A molten salt storage tank and a molten salt electro-thermal long-term energy storage device

Abstract

The present invention provides a molten salt storage tank and a molten salt electro-thermal long-duration energy storage device. This device can directly replace existing coal-fired and gas-fired industrial steam boilers, thereby promoting centralized gas supply and heating through molten salt heat storage in industrial parks; it can also directly provide centralized heating for residential areas. At the same time, when the molten salt electro-thermal long-duration energy storage device of the present invention is combined with a supercritical carbon dioxide power generation unit, a new type of efficient "electricity-thermal-electricity" technical mode is generated, which can replace existing pumped-storage power stations and chemical energy storage stations and become a large-capacity long-duration energy storage technology. The present invention has the advantages of compact structure, simple system, low heat storage cost, high heat release efficiency, small system heat loss, high electro-thermal and thermoelectric conversion efficiencies, can achieve modular, standardized, batch, and factory production, has safety, economy, and flexibility, and also has the characteristics of strong regulation ability, short construction period, low site selection requirements, and flexible construction scale.

Description

Technical Field

[0001] The present invention discloses a molten salt storage tank and a molten salt electro-thermal long-term energy storage device, belonging to the field of new energy storage technologies. Background Art

[0002] With the development of society, the deterioration of the global ecological environment is becoming increasingly severe, the control of carbon emissions is becoming more stringent, and the adjustment of the energy structure is accelerating towards green energy. Green energies such as solar energy and wind energy have received unprecedented attention. With the further increase in the proportion of new energy installed capacity, the main measures taken to ensure the consumption of new energy are as follows: First, deep peak shaving transformation of coal-fired power: reducing the minimum power generation; Second, building new energy storage facilities: storing the power generation of new energy and shifting it to the peak electricity consumption period for use. Deep peak shaving of coal-fired power is the short-term priority choice, while matching with an energy storage system is the inevitable choice for future long-term development! Currently, several relatively common energy storage methods are compared: Pumped storage: Although the technical route of pumped storage is mature, the cost is relatively high and it is difficult to reduce in the future. If there is no existing upper or lower reservoir available, the unit investment and moving cost of pumped storage will still be high; in addition, the construction period is generally 7-8 years, which is significantly behind the new energy construction cycle and far from being able to solve the immediate problem; secondly, the geographical conditions for building pumped storage in the northwest region are rare, and large-scale construction of pumped storage is not realistic. Therefore, although the technology of pumped storage is mature, the cost will only increase and will not decrease.

[0003] Electrochemical energy storage: Currently, electrochemical energy storage is restricted by high cost, short life, and low safety. The cost of the energy storage link per kilowatt-hour of electrochemical energy storage is relatively high; the operating life is generally 8-10 years before the battery needs to be replaced, and there are also environmental protection problems after scrapping; there are also frequent safety problems such as combustion or explosion. Whether the cost of electrochemical energy storage can continue to decline in the future is analyzed from two aspects: First, the cost reduction cannot be unlimited and endless; Second, the reserves of lithium are limited, and the future cost is difficult to predict. Therefore, electrochemical energy storage itself urgently needs a technological revolution to solve a series of problems such as "safety, environmental protection, cost, and life".

[0004] In the context of a high proportion of renewable energy installed capacity, in order to reduce wind and light curtailment, ensure the safe operation of the power grid, and meet the peak electricity consumption demand, there is an urgent need for a new type of energy shifting energy storage technology with low cost, large scale, high efficiency, high safety, and environmental protection and low carbon.

[0005] On the other hand, with the development of society and the improvement of people's living standards, the day-night gap in the electricity consumption structure in China is becoming increasingly large, the demand for peak shaving and valley filling is becoming more intense, and the phenomenon of curtailment of wind and solar power is becoming more serious; China's economic growth rate has shifted gears and entered a new normal of development, and the industrial structure is gradually transforming from the mid- to low-end to the mid- to high-end. The adjustment of the industrial structure has led to a continuous increase in the proportion of electricity consumption in the tertiary industry and urban and rural residents, and their electricity consumption characteristics determine that the peak-valley difference rate of the load curve is significantly higher than that of the secondary industry. The peak-valley difference rate on the electricity consumption side in China shows an upward trend, and the demand for electric energy storage and heat storage and the construction of regulating power sources on the user side continue to increase.

