Zinc-Bromine Flow Battery Technology for Energy Security
Energy security and reduced fuel consumption are key strategic objectives of the Department of Defense. Energy security for the DoD means having assured access to reliable supplies of energy to meet operational and mission needs. A properly integrated energy storage system (ESS) can improve the energy security of an installation by allowing “islanding” of a facility. This means that critical operations can be maintained during natural or manmade power interruptions, cyber attack or other unintended power outage situations associated with the commercial power grid. Zinc-bromine (Zn-Br) flow battery technology is an attractive energy storage technology for use in energy security applications because of its low cost, desirable energy storage capacity, transportability, cycle life, system lifetime and safety features.
Raytheon was recently selected for a government contract as the system integrator to manage renewable energy and energy security for a military installation. This involves integrating a Zn-Br battery and Raytheon Intelligent Energy Command and Control technology to provide emergency backup power and a secure islanding capability.
While the chemistry behind the Zn-Br cells has been employed as an energy storage technology for many years, the ability to scale up a Zn-Br system to utility-scale power levels is where much of the latest development has occurred. This innovation enables Zn-Br to be an appealing large-scale energy storage technology for applications requiring modest power output (hundreds of kilowatts to several megawatts) and large energy storage capacity (>2 MWh) at an affordable cost when compared to competing technologies. Moreover, Zn-Br based energy storage systems can be designed to work in parallel to accommodate higher power and energy loads.
In one instantiation of a Zn-Br ESS, electrolyte is pumped from two electrolyte reservoirs through a battery stack in two circuits, one for anode half-cells and the other for cathode half-cells. The electrolyte in the anode loop is commonly called anolyte; the electrolyte in the cathode loop is called the catholyte. Anolyte and catholyte are separated by a micro-porous cell membrane that allows ions to readily pass through, but prevents bulk mixing of anolyte and catholyte. The actual electrodes in the system do not participate in the chemical reaction, but in fact just act as a substrate for the reaction (Figure 1).
While at a zero-potential state, the electrolyte is a homogeneous aqueous solution of zinc bromide (ZnBr2) and various salts. As the system is charged, however, zinc ions (Zn2+) in solution are reduced (absorbs two electrons [2e–]) to zinc metal (Zn) on the anode, essentially plating metallic zinc across the electrode surface (Figure 2 top).
Concurrently, bromide ions (Br–) travel across the micro-porous membrane toward the cathode and are oxidized (release two electrons [2e–]) into molecular bromine at the electrode within the aqueous solution. The anolyte and catholyte gradually develop different compositions, which correspondingly changes the color of the electrolyte tanks. Elemental bromine (Br2) produced in the cathode half-cells combines with an oil and forms a polybromide complex with quaternary salts in the catholyte. The polybromide complex separates from the catholyte aqueous phase as a high-density oily liquid. This is collected in the bottom of the catholyte reservoir. Thus, the energy stored in the system is the chemical potential energy of the zinc metal plated across the anode and the polybromide complex that settles within the catholyte reservoir. As more zinc is plated across the anode and more polybromide complex is created, the total amount of energy stored in the system increases. The system is always ready for instantaneous power delivery by maintaining fresh electrolyte in the half cells at all times. Full power can be provided even in a standby mode when the pumps are off. Enough electrolyte is introduced intermittently to keep quality reactant present, enabling full power discharge when pumps are started. Once the pumps are started, they flow electrolyte for continuous operation.
During discharge these processes are reversed (Figure 2 bottom). When a load is applied to the cell, the zinc metal plated across the anode oxidizes, reforming the zinc ion (Zn2+); and bromine is reduced to bromide ion (Br–) at the cathode. Upon full discharge, both the anolyte and catholyte tanks are returned to a homogeneous aqueous solution of zinc bromide. The pumping system circulates the anolyte and catholyte, allowing the polybromide complex settled at the bottom of the catholyte reservoir to flow across the cathode to sustain appropriate concentrations of polybromide complex to maintain the reaction. This is what gives Zn-Br flow batteries their long-discharge capabilities.
The essential reactions in the zinc-bromine energy storage system are:
Anode: Zn(s)↔ Zn2+(aq) + 2e– -0.76V vs SHE
Cathode: Br 2(aq) + 2e– ↔ 2Br–(aq) +1.087V vs SHE
Overall: Zn(s) + Br2(aq)↔ Zn2+(aq)+ 2Br–(aq) +1.067V vs SHE (standard hydrogen electrode)
Arranging multiple Zn-Br cell stacks in series, forming them into battery strings, and arranging them in parallel determine the power rating of the ESS. One of the most desirable characteristics of flow batteries is that the amount of energy (watt hours) that the system can store is scalable by the volume of electrolyte available and the space available within the system to plate zinc (Figure 3).
Larger tank systems provide more energy storage, though the larger systems can be more complex to balance and maintain. Zn-Br companies have developed innovative methods for scaling up by using advanced safety measures, and developing sophisticated charging algorithms and control systems to give their batteries a long safe life and manage their power levels. Current Zn-Br systems are capable of megawatt hours of energy storage at moderate power levels (up to 500 kW) at a price point of $440–$485/kWh1. The low cost, desirable energy storage capacity, transportability, system lifetime and safety are the key features that caused Raytheon to select Zn-Br technology as the energy storage component in a secure, renewable, intelligently managed energy solution for our customer.
1Electricity Energy Storage Technology Options - A White Paper Primer on Applications, Costs and Benefits. (2010) Palo Alto: Electric Power Research Institute.
Contributors: Philip Carrigan, Alf Carroll, Gami Maislin, Peter Morico