“Next Generation Batteries” for residential & commercial applications
The resource-saving vanadium redox flow technology is an important component of VoltStorage’s multi-technology approach. With the continuous development of this storage technology, VoltStorage is making important contributions to the development of pioneering stationary battery solutions.
The storage medium is a vanadium-based liquid electrolyte. It is stored in two separate tanks takes on different oxidation states through a redox process. This process makes storing energy possible.
The electrolyte flows through the battery cells in two separate circuits. When the battery is charged, the electrolyte is reduced in the negative half cell and oxidized in the positive half cell. When discharging, this process is reversed again.
A special pump system flows the electrolyte into and through the battery cells. This makes it possible that not all the electrolyte has to be kept in the cells, enabling the decoupling of energy capacity and power. The pumping system is only activated on demand.
Discharged, the positive electrolyte circuit contains vanadium ions with an oxidation state +4. In this state the electrolyte exhibits a blue colour. During charging, the vanadium ions oxidize and take on the oxidation state +5.
When the storage system is fully charged, the positive electrolyte circuit only contains vanadium +5. The electrolyte then has a yellow colour.
Discharged, the negative electrolyte circuit contains vanadium ions with an oxidation state of +3. In this state the negative electrolyte exhibits a blue-green colour. During charging, the vanadium ions reduce and take on the oxidation state +2.
When the storage system is fully charged, the negative electrolyte circuit only contains vanadium +2. The electrolyte then has a purple colour.
The positive and negative electrolytes are pumped through special battery cells for the charging and discharging processes. Each battery cell consists of two half-cells separated by a selective membrane. The membrane is ion-permeable so that during charging and discharging, freed-up ions can migrate through the membrane into the other electrolyte circuit. However it is selective, so that the vanadium ions can not pass. Inside of each half cell, the electrolytes undergo redox reactions which ensure that electrical energy is converted into chemical energy and stored.
A pump system moves the positive and the negative electrolytes within each circuit, supplying the battery cells with fresh electrolyte from the storage tanks.
The bipolar plate electrically connects the cells of a redox-flow battery stack. It thus provides the electrical conductivity, which is increased from a material standpoint by using particularly conductive and corrosion-resistant materials, such as graphite.
Ion exchange membrane
The transparent and thin ion exchange membrane separates the positive and negative half cells. The membrane ensures selective ion exchange between the half-cells, preventing transfer of vanadium ions between half cells, which is necessary for efficient charging and discharging.
The electroactive area is located in the centre and is completely filled with a graphite felt. The fine-meshed felt has a very large surface area and thus improves the electrochemical redox process while at the same time allowing the electrolyte to flow through. Direct contact with the ion exchange membrane results in a charge exchange between the differently charged half cells.
Cell frame with electrolyte channels
The cell frame of the redox flow battery cells from VoltStorage contain many electrolyte-guiding channels. These channels have been specially designed to ensure uniform supply of electrolyte to the electroactive area of the battery cells.
The vanadium redox flow technology does not require any rare earths. The vanadium used in the storage medium is obtained as a by-product of the iron production – and thus without overexploitation of nature and the associated impact on our ecosystems.
Operational safety plays a key role in energy storage. The VRF technology offers a decisive advantage in this respect: The vanadium-based electrolyte consists exclusively of non-combustible components, and is mostly pure water. VRF storage systems are therefore 100 percent non-flammable and offer more operational safety than other storage solutions.
With the vanadium redox flow technology, we rely on a proven CO2-reducing battery technology. A study by the Technical University of Munich has shown that VRF batteries produced by VoltStorage result in up to 37% less CO2 emissions than the production of comparable lithium batteries.
Since the market launch of our residential flow battery system in 2019 , we have constantly increased the number of installed systems. Meanwhile, we have the largest fleet of operating flow batteries in the world.