Controls, Interviews + Opinion

PNNL’s Gabe Arnold Talks DC Building Microgrids

I recently had the opportunity to interview Gabe Arnold, PE, LC, Senior Engineer, Pacific Northwest National Laboratory for an article about DC building microgrids for an upcoming issue of ELECTRICAL CONTRACTOR, the official publication of NECA. Transcript follows. Check out PNNL’s new whitepaper on DC microgrids here.

DiLouie: How would you define DC microgrids?

Arnold: Microgrids are subsections of the larger electric grid with their own power sources such as solar photovoltaics (PV), wind, generators, and/or energy storage batteries. Microgrids can disconnect from the larger grid and continue to operate in what is called “island” mode. Microgrids improve resiliency because they enable the continued supply of electricity when there is an outage on the larger grid. Microgrids can be either direct current (DC), alternating current (AC), or a hybrid of the two. A direct current (DC) microgrid is one in which the power sources and loads within the microgrid are connected using DC rather than AC electricity.

DiLouie: What are the different types of DC microgrids and what types of equipment and connections are installed?

Arnold: Microgrids can be utility scale, community scale, or building scale. A utility scale microgrid may include large fields of solar panels, large battery facilities, or even small power plants that can disconnect from the larger grid and power a portion of the electric grid in the event of a grid outage. Utility scale microgrids are often high voltage distributing power in thousands of volts through transmission and distribution power lines. A community scale microgrid is smaller size and may include solar panels, generators, and/or battery facilities to power a neighborhood or campus in the event of a grid outage, usually at lower voltages than utility scale. A building scale microgrid is smaller further at the individual building or site level. It typically includes on-site solar panels, batteries, and/or generators that can provide power to the building in the event of a grid outage.

The equipment and connections can vary considerably depending on the scale and application of the microgrid. For a building scale microgrid, some type of inverter and controller is usually required to connect the PV and energy storage batteries to the building electrical system and the larger electric grid. These controllers can be off-the-shelf sometimes called “Power Servers”, or they can be assembled from individual components and controlled via software.

DiLouie: What are the prevalent standards? What approaches in terms of standards and equipment appear to show the strongest promise?

Arnold: An organization called the Emerge Alliance has defined standards for DC electricity distributed within buildings using a 380 V DC bus to power larger building loads and 24 or 48 V DC for smaller loads such as lighting and electronics. From a lighting perspective, PNNL’s market research identified 8 lighting manufacturers that offer lighting fixtures that accept these voltages at their input as a standard option, though many more manufacturers could offer fixtures at these voltages through a custom order.

The other relevant standard from a lighting perspective is IEEE 802.3bt for Power-over-Ethernet (PoE). Released in 2018, this standard enables up to 90 watts of power through PoE cabling to power building loads such as lighting. PNNL identified 17 lighting manufacturers offering a wide range of light fixtures that accept PoE at their input. PoE lighting has significant benefits including an existing supply chain and workforce that is already trained on how to install the PoE network cabling, high-speed data capability to support IoT applications, and integration benefits with other systems through the use of TCP/IP communication.

DiLouie: What are the benefits of DC building microgrids? What are ideal applications? Overall, what opportunity do they represent for buildings?

Arnold: The key benefit to microgrids is resiliency, and if it’s a DC building microgrid, there are additional potential benefits including energy savings, reduced costs, and improved equipment reliability. DC building microgrids reduce or eliminate the conversions from DC-to-AC and AC-to-DC required to connect PV 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, reduce the cost of the equipment by eliminating the conversion circuitry, and eliminate a common failure point within the equipment. For example, a recent DOE study that implemented accelerated stress testing on LED drivers found that 64% of the drivers failed within the 6,000 hour accelerated testing period, with all failures attributed to the Stage 1 driver circuitry that performs the AC-to-DC conversion. If the conversion circuitry within drivers is eliminated, drivers may cost less and last longer.

The ideal applications for DC building microgrids are buildings where resiliency and/or sustainability are important and where PV and/or energy storage batteries are being installed. The resiliency benefits can help avoid costly downtime for a business or critical facility, and the 10-18% energy savings can help achieve a Zero Energy Building and qualify for special LEED points available to DC power systems.