[0006] Thermal energy storage technology, compared with pumped-storage energy storage and compressed-air energy storage technologies, belongs to long-duration energy storage. However, the equipment technology of thermal energy storage is relatively simple and mature. The key is that thermal energy storage can absorb unstable and highly volatile renewable electricity, while pumped-storage energy storage and compressed-air technologies require stable off-peak electricity for support. But with the reduction of coal-fired power plants and the decline in peak-shaving capacity, once renewable energy becomes the main energy source and off-peak electricity becomes history, the pumped-storage energy storage and compressed-air energy storage equipment will be idle, and huge investments will become junk assets.

[0007] Compared with chemical battery energy storage, the prominent features of thermal energy storage are large energy storage capacity and low unit energy storage cost, which chemical battery energy storage cannot compete with. However, in terms of flexibility, thermal energy storage is obviously inferior to chemical battery energy storage. Therefore, the flexibility problem of thermal energy storage applications must be solved.

[0008] The main technical approach of traditional molten-salt energy storage is: adopting a heat storage mode with double storage tanks, namely a hot molten-salt storage tank, a cold molten-salt storage tank, multiple high-power dedicated molten-salt heaters, multiple high-temperature molten-salt pumps, multiple low-temperature molten-salt pumps, shell-and-tube molten-salt heat exchangers, and salt drainage tanks and other equipment, which are connected into a system through long molten-salt conveying pipelines. In order to prevent the freezing of the molten-salt conveying pipelines during use, electric heating tapes also need to be installed on the conveying pipelines, and an auxiliary salt drainage system is added. This system is used to collect the molten salt in equipment such as molten-salt conveying pipelines when the system shuts down. The entire system has large heat losses, high heat storage and heat release costs, and large overall investment. Once the required molten-salt heat storage temperature is above 800 degrees, the technical difficulty of this set of traditional molten-salt heat utilization technology will be greatly improved; in addition, the manufacture of the double molten-salt storage tanks must be completed on-site, which not only has a long construction period, but also is not easy to control the manufacturing cost and quality. In short, the component equipment in the traditional molten-salt double-tank energy storage system cannot be mass-produced and standardized, the heat storage cost is too high, the heat release efficiency improvement is limited, the system has large heat losses, the system is relatively bulky and not compact, and it is difficult to achieve modular, standardized, mass-produced, and factory-produced production.

[0009] Another technical approach for traditional molten salt energy storage is to adopt the heat storage form of a single storage tank. However, there is a technical problem of the "thermocline" that is difficult to overcome in a single storage tank. There is a thermocline with a relatively complex temperature distribution between the cold molten salt at the lower part of the tank and the hot molten salt at the upper part. This not only increases the control difficulty of the molten salt heat exchange process but also reduces the high-grade thermal energy accordingly. In particular, this single storage tank technology does not meet the technical requirements for long-duration energy storage.

[0010] The proposal of the strategic goals of carbon peak and carbon neutrality has pointed out the direction for the future development of green energy in China. Especially in the future, renewable energy will gradually shift from a supporting role to a leading role in China's energy field, and the advantages of the "electricity - heat - electricity" technology model are gradually emerging. Therefore, it is necessary to introduce the "electricity - heat - electricity" technology model based on long-duration energy storage technology in a timely manner.

[0011] In fact, the technology of molten salt energy storage heat utilization is not difficult in terms of basic principles. The key lies in whether the overall molten salt energy storage system has economy, safety, reliability, replicability, and the ability to be promoted in large quantities. This is the key to whether a technology can be widely promoted and applied on a large scale. Summary of the Invention

[0012] To solve the problems in the background technology, the present invention provides a molten salt storage tank and a molten salt electric heat storage long-duration energy storage device. This device can directly replace the existing coal-fired and gas-fired industrial steam boilers, thereby promoting centralized gas supply and heat supply through molten salt heat storage in industrial parks; it can also directly supply centralized heating for residential areas. At the same time, when the molten salt electric heat storage long-duration energy storage device of the present invention is combined with a supercritical carbon dioxide generator set, a new type of efficient "electricity - heat - electricity" technology model is generated, which can replace the existing pumped-storage power stations and chemical energy storage stations and become a large-capacity long-duration energy storage technology.

