Flow Battery

Image : pv magazine

Although its concept has been around for a while, flow batteries are a relatively new technology in the battery storage sector. The most promising, actively researched, and sought redox flow battery technology is the vanadium redox flow battery (VRFB). Redox flow batteries store the electrolytes in external tanks apart from the battery centre, which is a substantial difference from more conventional electrochemical batteries in terms of electrolyte storage. The main parts of a VRFB are an electrolyte, membrane, electrode, bipolar plate, gasket, collector plate, storage tank, and pump.

Operation

Vanadium ions in four different oxidation states (V2+, V3+, V4+, and V5+), each with its redox pair, are normally held in two of these tanks by RFBs. The positive side refers to the electrode and electrolyte that contain V4+ or V5+. Electrode and negative electrolyte are opposed. Separate half-cells are used to feed the electrolytes into the battery, and after use, they are returned to storage tanks for recirculation. An electrode and a bipolar plate make up each reactor half-cell, and two half-cells are separated from one another by a membrane to allow for selective ion exchange while avoiding electrolyte cross-contamination. This is a single cell, and subsequent cells share a bipolar plate to form a stack. The image shows the whole cell reaction.

While being charged, the negative electrolyte is reduced to V2+ while the positive electrolyte is oxidised to V5+ (VO2+), leaving the negative and positive electrolytes with just V3+ and V4+ (VO2+) correspondingly after they are fully discharged. When the battery is charged, hydrogen (H+) ions flow through the membrane to the negative side of the bipolar plate as electrons move from the positive to the negative side. The same procedure takes place in reverse during discharge.

Depth of Discharge

When compared to other battery technologies, a vanadium flow battery's DOD is often very high. Normally, the battery may be discharged up to 100% of its capacity without suffering serious harm. The deep discharge capability of VFBs, which enables a longer usage of the stored energy, is one of its key advantages. Due to its high DOD, vanadium flow batteries are particularly well suited for long-duration energy storage applications including grid-scale storage, load balancing, and the integration of renewable energy sources. The system's total efficiency and economic feasibility are increased when the battery can be discharged to a high percentage of its capacity, maximising the utilisation of stored energy.

Lifetime

Cycle life is the number of charge-discharge cycles a battery can withstand before experiencing a significant loss in capacity. Thousands of cycles may frequently be performed by VFBs without noticeably diminishing storage capacity. The nature of vanadium redox chemistry, which includes the passage of vanadium ions through electrolyte tanks without significantly degrading the electrodes or electrolyte, is primarily responsible for this improved cycle life. With routine maintenance and operation, a well-designed VFB can have a projected lifetime of 10 to 20 years or more. The real-world performance and durability, however, might differ based on a number of factors, including VFB chemistry, system design, and usage patterns.

Battery Efficiency and Maintenance

VFBs commonly achieve efficiency levels of 80–90% or more. As a result, a considerable part of the energy that is put into the battery is effectively stored, and a similar amount of the stored energy may be recovered when the battery is discharged. To guarantee optimum battery performance, regular electrolyte composition monitoring and maintenance, including maintaining proper vanadium ion concentrations and pH levels, is necessary. To maintain the required electrolyte characteristics, this may call for recurring chemical alterations or additions