Connecting up to DC: Direct current microgrids are making a comeback

Published On
Feb 15, 2021

Throughout the age of alternating current, few imagined that direct current would reemerge as a viable primary power distribution option at the grid, community or building level. DC and hybrid AC/DC microgrids, however, have gained interest in recent years due to the proliferation of renewable energy, demands for backup power during grid failures and potential efficiencies that can be gained by directly connecting DC power to DC end-use devices, such as LED lighting and controls.

If this nascent trend grows, it could present opportunities and challenges for ECs.

EMerge Alliance, Alexandria, Va., defines a building DC microgrid as an “electrical system that can efficiently distribute, consume, and potentially create and store direct current electricity to power a wide variety of electrical devices in and around buildings when connected to a utility grid or when disconnected as an island.”

These loads include LED lighting, controls, sensors, motors, fans, computers/IT and other electronics. Many of these loads are expected to grow significantly in coming years, particularly as more buildings adopt internet of things (IoT) strategies reliant on integration, networking and sensors.

“The key benefit to microgrids is resiliency, though there are additional potential benefits including energy savings, reduced costs and improved equipment reliability,” said Gabe Arnold, senior engineer for Pacific Northwest National Laboratory (PNNL), Richland, Wash. “DC building microgrids reduce or eliminate the conversions from DC to AC and AC to DC required to connect photovoltaic panels and energy storage batteries to the increasing number of DC loads in buildings. Eliminating these conversions can save 10–18% energy in a fully DC building microgrid.”

PNNL is interested in DC microgrids as a strategy to improve grid reliability and save energy while positioning a building for development of IoT strategies. In a September 2020 white paper, PNNL described the current state of DC microgrid adoption, including the factors inhibiting it from gaining traction.

One potential beneficiary of DC microgrids becoming more widely adopted is LED lighting. LED drivers include a rectifier that converts AC to DC. On a DC power supply, the rectifier component would no longer be required, reducing the materials, size and cost of LED products.

“There are already many LED lighting products available that accept DC at their input. If using power over ethernet, the networking is built in, and one of the key benefits of PoE is its integration and IoT potential,” Arnold said.

The major open standards covering building DC power distribution are the EMerge Alliance’s standards, produced by an industry consortium, and PoE, produced by the Institute of Electrical and Electronics Engineers (IEEE), New York.

The EMerge standards cover DC distributed over a 380V DC bus to power larger loads and 24V or 48V DC for smaller loads, such as lighting and control devices. PNNL identified eight lighting manufacturers offering luminaires that accept these voltages at their input as a standard option.

PoE (IEEE 802.3bt) enables up to 90W though PoE cabling to power loads such as lighting and controls. PNNL identified 17 lighting manufacturers offering numerous luminaires accepting PoE at their input. Though it’s less energy-efficient than the EMerge approach, significant advantages of PoE include high-speed data transfer over the same cable being used for power and simplified integration between building systems using TCP/IP communication, both of which are IoT building blocks.

Despite their benefits, DC microgrids have been slow to gain traction due to a lack of awareness, equipment availability and standardization. In this early adoption stage for the technology, applications with the most compelling value propositions will be first to adopt. These include buildings seeking greater resiliency against grid failures, a high level of sustainability or accommodation of advanced technologies such as LED lighting, controls and the IoT.

“As we move toward smarter and more sustainable buildings, these new technologies will be needed, and we’ll need trained contractors to install them,” Arnold said. “PoE is a great example. Electrical contractors would be well-served to become familiar with installing PoE technologies.”

Proficient contractors will also be positioned to offer integration and other services for the end-use systems.

A challenge for the electrical contracting community is that low-voltage installation may not require a licensed electrician. Following publication of the 2018 PoE standard, legislation was introduced in 22 states effectively requiring PoE to be installed by fully licensed electricians.

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