The idea of creating direct current (DC) microgrids to yield efficiencies is gaining ground in the U.S. and Europe as more and more DC devices are added to homes and businesses.
In fact, Current/OS Foundation, which is funded by Schneider Electric Foundation, Siemens and 82 other industry players, aims to expand the use of DC microgrids, arguing that DC microgrids at the grid edge could help integrate renewables, reduce power demand, lower costs and create efficiencies.
Current/OS Foundation argues that DC microgrids could regulate power for the grid, avoiding some of the complexities and inefficiencies of converting DC power to alternating current (AC) power. This would also allow the microgrid and the DC devices to go off grid, if necessary.
Some DC microgrid projects have or are moving forward.
But from a single building perspective, Karina Hershberg, associate principal at PAE Consulting Engineers, says she tried to design DC microgrids for two separate commercial clients interested in sustainable buildings and discovered they didn’t pencil out. Based on this early experience, she believes that DC microgrids don’t make sense unless they’re deployed in small homes.
The challenges to designing DC microgrids
Herschberg said she encountered many challenges trying to design the microgrids, including the need to lower voltage, purchase large quantities of cable and source components.
“I think that the concept of DC to DC buildings with a microgrid makes for a nice kind of academic white paper. But when you move that from theory to application and implementation, the numbers start to fall apart,” Hershberg said.
She’s not alone in questioning the wisdom of creating all-DC microgrids.
Do hybrid AC/DC microgrids make more sense?
Bob Kroon, founder and CEO of August Berres, which markets battery-powered workstations for DC-powered buildings, agrees that attempting to meet the DC microgrid dream can spark many challenges; hybrid AC/DC microgrids make more sense, he said.
“The market for mature and cost-effective AC components and appliances is vast compared to that of DC,” Kroon added. “Opting for a purely DC microgrid can limit equipment choices and potentially increase upfront costs and long-term maintenance complexities.”
A hybrid approach provides flexibility, allowing building owners to capitalize on readily available and often more affordable AC technologies while incorporating DC for specific applications like solar PV and battery storage, he added.
Hershberg said she worked on two DC microgrid projects, the first about 10 years ago and the second 7 years ago.
In the first project, there weren’t many DC components available. “We could kind of cobble together something that people could maybe plug their laptops into,” she said.
For the second project–an office building–instead of just providing a 120-volt receptacle, she looked at providing a DC source such as a USB-C outlet that employees could plug their laptops into, for example. That was fine for plug loads, but finding options for LED lighting was more challenging.
Adding–not eliminating–conversion steps
“The whole premise behind the idea of DC to DC is that you've eliminated those conversion steps. Right now, you go from DC to AC. Every time you do any sort of conversion, you have some efficiency losses in the form of heat. The idea was if you can eliminate those, you've improved the energy efficiency of your building,” she explained.
But with DC to DC, she found that voltage transformations were needed. PV and battery run at higher voltages–either 600 or 1,000 volts. But many devices run at 25 or 48 volts.
“Because there isn't really standardization in the DC world yet, we were having to go from 600 volts to 12 volts or 48 volts for some devices,” she said.
Lighting a challenge with DC
A second challenge: To get more lights on a DC circuit, many more data cables were needed. On the other hand, it’s possible to get most of one floor on one AC circuit.
“It created this fascinating question of whether the supposed efficiency gains are then just spent on manufacturing a whole bunch more cable,” Hershberg said.
Her conclusion: It made more sense to focus on energy efficiency and include PV and a battery to create a grid-interactive project, which would be more impactful than a DC to DC connection.
Others recognize the challenges of creating DC microgrids, but say that if designed properly, they can be more efficient.
DC microgrids yieled the most benefits under specific circumstances, said Phil Teague, founder of Rectify Solar, which is a DC and hybrid AC/DC microgrid developer.
“DC microgrids absolutely make sense in specific, high-impact scenarios today,” he said. That includes buildings with large on-site loads and generation potential, or places where infrastructure upgrades are too expensive. In a home, for example, if a DC-coupled solar, battery and EV charger can help avoid a full panel or service upgrade, the economics are in favor of a DC backbone. After that, it could be expanded to appliances and other loads, he said.
“The key to adoption and scaling is to drive down soft costs. Many engineering and architecture firms can implement microgrids, but few know how to design DC systems efficiently,” Teague said.
Microgrids becoming more important to many entities
While there’s increasing interest in DC microgrids, there’s also more interest in microgrids in general, Hershberg said.
“We're consulting engineers working mostly at the building side of the meter, working with single buildings up to campuses and districts. And I'm seeing interest in microgrids across all sectors, and for different reasons.” The interests include achieving resilience and sustainability and lowering energy costs.
“I think a lot of people are recognizing that a microgrid is a big part of the new energy solution as we move into the future,” she said.
