- purchased at full cost. Sending excess power back into a some-

June 2006

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