Distributed generation (DG) locates small-scale power generating units close to the load served. These small-scale generating units include traditional rotating machinery such as diesel generators and gas turbines as well as emerging technologies such as microturbines that produce 60-Hertz (Hz) alternating current (AC). Other emerging small-scale generating technologies include photovoltaics (PV) and fuel cells. Both PV and fuel cells produce direct current (DC) that is usually converted to AC to serve today’s building loads.
These DC power sources are very environmentally friendly and are becoming increasingly viable as the technologies advance, production costs decrease, and utility rates increase. As these technologies become more mainstream and an integral part of a building’s power supply strategy there may be a point where DC becomes the preferred distribution method. If DC power sources provide sufficient power to serve all or part of a building’s load there may be advantages to using DC to serve the entire building load or certain load classes such as lighting.
As noted above, it is common today to use an inverter to convert the DC from PV or fuel cells to 60 Hz AC to serve building loads through the existing distribution system or interconnect with the utility grid to sell excess power. At the load, AC is converted back to DC through switch-mode power supplies in electronic equipment or high-frequency pulsed DC in the case of ballasted lighting fixtures. Inverters and power supplies incorporated into many building loads could be eliminated or simplified using DC distribution. Today, there are DC incandescent, compact fluorescent and LED lamps available as well as DC ballasts that can be incorporated into standard fluorescent luminaires. DC lamps and ballasts are more expensive than standard AC lamps and ballasts because of limited demand. If demand increases in the future due to the use of DC distribution, the cost of DC lamps and ballasts would probably decrease.
Eliminating inverters and simplifying building load power supplies would increase the overall efficiency of the system because conversion losses would be eliminated using DC distribution. In addition, if batteries were used to store excess power from a PV installation for nighttime use, the stored DC power would not have to be converted back to AC to serve building loads. Also, if inverters could be eliminated and the cost of DC lighting and other equipment were comparable to AC equipment, the original installed cost of a PV or fuel cell system would be reduced. However, the cost reduction from the elimination of inverters might be offset by increased distribution system costs due to the need for higher ampacity equipment resulting from the use of lower DC voltages serving the same load. Additionally, if the building’s DC distribution system were to be supplemented or backed up by utility-supplied AC, there would need to be an inverter at the service to convert AC to DC which would also offset any savings resulting from the use of a DC distribution system and loads.
In commercial buildings of any size, mechanical equipment loads are significant and in the foreseeable future, it won’t be feasible or economical for these loads to be powered by alternative DC power sources. These loads include chillers, fans, pumps, compressors and other motor-driven equipment. Induction motors need an AC power supply to operate or an inverter to convert from DC to AC. DC motors are more expensive and require more maintenance than standard squirrel-cage induction motors. Therefore, commercial buildings of any size will probably require an AC distribution system for mechanical equipment. It would only be feasible for residential and small commercial buildings that have very small mechanical equipment loads to power this equipment from a DC sources.
DC distribution would also improve power quality by eliminating harmonics caused by nonlinear loads. Nonlinear loads chop up the AC current waveform by conducting only during a portion of the voltage cycle. Common equipment that causes harmonics include electric discharge lighting and electronic equipment like personal computers that have switch-mode power supplies. Harmonics can cause overheating of electrical conductors and equipment that can result in premature insulation failure and nuisance tripping of overcurrent devices. DC distribution would eliminate harmonics because the need to convert AC to DC or pulsed DC at the load would be eliminated.
There exists the possibility of a shift from AC distribution systems today to the use of DC distribution in tomorrow’s buildings. If this shift occurs, it will probably be driven by the integration of emerging DC generating technologies into buildings that will power all or a significant portion of the building’s load. Also, contributing to this shift is the trend toward smaller and increasingly efficient electronic equipment as well as the replacement of mechanical devices with solid-state electronics that will result in reduced building loads and energy demand. DC building distribution may be a future trend for which to prepare.
GLAVINICH is an associate professor in the Department of Civil, Environmental and Architectural Engineering at The University of Kansas and is a frequent instructor for NECA’s Management Education Institute. He can be reached at 785.864.3435 or firstname.lastname@example.org.