Direct Current Building-Scale Microgrid Platform

More and more energy efficient devices are internally utilizing Direct Current (DC) in both new and retrofit buildings. These devices include LED lights, industrial ceiling fans, and multi-speed HVAC. Similarly, local energy sources such as solar photovoltaic (PV), modern generators, fuel cells, and energy storage devices are using DC as well. Successful connection to the utility grid requires conversion of DC to Alternate Current (AC), which results in significant conversion losses.

ESTCP is currently funding a project titled “DC Microgrid Building Energy Management Platform for Improved Energy Efficiency, Energy Security, and Operating Costs” ( EW-201352). This project is working to demonstrate the substantial benefits offered by a DC Microgrid Building Energy Management Platform relative to conventional AC building systems. Two of these benefits are reduced total cost of ownership and increased energy security.

Conventional AC 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. These conventional systems result in excess life-cycle costs and energy security challenges, such as preventing the use of grid-tied renewable energy sources (solar PV) by standard inverters when grid power is lost, unless complex grid-forming inverters with expensive transfer switches and associated controls are utilized. In addition, these systems suffer from AC-to-DC conversion losses when powering many building devices, as well as DC-to-AC losses when using locally produced DC power.

 

Schematic of Conventional AC system versus DC microgrid

Schematic of Conventional AC system versus DC microgrid

The Bosch DC Microgrid architecture is a building-scale solution which maximizes the utilization of solar PV generated electricity in a facility by eliminating the need for the DC-to-AC and AC-to-DC conversion equipment required in typical AC-based systems. This solution makes the overall system more reliable, reduces the lifetime maintenance, and provides a built-in mechanism for seamlessly islanding critical DC loads during grid outages without the need for complex AC power transfer schemes and the associated grid synchronization. By transitioning the majority of a building’s AC loads to a DC microgrid, a commercial customer can expect up to a 30% lower total cost of ownership, higher reliability, improved resiliency, and optimized utilization of renewable generation when compared to a comparable AC solution.

The fitness center building at Fort Bragg, North Carolina was selected as the demonstration site, as it includes two high-bay areas. Phase 1 of this project focuses on energy comparison between the DC Microgrid and a conventional AC system. This phase included installation of 44, 260W high-bay DC lights, 4, 18 ft. diameter DC ceiling fans, and 45kW of rooftop solar PV. Preliminary results thus far indicates a reduction of 9.5% PV energy losses over a conventional AC system.  However, average savings requires monitoring of the performance over the course of a full year under varying weather conditions, which is planned for the remainder of the demonstration. Phase 2 of the project focuses on the use of an energy storage system to demonstrate resiliency and reliability. During this phase a 100KW/100kWh battery Energy Storage System and 100 kW of additional solar PV will be installed. This project is expected to be complete in early 2017.

On May 5, 2016, John Saussele, the Principal Investigator for this project, presented his findings as part of the SERDP and ESTCP Webinar Series. You can view the recording here.

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