This project demonstrated that microgrids with low-cost, large-scale Energy Storage Systems (ESS) have the potential to enhance energy security on military installations by facilitating integration of more renewable energy (RE) and reducing single-point-of-failure vulnerabilities associated with traditional electric service and back-up generators.
There were two main objectives of this project. The first objective was to demonstrate that an ESS enables the use of existing RE systems that normally are unavailable during a grid outage to “island” a building circuit without a diesel generator. The current large deployments of renewable photovoltaic (PV) systems that have been installed by the Department of Defense (DoD) give incentive to this objective. These systems have built-in safety features such as Underwriters Laboratory Inc.’s Standard for Safety for Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources (UL1741) that disable the systems in the event of a grid outage. This project intended to demonstrate that an ESS can provide voltage control capability in islanding operations that allows the functionality of existing PV systems in microgrid mode at high penetration levels.
The second objective was to demonstrate that an ESS can peak shave for demand charge avoidance. Many DoD facilities have been attempting to reduce their operational energy costs by implementing a variety of energy efficiency and RE programs. One of the biggest costs to many facilities is not the cost of energy purchases but the demand charge issued to the facility based on its load profile. This project was designed to allow the ESS to be programmed to charge/discharge according to a defined peak shaving schedule and assess how this changes the load profile seen at the meter of the circuit.
There were two types of technologies demonstrated in this project. The first was a novel design of a Zinc Bromide (Zn/Br) flow battery manufactured by Primus Power (Figure 1). The second was the Intelligent Power and Energy Management (IPEM) Microgrid Control System (MCS) developed by Raytheon.
Figure 1. Image of Installed ESS at MCAS Miramar Outside of Building 6311.
The traditional Zn/Br cell design uses carbon-coated felt paper as the electrode surface. The cells also have two separate electrolyte tanks for capturing the anolyte and catholyte separately during charge and discharge. Traditional Zn/Br cells need to be replaced after 1,500 cycles, which would constitute replacement every 4.1 years if cycled daily. Primus Power takes a different approach to their Zn/Br cells. Instead of using carbon-coated felt paper for their electrodes, Primus utilizes an activated solid titanium electrode for its Zn plating surfaces. Using a titanium electrode allows Primus to use a single flow loop of electrolyte as opposed to dual flow loops and to eliminate the need for an ion exchange membrane, an early failure mechanism in traditional Zn/Br cells. This reduces the number of tanks and pumps required for managing the electrolyte. The titanium electrodes also provide better energy density when compared to traditional Zn/Br: 3.1 kWh/ft2 compared to 1.7 kWh/ft2.
This project also demonstrated the use of Raytheon’s IPEM MCS, a model-based energy system planning and Command and Control (C2) technology, as a means to improve energy security and efficiency while reducing operational energy costs at Navy facilities. The IPEM MCS enables microgrid systems modelling in a Matlab Simulink environment during the design process, simulating normal operation as well as off-nominal conditions. Simulink auto-code generation is then used to generate target executables and link with external libraries. This allowed the control algorithms to be designed against component models and reduced system integration risk by making apparent the behavior of the system as its design matured over time.
This project conducted both grid-tied and islanding mode demonstrations to determine the capabilities of the microgrid against the performance objectives. A summary of the demonstration results is shown in Table 1 below.
Table 1. Summary of Demonstration Results
One of the key challenges of this demonstration was working with a technology that was still in its final development phases. Fielding technologies that have been breadboard validated in lab environments is always a challenge and requires iterations and lessons learned to optimize designs. When Primus was selected as the ESS supplier, the team had to manage a company that had promising technology despite their system being lower on the Technology Readiness Level (TRL) scale than the original proposed supplier (Primus was at TRL4). This required the team to simultaneously manage and scale up a promising technology that was in final development. The team was challenged with making hard decisions to continuously balance project performance, risk, and cost to meet the intent of the demonstration objectives within budget.
As this program spanned multiple years, the process of obtaining the Interconnect Agreement (IA) from San Diego Gas & Electric (SDG&E) took some patience and effort. The use of large-scale energy storage in microgrid capacities is new to the utility industry for behind-the-meter applications. Thus the IA process is changing in real time for utilities to adapt to how these systems will be deployed. This project was subject to some of the real-time changes as a few iterations of the application were required due to changing application requirements. Ultimately the IA and permit to operate were granted due to the hard work of multiple parties; however, it is still unclear if there is a well-defined process for getting IAs in place for microgrids.