The Vanadium Redox Battery for Emergency Back-Up Applications

M. Skyllas-Kazacos and C. Menictas
School of Chemical Engineering and Industrial Chemistry
The University of New South Wales, Sydney AUSTRALIA.
email: M.Kazacos@unsw.edu.au


Abstract

Secure and reliable power is essential in areas such as telecommunications and information technology to safeguard the vast computer networks which have been established. Uninterruptable power systems have incorporated battery technology to allow smooth power feeding switch-over in the case of a power failure. In such systems lead-acid batteries have commonly been used until generators come on-line or for safe computer shutdown.

A new battery system, the Vanadium Redox Battery (VRB) is under development at the University of New South Wales which provides many advantages over conventional batteries for emergency back-up applications. This system stores all energy in the form of liquid electrolytes which are recirculated around the battery system. The electrolytes can be recharged an indefinite number of times, or the system can be instantly recharged by mechanically exchanging the discharged solution with recharged solution.

A vanadium battery emergency back-up system has been designed and tested for operation between a restricted voltage operating range of 22-28V. The system achieved energy efficiencies above 80% and coulombic and voltage efficiencies as high as 96.4% and 92.2% respectively depending on the load.

The system demonstrated excellent performance characteristics with a capacity utilisation up to 94% of theoretical.


1. Introduction

While the development of new efficient and environmentally safe energy storage systems has been a critical issue for the more widespread use of renewable energies such as solar and windpower, the availability of safe and reliable batteries for emergency back-up applications would provide enormous benefits in the rapidly growing area of telecommunications and information technology.

Uninterruptable Power Systems (UPS) are widely used in the telecommunication and financial industries where secure power for vital computer installations and electrical equipment is a top priority. These systems rely on sealed lead-acid batteries to provide 10-15 minutes of power in the case of power failure, enabling an orderly shutdown of computer systems, or allowing sufficient time for a diesel generator to switch on.

Many problems are regularly encountered in the use of diesel generators, however, these problems often arising from failure of the generators to switch on due to flat starter battery problems.

The new vanadium redox flow battery currently under development at the University of New South Wales has many unique features which would make it ideal for emergency back-up applications, particularly as a diesel generator replacement which could also be integrated into the UPS system itself. The features and operating principles of the vanadium redox battery (VRB) are discussed below:


2. Basic Principles and Features of the VRB

The vanadium redox battery which was pioneered at the University of NSW, was proposed to overcome the deficiencies of other energy storage systems. Briefly, the VRB employs V(II)/V(III) and V(IV)/V(V) redox couples in the negative and positive half-cell electrolytes therefore, there are no problems of cross-contamination and the solutions have an indefinite life [1-3]. Additional features of the vanadium battery which would make it ideally suited for a wide range of energy storage applications[4,5] are given in Table 1 while a schematic diagram of the system is given in Figure 1.

In the vanadium redox cell, the following half-cell reactions are involved:

At the negative electrode:

V3+ + e - <----> V 2+ E0 = -0.26V (1)

At the positive electrode:
VO2+ + 2H+ + e- <----> VO2+ + H2O E0 = 1.00V (2)

The standard cell potential is thus E0 cell = 1.26 Volts at concentrations of 1 mole per litre and at 250C. Under actual cell conditions, an open circuit voltage of 1.4 Volts is observed at 50% state of charge, while a fully charged cell produces over 1.6 Volts at open-circuit. The electrolyte for the vanadium battery is 2M vanadium sulphate in 2.5M H2SO4, the vanadium sulphate (initially 1M V(III) + 1 M V(IV) being prepared by chemical reduction or electrolytic dissolution of V2O5 powder[6]. The basic components of the redox cell are illustrated in Figure 2.

Table 1. Benefits of Vanadium Redox Battery for Large Scale Energy Storage.



Energy stored in tanks separate from the cell stack