[0013] At the same time, the flexibility problem of traditional thermal energy storage applications is solved. This technology has safety, economy, and flexibility, and also has the characteristics of strong regulation ability, short construction period, low site selection requirements, and flexible construction scale. The molten salt electric heat storage long-duration energy storage technology has opened up a new application scenario in the fields of industrial steam, residential heating, and "electricity - heat - electricity" energy storage.

[0014] To achieve the above object, the technical solution adopted by the present invention is as follows: A molten salt storage tank, characterized in that a molten salt output branch pipe is provided at the bottom of the molten salt storage tank, and the outer port of the molten salt output branch pipe is the first molten salt outlet, and an outlet shut-off valve is provided at its inner port. The molten salt storage tank is provided with a molten salt return branch pipe extending from the upper part to the bottom of the storage tank, and the outer end of the molten salt return branch pipe is the first molten salt inlet, and the first molten salt inlet is above the molten salt liquid level line. An inlet shut-off valve is provided at the inner port of the molten salt return branch pipe; the outlet shut-off valve and the inlet shut-off valve have the same structure, which are, from bottom to top, a valve seat, a valve flap, a connecting skeleton, a valve rod, and an electric controller. Among them, the valve seat communicates with the inner port of the molten salt output branch pipe or the molten salt return branch pipe. The valve rod is vertically arranged, and its bottom end is assembled with the valve flap through the connecting skeleton. The upper end of the valve rod extends above the molten salt liquid level control line, and the electric controller is provided at the top end. The motor part on the electric controller is arranged outside the top of the molten salt storage tank and can drive the valve rod to move up and down to realize the opening and closing of the outlet shut-off valve and the inlet shut-off valve. In the traditional molten salt energy storage technology, the electric molten salt valve is a key component that is relatively difficult to handle. Especially in the case of ultra-high temperatures above 700 degrees Celsius, the heat resistance, sealing performance, and heat preservation performance of the traditional electric molten salt valve all face great technical problems. It has a high cost and great manufacturing difficulty. It is prone to leakage, large heat loss, easy to freeze and block, short service life during operation, difficult to repair and replace, and requires shutdown for maintenance. Each shutdown for maintenance has a relatively high economic cost. Especially, once the valve is frozen and blocked, it may even cause irreversible damage to the entire system. The present invention creatively integrates the molten salt shut-off valve into the molten salt storage tank and is set at the molten salt outlet and the molten salt inlet. Its valve rod exceeds the height of the molten salt liquid level. In the molten salt storage tank, the shut-off valve becomes a normal pressure shut-off valve, there is no sealing problem of the valve rod, no freezing and blocking problem of the valve, and there is not much part that needs heat preservation. It is easy to process and manufacture. Except for the electric controller at the top of the valve, almost no other components need special maintenance, and its service life is greatly improved. Usually, the overall dimensions of the molten salt storage tank product of the present invention are controlled within the dimensions allowed by the road transportation regulations, so that the molten salt storage tank can be mass-produced in a factory, standardized, and modularized, greatly reducing the manufacturing cost and manufacturing difficulty, ensuring the product quality, and greatly shortening the production cycle. Usually, the top cover of the molten salt storage tank is detachable, which is convenient for the daily maintenance and repair inside the storage tank. Usually, the valve rod is made of precision-drawn seamless stainless steel pipe, which can effectively reduce the weight of the valve rod. At the same time, a number of through holes are provided on the tubular valve rod to facilitate the entry of molten salt and reduce the volume occupied by the valve rod.

[0015] Further, a guiding rod is provided at the center of the bottom of the valve seat, and a guiding sleeve matching the guiding rod is provided at the center of the bottom surface of the valve flap. The precise cooperation between the two ensures the opening and closing cooperation of the valve seat and the valve flap when the valve stem moves up and down; the connecting framework is a universal coupling. Generally, such a design can effectively ensure the sealing performance and convenience of the opening and closing of the valve seat and the valve flap. Especially for valves with an extremely long valve stem, connecting the valve stem and the valve flap through a universal coupling can greatly ensure the horizontal sealing cooperation between the valve flap and the valve seat, and avoid the influence of the central deviation caused by the long valve stem on the sealing cooperation degree between the valve flap and the valve seat.

