Zinc-bromine flow battery backs up solar rooftop PV system at SA factory



Flow batteries have been around for several decades but until recently have been scaled for utility sized operation. Recent developments have produced smaller units which are finding application in industrial networks. A bank of such units manufactured by Redflow, was recently installed at the premises of Bosco in Johannesburg.

The ZBM2 flow battery unit, which has a capacity of 10 kWh and a maximum output of 5 kW peak (3 kW average), at a nominal 48 V DC, uses as a hybrid zinc-bromine flow process. The net DC-DC efficiency of the battery is reported to be 80%.

The zinc-bromine flow battery

The battery is a hybrid redox flow battery, because much of the energy is stored by plating zinc metal as a solid onto the anode plates in the electrochemical stack during charge. The total energy storage capacity of the system is dependent on both the stack size (electrode area) and the size of the electrolyte storage reservoirs. As such, the power and energy ratings of the zinc-bromine flow battery are not fully decoupled.

Fig. 1: Zinc-bromine flow battery [1].

In each cell of a zinc-bromine battery (ZBB), two different electrolytes flow past carbon-plastic composite electrodes in two compartments, separated by a micro-porous polyolefin membrane. The electrolyte on the anode (negative) side is purely water-based, while the electrolyte on the positive side also contains an organic amine compound to hold bromine in solution.

When the ZBB is charged, the overall chemical reaction involves the reduction of zinc and evolution of bromine, as simplified form in Eqn.  1.

ZnBr2 → Zn + Br2          (1)

Similarly, zinc and bromine recombine to form ZnBr2 , when the ZBB is discharged,

Zn + Br2 →Zn Br2           (2)

Fig. 2: Diagram of a multicell ZBM2 battery (Redflow).

During charge, metallic zinc is plated  as a thick film on the anode side of the carbon-plastic composite electrode, and bromide ions are oxidised to bromine and evolved on the other side of the membrane. During discharge, the zinc metal is oxidised to Zn2+ ion and dissolved into the aqueous anolyte. Two electrons are released at the anode the external circuit. The electrons return to the cathode and reduce bromine molecules (Br2) to bromide ions, which are soluble in the aqueous catholyte solution. The bromine in the catholyte is converted into two bromide (Br-) ions at the cathode, balancing the Zn2+ cation and forming a zinc bromide solution. The chemical process used to generate the electric current increases the zinc-ion and bromide-ion concentration in both electrolyte tanks.

In the Redflow battery, both electrolyte tanks are combined in a single structure, a unique design with the outer container holding the zinc-based electrolyte while the inner container holds the bromine complex electrolyte, isolating the bromine complex from the outside by two separate walls. Fig. 2 shows a diagram of the battery system.

Fig.3 shows the complete unit.

Fig. 3: The ZBM2 flow battery (Redflow).

The zinc-bromine redox battery offers one of the highest cell voltages and highest energy density of all flow batteries.  However, there are problems with the zinc plated on the anode, which is not entirely removed during discharge, and the formation of zinc dendrites within the membrane, which affects the discharge capacity, and must be removed regularly. The process involves a very high rate discharge, according to the suppliers. This is the only regular maintenance that needs to be carried out.

South African case study: Bosco printed circuits

The production line at Bosco printed circuits uses a lot of energy and the direct and indirect consequences of losing energy supply to the line are substantial. The area has significant issues with energy supply, both in terms of reliability and energy cost. The company has an extensive solar array installation, but this not sufficient on its own to address the callenges of supply cost and security. The cost of grid energy is time-of-usage based, and is very high during morning peak and late afternoon periods, which are also periods of high energy demand for the factory. These peak times are also outside the solar peak generation periods, and coincide with the grid outages.

Fig. 4: Installation at Bosco.

Bosco had a variety of business aims and objectives across their daily operating cycle that their energy system had to address:

  • To ride over transient periods of grid loss seamlessly using battery energy.
  • To support the operation of the production line for an extended period (hours, not merely minutes) in the face of longer periods of grid outage, using battery energy augmented with any available solar energy.
  • In cases of an extended grid outage, to allow the production line to be closed down with advance warning of at least an hour.
  • To time-shift energy obtained from the low cost overnight off-peak period into the morning and afternoon peak period, prioritising battery energy usage in order to minimise the use of expensive grid energy.
  • To use the residual battery energy, harvested from overnight off-peak charging and from any excess of daytime solar power, to supply the background energy needs of the building into the evening. To recharge the battery array again using off-peak power ready to commence the next daily cycle.

This set of objectives required a battery energy system capable of consistent hard work and capable of daily 100% energy discharge, working in a hot environment, and without loss of output capacity over time. The solution uses 14 Redflow ZBM2 batteries, with a total capacity of 140 kWh, interfaced to a large array of Victron energy inverters/chargers and a large solar array. The system controls this complex daily cycle of energy optimisation using the Victron CCGX and the Redflow BMS, to achieve the aims and objectives. Fig. 4 shows the installation.

Fig. 5: Bosco printed circuits energy consumption (Redflow).

Fig. 5 shows a typical day in the life of this system, in terms of the sources of energy to run the plant.

Reference

[1] M Saleem et at “Progress in Flow Battery Research and Development” Journal of The Electrochemical Society, June 2011.

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