VoltStorage, a German energy storage startup founded in 2016, has developed vanadium redox flow battery technology for industrial and agricultural sectors to meet their energy requirements during periods of low wind and low sun. Meanwhile, the company develops a new cost-effective iron redox flow battery system with a 70% energy efficiency and 20-year lifespan. Its iron flow battery is also temperature-resistant, making it a promising energy storage solution that can be used globally to ensure renewable base load provision.
Challenges: Store renewable energy at a low cost
Redox flow batteries for renewable energy storage
By 2022, the world had about 1,185 GW of installed solar capacity and about 906 GW of installed wind capacity. Solar power has grown at a 24% annual rate over the last decade. The US solar industry installed 6.1 GW of capacity in the first quarter of 2023, 47% more than in the first quarter of 2022. On the other hand, the average power of new wind turbines installed in the US in 2022 was 3.2 MW, 7% higher than in 2021.
As more and more solar cells and wind turbines are installed, the ability to store this clean electricity would significantly increase the efficiency and reliability of these intermittent renewable energy sources.
Flow batteries store and release electrical energy through the movement of electrolytes. A typical flow battery consists of two half-cells separated by a membrane. Each half-cell contains an electrode and an electrolyte. The anolyte and catholyte contain redox-active species dissolved in an aqueous or non-aqueous solvent. They are stored outside of the battery. When the battery is charged or discharged, the electrolytes flow through the half-cells in contact with the electrodes via external pumps. This is very different from lithium-ion batteries.
The majority of flow battery systems are based on vanadium, Fe—Cr, and Zn—Br. They are simple systems and easy to scale up. They can have a long lifespan of more than 20 years. This makes them good candidates for storing renewable energy.
However, the high cost of vanadium salts, as well as the capacity decay and toxicity of the last two systems, has limited their application worldwide. The capital cost of flow batteries is around $800 per kilowatt-hour, which is more than twice as much as lithium-ion batteries.
Low-cost, eco-friendly iron flow batteries
The low-cost, eco-friendly iron flow battery uses iron and cheap aqueous electrolytes that are made of earth-abundant iron salts. The anolyte contains Fe²⁺ / Fe³⁺ redox couple and the catholyte contains Fe²⁺. To make the conductivity even better, the electrolytes also have chloride salts like potassium chloride (KCl), ammonium chloride (NH₄Cl), and sodium chloride (NaCl).
However, an iron flow battery generally has a parasitic reaction that releases hydrogen gas during its operation. This hydrogen gas evolution reaction causes an increase of the pH value of the catholyte and an imbalance in the ratio of iron oxidation states in the catholyte to anolyte. Specifically, the state of charge (SOC) of the anolyte is higher than the SOC of catholyte. The capacity and life of a battery go down when the pH changes and the SOC of both electrolytes is out of balance.
VoltStorage Technology
VoltStorage develops a new iron flow battery system that can rebalance the SOC of anolyte and catholyte and restore the aqueous electrolytes to their initial state using simple and inexpensive means. This iron flow battery system has a primary flow cell and a rebalancing flow cell. The primary flow cell operates in charge and discharge modes, whereas the rebalancing flow operates only in discharge mode to reduce the SOC of the anolyte when needed. To restore anolyte and catholyte, the iron flow battery system also has additional tanks of HCl solution, base solution, iron powder, and reducing solution.
VoltStorage iron flow battery
The diagram below depicts the VoltStorage iron flow battery system.
Sorry, you need to purchase an energy storage member to view the following content. Already an energy storage member? Please login to view the full article.