Abstract:
This paper discusses the current state of energy storage, elucidates the technical advantages and challenges faced by zinc-iron flow batteries, and provides an in-depth analysis of their application advantages in the field of energy storage, along with future prospects. Zinc-iron flow batteries, with their low cost, excellent performance, and abundant raw material sources, are poised for large-scale application in the energy storage sector, significantly enhancing energy use efficiency.
Keywords: Flow Batteries; Zinc-Iron Flow Batteries; Energy Storage Technology
1. Introduction
Energy storage has emerged as a crucial component in the transition towards a sustainable energy future. As of the end of 2022, the cumulative installed capacity of global power generation and energy storage projects (including physical energy storage, electrochemical energy storage, and molten salt thermal storage) stood at approximately 237.2 GW, with an annual growth rate of 15%. China, in particular, has seen its cumulative installed capacity exceed 59.8 GW, marking an annual growth rate of 38%. Notably, the cumulative installed capacity of new energy storage projects in China surpassed the 10 GW milestone, reaching 13.1 GW/27.1 GWh, with annual growth rates of 128% for power scale and 141% for energy scale. Lithium-ion batteries dominate the electrochemical energy storage sector, accounting for 98% of the market share. However, other technologies such as compressed air energy storage, flow batteries, sodium-ion batteries, and flywheels have also witnessed breakthroughs in scale. Among these, electrochemical energy storage has emerged as one of the fastest-growing directions in the energy storage field.
2. Overview of Zinc-Iron Flow Battery Energy Storage Technology
2.1 Structure and Operating Principle
A zinc-iron flow battery cell consists of a positive electrode, a negative electrode, and a separator. The positive electrode undergoes the interconversion between ferrous and ferric ions, while the negative electrode undergoes the deposition and dissolution of zinc. Internal ion exchange is facilitated through the separator. The battery operates by pumping energy storage substances stored in electrolyte tanks to the reaction site—the stack—where chemical energy is converted to electrical energy and vice versa. The stack, serving as the conversion site for chemical and electrical energy, is composed of several cells connected in series, which are further connected in series and parallel to form the energy storage system.
2.2 Advantages of Flow Batteries
One of the greatest advantages of flow batteries over traditional batteries is the independence of their energy unit from the power unit. This allows for independent scaling of either the energy unit or the power unit in established projects to adjust system scale according to project requirements.
Table 1: Comparison of Various Energy Storage Technologies
| Type | Typical Power Rating | Typical Energy Capacity | Full Response Time | Cycle Life (times) | Cycle Efficiency (%) | Advantages | Disadvantages | Application Scenarios |
|---|---|---|---|---|---|---|---|---|
| Mechanical Energy Storage | 100-200 MW | 4-10 h | Minutes | Unlimited (device lifespan) | 70-85 | Suitable for large scale, technologically mature | Slow response, requires geographical resources | Daily load regulation, frequency control, and system backup |
| Compressed Air Energy Storage | 10-300 MW | 1-2 h | Minutes | Unlimited (device lifespan) | ≥70 | Long lifespan, suitable for large scale | Slow response, requires geographical resources | Peak shaving, system backup |
| Flywheel Energy Storage | 5 kW – 1.5 MW | 15 s – 15 min | Ten milliseconds | ≥20,000 | 85-90 | High specific power | High cost, noisy | Peak shaving, frequency control, UPS, and power quality |
| Electrochemical Energy Storage | Lead-Acid Batteries | kW-50 MW | 1 min – 3 h | Hundreds of milliseconds | 500-1200 | Technologically mature, cost-effective | – | – |
(Note: Additional rows for other technologies can be added as needed, but for brevity, only lead-acid batteries are shown here as an example from the electrochemical category.)
2.3 Advantages of Zinc-Iron Flow Batteries
Zinc-iron flow batteries offer numerous advantages, especially when compared to vanadium redox flow batteries (VRBs). These include:
- System Aspects: Zinc-iron flow batteries operate in an alkaline system, while VRBs operate in an acidic system. The sealing and stability of zinc-iron flow batteries are more conducive to industrialization.
