Development Prospects Of Long-Term Energy Storage Technology

1. Redox flow battery technology
   
   1. All vanadium redox flow battery
   2. Zinc bromine redox flow battery
2. Metal-air battery
3. Regenerative energy storage technology and mechanical energy storage technology

At present, the industry refers to energy storage systems that can last for more than 4 hours or several days or months of charge and discharge cycles, which are collectively referred to as long term energy storage. The higher the penetration rate of renewable energy generation, the longer the required energy storage time.

On the whole, long-term energy storage can adjust new energy generation fluctuations in a longer time dimension by virtue of its long-term and large-capacity characteristics, avoid grid congestion when clean energy is in excess, and increase clean energy consumption during peak loads.

In the long run, because the high output time of wind power and photovoltaic power station does not strictly correspond to the peak time of user demand, and the short-term energy storage does not have the ability to absorb power generation for several hours or even days, it will affect the stability of the power system.

Therefore, renewable energy + long-term energy storage has become one of the important solutions to accommodate renewable energy and replace traditional thermal power plants.

There are already a series of long-term energy storage technologies under development and application, among which redox flow batteries, metal-air batteries, regenerative energy storage technologies and mechanical energy storage technologies are more prominent. The technical maturity and market maturity of these energy storage technologies vary.

Although these energy storage technologies are currently promising, efforts should be made to reduce costs and achieve large-scale applications in order to accelerate the commercialization of such technologies.

Redox flow battery technology

Redox flow batteries are characterized by independent design of power and capacity, strong scalability, easy scale-up, long service life, and high safety in use. Therefore, they are suitable for use in the field of stationary energy storage.

The key components of this type of battery are the flowing electrolyte and the ion-selective membrane. However, this type of battery also has limitations, generally because the energy density is lower than that of lithium-ion batteries.

Among the chemical properties of redox flow batteries, the most studied ones are all-vanadium redox flow batteries (VRFB) and zinc-bromine redox flow batteries (Zn-Br).

The advantages of flow batteries include: flexibility, longer energy storage time, lower levelized energy storage cost, longer life, and fewer environmental temperature problems.

But on the other hand, so far, neither the all-vanadium redox flow battery nor the zinc-bromine redox flow battery has achieved large-scale commercialization, and both face some technical and commercial challenges.

All vanadium redox flow battery

The all-vanadium redox flow battery is the largest redox flow battery technology applied so far, but its application proportion is still very low in various energy storage systems under development.

The main obstacle to the rapid marketization of all-vanadium redox flow batteries is cost, among which, its upfront investment cost is relatively high-this situation cannot be reversed in 2022. Also, the cost of the electrolyte and ion-exchange membrane it uses is high—vanadium is not only expensive, but also fluctuates in price, which is also a major problem for the battery technology.

Rapid scaling is needed to reduce the cost of this battery technology and realize its application potential. Although some flow battery developers are currently developing vertically integrated business models, for example, entering certain links in the raw material supply chain, this trend needs to be accelerated.

In terms of technology, it is necessary to develop new ion-exchange membranes with higher ion selectivity, higher conductivity, and more cost-effective. In addition, energy efficiency and energy density can also be improved by optimizing the battery design.

Zinc bromine redox flow battery

Zinc-bromine redox flow batteries allow aqueous battery systems to achieve high voltages, and their energy density is relatively high among flow batteries.

But this battery technology also faces at least two technical difficulties. One is the need to mitigate the generation of hydrogen gas and the formation of zinc dendrites, which degrade battery performance and pose safety concerns. Second, the current zinc-bromine battery system uses expensive chemicals in order to reduce toxic bromine vapor emissions.

In terms of commercialization, since the commercialization of redox flow batteries in the past decade has been carried out around the all-vanadium redox flow battery technology, more efforts need to be made in the commercial development of zinc-bromine redox batteries.

Metal-air battery

Metal-air batteries are used in scenarios that require energy storage for several days, and their theoretical energy density is much higher than that of commercial lithium-ion batteries.

The materials used in this battery, such as zinc and iron, are safe and economical, are very abundant on earth, and the battery uses an aqueous electrolyte, so the cost of materials and related system-level energy costs are low.

However, metal-air batteries face challenges in terms of metal anodes, air cathodes, and electrolytes, and have yet to realize their full potential. The energy conversion efficiency of metal-air batteries is also lower than that of lithium-ion batteries and flow batteries.

Regenerative energy storage technology and mechanical energy storage technology

Regenerative energy storage, such as hot rock storage, has the advantage over most other forms of energy storage that the materials used are low-cost and plentiful. The main challenge for thermal energy storage technology is how to efficiently and economically convert thermal energy into electrical energy.

Among mechanical energy storage technologies, the two most discussed technologies are pumped hydro storage technology and compressed air energy storage technology. The energy density of mechanical energy storage technology is much lower than that of electrochemical energy storage technology and chemical energy storage technology.

In addition, the site selection for most mechanical energy storage technologies must have suitable topographical conditions. For example, a pumped storage hydropower station needs to have two reservoirs with different elevations, and there must be a certain slope between the two reservoirs.

The reliability and scalability of new thermal and mechanical energy storage technologies also need to be improved urgently. Long-duration energy storage technologies also currently face various financing, market, and policy hurdles in many parts of the world, limiting their adoption.

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