This week we look at using batteries for temporary storage of renewable electricity. The installed battery capacity is still small compared to pumped hydro, but it is the fastest growing solution, and the most flexible.
Battery is a generic term for electrochemical storage devices that store electricity through reduction and oxidation of metals or other materials. These ‘redox’ processes occur all around us, from the corrosion of metals to the fundamentals of life. Redox processes pull electrons from one material (oxidation) and pass them to another material (reduction). There is an energy difference between the reduced and oxidised states. A chemical potential difference, harvestable as energy from a battery or the electron transport chain of mitochondria found in the cells of complex life.
What are batteries?
In 1799, Alessandro Volta developed the first portable electrical source by stacking copper and zinc discs separated by cardboard or felt spacers soaked in salt water. The battery works because zinc easily gives up electrons (as a reducing agent), and copper takes up electrons (as an oxidising agent). We have all seen the school science experiment of sticking to dissimilar metals into a lemon, getting a voltage difference and seeing a current flow. Since Volta, a wide range of different battery chemistries have been used in a bewildering variety of physical forms, but the core idea is the same – use a chemical potential difference to drive electrons through an external circuit.
Some battery chemistries are one way. Once the chemical reaction has completed, you can’t reverse it. Other chemistries can be driven both forwards and backwards, and that allows the rechargeable batteries we are interested in for energy storage.
Rechargeable Battery Mechanism
All types of rechargeable battery are built out of the same simple components. There are two conducting electrodes. A negatively charged anode, and a positively charged cathode, separated by a permeable barrier such as a polymer film. An electrolyte that allows charged ions to move through it fills the rest of the gap between the electrodes. A charging circuit pushes electrons into the anode, withdrawing them from the cathode. Ions move from one side of the cell to the other, balancing the charges. Once you disconnect the charging circuit, there is an imbalance in the charges across the cell, creating a potential difference between the two electrodes. Connect the cell to an external circuit, and the electrons and ions will flow back in the opposite direction, releasing the stored energy. You can stack individual cells together to get a higher voltage or to store more electricity.
Flow batteries are an interesting variant of conventional rechargeable batteries. In flow batteries, the electroactive materials are not stored within the electrode compartments, but dissolved in electrolyte solutions kept in separate tanks. The power generation stack consists of positive and negative electrode compartments divided by an ion-permeable separator. Electrolyte solutions flow through this stack when the battery is being charged or discharged. Because energy and power are decoupled, flow batteries have high-capacity, long life, high efficiency, and can be fully discharged. Flow batteries are suitable for large-scale applications because they are modular and scalable.
Where are batteries used in energy storage?
Battery storage systems are both scalable and flexible. They have pretty good energy density in Wh/kg, reasonably fast charge and discharge times, and have become a lot cheaper over recent years. Batteries have found many applications in energy storage, from portable electronics all the way through battery electric vehicles to grid-scale systems. This makes them the fastest growing energy storage technology; vital to plans for delivering zero-carbon electricity grids. Most of the current commercial applications are for lithium ion (Li-ion) batteries.
Falling Price of Li-ion Batteries
With growing demand and improving technology, the cost of Li-ion batteries has fallen from $732/kWh in 2013 to $141/kWh in 2021. Because of supply constraints, prices increased in 2022 to $151/kWh but with new manufacturing capacity coming on stream, prices will drop again by 2024.
Battery Stacks at Moss Landing Battery Energy Storage Facility, California – image LG Energy Systems
Lower prices mean wider use, and Li-ion batteries find application at every scale from a single home (e.g. the 13.5 kWh Tesla Powerwall) to the current world’s largest battery storage facility at Moss Landing in California, capable of storing 1,600 MWh and delivering 400 MW.
The Johan Cruyff Arena in Amsterdam is an example of medium scale implementation of battery storage. The roof has over 4000 PV panels to provide renewable energy for the arena services. Buried under the playing area are 148 second hand Nissan Leaf EV batteries. These store 2.8 MWh and power the arena in the evenings or at night. The system can provide peak shaving services to the local grid, support other local businesses and EV charging in the car park. Using ‘second-life’ EV batteries will be common, as multiple TWh of older batteries come onto the market in the next few years.
Li-ion batteries don’t have it all their own way. China has just commissioned a 100 MW/400 MWh vanadium redox flow battery, with further systems both smaller and larger in planning and development.
Growth in Grid-Scale Battery Storage
Although starting from a small base, grid scale battery storage has been booming. An extra 6.5 GW was added in 2021. The IEA Net Zero Scenario projects 680 GW battery storage added by 2030. In this scenario, battery storage will overtake pumped hydro somewhere between 2025 and 2030.
Despite the rapid growth, batteries on their own are not the complete answer to grid scale energy storage. Batteries cannot be charged and discharged fast enough to meet grid requirements such as frequency stabilisation. The lifetime and performance of batteries suffers if they are rapidly cycled, allowed to overheat, or completely drained. This has led to a lot of interest in hybrid energy storage systems (HESS). Combining flywheels, supercapacitors or superconducting magnets with batteries provides the high power and fast response of the first group of technologies with the greater capacity of batteries.
Next steps for batteries
Everybody wants to make batteries cheaper, safer, longer life and with greater energy density. There are two research approaches; take an existing chemistry, such as Li-ion, and improve it, or commercialise an alternative chemistry.
The Li-ion battery has three problems; they don’t like being charged too fast or overcharged, and current electrolytes are flammable.
Current research focuses on new electrode materials and ways of manufacturing the electrochemical cells to increase battery capacity and life. New types of non-flammable electrolytes based on silicone polymers are being commercialised, and many believe that solid electrolytes with high lithium ion mobility will provide the next generation of batteries.
Sodium, potassium, aluminium, and zinc ion batteries and lithium-sulphur batteries are all actively researched as alternative chemistries to the current dominant Li-ion. Each has their advantages and disadvantages, and their enthusiasts and champions. Which battery technologies will ultimately win out is unclear, but something will challenge Li-ion.
Flow batteries have the advantages of high capacity, long life, high efficiency and can be completely discharged. However, they are expensive, have a low energy density, and many chemistries are both toxic and aggressive. Better electrode materials and separators are key research targets for aqueous electrolyte systems. Others are looking at non-aqueous electrolyte systems with higher energy densities (StorTera), or suspended nanoparticles (Influit Energy). Flow batteries have so much potential that we can expect multiple new technologies to be commercialised soon.
Battery energy storage systems will continue to spread rapidly, and new technologies will reduce cost, and increase lifetime, capacity and safety. Somewhere in the second half of this decade, batteries will become the dominant electricity storage technology.
Next up – Power to X: other forms of chemical storage
- Electricity Storage – the Options
- Electricity Storage Options 2 – Using Gravity
- Electricity Storage Options 3 – Springs and Tops
- Electricity Storage Options 4 – Turning up the Heat
- Electricity Storage Options 5 – Pure Electricity
- Electricity Storage Options 7 – Power-to-X
- Electricity Storage Options 8 – So What Have We Learned?
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