[0016] Further, a horizontal cover plate is provided above the set molten salt liquid level line of the molten salt storage tank. Two through holes for the valve stem to pass through are provided on the cover plate. A limiting block with a diameter larger than that of the valve stem is provided at the top of the valve stem. A compression spring is provided on the valve stem between the cover plate and the limiting block. A groove is provided on the upper part of the limiting block, and a heat insulation pad is provided in the groove. The electric controller is an electric push rod, and the top of its push rod abuts against the heat insulation pad. When the push rod of the electric push rod extends, its thrust compresses the compression spring and makes the valve stem move downward to achieve the close-fitting cooperation between the valve flap and the valve seat, that is, the shut-off valve is closed at this time. When the push rod of the electric push rod retracts, the elastic force of the compression spring directly pushes the valve stem upward, and the shut-off valve is opened at this time. In the present invention, the electric push rod at the top of the valve stem is not mechanically connected to the valve stem, and a heat insulation pad is used to isolate at the abutting position between the top of the valve stem and the electric push rod, blocking the heat transfer to the electric push rod and effectively reducing the heat loss; the self-weights of the valve stem, the valve flap, the connecting framework, etc. are all supported by the provided compression spring. When the electric push rod does not act, the shut-off valve is in an always-open state. When the elastic force of the compression spring is adjusted to a suitable value, the rated thrust requirement of the electric push rod can be reduced, so that the rated power of the electric push rod can be as small as possible, reducing the unnecessary parasitic power consumption. Generally, the rated power of the electric push rod is only a very low few hundred watts; generally, the upper surface of the cover plate is coated with a heat insulation coating.

[0017] Further, a plurality of electric heating tubes are provided in the molten salt storage tank. A plurality of electric heating tube installation openings are provided on the cover plate. The electric heating tubes are of a single-head flange structure and match the electric heating tube installation openings; the rated voltage of the electric heating tubes is 6.6 KV or 10 KV. By directly arranging electric heating tubes inside the molten salt storage tank to perform static heating on the molten salt, compared with the traditional heating method of a special molten salt heater outside the storage tank, not only the investment cost is saved, but also the heat loss during the flowing heating of the molten salt is reduced, improving the overall energy efficiency. Especially, using high-voltage electric heating, its electro-thermal conversion rate exceeds 98%, not only saving a large amount of capital investment such as transformers, but also increasing the electro-thermal conversion rate by at least three percentage points compared with the traditional low-voltage electric heating.

[0018] A molten salt electro-thermal long-term energy storage device includes a molten salt storage tank, molten salt, a molten salt output main pipe, a molten salt return main pipe, a molten salt pump station, a molten salt heat extractor, and an electric control system. It is characterized in that the molten salt storage tank has the characteristics described in the above several articles, and the number of molten salt storage tanks is set to more than three, and it is ensured that one of the molten salt storage tanks is empty, and the outlet shut-off valve in the empty molten salt storage tank is in the closed state while the inlet shut-off valve is in the open state. The remaining molten salt storage tanks are filled with the molten salt. The first molten salt outlet on each molten salt storage tank is connected to the molten salt output main pipe, and the first molten salt inlet on each molten salt storage tank is connected to the molten salt return main pipe; the molten salt pump station includes a receiving tank and a molten salt pump arranged in the receiving tank. The overall height of the receiving tank is not lower than the height of the molten salt storage tank, which ensures that the installation position of the motor on the molten salt pump is always above the molten salt liquid level line. A molten salt total inlet is arranged at the bottom of the receiving tank, and the molten salt total inlet is connected to the molten salt output main pipe. The molten salt pump is provided with a molten salt total outlet; the molten salt heat extractor is provided with a second molten salt inlet, a second molten salt outlet, a heat extraction medium inlet, and a heat extraction medium outlet. Among them, the second molten salt inlet is connected to the molten salt total outlet, and the second molten salt outlet is connected to the molten salt return main pipe; the electric control system has a linkage control relationship with the electric heating tubes, molten salt pumps, outlet shut-off valves, and inlet shut-off valves in each molten salt storage tank. Usually, in the traditional heat storage mode of double storage tanks, that is, one hot molten salt storage tank and one cold molten salt storage tank, the empty storage tank accounts for 50%. In the present invention, at least three or more molten salt storage tanks are adopted. The more the number, the less the proportion of the empty storage tank, and the funds occupied by the molten salt storage tank part are greatly saved. By adopting such a multi-storage tank method, the more important purpose is that the size of the molten salt storage tank can be fixed and become a single product specification, which can more conveniently control the size of a single molten salt storage tank within the size range stipulated by the national road transportation law. Usually, the diameter of the storage tank is not greater than 2.5 meters, and the height of the storage tank is not greater than 14 meters. In this way, the molten salt storage tank can be mass-produced in a standardized and industrialized manner. At the same time, the energy storage capacity of the long-term energy storage device can be easily and flexibly expanded. On the other hand, in the whole set of long-term energy storage device, only one molten salt pump is required, and all the equipment in the system shares this molten salt pump. And a slender cylindrical receiving tank is cleverly configured for the molten salt pump. The lift of the molten salt pump is almost equal to the height of the molten salt storage tank. In this way, the rated power of the motor of the molten salt pump can be reduced, and the parasitic power consumption is reduced. The receiving tank supporting the molten salt pump is even a small desalting tank, which can receive the molten salt flowing back from the molten salt heat extractor when the molten salt pump stops, achieving the purpose of desalting the equipment and pipelines when stopping, emptying the molten salt in time, preventing the molten salt from staying in the molten salt heat extractor and pipelines and causing freezing blockage, saving the standard desalting tank system in the traditional double-storage-tank molten salt energy storage system, and further saving the capital investment.