- Electrode Materials: Zinc-iron flow batteries use various materials as electrodes, while VRBs use carbon felt for electrodes and conductive plastics for bipolar plates. This allows zinc-iron flow batteries to have a higher power density and superior rate performance compared to VRBs.
- Electrolyte Raw Materials: Vanadium accounts for only 0.02% of the global crustal content, while iron and zinc account for 4.75% and 0.5%, respectively. Vanadium resources could become a bottleneck restricting the large-scale development of VRBs.
- Safety: The elements zinc and iron used in zinc-iron flow batteries are essential trace elements for the human body, whereas vanadium in high valence states can affect the respiratory system, nervous system, gastrointestinal tract, and skin.
Table 2: Advantages, Disadvantages, and Open Circuit Voltages of Various Flow Batteries
| Flow Battery Type | Advantages | Disadvantages | Open Circuit Voltage (V) |
|---|---|---|---|
| Zinc-Iron Flow Battery | Flexible capacity ratio, good safety performance, high system rate performance | – | – |
| Vanadium Redox Flow Battery | Proven technology, long cycle life | High cost due to vanadium resource scarcity | – |
| Other Flow Batteries (e.g., Zinc-Bromine) | – | Various challenges depending on specific technology | – |
3. Application of Zinc-Iron Flow Batteries in Energy Storage Systems
Energy storage can be applied in various aspects of the power system, including the grid side, generation side, and user side. As one of the important equipment in energy storage systems, zinc-iron flow batteries play a significant role due to their safety, environmental friendliness, and excellent rate performance.
3.1 Application on the Grid Side
Integrating energy storage devices into the transmission grid can increase its transmission capacity. Similarly, in the distribution grid, energy storage systems can further enhance safety and economic benefits. They can serve as alternative measures for transmission grid upgrades, enhancing the transmission capability of critical transmission pipelines and sections or improving the safety and management level of national grid operations. However, due to land and environmental constraints, the increasing demand for renewable energy generation and load expansion has outpaced the capacity of transmission and distribution infrastructure. Large-scale energy storage devices can be configured in the transmission grid to enhance its output capacity, thereby reducing investments in transmission and substation equipment.
3.2 Application on the Generation Side
On the generation side, energy storage technologies can address issues such as wind and solar curtailment, while also tracking planned output and smoothing output fluctuations. For example, the integration of zinc-iron flow batteries can help balance power output fluctuations and enhance the friendliness of renewable energy generation.
3.3 Application on the User Side
Energy storage technologies can be widely used to improve power efficiency and safety, as well as to achieve local consumption of distributed resources. For business customers with renewable energy facilities, integrating energy storage systems can flatten the oscillation caused by renewable energy generation, improve energy quality, and realize peak-shaving and valley-filling to capitalize on price differences. For residential users with photovoltaic installations, home energy storage can effectively utilize photovoltaic power generation, even achieving self-sufficiency. Additionally, during distribution grid faults, home energy storage can continue operation, reducing power outages and enhancing grid stability.
4. Prospects for the Application of Zinc-Iron Flow Batteries in Energy Storage Systems
The advantages of zinc-iron flow batteries in energy storage systems are apparent, mainly manifested in the following three points:
- Cost Reduction Potential: Due to the extremely low cost and safety of the materials used in flow batteries, there is significant potential for commercialization on a large scale.
- Excellent Rate Performance: By selecting electrode materials with high conductivity, thermal conductivity, and corrosion resistance, zinc-iron flow batteries can operate at high current densities for extended periods. This makes them well-suited for various energy storage applications, such as grid peaking and load following.
- Ease of Maintenance: The variety of electrode materials used in zinc-iron flow batteries facilitates easier maintenance and repair, paving the way for large-scale operation.
Conclusion
As a promising energy storage battery capable of large-scale application, zinc-iron flow batteries are poised for large-scale commercialization due to their low cost, excellent system performance, and abundant raw material sources. They are expected to receive increasing attention and will likely be widely used in various fields of energy storage in the near future, significantly enhancing energy use efficiency.