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DC Power Production, Delivery and Utilization

The reduction in lamp life is unknown. Additional research is needed to determine whether the energy savings over the life of the lamp would compensate for the increased cost due to premature lamp failure.

Electric baseboard and water heating

DC voltage can be used to run almost any device utilizing an electric heating element, including resistive baseboard and electric water heaters. In these applications, electrical current flowing in a heating element produces heat due to resistance.

The chief concern of using DC in such applications is not in the heating element itself, but in the contactors, switches, and circuit breakers used for such circuits. Since DC is more dif- ficult to interrupt, the interrupting devices must be capable of clearing any faults that develop. There are no DC equivalents to ground fault circuit interrupters (GFCIs), which are com- monplace electrical devices used in AC systems to prevent electric shock.

Uninterruptible Power Systems

Uninterruptible power systems (UPS) are excellent candidates for DC power support. A UPS is composed of an inverter, a high-speed static switch, various controls, and battery energy storage. The functional objective of the UPS is to provide high reliability and power quality for connected loads that may be susceptible to voltage sags or short duration power interrup- tions. Most UPS systems have anywhere from a few minutes up to about 30 minutes of battery storage. For larger UPS units (>30 kVA), it is typical to have a backup generator that starts and picks up load a few minutes after the utility power is inter- rupted, which, today, is lower cost than having several hours of battery energy storage onsite.

Since a UPS has an inverter and an internal DC bus, it already has many of the elements needed to operate with DC energy.

Variable speed motors

Motors are very important electrical devices, and represent a significant portion of power use in the U.S. In industry, for

June 2006

An EPRI White Paper

instance, approximately two-thirds of the electricity use is at- tributable to motors.

Most AC motor loads still use the same basic technology as the Tesla induction motor. These omnipresent motors convert AC power for applications such as air handling, air compression, refrigeration, air-conditioning, ventilation fans, pumping, machine tools, and more.

A workhorse of modern society, these motors can only oper- ate with AC power. In fact, if subjected to DC power, an AC motor could burn up quickly. In addition, without alternating current, the magnetic vectors produced in the induction mo- tor powered with DC would not be conducive to rotation and the motor would stall—so an induction motor simply will not operate directly on DC power.

But DC can be used if a variable frequency drive is part of the system. A variable frequency drive allows for adjusting the motor speed, rather than operating it either on or off. By vary- ing the frequency of power over a wide range, motor speed can be adjusted to best match the mechanical process, such as circulating air with a fan. This ability to adjust speed can translate into significant energy savings, as a CEO for a major manufacturer explains:

And 85% of those motors are electro-mechanically actuated—either they’re on or they’re off.They’re dumb, wasteful motors. If we could convert all of those motors to variable speed, we could cut their [power] consumption by half, which amounts to 10% of total global energy consumption.

Imagine if you drove your car with your foot all the way on the gas, then your foot all the way on the brake.Youd get re- ally crappy mileage, and your car wouldn’t last as long. Many electric motors work this way: either off or full-speed on. So the motors in most refrigerators, for example, control electric- ity with a switch— you can hear it go on and off.When it’s on, a little motor turns furiously to drive a compressor to cool the refrigerator, then shuts off completely when the tempera- ture falls.That’s not efficient.A motor drive that turns exactly

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