Based on the form of energy storage, energy storage includes electrical energy storage, thermal energy storage, and hydrogen energy storage, with electrical energy storage being the most important. Electrical energy storage is further divided into electrochemical energy storage and mechanical energy storage based on different storage principles.
Different technical approaches have their own advantages and disadvantages, and are suitable for different application scenarios.
Electrochemical energy storage offers greater flexibility in rated power and stored capacity, but it generally presents safety or environmental challenges. It is primarily used for renewable energy integration, peak-valley price arbitrage, power system peak shaving and frequency regulation, and UPS systems. Mechanical energy storage typically has a longer lifespan, but its response time is significantly slower than electrochemical and electromagnetic energy storage. It is mainly used for power system peak shaving.
Solar power and energy storage are a perfect match. With the increasing proportion of renewable energy being integrated with energy storage, and the energy storage market poised for explosive growth, it is recommended that everyone in the solar power industry pay attention to and learn the basics of energy storage.
Today's article will share a comparison of the three major technological paths for energy storage.
I. Hydrogen Energy Storage
The basic principle of hydrogen energy storage is to electrolyze water to obtain hydrogen and store it. When electricity is needed, the stored hydrogen is converted into electricity through fuel cells or other means and transmitted to the grid.
Electrolysis of water to produce hydrogen requires a large amount of electricity, and the cost is much higher than that of traditional hydrogen production methods. However, due to the instability of renewable energy grid connection, my country has serious problems of wind and solar power curtailment. Using surplus electricity generated by wind and solar power to produce hydrogen can effectively solve the cost problem of electrolysis of water to produce hydrogen and solve the problem of wind and solar power consumption. Therefore, hydrogen energy storage is gradually becoming the focus of my country's energy technology innovation.
However, my country currently lacks convenient and effective hydrogen storage materials and technologies, and the energy conversion efficiency of hydrogen storage is relatively low. Therefore, its application is currently limited. Solving these two problems will be the key to whether hydrogen storage can gain a larger market share in the future.
II. Mechanical Energy Storage
Mechanical energy storage stores energy through physical methods and converts mechanical energy into electrical energy when needed. Mechanical energy storage mainly includes gravity storage, pumped hydro storage, flywheel energy storage, and compressed air energy storage.
1. Gravity energy storage
Gravity energy storage media are mainly divided into water and solid materials. The charging and discharging process of the energy storage system is realized by raising and lowering the energy storage media based on the height difference.
Besides the relatively mature pumped hydro storage, the mainstream gravity energy storage method is the energy storage tower proposed by Energy Vault (EV). This method uses cranes to stack concrete blocks into a tower, storing and releasing energy through the lifting and lowering of the concrete blocks. According to EV's official website, its energy storage tower has an energy efficiency of up to 90% and can continuously discharge at 4-8MW for 8-16 hours, achieving a high-speed response to grid demand.
2. Pumped storage
Pumped storage power stations consist of two reservoirs, an upper reservoir and an lower reservoir. During periods of low electricity demand, excess electricity is used to pump water into the upper reservoir. During peak periods, the water is released and used to generate electricity by utilizing the mechanical energy generated as the water flows from the upper reservoir to the lower reservoir, thus achieving peak shaving.
Pumped hydro storage enables large-scale energy storage and is therefore widely used for peak shaving in power systems. However, its future development potential is limited due to its slow response time, high initial investment, and geographical location constraints.
3. Flywheel energy storage
When storing energy, the flywheel drives the motor to run, and the motor drives the flywheel to rotate faster. The flywheel stores energy in the form of kinetic energy. When releasing energy, the high-speed rotating flywheel drives the motor to generate electricity, completing the conversion of mechanical energy into electrical energy.
Flywheel energy storage boasts a high power density, a lifespan of 15-30 years, and millisecond-level response speeds. Therefore, it is primarily used for frequency regulation and UPS systems. However, due to its low energy density and limited backup time (no more than 30 minutes), it cannot be applied to large-scale energy storage power plants.
4. Compressed air energy storage
Compressed air energy storage technology originates from gas turbine technology. During off-peak hours, an electric motor drives a compressor to compress air and store it in the storage chamber, converting electrical energy into the internal energy of the air for storage. During peak hours, the high-pressure air is released from the storage chamber and enters the fuel chamber to burn with fuel, driving the turbine to work and powering the generator to generate electricity.
Compressed air energy storage is another technology suitable for large-scale power storage at the GW level, following pumped hydro storage. In addition to high energy storage capacity, it also has advantages such as high energy density and power density, low operating costs, and long service life. However, similar to pumped hydro storage, compressed air energy storage is also limited by geographical conditions, requiring highly airtight caves as storage chambers, which further restricts the development of compressed air energy storage.