[0019] When the long-duration energy storage device is in the electro-thermal energy storage working state, the inlet shut-off valves and outlet shut-off valves on an empty molten salt storage tank and the other molten salt storage tanks filled with molten salt are all in the closed state. At this time, the electric heating tubes in the empty molten salt storage tank do not generate heat, and the molten salt in the other molten salt storage tanks filled with molten salt is statically heated by the electric heating tubes to the set upper temperature limit, completing a complete molten salt electro-thermal energy storage process.

[0020] When the long-duration energy storage device is in the heat release working state, the outlet shut-off valve on an empty molten salt storage tank is in the closed state while the inlet shut-off valve is in the open state, and the inlet shut-off valves on the other molten salt storage tanks filled with molten salt are all in the closed state, and only the outlet shut-off valve on one molten salt storage tank waiting to be emptied by the molten salt pump is open. At this time, the heated molten salt is pumped into the molten salt heat exchanger by the molten salt pump to release heat, and the molten salt after heat release flows back to the empty molten salt storage tank through the molten salt return main pipe until all the molten salt in this molten salt storage tank is emptied into the empty molten salt storage tank. At this time, this molten salt storage tank becomes a new empty molten salt storage tank, so that the other molten salt storage tanks filled with molten salt will be emptied in turn, completing a complete molten salt heat release cycle. In each round of molten salt heat release process, there is no fixed molten salt storage tank that is empty, and each molten salt storage tank filled with molten salt will experience being empty once, and the originally empty molten salt storage tank will also be filled with molten salt pumped from other storage tanks. Then, after one round of heat release, it enters the next round of electro-thermal energy storage process, and thus the cycle of heat storage - heat release - heat storage continues. Usually, the molten salt is a binary mixed molten salt composed of 60% sodium nitrate and 40% potassium nitrate; or a ternary mixed molten salt composed of 53% potassium nitrate, 40% sodium nitrite, and 7% sodium nitrate; or a chloride mixture made from a certain proportion of sodium chloride, potassium chloride, and calcium chloride after purification and treatment, and the oxygen content in the mixture is less than 300 ppm.

