1. Mainstream Energy Storage Features and Technological Status

Pumped hydro storage is currently the most mature electrical energy storage technology, holding the largest installed capacity, accounting for 99% of the total domestic energy storage installations. However, its future development is limited due to geographical siting and construction challenges. Compressed air energy storage, flywheel energy storage, and electrochemical energy storage technologies are high-growth areas for the future. The diversity and flexibility of these technologies will play an important role in the optimization and intelligent development of our power systems. Mainstream energy storage technologies, aside from pumped hydro storage, include:

(1)Compressed Air Energy Storage

Compared to other storage technologies, compressed air energy systems have large capacity, long operational duration, numerous charge-discharge cycles, and extended lifespan. They have proven commercial operation projects and can be implemented on a large grid scale. However, they have relatively low efficiency and demanding site requirements, with traditional technologies relying on fossil fuels. To address these issues, advanced adiabatic compressed air energy storage, liquid air energy storage, supercritical compressed air energy storage, and renewable energy-coupled compressed air energy storage have become key research and development areas domestically and internationally.

(2)Flywheel Energy Storage

Flywheel energy storage, especially high-speed systems, offer high power density, long lifespan, real-time charge status monitoring, and insensitivity to environmental temperatures. A major downside is significant self-discharge. Furthermore, flywheels are expensive for energy-type applications, which restricts their development in these areas.

(3)Lead-Acid Batteries

Traditional lead-acid batteries are maintenance-free, perform well under extreme temperatures, withstand overcharging, and have excellent charge acceptance and battery uniformity. Lead-carbon batteries, evolved from traditional lead-acid technology, incorporate activated carbon in the negative electrode, offering several advantages over traditional lead-acid batteries:

l Faster charging, increasing charge speed by eight times;

l Triple the discharge power;

l Six times the cycle life;

l Higher cost-effectiveness;

l Safe and stable for widespread use in various new energy and energy-saving fields.

(4)Lithium-Ion Batteries

Compared to other traditional batteries, lithium-ion batteries have several advantages:

l High safety and strong environmental adaptability;

l Environmentally friendly;

l High energy storage efficiency, with excellent energy retention and recovery;

l Long lifespan, reducing maintenance costs;

l Stable discharge platform, providing consistent output voltage;

l Abundant raw material resources.

(5)Sodium-Sulfur Batteries

Sodium-sulfur batteries have high energy density, good power characteristics, and a long cycle life. They have become one of the most mature large-scale megawatt-level chemical storage technologies, already achieving commercial operation. However, manufacturing costs, long-term reliability, and scalability remain significant challenges for widespread application.

(6)Flow Batteries

Flow batteries, unlike conventional batteries with solid electrodes, use liquid electrolytes. Due to the large volume of electrolyte that can be stored externally and pumped into the battery, flow batteries offer flexible power and capacity configurations, allowing for significant scaling. Vanadium redox and zinc-bromine are two types of flow batteries widely used in energy storage. Vanadium redox batteries have a long lifespan, with over 10,000 cycles, but lower energy and power density compared to other batteries. Zinc-bromine batteries offer higher energy density and lower cost, but suffer from high self-discharge rates due to electrode reaction by-products.

(7)Supercapacitors

Supercapacitors feature fast charging and discharging, high power density, long cycle life, high efficiency, and safe operation, making them excellent for power-type storage systems suitable for frequency regulation and improving power quality. However, their relatively low energy density means they are often used in conjunction with other storage technologies.

2. Energy Storage Technology Development Trends

Currently, energy storage technology is primarily focused on lithium-ion, lead-acid, sodium-sulfur, and flow batteries. These technologies have seen megawatt-scale demonstration applications in renewable energy integration, distributed generation, and microgrids, as well as in frequency regulation services, power transmission and distribution, and electric vehicles.

From 2016 to 2020, technological developments are expected to continue to focus on these batteries, maximizing their value in respective applications. At the same time, demonstrations and commercial applications of supercritical compressed air storage, flywheel storage, and supercapacitors will be conducted.

Post-2020, lithium-ion, lead-acid, sodium-sulfur, and flow batteries are expected to achieve commercial application, while new generations of improved storage technologies are developed and industrialized.

3. Safety and Reliability of Energy Storage Systems

Evaluation indicators for energy storage systems include safety, economy, reliability, efficiency, and ease of operation and maintenance. Safety is the most important, encompassing electrical safety, battery safety, functional safety, transport safety, electromagnetic compatibility, environmental protection, and grid connection protection. Here are some key aspects:

Electrical Safety

As energy storage technology advances, individual system capacity increases, and system voltages have shifted from safe low-voltage systems (≤60 Vd.c.) to higher voltages, reaching 1000Va.c. and 1500Vd.c. While this increase brings cost and efficiency advantages, it also highlights electrical safety issues.

Battery Safety

The high reactivity of lithium ions makes lithium-ion batteries prone to fires and explosions. With large capacities and high voltages, energy storage systems must ensure battery safety through design, manufacturing, quality control, and extensive testing.

Functional Safety

Due to the unique nature of energy storage products, their safety requires multiple protective features, including voltage protection, current protection, temperature protection, communication detection, grid interface protection, and on/off-grid switching functions.

Electromagnetic Compatibility

Energy storage systems contain numerous electronic components, leading to electromagnetic compatibility issues. These systems must function properly within a certain electromagnetic environment without causing unacceptable electromagnetic disturbances.