DC Microgrids Gain Popularity in Commercial Buildings

DC Microgrids Gain Popularity in Commercial Buildings

This piece was originally published in the March 2016 issue of ei, the magazine of the electroindustry.

By Robert Wills, PhD, President, Intergrid

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Will direct current (DC) microgrids for commercial buildings be our next billion-dollar industry? This question was discussed at NEMA’s Emerging Opportunities Forum at the NEMA Conference Center in January.

DC microgrids are the most efficient way to use DC electricity from on-site power generation, such as photovoltaic (PV) and fuel-cell power. Many loads now are DC in nature.

Microgrids have many definitions, but in general there is agreement that a microgrid is a group of interconnected loads and local generation that can act as a single controllable entity. While alternate current (AC) microgrids are becoming commonplace, as states and municipalities support them as a solution to grid resiliency, DC microgrids are like a return to the war of currents.

The War of Currents

In the late 1880s, Thomas Edison and Nikola Tesla battled for the adoption of DC and AC power distribution systems, with Mr. Edison arguing that AC was fundamentally more dangerous. The war was won in 1886 when Westinghouse began using AC transformers to convert electric power to high voltage for transmission. Nowadays, with high-voltage power electronics, many utility transmission links are direct rather than alternating current.

In today’s commercial buildings, with the exception of some emergency lighting and uninterruptible power source (UPS) systems, most electrical distribution is alternating current. In the 1980s, researchers at the University of Massachusetts Lowell experimented with PV assisted lighting systems, where DC solar electricity was fed directly to fluorescent ballasts. Backup for cloudy periods and nighttime was provided by a rectifier from grid power.

A U.S. Department of Energy report from that time found that these systems combine the efficiency of modem electronic ballasts with the use of clean solar energy while avoiding the cost, complexity, and unreliability of power conditioning systems usually required to match PV to the utility grid. While the concept held promise, it had not been implemented, until recently, on a significant scale.

 

dc microgrids

The reasons for slow adoption were a lack of cost-effective generation and utilization equipment, few applicable codes and standards, and the relatively low cost of grid electricity. All of these have now changed. PV modules are available for $500/kW, many loads are inherently DC-based, and electricity costs have risen significantly.

The widest adoption of DC microgrids to date has been in data centers (U.S. data centers account for about 20GW of electric demand in the U.S.). Data center operators can increase efficiency by up to 30 percent by feeding solar power to servers without intermediate conversion to AC and back to DC.

Other applications for DC microgrids such as lighting and HVAC are emerging, however. Bosch USA has installed a real-world test system at a commercial warehouse in Charlotte, North Carolina.

Two identical systems (in terms of photovoltaic arrays and loads) feed building light-emitting diode (LED) lighting and ventilation fans. One, however, uses direct DC coupling, while the other uses conventional conversion to AC power. The DC microgrid system, operating now for more than a year, has nine-percent lower energy costs, lower capital costs, and significantly higher projected reliability.

It may not be obvious at first, but LED light ballasts operating from 380 Vdc (volts of direct current) may have fewer parts and are more reliable than their AC counterparts. In particular, they do not need electrolytic capacitors, often the least reliable components in electronic assemblies.

HVAC and pumping are also emerging applications for DC power in buildings. As lighting efficiency improves, HVAC is becoming the largest contributor to many building electric bills. The efficiency gains from using variable frequency drives (VFDs) for fans and pumps are widely known.

Most VFDs are two-stage devices: an AC-to-DC rectifier followed by an output inverter. Most, with appropriate certification, can be fed directly from DC sources. Direct DC coupling in HVAC and water pumping systems can improve the efficiency of solar electric utilization by five percent, while eliminating the cost of the solar inverter.

Barriers are also falling in the codes and standards area. Proposals for DC microgrid requirements in the next edition of the National Electrical Code® (NEC) were accepted last year. Electrical designers and inspectors will be able to turn to a new Article 712 (DC Microgrids) in the 2017 edition of the NEC.

 

 

Building Resiliency

DC microgrids can also improve buildings’ resiliency to utility power loss. DC microgrids lend themselves to integration of battery storage on the bus, so emergency and backup power functions can easily be added. There are, however, code issues related to legally required emergency power systems; NEC Article 700 is based on the technology of generators and transfer switches. We might have to wait until the 2020 NEC before DC microgrids become an acceptable alternative.

Other barriers for implementation include DC wire marking requirements, per NEC 210.5(C)(2), and fault-current requirements for DC branch circuit breakers. The former is addressed in the 2017 NEC. Fuse and circuit breaker manufacturers are now developing solutions for overcurrent protection in 380 Vdc systems.

Will DC microgrids become a billion-dollar business? If they do, there will be opportunities for manufacturers of inverters, switchgear, wire and cable, DC appliances and luminaires, energy storage systems, IT power supplies, EV charging stations, VFDs and HVAC equipment, and more.

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