Hybrid Use of AC and DC

May 30, 2012
Incorporating DC power can improve your energy efficiency.

Varying views exist on using DC (direct current) power vs. AC (alternating current) power, but the two aren’t necessarily in opposition. Depending on the applications, utilizing both AC and DC power supplies can improve efficiencies.

“We ought to stop fighting,” says Brian Patterson, chairman of EMerge Alliance, an open industry association developing strategies to encourage DC power distribution. “The power community is well-prepared to service the entire industry with hybridized energy and, where it’s appropriate, use DC. Where it’s not, use AC.”

Aggregating DC at One Point
Many devices – from computers to electronic lighting ballasts – are “DC-native” and require the incoming AC power supply to be converted to DC, and that takes a toll on efficiency, Patterson says.

Companies like Armstrong World Industries and Nextek Power Systems have developed strategies to address this inefficiency.

“Instead of converting at every device, you can aggregate the rectification of AC to DC with a small DC power system called a microgrid,” Patterson says, adding that this addresses energy waste and improves efficiency in single-digit percentages.

However, the gain isn’t enough to initiate a retrofit, he says.

“Be opportunistic. If you’re already going to refurbish ceilings and lighting, consider DC. Don’t just put the old stuff in again,” he advises. “The initial shift due to inefficiencies is the tip of the iceberg. We’re taking it to the next level.”

Generating DC On-Site
The second driver of incorporated DC use involves renewable clean energy sources like solar, wind, and fuel cells, which are natively DC. The potential of a DC system is maximized when DC energy is produced at the facility.

“If you want to power your building with renewable energy like photovoltaic (PV) cells, you can blend that in on the DC side of the system,” Patterson says, stressing that current AC-only systems must convert a renewable DC energy source to AC for distribution, only to have it converted back to DC for certain devices and applications.

“The microgrid takes 100% of the power generated from the renewable source and couples it directly into the application, so you maximize the investment,” he explains, adding that conversion to AC wastes about 7-10% of renewable energy production.

Armstrong World Industries developed the DC FlexZone Grid to distribute power in the ceiling plane. The University of California, San Diego’s Sustainability Resource Center recently installed four rooftop PV panels, DC-driven lighting and controls, and the FlexZone. The solar power source is connected directly to the ceiling grid, eliminating multiple conversions required in a traditional AC system.

The facility earned LEED Gold certification and a special credit for its high-efficiency DC microgrid.

“We’re at the tipping point where buildings are bringing more DC to the building in renewables and so many devices are already DC,” says Susan Rhoades, marketing manager at Armstrong. “It’s time to rethink energy infrastructure inside the building.”

Vision for the Future
The hybrid use of a DC power system is new and relatively complicated, but it’s a resource that needs to be tapped, Patterson says.

“People ask why DC hasn’t taken over yet. The answer is because it’s hard to make these kinds of transitions. It’s a bit of chicken before the egg – where do you start?” Patterson says. “The idea is to be progressively or modularly transformative.”

When you’re retrofitting air handlers, consider a DC-input variable frequency drive. When you’re installing solid state lighting, consider how DC can maximize its potential, he recommends.

“Take a hybrid approach next time you renovate,” Rhoades agrees. “It’s not any one thing. It’s about looking at managing the building as an asset in a different way.”

Chris Curtland ([email protected]) is assistant editor of BUILDINGS.

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