Chemical energy is provided by two components dissolved in liquids and separated by a membrane.
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RFBs are secondary batteries, composed of two parts that are connected through pumps: the battery stack, where electrochemical reactions take place, and the external tanks, where electrolytes are stored. The battery stack is normally composed of two sets of electrodes, bipolar plates, and current collectors that sandwich a membrane between two electrodes. The membranes serve as charge-carrier conductors while blocking the electrolytes of the two sides from mixing.
True flow batteries have all the reactants and products of the electro-active chemicals stored external to the power conversion device. Systems in which all the electro-active materials are dissolved in a liquid electrolyte are called redox (for reduction/oxidation) flow batteries (RFBs). Other true flow batteries might have a gas species (e.g., hydrogen, chlorine) and liquid species (e.g., bromine).
Since the 1970s, various RFBs have been introduced but only all-vanadium, vanadium-polyhalide, zinc-bromine, zinc-cerium and bromine-polysulfide batteries have been tested or commercialized on a large-scale. The VRFB has the highest efficiency and the largest life cycle, while zinc-cerium and bromide-polysulfide systems have advantages in power density and cost, respectively.
Iron–Chromium Flow Battery was the first prototype with a standard theoretical potential of 1.18 V. The development of ICBs is ultimately hindered by intrinsic problems.
All-Vanadium Redox Flow Battery are the most well-established and promising RFB system. A VRB uses four oxidation states of vanadium to undergo the following reaction during cell operation
Aqueous Metal-Based Redox Flow Chemistries RFBs with aqueous metal-based electrolytes are still the most well-developed and advanced system with large-scale industrial installations. However, the cost of vanadium has severely limited its potential to further lower the overall system cost.
The vanadium flow battery (VFB) consists of an assembly of power cells in which the two electrolytes are separated by a proton exchange membrane or ion transfer membrane. The standard cell potential is E = 1.259V at concentrations of 1M at 25° C. During the charge and discharge process, hydrogen cations are consumed or produced, which means that the pH of the positive or the negative electrolyte can be expected to change over time. To maintain the balance within the different half-cells, there is a mechanism for the transit of hydrogen ions from the negative half-cell to the positive half-cell, which is generally accomplished by placing an ion-conducting or ion-transferring membrane between the two half-cells and vice versa.
VFBs is unsuitable for mobile and portable power, main applications are in large fixed installations and products for renewable energy storage.
Load levelling Large-scale energy storage systems allow the use of excess power and avoid high cost power plants to cycle on and off. By employing an RFB, bottlenecks within the transmission system can be relieved; this can result in reduced power transmission losses and more reliable electrical power.
Power quality control applications RFBs are favourable in electrical power system failures as their response time to power demand can be less than 1 min and the maximum short-time overload output can be several times that of the rated capacity. This makes RFBs attractive for both voltage and frequency control. Thanks to the advantage of longer discharge times, RFBs can be used as a battery-backed UPS system for protection against faults on the power utility transmission and distribution systems, providing electricity during shutdown of computer systems or switch-on of the backup generator.
Coupling with renewable energy sources Due to the large capacity and the long discharge time, flow batteries are attractive when coupling with renewable energy sources, as the renewable energy production is usually intermittent and not connected to the grid.
Solar panels are traditionally connected to conventional lead-acid batteries, which only use 25–75% of their state of charge, thus using only 50% of the actual capacity. RFBs have advantages over conventional lead acid batteries in high efficiency, long cycle life and low cost.
Coupling RFBs with wind turbines is useful in removing fluctuations and maximizing the reliability of power.
Electric vehicles Since energy is stored in the electrolyte, 24 h operation of an electric vehicle is possible if the electrolyte is refuelled every 4 or 5 h. However, the low energy density of VRFB remains the main challenge. Therefore, the recent larger specific energy density of vanadium-air and lithium flow batteries could be promising for longer mileages of electric vehicles but the safety conditions of using a lithium flow battery still need to be identified.
Energy is stored in solution so no solid-phase changes occur, this eliminates the possibility of short-circuiting or shedding of the active material. FBs present simple design and ease of operation because the same electrolyte is used for both the positive and the negative side. The active materials in VFBs are vanadium ions with different valences in the electrolyte: during the charge and discharge process, the vanadium ions only change valence and thus do not degrade, additionally the solid electrodes used in flow batteries are inert. Thus, Flow batteries are free of the processes that lead to mechanical breakdown of the active material and therefore offer long cycle life under deep discharge operation.
