The working principle and equivalent circuit of Vanadium redox flow battery (VRB)
C.1 Introduction
The battery technology has been developed mainly for two purposes. One is for electric vehicles and the other is for the stationary applications. The details of battery technology and their use in different applications are given in the following manner.
C.1.1 Batteries for electric vehicles
Internationally, there is an increased interest in the development of modern battery powered electric vehicles. Batteries are the enabling technology for the electric drive vehicles. In 19th century almost of all the electric vehicles were powered with lead acid batteries [111]. In 20th century nickel iron battery has been developed with slight higher energy density. This battery has achieved an improved performance over lead acid battery. In 20th century, varieties of batteries have been developed for the electric vehicles such as:
Nickel-cadmium batteries, Nickel-zinc batteries, Sodium-metal chloride batteries, Sodium- sulfur batteries, Zinc-bromine batteries, Zinc-chlorine batteries and Li-ion batteries. All these batteries support energy to the electric vehicle less than 100 miles. The large lithium-ion batteries were developed by Sony and Hitachi for electric vehicle applications in the early 1990s with high energy ratings. They designed thin electrodes in the lithium-ion battery to provide high power. With the introduction of lithium-ion battery the serious safety issues were mitigated and the battery technology has been improved to a significant level.
C.1.2 Batteries for stationary storage applications
The extended role of the energy storage systems in the transmission and distribution side of the electric grid started in late 1980s and in early 1990s [111]. These batteries were developed for the national benefits. The batteries for stationary energy storage applications can be divided into two categories. In the first category: the Sodium-beta high temperature battery and the Sodium/sulfur batteries were developed. The limitations with these batteries are: safety issues, durability and the thermal management. In the second category: the flowing electrolyte batteries were developed. The flow type battery (zinc/chlorine hydrate battery) is invented in 1968. Thereafter, the work on this type of batteries has been started.
The flow type batteries are of two types. First one is Zinc/bromine flow type batteries.
These batteries have the following disadvantages such as: the temperature control issues, safety issues and improvements are needed to moderate power capability. Second one is Vanadium redox flow type batteries. NASA has conducted a development program to develop the redox flow batteries using ferric chloride (FeCl3) as the oxidizing agent (positive) and chromium chloride (CrCl2) as the reducing agent (negative)) for stationary energy storage applications. Thereafter, the work on the redox flow battery has been started.
The words ‘reduction’ and ‘oxidation’ were decreased to the term ‘redox’. These batteries are used in the electrochemical systems (oxidation and reduction involves in ionic species solution and the reactions take place on the inert electrodes). The active materials in these batteries are stored externally from the electrode cells.
The Vanadium Redox flow Battery (VRB) was introduced in the year 1988. Thereafter, a significant development has been achieved for the higher level electrical energy storage. The VRB is well suited for the large scale energy storage applications. VRB has the following features: high capacity, long life span, minimum maintenance and quick response to rapid changes [153, 154]. VRB is an electrochemical cell divided into two components. One is the ionic membrane (battery reactions exist) and the other is storage tanks (vanadium electrolyte (positive and negative) is stored).
VRB provides the voltage support and peak load relief to the load served by the distribution feeder and also, improves the power quality in the distribution grid. The working model of VRB is shown in Fig. C.1. The components that are involved in the VRB model are: the electrolyte (electrolyte stays at different valance states of vanadium sulfate in the positive and negative electrode components of VRB. The sulfuric acid is the supporting electrolyte for this solution and stays at 2 molar in concentration), the electrolyte storage tanks, cell stacks (electrodes), control unit and the converter unit. Electrolyte is the solution of Vanadium in dilute sulfuric acid and this solution is electrochemically oxidized or reduced to store energy. The nitrogen air pads are used to prevent the oxidation.
The VRB works in the following manner. The electrolyte is pumped from storage tanks through cell stacks (positive and negative electrodes) and the electrical energy is added to the cell stacks to charge the electrolyte. The charged electrolyte is pumped through cell stacks when the energy required as shown in Fig. C.1 [153, 154]. The converter unit is used to
convert ac-dc when charge energy from the source and dc-ac when discharge energy to the load. The control panel is used to operate the VRB remotely. The applications of VRB are:
renewable energy and remote area power supplies, wind and solar energy storage coupled with diesel generation for fuel and emission reduction, commercial and industrial facilities, ancillary services for utilities, telecommunications and backup power systems.
Source
Negative electrode Positive
electrode
Ionic membrance
Negative electrolyte V2++++/V3++++ Positive electrolyte
V5++++/V4++++
Pump Pump
Converter unit
Load Fig. C.1 The working model of VRB
The power availability in VRB depends on the size of the electrodes in the cell stacks. The amount of energy stored in VRB is the function of active chemical substances and the state of charge (SOC). The chemical reactions that are taken place in VRB cell during charging and discharging operations are expressed as
4+ − 5+
− ↔
V e V (C.1)
3+ − 2+
+ ↔
V e V (C.2) (C.1) is for positive electrode and (C.2) is for negative electrode.