[0021] Furthermore, the total number of the molten salt storage tanks is a multiple of two, and they are evenly divided into two columns and arranged in a matrix; the main molten salt output pipe is arranged near the bottom in the middle of the two columns of molten salt storage tanks, and the main molten salt return pipe is arranged near the top in the middle of the two columns of molten salt storage tanks. Both of them are welded into one body by a number of three-way or four-way welded pipe joints of the same specification and segmented molten salt pipe short joints with U-shaped joints. The first molten salt outlet on each molten salt storage tank is respectively connected to the corresponding three-way or four-way welded pipe joint on the main molten salt output pipe by welding, and the first molten salt inlet on each molten salt storage tank is respectively connected to the corresponding three-way or four-way welded pipe joint on the main molten salt return pipe by welding; a certain space is left above the set molten salt liquid level line in each molten salt storage tank, and the spaces of two adjacent such spaces are communicated by a short connecting pipe, and the space at the top inside the molten salt storage tank is penetrated by this short connecting pipe to facilitate the balance of the air pressure in each molten salt storage tank; two adjacent molten salt storage tanks are communicated by a short connecting pipe at a position near the set molten salt liquid level line below, and the uppermost parts inside the molten salt storage tanks are penetrated by this short connecting pipe to facilitate the balance of the molten salt liquid level height in each molten salt storage tank; a corrugated expansion joint is arranged in the middle section of this short connecting pipe; all the molten salt storage tanks, the main molten salt output pipe, the main molten salt return pipe and the molten salt pumping station, which are connected into one body by welding, are integrally arranged in a rectangular trough-shaped heat preservation housing. A heat preservation cover is arranged on the top of this heat preservation housing, and only the motor on the electric controller and the motor on the top of the molten salt pump are exposed outside. At the same time, the main molten salt outlet on the molten salt pump and the external interface of the main molten salt return pipe both extend out of one side wall of the heat preservation housing and are connected to the molten salt heat extractor. Usually, with such a design of the present invention, the standardization and modularization degree of equipment and various components such as molten salt storage tanks will be further improved, so that each three-way or four-way welded pipe fitting can have the same specification and the same standard, and it is ensured that there are no any sealed connection points in the part below the molten salt liquid level line in the heat preservation housing, and all are fixed and sealed by welding, eliminating any possible leakage. U-shaped expansion joints or corrugated expansion joints are arranged for all the molten salt pipelines to eliminate the influence of thermal stress on the welded joints. The molten salt expansion spaces reserved in the upper parts of all the molten salt storage tanks are communicated with each other by a short connecting pipe with a corrugated joint in the middle, including one empty molten salt storage tank, which is filled with inert gas inside. The motor on the molten salt pump is usually a variable frequency motor.

[0022] Furthermore, the installation heights of the molten salt heat exchanger, its second molten salt outlet and second molten salt inlet all exceed the height of the total molten salt outlet on the molten salt pump. The installation heights of the molten salt return main pipe and the first molten salt inlet both exceed the liquid level height of the molten salt in the molten salt storage tank, and the molten salt return main pipe has a downward slope in the direction of the first molten salt inlet. Generally, such a design ensures that when the molten salt pump stops working, the molten salt in the molten salt heat exchanger and the molten salt in the molten salt return main pipe automatically flow back into the molten salt storage tank or the molten salt pump containment tank, eliminating the risk of freezing and blockage caused by the temperature drop of the molten salt remaining in the molten salt heat exchanger or the molten salt return main pipe when the system shuts down. At the same time, compared with the traditional double-tank technology, there is no need to additionally set up a salt drainage tank system, further saving capital investment.

[0023] Furthermore, the outer layer of the thermal insulation housing is a rectangular trough-shaped reinforced concrete structure, and thermal insulation layers are provided on its inner bottom and four inner vertical surfaces. High-temperature resistant thermal insulation bricks are also laid on the thermal insulation layer at the bottom. The molten salt storage tank and the molten salt pumping station are both placed on these thermal insulation bricks. A high-temperature resistant thermal insulation coating layer is applied to the outer surfaces of all metal tanks and pipelines in the thermal insulation housing, and high-temperature resistant solid particle heat storage materials are filled in the gaps. Generally, the designed service life of molten salt long-term energy storage devices is more than 30 years, and the safety requirements are extremely high. Therefore, the present invention adopts the outer layer design of reinforced concrete to minimize the possibility of harm caused by accidental leakage of molten salt, improving safety. Although the cost of the thermal insulation outer layer increases due to the use of concrete, the thermal insulation layer is arranged on the inner wall of the reinforced concrete outer layer, and the area of the thermal insulation layer is much smaller than that of thermal insulation for each molten salt storage tank and molten salt pipeline. At the same time, the thermal insulation construction difficulty is also greatly reduced. Solid particle heat storage materials are filled in the gaps outside the molten salt storage tank, the containment tank, and the molten salt pipeline, which not only increases the total heat storage capacity but also has a thermal insulation and heat tracing effect on the molten salt in the molten salt storage tank or the molten salt pipeline. Especially, the molten salt output main pipe at the lower part of the storage tank is entirely buried in the solid particle heat storage materials, eliminating the need to additionally set up an electric heat tracing belt on the outer surface of the molten salt output main pipe, further saving expenses and reducing parasitic power consumption. On the other hand, the reinforced concrete thermal insulation housing outer layer is sufficient to withstand the outward extrusion pressure from the inside, and the entire molten salt storage tank is wrapped by solid particle heat storage materials, making it possible to reduce the wall thickness of the molten salt storage tank and correspondingly reducing the overall cost of the molten salt storage tank. Generally, the solid particle heat storage materials are silicon carbide particles, sand particles, or magnesia inorganic particles. In short, by adopting the reinforced concrete thermal insulation outer layer and adding solid particle heat storage materials, the safety, economy, thermal insulation performance, and heat storage capacity are all greatly improved.

