The application of Battery Energy Storage System (BESS) in the Power System environment has shown remarkable results in the fields of ancillary services providing Spinning and Frequency Reserve services while improving Power Quality and managing peak shaving operations. One case of success is the Hornsdale Power Reserve, the 100MW battery system installed in Mid North region of South Australia by Tesla, which in only six months after beginning of operations, was responsible for 55% of frequency control and ancillary services in South Australia and reduced the service costs by 90% [1].
With the ever-growing renewable energy sources and their consequent intermittency, the demand for high rated BESS will grow proportionally. The most commonly used technology for these systems is the Lithium-ion battery that depends heavily on minerals like Lithium, Cobalt, Nickel and other specific metals. These materials are not abundant resources, restricted to geographical availability and might be dominated by small groups of suppliers. Recent demand due to soaring production of electrical vehicles could cause the cost of a BESS to be unsustainable. Due to this possible limitation on the development of new BESS, the R&D community has been putting a lot of focus into developing alternatives to the Lithium-ion battery.
Such technologies being developed are on the field of Redox Flow Batteries. There is a large variety of active materials from which flow batteries can be built (redox couples). From a cost point of view, the ideal flow battery must show a high power density, a high energy density and must be designed around low cost active materials. The combination of these 3 factors is to a large extent decisive for the storage cost per kWh and therefore fundamental when creating a sustainable product affordable to be installed at large scale, although in the case of static batteries the Energy Density becomes the least important factor.
One such promising redox couple is the hydrogen and bromine. Hydrogen and bromine are abundantly available on a global scale. Figure 1 shows the electrochemical cell configuration of an HBr Flow battery.
Figure 1 - electrochemical cell configuration of a HBr Flow battery [2]
The membrane in this cell is on one side in contact with the electrolyte circuit, an aqueous solution of HBr and Br2, and on the other side with a hydrogen (H2) gas circuit. Both active materials circulate in a closed loop along their own respective side of the cell. The electrolyte (HBr/Br2 solution) and hydrogen (H2) circuits can be separated by a proton-conductive membrane. A complete battery is comprised of several of this cells stacked together in series.
Figure 2 - Charging and discharging process for the HBr flow battery [3]
During the charging process, when an electrical current is established and breaks the HBr molecule into an H+ ion (= proton) and a Br- ion:
- The H+ ion crosses the membrane, absorbs an electron to form hydrogen (H2), stored at the right part of Figure 1.
- The Br- ion releases an electron to form Br2 and remains stored in the reservoir at the left side of Figure 1.
During the discharge process, when applying an electrical load, H2 and Br2 molecules are combined into hydrobromic acid (HBr):
- The H2 molecule releases 2 electrons and forms 2 H+ ions (protons).
- These protons cross the membrane and form HBr in the electrolyte circuit and remain stored in the reservoir until it is re-charged.
This makes process 100% reversible meaning no materials are consumed and therefore need no replenishment other than electric power. The system is however dependent of H2 and HBr reservoirs and their respective apparatus that make for a complex build of plant (BoP) when comparing to other types of batteries.
Notably, as membranes can account for 22% to 40% of the total cost there are some studies that apply membrane-less cells using the concept co-laminar streams of liquid flowing at low Reynolds numbers [3]. This could even further decrease the cost of the batteries while increasing their life cycle, as HBr environments are typically corrosive for physical membranes.
Although The HBr battery have lower energy density (less than that of lithium-ion batteries) and a complex BoP that actually prevent the use of H2-Br2 flow batteries in transportation applications, for BSSE applications, these drawbacks are dismissible.
Overall, there are some commercial applications of this technology where storage costs per unit of electricity are 95% lower than those of Lithium batteries [2], meaning it could be a viable solution for keeping future BESS projects affordable.
[1] - https://electrek.co/2018/05/11/tesla-giant-battery-australia-reduced-grid-service-cost/
[2] - https://www.elestor.nl/technology-the-elestor-solution/
[3] - G. Lin, P.Y. Chong, V. Yarlagadda, T.V. Nguyen, R. J. Wycisk, P. N. Pintauro, M. Bates, S. Mukerjee, M. C. Tucker, and A. Z. Weber - Advanced Hydrogen-Bromine Flow Batteries with Improved Efficiency, Durability and Cost
Journal of the Electrochemical Society 2015 163: A5049-A5056.
[4] - Braff, W., Bazant, M. & Buie, C. - Membrane-less hydrogen bromine flow battery. Nat Commun 4, 2346 (2013). https://rdcu.be/b0OMj