Self-discharge mechanisms in the vanadium redox battery
There is a Worldwide requirement for reliable, long life and efficient electrical energy storage devices to back up the increasing number of regenerative power supplies (wind, water and solar). These devices are being implemented on = 5 kW scale for single household and village use, up to multi-MW installations connected to the main grid. However, regenerative power supplies are intermittant by their very nature and therefore require some energy buffering (load levelling) or storage. Redox flow batteries present a most promising solution to this problem. In addition redox flow batteries may be used to ensure uninterruptible power supplies (UPS application) for critical users, e.g. tunnels, hospitals, mainframe computer installations.
Contents and objectives
The weak point of the state-of-the-art vanadium redox battery (VRB) is its relatively high self-discharge rate, which is the sum of several independent processes, and leads to a loss in recovering the stored energy. The content of this project is to describe and quantify the self-discharge mechanisms and work on solutions to reduce them. Furthermore proposals for the implementation of the successful solutions should be advanced.
The overall objective of this project is to reduce the self-discharge of the VRB and therefore increase the overall efficiency.
- Improvement of scientific understanding of the self-discharge mechanisms in the VRB
- Working on solutions to minimise the self-discharge and improve the efficiency
- Construction of a 100-cell series battery to verify the operation of a high voltage (laboratory-based) VRB
- Converting the proposed solutions for reduce the self-discharge into realisable components
The methods used to study the various self-discharge mechanisms are a combination of standard electrochemical techniques and those developed specifically for study of the VRB by the R & D partner (e.g. salt tracer methods to study the electrolyte flow, parallel OCV cell method to measure shunt currents, single-cell test-rig to study hydrogen evolution, module test-rig to study the temperature dependent behaviour, test-rig to gravimetrically determine transport properties of the membranes).
This project aimed to improve the efficiency and economic viability of the vanadium redox flow battery. Causes of the self-discharge were addressed. Specifically, the results of this research were:
- A more efficient and higher flow rate alternative to the currently used pumps has been found (>20% pumping efficiency for lower flow rates, >15% for higher), and several pumps and pump drivers characterised. This should reduce the power consumption of the peripheral devices.
- An optimum structured electrode has been tested and found to be beneficial in reduce the flow resistance of the modules, but caused a slight increase in the electrical resistance. Modules employing structured felts are suggested for applications which would otherwise have high pumping losses in comparison to resistive losses.
- Experimental results have shown the hydrogen evolution rate dependence on potential. From these measurements a new maximum cell voltage has been advised.
- An optimised membrane has been shown to give comparable performance to existing membranes, but at a reduced cost.
- Development has lead to a module that is thoroughly tight over the temperature range 2 - 45°C. This has been the result of improved bipolar plate, frame, distributor plate and spacer design. The need for expensive aluminium presser plates is now removed except where transport conditions are likely to include high temperatures.
- Various techniques for measuring the shunt currents have been demonstrated and a mathematical model developed, which agrees well with the experimental data. A test-rig for measuring the effectiveness of "shunt killer" devices has been built and the tests conducted show exceptionally good initial performance, but with a gradual increase in shunt-currents during operation.
The improvements described above will lead to a more competitive product in economical and performance terms. High voltage and high power markets can now be addressed, including UPS (Uninterrupted Power Supply) applications. The UPS segment is of growing importance due to recent high profile power failures in the US and Europe. The vanadium redox battery will open opportunities to satisfy customer requirements for high reliability and low maintenance.
EN-o-DE Energy on Demand Production and Sales GmbH
Project and cooperation partners
- Funktionswerkstoffe Forschungs- und Entwicklungs GmbH
- Treibacher Industrie AG
- FuMA-Tech GmbH