Separation of energy (kilowatt-hour) and power (kilowatt) rating provides great ﬂexibility in system design: number of cells or cell stacks (that deﬁnes system voltage) and the electrode area (that determines current) determines the power rating, while the electrolyte’s volume and concentration give the battery its storage capacity. Thanks to the independence of power and energy, FBs can be upgraded for more energy by adding tanks and electrolyte, and upgraded for more power by adding cell stacks, this is particularly important in large-scale energy storage applications that require energy capacities of several hours.
In flow batteries, the flowing electrolyte circulates between the tank and the stack and acts as a coolant, effectively becoming an active cooling system for the device. Avoiding heat accumulation inside the stack results in longer lives for electrochemical components.
The batteries can be fully charged and discharged without the risk of damage, loss of battery life, or loss of capacity. Flow batteries are less susceptible and more robust than conventional batteries to conditions such as overcharge, deep discharge, and partial state-of-charge (SOC) cycling, they can be left completely discharged for long periods with relatively no self-discharge occurring and they can endure long standby times without any capacity decay of the electrolyte.
In conventional batteries, electrolytes must be managed at the cell level, This can be costly and time-consuming. In flow batteries, the electrolytes can be monitored and managed for the entire battery at once, since all cells share the same electrolyte.
The capacity of an electrochemical battery will decay over time while the capacity of a VFB can be recovered. For vanadium redox flow batteries (VFBs), the capacity decay is mainly because of the imbalance of positive and negative ion valence, therefore, using chemical or electrochemical reaction methods, the positive and negative ion valence can be effectively rebalanced and the capacity recovered.
State-of-charge determination is readily achieved using an open-circuit cell either before or after the cell stack. This can continuously monitor the potential between the two half-cell solutions and convert this into an SoC reading using the Nernst equation.
VFBs have a relatively long cycle life. The solid electrodes used in conventional batteries not only suffer mechanical and thermal stress with cycling but also may have phase changes during the charge and discharge process, shortening their lives.
The main disadvantage of VFBs is their relatively poor energy-to-volume ratio, limited by the solubility of the vanadium ions in the electrolyte. For electrolyte with sulfuric acid as the supporting electrolyte, the energy density is only about 15– 25 Wh/L, much lower than the energy density of lithium-ion batteries.
Another disadvantage of VFBs is the need to circulate liquid electrolytes, which is somewhat cumbersome accounting for technical limitations for assembling large cell stacks, liquid seepage and leakage, uneven distribution of voltage and current, non-uniform liquid flow through single cells, and instability of materials. Additionally, cross-contamination of the electrolyte in the operation process may result in huge losses of capacity and energy along with lifetime and safety reduction of the RFB system. The system complexity and the aqueous electrolyte, make VFB systems heavy in comparison with other solid battery storage systems.
Critical aspects of RFB commercialization regard the costs of all RFBs, currently higher than the near-term target (within five years; capital cost under USD 250 per kWh) and longer-term target (USD 100–150 per kWh) of battery storage technologies set by the US Department of Energy. Both the active materials and the battery compartments contribute to the high cost of RFBs: the expensive active redox couples and membrane (i.e. the cost of a Nafion membrane is 30%–40% of the total cost), some expensive noble catalysts required to enhance the reversibility and the kinetics of redox reactions, active temperature management in order to retain the solubility of active redox couples and maintenance costs related to the life of cell stacks and electrolytes.
Key websites to get an overview:
Aug 01, 2020
It is the size of a small fridge, doesn't use unsustainable rare-earth metals, won't blow up and can power your house for 20 years.
Jul 24, 2020
A six-month feasibility study at Portsmouth, UK, port has demonstrated how flow batteries offer a more cost effective solution to reducing the shipping industry’s emissions than conventional lithium-ion and lead-acid batteries.
Jul 03, 2020
Researchers at MIT propose chitin, a cellulose-like polysaccharide found in shrimp shells, as an ingredient for vanadium redox flow batteries.
Dec 11, 2019
Elon Musk and GM are making big bets on lithium-ion battery technology for energy storage and auto markets in the climate change era, but iron-flow batteries, which are backed by Bill Gates, Jeff Bezos, Jack Ma and Michael Bloomberg, could play a big role in the future.
Nov 11, 2019
Researchers at the Lawrence Berkeley National Lab say they have devised a new membrane for flow batteries that is durable and inexpensive. Just the ticket for low cost grid scale energy storage.