As the world's power needs grow, the demand for power from renewable resources, such as wind or solar, is increasing. One major drawback associated with these renewable resources is that the power output is dependent on environmental factors, such as cloud cover and wind speeds. This allows the possibilities of either power output exceeding or falling short of forecast levels which may lead to grid instabilities. Therefore, Large Scale Energy Storage (LSES) systems are critical to store excess power when the output exceeds demand in order to supplement output power when it falls short of demand.1 Some promising LSES contenders are: 1) Zinc bromide flow cells (FC), 2) Vanadium redox FC, 3) Sodium sulfur batteries, and 4) Lead acid batteries.2 Of these available technologies the zinc bromide technology was selected for further investigation because of previously reported long cycle-life (CL) and high efficiencies.3
In this work, the cell efficiency dependence on temperature of a bench scale Zinc Bromide battery manufactured in our lab was investigated to confirm the theoretical efficiency. Efficiency cycling was voltage limited on the discharge cycle at a 1.5 / V cell cut-off potential. Standard method of measuring columbic efficiency was implemented. At room temperature, ~80% efficiency was achieved when cycling the system between 20 - 100% State of Charge (SOC), which is in good agreement with published values.4 The cell efficiency was demonstrated at over 90% at 50°C and cycling between 20 - 100% SOC.
In addition, given that deep discharge is perceived as highly damaging to electrochemical energy storage, the battery was cycled through the Zero-Point (ZP) 10,000 times. The cell was continuously cycled between 0 - 5% (SOC). Battery performance was evaluated from transfer rate and exchange current density, determined using Tafel scans. No degradation of the electrodes was found, and in fact a slight increase in the exchange current density was observed. We interpret this exchange current increase as a result expected for electrode conditioning of the polished carbon electrode substrates used in our test cells.5
These along with further more recent results will be presented.
1. Dell, R. M.; Rand, D. A. J. Energy storage: a key technology for global energy sustainability. J. Power Sources 2001, 100, 2-17.
2. (a) Schoenung, S. M. Characteristics and Technologies for Long- vs. Short-Term Energy Storage: A Study by the DOE Energy Storage Systems Program. Sandia National Laboratories SAND2001-0765 2001 p 59; (b) Schoenung, S. M.; Eyer, J. Benefit/Cost Framework for Evaluating Modular Energy Storage. Sandia National Labratories SAND2008-0978 2008 p 40.
3. Manla, E.; Nasiri, A.; Hughes, M. Modeling of zinc energy storage system for integration with renewable energy. Industrial Electronics 2009, IECON '09 35th Annual Conference of IEEE, 3-5, p 3987-3992.
4. Lex, P.; Jonshagen, B. The zinc/bromine battery system for utility and remote area applications. Power Engineering Journal 1999, 13, 142-148.
5. Kautek, W.; Conradi, A.; Fabjan, C.; Bauer, G. In situ FTIR spectroscopy of the Zn-Br battery bromine storage complex at glassy carbon electrodes. Electrchimica Acta 2001, 47, 815-823.
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