DiLouie: How does LED lighting, networked lighting controls, and sensors/integration between lighting and other building systems/IoT fit in with DC building microgrids?

Arnold: LED lighting is an inherently DC technology that is well suited to DC microgrids. There are already many LED lighting products available that accept DC at their input. If using PoE, the networking is built-in and one of the key benefits of PoE is its integration and IoT potential. You don’t have to use POE though, and a wireless networked lighting control system can work just as well with DC LED lighting.

DiLouie: There are currently two approaches, one being PoE to move power and data and the other low-voltage distribution (e.g., EMerge Alliance) to move power. Do you see these approaches continuing to coexist, or do you believe they will congeal around a single standard to simplify the market?

Arnold: There are benefits both for the EMerge Alliance and the PoE approaches, and I expect we’ll continue to see them both for the foreseeable future. Though PoE has a lot of momentum and some unique benefits, it may not be the best solution from an energy efficiency perspective. Studies have found that there can be a lot of power losses within the CAT 5/6/7 network cabling and standby power losses within the PoE networking switches. These can be mitigated through larger gauge wire within PoE cabling, shorter cable runs, and improved switch design.

DiLouie: What are current barriers to adoption of DC building microgrids, and how they should they be addressed?

Arnold: This is still an early stage with these technologies, and it will take time to raise awareness, improve the equipment availability, and then educate the workforce in how to apply and install it. Among the biggest barriers we heard is the lack of available equipment that accepts DC at the input – even though the loads are inherently DC. Equipment manufacturers continue to include AC-to-DC converters at the equipment input. This is a chicken-and-egg scenario whereby equipment manufacturers are unwilling to divert resources to develop DC input equipment without a significant market to support it, and that market will not develop without more equipment choices and competition. The way forward is to start with the technologies and applications with the most compelling value propositions, or where there is already available equipment. LED lighting is a great example. Electric vehicle chargers are another. These can help build the case and gain experience towards fully DC building microgrids.

DiLouie: What’s in it for the electrical contractor? What kind of opportunities are developing for them?

Arnold: As we move towards smarter and more sustainable buildings, these new technologies will be needed, and we’ll need trained contractors to install them. PoE is a great example, I expect we’ll see a lot more of it. Now that PoE can power building loads up to 90 watts, there are a lot more loads within the building that may move in this direction over conventional AC wiring. Electrical contractors would be well-served to become familiar with installing PoE technologies.

DiLouie: What is the legislative climate for requiring a licensed electrician for installing PoE systems, and what does this mean for the electrical contracting community?

Arnold: Since the finalization of the 2018 PoE standard that enabled 90 watts to be carried through PoE cabling, a turf war of sorts has emerged. PoE is considered Class 2 low-voltage by the National Electric Code. It is safer, does not require conduit, and may have lower licensing requirements for its installation in some jurisdictions. Industry advocacy groups have gotten involved and in 2019 legislation was introduced in 22 states that would effectively require PoE to be installed by fully licensed electricians. Most of this legislation stalled or changed and it remains to be seen if or how licensing requirements may change for PoE installations. Regardless of the outcome, this indicates the potential growth expected with PoE systems in powering other loads within buildings, including lighting.

DiLouie: What extension opportunities are available for electrical contractors, such as integration and IoT?

Arnold: We’ll continue to see more movement to smart buildings and you can’t have a smart building without integrating your building systems together. We expect integration of lighting with HVAC and building management systems, security systems, industrial systems, and with new applications enabled by IoT. In fact, the Department of Energy has just launched a national campaign around this called the Integrated Lighting Campaign. I encourage readers to check it out at The need and demand for system integrators to integrate these systems will continue to grow and this may be a good opportunity for electrical contractors to pursue.

DiLouie: If you could tell the entire electrical industry only one thing about integrating lighting with DC microgrids, what would it be?

Arnold: Though it is relatively early stages of these technologies, I would encourage industry to collaborate in the development of standards where possible. The proliferation of proprietary solutions can work against the integration opportunities and slow the overall market adoption of these beneficial technologies.

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Craig DiLouie


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