| Specific energy | 10–20 Wh/kg (36–72 J/g) |
|---|---|
| Energy density | 15–25 Wh/L (54–65 kJ/L) |
| Charge/discharge efficiency | 75–80%<. |
| Time durability | 20-30 years |
| Cycle durability | >12,000-20,000 cycles |
| Nominal cell voltage | 1.15–1.55 V |
Schematic design of a vanadium redox flow battery system
1 MW 4 MWh containerized vanadium flow battery owned by Avista Utilities and manufactured by UniEnergy Technologies
A vanadium redox flow battery located at the University of New South Wales, Sydney, Australia
The vanadium redox battery (VRB), also known as the vanadium flow battery (VFB) or vanadium redox flow battery (VRFB), is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy. The vanadium redox battery exploits the ability of vanadium to exist in solution in four different oxidation states, and uses this property to make a battery that has just one electroactive element instead of two. For several reasons, including their relative bulkiness, most vanadium batteries are currently used for grid energy storage, i.e., attached to power plants or electrical grids.
The possibility of creating a vanadium flow battery was explored variously by Pissoort in the 1930s, NASA researchers in the 1970s, and Pellegri and Spaziante in the 1970s,
but none of them were successful in demonstrating the technology. The
first successful demonstration of the all-vanadium redox flow battery
which employed vanadium in a solution of sulfuric acid in each half was
by Maria Skyllas-Kazacos at the University of New South Wales in the 1980s. Her design used sulfuric acid electrolytes, and was patented by the University of New South Wales in Australia in 1986.
The main advantages of the vanadium redox battery are that it can
offer almost unlimited energy capacity simply by using larger
electrolyte storage tanks; it can be left completely discharged for long
periods with no ill effects; if the electrolytes are accidentally
mixed, the battery suffers no permanent damage; a single state of charge
between the two electrolytes avoids the capacity degradation due to a
single cell in non-flow batteries; the electrolyte is aqueous and
inherently safe and non-flammable; and the generation 3 formulation using a mixed acid solution developed by the Pacific Northwest National Laboratory operates over a wider temperature range allowing for passive cooling.
VRFBs can be used at depth of discharge (DOD) around 90% and more, i.e.
deeper DODs than solid-state batteries (e.g. lithium-based and
sodium-based batteries, which are usually specified with DOD=80%). In
addition, VRFBs exhibit very long cycle lives: most producers specify
cycle durability in excess of 15,000-20,000 charge/discharge cycles.
These values are far beyond the cycle lives of solid-state batteries,
which is usually in the order of 4,000-5,000 charge/discharge cycles.
Consequently, the levelized cost of energy (LCOE, i.e. the system cost
divided by the usable energy, the cycle life, and round-trip efficiency)
of present VRFB systems is typically in the order of a few tens of $
cents or € cents, namely much lower than the LCOEs of equivalent
solid-state batteries and close to the targets of $0.05 and €0.05,
stated by the US Department of Energy and the European Commission
Strategic Energy Technology (SET) Plan, respectively.
The main disadvantages with vanadium redox technology are a relatively poor energy-to-volume ratio in comparison with standard storage batteries (although the Generation 3 formulation has doubled the energy density
of the system), and the aqueous electrolyte makes the battery heavy and
therefore only useful for stationary applications. Another
disadvantage is the relatively high toxicity of oxides of vanadium.
Numerous companies and organizations involved in funding and developing vanadium redox batteries include Avalon Battery, Vionx (formerly Premium Power), UniEnergy Technologies and Ashlawn Energy in the United States; Renewable Energy Dynamics Technology in Ireland; Enerox GmbH (formerly Gildemeister energy storage) in Austria; Cellennium in Thailand; Rongke Power in China; Prudent Energy in China; Sumitomo in Japan; H2, Inc. in South Korea; redT in Britain, Australian Vanadium in Australia, and the now defunct Imergy (formerly Deeya).
Lately, also several smaller size vanadium redox flow batteries were
brought to market (for residential applications) mainly from StorEn
Technologies (USA), Schmid Group, VoltStorage and Volterion (all three from Germany), VisBlue (Denmark) or Pinflow energy storage (Czechia).
Operation
Diagram of a vanadium flow battery
A vanadium redox battery consists of an assembly of power cells in which the two electrolytes are separated by a proton exchange membrane.
The electrodes in a VRB cell are carbon based; the most common types
being carbon felt, carbon paper, carbon cloth, and graphite felt.
