DC Power Production, Delivery and Utilization
that energy. That amount represents 32 billion kWh/year, or a savings of $2.5 billion. If powered by DC, conversion losses could be reduced and significant savings achieved.
EPRI Solutions conducted tests to assess the ability of two standard SMPS-type computer power supplies to operate on DC; a 250 watt ATX type typically used for desktop computers, and a portable plug-in power module for laptop computers. In both cases, tests revealed that the power supplies would operate properly when supplied with DC power of the right magnitude, although no tests were done to determine power supply operation and performance when connected to the computer loads.
For the desktop computer power supply, sufficient output was provided when supplied with 150 V of DC or greater. For the laptop SMPS unit, 30 V DC was required to “turn on” the out- put, which begins at 19.79 V and continues at that output un- less DC supply drops to 20 volts DC or below.
Fluorescent lighting with electronic ballasts
The key to DC operation of fluorescent lights lies in the use of electronic ballasts. The ballast is used to initiate discharge and regulate current flow in the lamp. Modern electronic bal- lasts function in much the same manner as a switched-mode power supply thus making it potentially possible to operate them from a DC supply.
Virtually all new office lighting systems use electronic ballasts, which are more efficient and capable of powering various lights at lower costs. Only older installations are likely to have the less-efficient magnetic ballasts in place.
For electronically ballasted applications, several manufactur- ers make ballasts rated for DC. Lighting systems could be ret- rofitted with DC-rated ballast units for DC operation. All light switches and upstream protection in line with DC current flow would also need to be rated for DC.
An EPRI White Paper
Compact fluorescent lamps
Compact fluorescent lamps (CFL) are energy-efficient alterna- tives to the common incandescent bulb. A new 20-watt com- pact fluorescent lamp gives the same light output as a standard 75-watt incandescent light bulb, and also offers an average op- erating life 6 to 10 times longer.
A compact fluorescent lamp has two parts: a small, folded gas- filled tube and a built-in electronic ballast. As with the fluores- cent tubes used in commercial lighting, the electronic ballast enables DC operation of CFLs. EPRI Solutions’ testing of a 20- watt CFL unit with DC power supply revealed that while the CFL could operate on DC power (see Figure 6), it required a much higher DC input voltage. With AC supply, the CFL provided con- stant light at 63 V, but with DC supply, 164.4 V DC was required.
After speaking with CFL manufacturers, EPRI Solutions re- searchers determined that the CFL used a voltage doubling circuit on the input to the electronic ballast. However, the volt- age doubling circuit does not operate on DC voltage. Hence, the DC voltage must be twice the magnitude of the AC voltage to compensate for the non-functioning doubling circuit. This resulting overvoltage on the capacitors could result in short- ened lamp life, depending on the ratings of certain input ele- ments in the circuit.
Figure 6. Compact fluorescent lamp running on DC power
Tests showed that some modifications may be required in CFL units for operation on DC power.