Requirements for electrical devices in buildings to operate on alternating current (AC) is largely historic, and results in wasted energy through many unnecessary conversions back and forth to direct current (DC) within a building, as well as inherent resiliency and reliability limitations due to the additional conversion/switching devices needed to interoperate with the AC supplied from the utility grid. Conventional alternating current building-level electrical power distribution systems require a reliable utility grid connection and do not use locally generated renewable energy in the most cost-effective manner, resulting in excess life-cycle costs and energy security concerns. These systems suffer from AC-to-DC conversion losses when powering many common devices, as well as DC-to-AC losses when using locally produced DC power. They also require an unreliable and expensive grid-tie inverter, which prevents the use of solar photovoltaic (PV) power when grid power is lost. This project demonstrates the performance and value of the DC Microgrid by overcoming these limitations through real-world demonstration at Fort Bragg, North Carolina. The DC Microgrid applies a novel approach that utilizes mature and reliable DC technology to dynamically manage power sources, loads, and energy storage system interaction to minimize total cost-of-ownership and grid reliance.
Two DC Microgrid architectures were demonstrated: (1) a core system that most effectively uses solar PV energy for common high energy use building loads, and (2) an enhanced system that integrates energy storage to dramatically increase the facility’s energy security and load-leveling capabilities.
The core system includes a solar PV array, DC power supply, DC high-bay induction lighting system, and large-diameter DC ceiling fans (to reduce heating and cooling system use). This core system is relatively small and is sized to match the DC loads, such that all solar PV power production is immediately used, eliminating the need for a grid-tie inverter and utility interconnection study. A Bosch Energy Management Gateway remotely manages each component to minimize utility grid energy usage. Historical energy usage data and lighting output measurements will be collected to quantify the DC Microgrid’s advantages relative to the base case and a small reference AC lighting system representing current state-of-the-art AC lighting.
The enhanced system includes a GreenStation battery energy storage system from Green Charge Networks and additional solar PV elements to improve the facility’s energy security and mission assurance capabilities during power outages. The GreenStation also performs building electrical load leveling/peak load reduction under normal operation to reduce utility usage demand and associated charges. The performance assessment will include the same data collection and analysis protocols used for the core system. The testing will also assess the DC Microgrid’s load-leveling, islanding abilities (by simulating blackout situations), and energy-security benefits resulting from the energy storage and additional solar PV capacity.
The energy efficiency advantages of the DC microgrid are due to the additional conversion losses which occur in a conventional AC system. The DC lighting fixtures which were originally installed at the beginning of the project in 2015 were 250 Watts induction lights with DC lighting driver electronics (called ballasts for induction lights) modified to operate on DC. The DC induction lights were available for the demonstration project much earlier, but the DC version was never commercialized, since development efforts by Bosch were focused on the faster-growing light emitting diode lighting market. Nevertheless, some limited lab comparison testing was done on the AC and DC versions of the induction lights, showing a very similar advantage (96.9% efficiency for DC induction light ballast vs. 92.7% efficiency for AC induction light ballast, or 4.2% additional loss for the AC conversion). It was expected that the AC inverter would experience higher losses in cloudier environments, since the inverter would spend more time at lower efficiency levels. The National Renewable Energy Lab also found significant energy efficiency advantage of the DC microgrid which was not initially anticipated. Electrical energy losses which occur in the lighting electronics turn into heat energy.
Since the light fixtures are located inside the building, these losses are effectively heating up the building. The AC lighting electronics exhibit higher losses than the DC lighting electronics, and therefore contribute more heat to the building. The DC system had advantage over the AC system in all climates. However, the additional savings in air conditioning energy provided by the lower lighting losses in the DC system results in even higher savings in climates with high air conditioning usage.
The demonstration project at Fort Bragg is relatively small scale (only 44 DC lights), was the first of its kind, and was designed primarily to develop and demonstrate the feasibility and functions of a DC microgrid. Therefore, it is not the most representative project to use for a commercial cost model. However, in considering commercialization of the technology, Bosch studied all the costs of a marketable commercial-scale system in detail and found the lifetime savings advantage of DC technology to be up to 30% less than a comparable AC system, while also having additional resiliency advantages.
No major barriers to implementation of DC microgrids in buildings were found during the demonstration project, and the building devices (lights and fans) operated identically to conventional technology, requiring no special training for the facility managers or building occupants.
A majority of the devices used in the demonstration were commercial off-the-shelf components. Standard building wiring is rated for both AC and DC, and other components needed in the building electrical distribution systems (panels, circuit breakers, fuses, etc.) are readily available in DC versions. A potential path to more widespread adoption of DC microgrids in Department of Defense applications is through financing mechanisms offered by Energy Service Companies or other similar organizations.