Recently, carbon nanotube based electrodes have gained marked interest from the scientific community. Both electrolytes are vanadium-based, the electrolyte in the positive half-cells contains VO2+ and VO2+ ions, the electrolyte in the negative half-cells, V3+ and V2+ ions. The electrolytes may be prepared by any of several processes, including electrolytically dissolving vanadium pentoxide (V2O5) in sulfuric acid (H2SO4). The solution remains strongly acidic in use.
In vanadium flow batteries, both half-cells are additionally
connected to storage tanks and pumps so that very large volumes of the
electrolytes can be circulated through the cell. This circulation of
liquid electrolytes is somewhat cumbersome and does restrict the use of
vanadium flow batteries in mobile applications, effectively confining
them to large fixed installations.
When the vanadium battery is being charged, the VO2+ ions in the positive half-cell are converted to VO2+
ions when electrons are removed from the positive terminal of the
battery. Similarly in the negative half-cell, electrons are introduced
converting the V3+ ions into V2+. During discharge this process is reversed and results in a typical open-circuit voltage of 1.41 V at 25 °C.
Other useful properties of vanadium flow batteries are their very
fast response to changing loads and their extremely large overload
capacities. Studies by the University of New South Wales have shown that
they can achieve a response time of under half a millisecond for a 100%
load change, and allowed overloads of as much as 400% for 10 seconds.
The response time is mostly limited by the electrical equipment. Unless
specifically designed for colder or warmer climates, most sulfuric
acid-based vanadium batteries only work between about 10 and 40 °C.
Below that temperature range, the ion-infused sulfuric acid
crystallizes. Round trip efficiency in practical applications is around 65–75 %.
Proposed improvements
Second generation vanadium redox batteries (vanadium/bromine) may approximately double the energy density and increase the temperature range in which the battery can operate.
Specific energy and energy density
Current
production vanadium redox batteries achieve a specific energy of about
20 Wh/kg (72 kJ/kg) of electrolyte.
More recent research at UNSW indicates that the use of precipitation
inhibitors can increase the density to about 35 Wh/kg (126 kJ/kg), with
even higher densities made possible by controlling the electrolyte
temperature. This specific energy is quite low compared to other rechargeable battery types (e.g., lead–acid, 30–40 Wh/kg (108–144 kJ/kg); and lithium ion, 80–200 Wh/kg (288–720 kJ/kg)).
Mechanisms of Electrode Permeation by Electrolyte
A
number of research groups worldwide have reported capacity loss in
VRFBs over prolonged periods of use. While several causes have been
considered, the influence of electrode microstructure on cell
electrochemistry within the electrode is poorly known. Electrolytic
wetting of carbon electrodes in VRFBs is important for overcoming
sources of degradation and applying appropriate operational procedures.
Recently, it appears that electrolytic wetting behaviour within the
electrode may be influenced by local concentration effects as well as
capillary action. Rapid wetting or permeation may also leave behind undissolved gases which could cause electrode degradation.
Applications
The
extremely large capacities possible from vanadium redox batteries make
them well suited to use in large power storage applications such as
helping to average out the production of highly variable generation
sources such as wind or solar power, helping generators cope with large
surges in demand or leveling out supply/demand at a transmission
constrained region.
The limited self-discharge characteristics of vanadium redox
batteries make them useful in applications where the batteries must be
stored for long periods of time with little maintenance while
maintaining a ready state. This has led to their adoption in some
military electronics, such as the sensor components of the GATOR mine system.
Their ability to fully cycle and stay at 0% state of charge makes them
suitable for solar + storage applications where the battery must start
each day empty and fill up depending upon the load and weather. Lithium ion batteries,
for example, are typically damaged when they are allowed to discharge
below 20% state of charge, so they typically only operate between about
20% and 100%, meaning they are only using 80% of their nameplate
capacity.
Their extremely rapid response times also make them superbly well suited to uninterruptible power supply (UPS) type applications, where they can be used to replace lead–acid batteries
and even diesel generators. Also the fast response time makes them
well-suited for frequency regulation. Economically neither the UPS or
frequency regulation applications of the battery are currently
sustainable alone, but rather the battery is able to layer these
applications with other uses to capitalize on various sources of
revenue. Also, these capabilities make vanadium redox batteries an
effective "all-in-one" solution for microgrids that depend on reliable
operations, frequency regulation and have a need for load shifting (from
either high renewable penetration, a highly variable load or desire to
optimize generator efficiency through time-shifting dispatch).
Largest vanadium grid batteries
A 200 MW, 800 MWh (4 hours) vanadium redox battery is under construction in China; it was expected to be completed by 2018 and its 250 kW/ 1MWh first stage was in operation in late 2018.
