Energy storage is a key component of the modern power system, as it can provide flexibility, reliability, and resilience to the grid. Energy storage can help balance the supply and demand of electricity, integrate renewable energy sources, reduce greenhouse gas emissions, and enhance power quality and security .
However, not all energy storage technologies are created equal. Some of them have limitations such as low energy density, high cost, short lifetime, or environmental impact. Therefore, there is a need for a new generation of energy storage devices that can overcome these challenges and meet the growing demand for clean and sustainable energy.
One of the most promising candidates for this role is the flow battery. A flow battery is a type of solar battery that uses liquid electrolytes stored in external tanks to store and release energy. Unlike conventional batteries, which have fixed amounts of energy and power, flow batteries can decouple these two parameters by adjusting the size of the tanks and the flow rate of the electrolytes .
Types of Flow Batteries
Flow batteries can be classified into different types based on the chemistry and configuration of their electrolytes. The most common types of flow batteries are:
- Vanadium flow batteries (VFBs): VFBs use vanadium ions in different oxidation states as the active species in both the positive and negative electrolytes. This eliminates the risk of cross-contamination and ensures high coulombic efficiency. VFBs have high energy density, long cycle life, and good stability, but they also have high cost, low power density, and limited temperature range.
- Zinc-bromine flow batteries (ZBBs): ZBBs use zinc and bromine as the active species in the negative and positive electrolytes, respectively. ZBBs have high energy and power density, low cost, and wide temperature range, but they also have low coulombic efficiency, high self-discharge, and high toxicity. ZBBs also require periodic electrode regeneration to prevent zinc dendrite formation and bromine depletion.
- Organic flow batteries (OFBs): OFBs use organic molecules, such as quinones or ferrocenes, as the active species in the electrolytes. OFBs have low cost, high abundance, and high tunability, but they also have low energy density, low stability, and low solubility. OFBs also face challenges such as complex synthesis, degradation, and crossover.
- Other emerging flow batteries: There are also other types of flow batteries that are under development, such as hydrogen-bromine, iron-chromium, iron-air, and all-iron flow batteries. These flow batteries aim to improve the performance, cost, and sustainability of flow batteries by using novel materials and mechanisms .
Applications of Flow Batteries
Flow batteries have a wide range of applications in different sectors and scenarios, such as:
- Utilities and grid-scale energy storage: Flow batteries can provide various services to the grid, such as frequency regulation, voltage support, spinning reserve, black start, and congestion relief. Flow batteries can also enable the integration of intermittent renewable energy sources, such as wind and solar, by smoothing out their fluctuations and storing their excess energy. Flow batteries can also reduce the peak demand and lower the electricity costs for both the utilities and the consumers.
- Microgrids and renewable energy integration: Microgrids are small-scale power systems that can operate independently or in coordination with the main grid. Microgrids can provide reliable and resilient power supply to remote areas, critical facilities, or communities. Flow batteries can enhance the performance and stability of microgrids by providing backup power, load shifting, and renewable energy integration. Flow batteries can also reduce the dependence on diesel generators and fossil fuels, and lower the carbon footprint of microgrids.
- Electric vehicles and transportation: Flow batteries can potentially power electric vehicles and other modes of transportation, such as buses, trains, ships, and airplanes. Flow batteries can offer long driving range, fast refueling, and high safety for electric vehicles. Flow batteries can also reduce the weight and volume of the battery pack, and extend the lifetime of the vehicle. Flow batteries can also enable the vehicle-to-grid (V2G) concept, where electric vehicles can supply power to the grid or other loads when parked or idle.
- Other potential applications: Flow batteries can also be used for other purposes, such as power quality improvement, uninterruptible power supply (UPS), military and defense, space exploration, and education and research. Flow batteries can also be combined with other energy storage technologies, such as supercapacitors, flywheels, or thermal storage, to create hybrid energy storage systems that can optimize the performance and cost of energy storage.
Flow Batteries vs. Lithium-ion Batteries
Lithium-ion batteries are the dominant energy storage technology in the market, as they are widely used in consumer electronics, electric vehicles, and grid-scale energy storage. Lithium-ion batteries have high energy and power density, high efficiency, and low self-discharge, but they also have high cost, limited lifetime, safety issues, and environmental impact.
Flow batteries and lithium-ion batteries have different strengths and weaknesses, and the comparison of them depends on the specific application and scenario. Here are some of the main aspects to consider when comparing flow batteries and lithium-ion batteries:
- Performance: Flow batteries have lower energy and power density than lithium-ion batteries, which means they need more space and weight to store the same amount of energy and power. However, flow batteries have higher energy capacity and longer duration than lithium-ion batteries, which means they can store and deliver more energy for longer periods of time，more suitable for long-term energy storage. Flow batteries also have higher coulombic efficiency and lower self-discharge than lithium-ion batteries, which means they can retain more of the stored energy and lose less energy over time.
- Efficiency and cost: Flow batteries have lower round-trip efficiency than lithium-ion batteries, which means they lose more energy during the charge and discharge cycles. However, flow batteries have lower levelized cost of energy (LCOE) than lithium-ion batteries, which means they have lower total cost per unit of energy delivered over their lifetime. Flow batteries also have lower capital cost and operating cost than lithium-ion batteries, as they have simpler and cheaper materials, components, and maintenance.
- Lifetime and degradation: Flow batteries have longer cycle life and calendar life than lithium-ion batteries, which means they can operate for more cycles and years without losing performance. Flow batteries also have minimal degradation and capacity fade than lithium-ion batteries, as they do not suffer from issues such as lithium plating, solid electrolyte interphase (SEI) formation, or dendrite growth. Flow batteries also have higher recyclability and reusability than lithium-ion batteries, as they can be easily refurbished or repurposed by replacing or replenishing the electrolytes.
- Safety and environmental impact: Flow batteries have higher safety and environmental friendliness than lithium-ion batteries, as they use non-flammable and non-toxic electrolytes that pose minimal risk of fire, explosion, or leakage. Flow batteries also have lower greenhouse gas emissions and water consumption than lithium-ion batteries, as they do not require high-temperature or high-pressure processes for manufacturing or operation. Flow batteries also have lower dependence on scarce and critical materials than lithium-ion batteries, such as cobalt, nickel, or lithium, which are subject to geopolitical and ethical issues.
Flow batteries and lithium-ion batteries have different pros and cons, and the best choice depends on the specific application and scenario. Flow batteries are more suitable for applications that require high energy capacity, long duration, long lifetime, low cost, and high safety, such as grid-scale energy storage, microgrids, or backup power. Lithium-ion batteries are more suitable for applications that require high energy and power density, high efficiency, and low weight and volume, such as consumer electronics, electric vehicles, or power quality improvement.
Flow batteries and lithium-ion batteries are not mutually exclusive, but rather complementary, as they can work together to create hybrid energy storage systems that can optimize the performance and cost of energy storage.
Conclusion
Flow batteries are a promising and emerging energy storage technology that can contribute to a greener and cleaner future. However, flow batteries still need more research and development, innovation and collaboration, and education and awareness to realize their full potential and overcome their limitations.