III. Electrochemical Energy Storage
Electrochemical energy storage involves converting electrical energy into chemical energy through electrochemical reactions, thereby enabling the storage and release of electrical energy. Currently, the main energy storage batteries used include lead-acid batteries, flow batteries, and lithium-ion batteries. In the future, sodium-ion batteries will also be gradually applied to energy storage as the industry chain matures.
1. Lead-acid batteries
Lead-acid batteries are a type of secondary battery that uses lead dioxide as the positive electrode, metallic lead as the negative electrode, and sulfuric acid solution as the electrolyte. They have been developed for more than 150 years and are the earliest secondary batteries to be used on a large scale.
Lead-acid batteries offer low energy storage costs, high reliability, and high efficiency, making them widely used in UPS systems and the dominant technology for large-scale electrochemical energy storage in my country in its early stages. However, their future applications will be severely limited due to their short cycle life, low energy density, narrow operating temperature range, slow charging speed, and the significant environmental impact of lead metal.
2. Flow battery
The technology pathways for flow batteries include vanadium redox flow batteries, iron-chromium flow batteries, and zinc-bromine flow batteries. Among them, vanadium redox flow batteries have the best overall performance and the highest degree of commercialization.
In a flow battery, the positive and negative electrolyte storage tanks are separate and placed outside the stack. Two circulating pumps pump the positive and negative electrolytes into the flow battery stack through pipelines, where they continuously undergo electrochemical reactions. This process of converting chemical energy into electrical energy enables the storage and release of electrical energy. The power of a flow battery depends on the size of the electrode reaction area, while its storage capacity depends on the volume and concentration of the electrolyte. Therefore, the design of flow batteries offers greater flexibility in scale and design.
We believe that vanadium redox flow batteries will have a cost advantage in long-term energy storage, giving them a differentiated competitive advantage over other technologies such as lithium batteries.
3. Lithium-ion batteries
Lithium-ion batteries store energy through the insertion and extraction of lithium ions into the positive and negative electrode materials. Lithium-ion batteries have high energy density and long lifespan, and are therefore gradually becoming the mainstream approach for electrochemical energy storage. Depending on the cathode material, lithium-ion batteries are further classified into lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, and ternary lithium batteries, among others.
Lithium iron phosphate (LFP) batteries have significant advantages in the energy storage field. They have moderate energy density, better safety and lifespan than other battery types, and lower cost. Lithium cobalt oxide (LCO) batteries are much more expensive than other batteries due to the scarcity of cobalt, and have poor cycle life and safety, so they are rarely used in energy storage. Lithium manganese oxide (LCO) batteries have similar energy density to LFP batteries, and although they are cheaper, their shorter lifespan results in a higher levelized cost of electricity (LCOE) over their entire lifespan, so they are less commonly used. Ternary lithium batteries have much higher energy density than other battery types and can also last for 8-10 years, but their safety is relatively poor and their cost is much higher than that of LFP batteries. Therefore, in energy storage fields that do not require extremely high energy density, their application prospects are weaker than those of LFP batteries.
4. Sodium-ion batteries
Sodium-ion batteries work on a similar principle to lithium-ion batteries, utilizing the insertion and extraction of sodium ions between the positive and negative electrodes to achieve charging and discharging. Compared to lithium iron phosphate batteries, sodium-ion batteries offer superior safety, low-temperature performance, and fast-charging capabilities, while also being less expensive. Furthermore, sodium resources are far more abundant than lithium resources and are distributed globally. If sodium ions can be widely applied, my country will be able to largely overcome its current situation of limited lithium resources.
The main disadvantages of sodium-ion batteries lie in their lower cycle life and immature industry chain. Currently, the cycle life of sodium batteries is generally between 2,000 and 3,000 cycles, and the immature industry chain leads to higher upstream prices, preventing sodium batteries from demonstrating their cost advantage.
IV. Summary
In summary, pumped hydro storage, lithium batteries, sodium batteries, and vanadium redox flow batteries all have significant room for development.
Specifically, in terms of large-scale peak shaving, pumped hydro storage has a cost advantage throughout its entire life cycle and will continue to be the mainstream choice; the latter three will be widely used in conjunction with wind power and photovoltaics; vanadium redox flow batteries are mainly used for long-term energy storage of more than 4 hours; sodium batteries will replace lithium batteries to some extent in large-scale energy storage power stations; and lithium batteries will still dominate in industrial, commercial and residential energy storage where energy density is highly sensitive.