[0024] Furthermore, the molten salt electro-thermal long-term energy storage device further includes an independent molten salt electro-heater, which is arranged between the molten salt pumping station and the molten salt heat extractor. There is a molten salt branch pipe connecting the molten salt output main pipe and the molten salt return main pipe, and a stop valve is arranged on the molten salt branch pipe. In this way, a small circulation loop is formed by the molten salt pumping station, the independent molten salt electro-heater, the molten salt heat extractor, the molten salt branch pipe and the stop valve. Such a design is mainly aimed at the application scenario of using valley electricity for energy storage. Usually, there are about 8 hours of low electricity consumption periods in 24 hours a day, and the electricity price during this period is very low. Storing energy during this period can not only play the role of peak shaving and valley filling for the power grid, but also generate huge economic benefits in the utilization of heat energy. Heat energy is generally used 24 hours a day. Therefore, in order to reduce the energy storage scale, the set energy storage capacity only supplies the heat energy demand during non-valley electricity periods, about 16 hours of non-valley electricity time. When the valley electricity period arrives, the previously stored heat energy has been released and enters the re-energy storage stage during the valley electricity period. The energy supply during the valley electricity period is directly heated by the independent molten salt electro-heater, saving the heat storage stage. This not only reduces the energy storage scale by 8 hours, but also the direct energy supply during the valley electricity period is more energy-efficient. At the same time, the long-term energy storage device can fully enter the energy storage working state again, and the energy supply during the 8-hour valley electricity period is completed by this small circulation loop. Usually, the molten salt heat extractor at this time is actually a molten salt steam generator, which specifically provides the steam demand for the user side. If a plate heat exchanger is connected behind the steam generator, heating and heating services can be provided for the user side.

[0025] Furthermore, the molten salt electro-thermal long-term energy storage device further includes a supercritical carbon dioxide power generation unit, which includes a generator, a turbine, a secondary compressor, a primary compressor, a precooler or condenser, a low-temperature recuperator, a high-temperature recuperator, and a supercritical carbon dioxide pipeline. A complete supercritical carbon dioxide Brayton cycle power generation system is formed by connecting through the supercritical carbon dioxide pipeline and the heat extraction medium outlet and heat extraction medium inlet on the molten salt heat extractor. The long-term energy storage device combined with the supercritical carbon dioxide power generation unit is a new and efficient "electricity-thermal-electricity" technology model. Without a doubt, the "electricity-thermal-electricity" technology model targets application scenarios of unstable and discontinuous renewable energy and electricity. In particular, wind power and photovoltaic power are very unstable and of low quality, the investment cost of solar thermal power generation is high, and the sun does not always shine brightly in the sky. Only by giving full play to their respective advantages and avoiding disadvantages can the defects existing in solar thermal power generation, wind power, and photovoltaics be fundamentally overcome, and ultimately the ultimate goal of replacing fossil energy with renewable energy can be achieved. The combination point between them is achieved through electric heating and heat storage. In this technology model, the energy efficiency of the long-term energy storage device and the energy efficiency of the power generation unit are the key points. The present invention effectively solves the energy efficiency problem of the long-term energy storage device, and the total energy efficiency of electro-thermal conversion is nearly 98% or more. The technology of supercritical carbon dioxide power generation is about to be commercialized, and its power generation energy efficiency is above 50%. The advantages of the "electricity-thermal-electricity" technology model are gradually emerging. The long-term energy storage device of the present invention is particularly suitable for the small-scale supercritical carbon dioxide power generation requirement of about 10 MW.

[0026] Compared with the traditional molten salt heat storage and heat utilization device, the present invention has the advantages of compact structure, simple system, low heat storage cost, high heat release efficiency, small system heat loss, high electro-thermal and thermoelectric conversion efficiency, and can achieve modular, standardized, batch, and factory production. It has safety, economy, flexibility, and also has the characteristics of strong regulation ability, short construction period, low site selection requirements, and flexible construction scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts. The drawings are only for reference and illustration, and are not used to limit the present invention.

