Molten salt energy storage-definition, composition and application

Long term energy storage generally refers to the technology of energy storage for more than 4 hours. The characteristics and cost reduction of energy storage technology are different. According to the different application scenarios, the long-term energy storage technology will present a pattern of simultaneous development of multiple lines.

Generally speaking, long-term energy storage technology can be divided into three main lines: mechanical energy storage, thermal energy storage and chemical energy storage. Among them, thermal storage is mainly molten salt energy storage. This article will introduce the definition, composition and application scenarios of molten salt energy storage.

What is molten salt energy storage

Molten salt energy storage technology is to store renewable energy or low valley electric energy by heating molten salt with light and heat, wind power, photovoltaic, night low valley electricity, industrial waste heat, etc. Then, when the useful energy is needed, it can exchange heat with water to generate hot water or steam for direct heating, or use steam to drive the turbine for external power generation.

What is the composition of molten salt energy storage

Main materials/equipment of molten salt energy storage system include:
● Molten salt
● Molten salt storage tank
● Molten salt pump
● Electric heater
● Steam generator
● Superheater
● Steam water heat exchanger

Molten salt is a medium with low cost, long life, good heat transfer and storage performance, high temperature, high heat flux and low operating pressure. Molten salt used in heat storage is usually a mixture of nitrate. For example, the most commonly used binary molten salt in photo thermal power plants is a mixture of 60% sodium nitrate and 40% potassium nitrate.

Molten salt energy storage has the characteristics of flexible power generation, large energy storage scale, long time, long service life, environmental protection and safety, etc., and its advantages in heat storage and peak shaving are particularly prominent.

Based on these characteristics, molten salt heat storage technology has gradually emerged in the past two years, and entered the vision of more people, and the application scenarios of molten salt energy storage have also been enriched.

What is molten salt energy storage used for

Photothermal power generation

In the scenario of conventional solar thermal power plant, molten salt energy storage is realized in the double tank cycle of hot salt tank and cold salt tank. The molten salt in the cold molten salt storage tank is transported to the solar collector by the molten salt pump. The molten salt absorbs the heat energy and heats up before entering the hot molten salt storage tank.

Then, the high-temperature molten salt flows into the molten salt steam generator, generating superheated steam to drive the steam turbine to operate and generate electricity, while the molten salt flows back to the cold molten salt storage tank after the temperature decreases.

In this scenario, the characteristics of high heat capacity of molten salt can be brought into full play, so that the power plant can use the high temperature molten salt in the thermal storage tank to generate electricity even at night.

Up to now, China has built many such projects, and the total installed capacity of large-scale commercial solar thermal power generation projects has exceeded 500MW.

Thermal power flexibility transformation

Thermal power flexibility usually refers to the operational flexibility of thermal power generator sets, i.e. the ability to adapt to large fluctuations in output and quickly respond to various changes. In China, there are top 10 thermal power storage system integration companies do well in this field.

The main indicators include peak shaving amplitude, climbing speed, start and stop time, etc. In the process of low load operation of thermal power units, the filling degree of flame in the furnace will be different from that of high load.

When the load decreases to a certain extent, due to the temperature drop in the furnace, the ignition distance of each branch of gas flow will increase, and the radiation loss of flame on the furnace wall will increase relatively, so it is easy to have unstable combustion, or even boiler flameout.

In the scenario of thermal power flexibility transformation, through molten salt energy storage technology, the deep peak shaving capacity of thermal power units has been greatly increased, that is, under extremely low load, the molten salt heat storage system can flexibly release heat and release heat as required, which can greatly reduce fuel consumption.

In addition, molten salt has the characteristics of low cost and long service life. Compared with the existing peak shaving technology of thermal power units, the thermal power peak shaving technology of steam heating molten salt energy storage has the following advantages:

● Lower energy consumption
● More energy saving and reliable operation
● Low transformation cost

Heat supply in industrial park

In the heat supply scenario of the industrial park, the park uses large-scale green power and molten salt energy storage technology to store excess green power (wind power or photovoltaic) in high-temperature molten salt during the low grid hours, and uses the stored heat energy to exchange heat for external heating or power generation during the peak and peak periods.

The molten salt energy storage heating system and steam turbine power generation system can provide heat and power to the park, and can respond to the power grid demand at the peak of full load.

On October 20, 2022, the largest molten salt energy storage project under construction in China (also the largest user side molten salt energy storage project in Zhejiang Province, China) – Shaoxing Green Electric Molten Salt Energy Storage Demonstration Project officially started steam production and trial operation in Shaoxing, China.

The project uses large-scale green power and molten salt energy storage technology to store excess green power (wind power or photovoltaic) in high temperature molten salt during the low grid hours, and use stored heat to exchange heat for external heating or power generation during peak and peak periods.

According to calculation, the project can generate 63.7 million kWh per year, provide 840000 tons of steam per year, save 155000 tons of standard coal per year, and reduce carbon dioxide by about 290000 tons. It has the advantages of high parameters, high efficiency, long storage time and low cost while meeting the power grid demand at the peak of full load.

Emerging markets that molten salt energy storage is expected to conquer

Distillery

In the brewing industry, the brewing efficiency is closely related to the production capacity of the enterprise. With the same raw materials and processes, “electric brewing” can increase the liquor yield by 6%, and the liquor quality can reach the top grade.

However, “electric brewing” has problems such as high heat loss, inflexible control of steam pressure, and tight heating energy in winter. How to reduce heat loss, flexibly control steam pressure, and reduce the use of heating energy is a difficult point for each distillery.

At present, China’s leading liquor enterprises, such as MOUTAI and FENJIU, have begun to try to layout molten salt heat storage technology. Recently, China also plans to introduce and organize the development of customized electric boilers for brewing, electric thermal storage steam system for molten salt valley and other new technologies to solve the problem of electric energy substitution for brewing companies.

Iron and steel plant

Relevant data shows that only 30%~50% of the energy in the steel manufacturing process of the steel industry is effectively used, and a large amount of remaining energy exists in the form of waste heat. Waste heat resources are widely distributed in the production process of each process, with huge recovery potential.

In this scenario, the use of molten salt energy storage technology can convert intermittent high-temperature waste heat resources into continuously adjustable and controllable high-temperature steam heat energy, greatly increase power generation power and efficiency, improve the economy of waste heat power generation, and also improve the flexibility of waste heat power generation to meet load tracking and grid peak shaving requirements.

Shared energy storage power station

In July last year, relevant Chinese departments clearly encouraged the exploration and construction of shared energy storage. Shared energy storage power plants based on molten salt thermal storage technology also see opportunities for development in energy storage market.

More than 20 local energy authorities in Shandong, Hunan, Zhejiang, Qinghai, Henan, Inner Mongolia and other provinces in China successively issued supporting policies, taking shared energy storage as an important direction for the development and construction of energy storage power stations.

At present, the planning of shared energy storage power plants based on molten salt thermal storage technology is also slowly being carried out. CSPBH is actively deploying molten salt shared energy storage power plants.

At the end of May this year, the wholly-owned subsidiary of CSPBH successfully signed a 600MW/3600MWh high-temperature molten salt shared energy storage, peak shaving and frequency modulation power station and a biomass comprehensive utilization project with an annual output of 40000 tons of activated carbon.

Summary

To sum up, under the background that all parties actively implement the dual carbon goals, molten salt heat storage technology has great potential in reducing production costs, improving enterprise efficiency, saving non renewable energy, reducing carbon emissions, recycling waste heat energy, electric energy, etc. It is also expected to cooperate with more industries in the future to build a more green energy system.