Flow battery
The ( redox ) flow battery or redox flow cell ( Red for reduction = electron uptake, Ox = oxidation electron donation ) - general also called liquid or wet cell battery - is an embodiment of an accumulator. Stores electrical energy into chemical compounds, the reactants are present in a solvent in dissolved form. The energy-storing two electrolytes circulate into two separate circuits, between which is carried out in the cell by means of a membrane of the ion exchange.
Due to the elaborate compared to a simple battery design that requires at least two pumps for the circulation of the electrolytes to flow batteries are not suitable for storing electricity for small consumers, ie not for mobile electronics and not for home use. For larger storage facilities, such as in association with a wind farm, but they are believed to be more appropriate than, for example, Lithium -ion batteries containing more expensive organic electrolytes, while flow cells with inexpensive aqueous solutions can be constructed. The suitability of the redox flow battery for electric mobility is still being researched; the advantage here would be that a refueling the battery with liquid electrolyte would be possible, similar to the modern refueling with fuels. However, the energy density of such systems for electric cars is not yet high enough so that they were at best suitable for bus fleets. The hitherto most commonly used and therefore most important type of flow cell is vanadium redox battery.
The redox flow cell is basically the fuel cell, but also with the batteries used ( electrochemical reversibility ). The cell voltage is given by the Nernst equation and is practical systems from 1.0 to 2.2 V.
- 3.1 Previous use
- 3.2 future
History
The basis for redox flow cells were developed mid-20th century in Germany, as the energy storage options were tested with redox couples for the first time. In the 1970s, NASA was concerned with the development of technology. The pure vanadium solution was first proposed in 1978, developed in the 1980s at the University of New South Wales from Mary Skyllas - Kazacos and their employees. This solution was patented in 1986 and is so far the most widespread. She learned a development of vanadium - bromide -based cell that allows twice as high energy densities.
Technology
Construction
The energy-storing electrolytes are stored outside the cell into separate tanks. Thus, the redox flow cell - as the fuel cell - an electrochemical energy storage in the amount of energy and power can be scaled independently of each other. The tanks could be filled manually and the battery are thus charged; in practice, however, the systems are designed with closed circuits as possible. By replacing the electrolyte liquid they can be charged and discharged in separate batteries; Thus, not the entire battery converter technology and the housing, but only the actual energy between the loading and unloading need not be replaced.
The galvanic cell is divided by a membrane into two half cells. On the membrane, the electrolyte flows over. The half- cell is defined by an electrode where the actual chemical reaction ( reduction or oxidation ) occurs.
The membrane is either a microporous separator which allows all ions to pass, or a selective anion or cation exchange membrane. The membrane is to prevent the mixing of the two electrolytes.
The electrodes are due to their high electrochemical window in aqueous solutions usually made of graphite. For a very high specific power graphite felts can be used with a high specific surface area as the electrode material.
Electrolyte
The electrolyte is dissolved in a solvent, salts. The composition of the electrolyte, specifically the concentration largely determines the cell voltage, the energy density of redox flow battery. Either inorganic or organic acids are used as solvent. Useful as redox compounds of titanium, iron, chromium, vanadium, cerium, zinc, bromine and sulfur are well known.
Properties
The redox flow cell can provide services from a kilowatts to several megawatts. Compared to other memory technologies, it has high efficiency, good preventability of self-discharge rate and high life expectancy. The latter is based on that the electrode material in the reaction of the electrolyte itself does not react chemically, and thus does not degenerate. In contrast, the energy density is comparatively quite small (up to 70 watt hours of energy per liter of electrolyte in a liquid vanadium bromide compound as the previously most effective chemical).
In the development of the following properties are desired:
- Energy density similar to that of conventional car batteries
- No or minimal self-discharge
- Efficiency well above 75 %
Applications
Previous use
Due to the properties, the redox flow cell is previously used in particular as a reserve source, backup battery and uninterruptible power supply. Thus, redox flow cells are used in the form of the vanadium redox battery as a backup source for mobile phone base stations or backup battery for wind turbines. The largest system of this type is used in a Japanese wind turbine and has a capacity of 4 MW and a storage capacity of 6 MWh
Future
The largest under construction flow cell battery is made by a Japanese company (Sumitomo Electric Industries Ltd.. ) For a Japanese power company ( Hokkaido Electric Power Co.) built in 2015 to go into operation and then can store 60 MWh of energy. The nominal capacity of the battery that is installed in a new building on two floors should be 15 MW. In Germany, a 20 - MWh -scale battery storage with 2 MW capacity, directly coupled to the DC link of a wind turbine, built, where the tanks are located in Japanese investment in the basement of the battery building, the cells on the upper floor.
The redox flow cell has been proposed also as energy storage for future electric cars; A prototype equipped with a redox flow battery sports sedan was introduced in 2014. In conventional electric vehicles in service, fast charging a significant challenge for redox flow batteries on the other hand, the " charging" simply by replacing the fluids done, ie by about refueling at a specially equipped gas station. The actual loading process then takes place outside the vehicle, and the handling would be similar to today's refueling at the pump. Against such a process is currently still speaks the relatively high power to weight ratio as compared to modern lithium - ion batteries, but the good suitability of an appropriate infrastructure for energy storage on the grid.
Another application arises from the fact that all cells use the same electrolyte, thus loading and unloading can be done with different number of cells. This allows powerful voltage converter ( DC-DC converter ) to build, or even the power output depending on the interconnection of existing cells, such as vehicles, control.
In January 2014, researchers from Harvard University presented a redox flow cell based on organic quinones, the rare and therefore relatively expensive manages substances without use. Prototypes with power densities of 600 milliwatts per square centimeter were measured. The main advantage of this technique the lower costs were mentioned, which are intended to be in mass production at a third of vanadium- based cells. Currently, such cells being researched at the long-term stability.