Flow batteries are emerging energy storage devices that can serve many purposes in energy delivery systems. They can respond within milliseconds and deliver power for hours. They operate much like a conventional battery, storing and releasing energy through a reversible electrochemical reaction with an almost limitless number of charging and discharging cycles. They differ from a conventional battery in two ways 1) the reaction occurs between two electrolytes, rather than between an electrolyte and an electrode and 2) they store the two electrolytes external to the battery and the electrolytes are circulated through the cell stack as required. The great advantage that this system provides is the almost unlimited electrical storage capacity (MWh), the limitation being only the capacity of the electrolyte storage reservoirs.
Flow batteries are essentially comprised of two key elements: cell stacks, where power is converted from electrical form to chemical form, and tanks of electrolytes where energy is stored. These two elements are supplemented with circulation and control systems. An individual cell consists of a negative electrode and a positive electrode separated by an ion exchange membrane. The battery uses electrodes that cannot and do not take part in the reactions but merely serve as substrates for the reactions. There is therefore no loss of performance, as in most rechargeable batteries, from repeated cycling causing electrode material deterioration. Banks of these cells can then be linked together to create a bipolar module ‘cell stack’ where the electrodes are shared between the adjacent cells, with the cathode of the first cell becoming the anode of the next cell, etc. Linked in series, sufficient cells in a string can then form the desired voltage for the cell stack.
During operation, the two electrolytes flow from the separate storage tanks to the cell stack for the reaction, with ions transferred between the two electrolytes across the ion exchange membrane; after the reaction, the spent electrolytes are returned to the storage tanks. During recharging, this process is reversed.
Flow batteries can be used for a very high number cycles, they are also more efficent and have a longer discharge time than lead acid batteries which are their main competition. They have an efficiency of 75% to 85% compared to 70% to 75% for lead acid batteries and a discharge time of several hours compared to less than an hour for lead acid batteries at their rated power. Cost per unit power ($/kW) is less than for conventional batteries and cost per unit energy ($/kWh) is similar or less than that of batteries. Lithium ion batteries have a higher efficiency than flow batteries and a shorter discharge time than lead acid batteries and a higher cost than either lead acid or flow batteries and are therefore not usually competitive.
The power (MW) and energy (MWh) ratings of the flow battery are independent of each other. Therefore, increasing the power from the facility necessitates additional cell stacks, while expanding the energy capacity of the system is accomplished by expanding the amount of electrolyte stored within the unit. Flow batteries are currently available in single digit kW to 10's of kW in size, with sizes in the 100's of kW expected to be available in the future.
A flow battery can be optimized for either energy or power delivery, can respond (from stand-by mode or from charging to discharging) within milliseconds, and can ramp-up from a full shutdown to full operation within a few minutes. These capabilities make them suitable for a variety of applications including: UPS (uninterruptible power supply), power quality, renewable energy (power quality and mitigating surges and intermittency) and peak shaving.
Flow batteries are catagorized by the chemistry of the electrolytes. Four chemistries have evolved; Vanadium Redox, Zinc Bromine, Polysulfide Bromide and Cerium/Zinc. Some of these types are now available commercially and some are just emerging from development.
Each of these chemistries will be examined in detail in future posts.