[0028] Figure 1 is a schematic structural diagram of a molten salt storage tank provided by the present invention; Figure 2 is provided by the present invention Figure 1 Partial enlarged view of the top; Figure 3 is provided by the present invention Figure 1Partial enlarged view of the shut-off valve at the bottom; Figure 4 Schematic structural diagram of a molten salt storage tank with an electric heating tube provided by the present invention; Figure 5 Provided by the present invention Figure 4 Enlarged structural diagram of the top; Figure 6 Provided by the present invention Figure 4 Enlarged structural diagram of the bottom; Figure 7 Schematic structural diagram of a molten salt electro-thermal long-term energy storage device provided by the present invention; Figure 8 Provided by the present invention Figure 7 Corresponding top view; Figure 9 Schematic structural diagram of an integrated thermal insulation of a molten salt electro-thermal long-term energy storage device except for the molten salt heat extractor provided by the present invention; Figure 10 Provided by the present invention Figure 9 Top view after removing the top thermal insulation cover in; Figure 11 Schematic structural diagram of a molten salt pump station provided by the present invention; Figure 12 Schematic structural diagram of a molten salt output main pipe and a molten salt return main pipe provided by the present invention; Figure 13 Schematic structural diagram of a molten salt electro-thermal long-term energy storage device with an additional independent molten salt heater provided by the present invention; Figure 14 Schematic structural diagram of a molten salt electro-thermal long-term energy storage device with an additional set of supercritical carbon dioxide generator sets provided by the present invention.

[0029] The reference numerals are as follows: molten salt storage tank 1, molten salt output branch pipe 2, first molten salt outlet 3, outlet shut-off valve 4, molten salt return branch pipe 5, first molten salt inlet 6, inlet shut-off valve 7, valve seat 8, guide rod 8a, valve flap 9, guide sleeve 9a, connecting skeleton 10, valve rod 11, electric controller 12, motor 12a, cover plate 13, through hole 14, limit block 15, groove 15a, compression spring 16, heat insulation pad 17, electric heating tube 18, electric heating tube installation port 19, molten salt 20, molten salt output main pipe 21, molten salt return main pipe 22, molten salt pump station 23, accommodation tank 23a, molten salt pump 23b, molten salt total inlet 23c, molten salt total outlet 23d, molten salt heat extractor 24, second molten salt inlet 24a, second molten salt outlet 24b, heat extraction medium inlet 24c, heat extraction medium outlet 24d, electric control system 25, short connecting pipe 26, thermal insulation housing 27, outer layer 27a, thermal insulation layer 27b, heat insulation brick 27c, thermal insulation cover 28, heat insulation coating layer 29, solid particle heat storage material 30, independent molten salt electric heater 31, molten salt branch pipe 32, stop valve 33, supercritical carbon dioxide generator set 34, generator 34a, turbine 34b, secondary compressor 34c, primary compressor 34d, pre-cooler or condenser 34e, low-temperature regenerator 34f, high-temperature regenerator 34g, supercritical carbon dioxide transmission pipeline 34h. Detailed implementation manners

[0030] The following are specific embodiments of the present invention. In combination with the accompanying drawings, the technical solutions of the present invention will be further described, but the present invention is not limited to these embodiments.

[0031] Embodiment 1: As shown in Figure 1 , Figure 2 , Figure 3 , a molten salt output branch pipe 2 is provided near the bottom of a molten salt storage tank 1. The outer port of the molten salt output branch pipe 2 is a first molten salt outlet 3, and an outlet shut-off valve 4 is provided at its inner port; the molten salt storage tank 1 is provided with a molten salt return branch pipe 5 extending from the upper part to the bottom of the storage tank, and the outer end of the molten salt return branch pipe 5 is a first molten salt inlet 6. The first molten salt inlet 6 is above the molten salt liquid level line, and an inlet shut-off valve 7 is provided at the inner port of the molten salt return branch pipe 5.

[0032] The outlet shut-off valve 4 and the inlet shut-off valve 7 have the same structure. From bottom to top, they are a valve seat 8, a valve flap 9, a connecting skeleton 10, a valve stem 11, and an electric controller 12. Among them, the valve seat 8 communicates with the inner port of the molten salt output branch pipe 2 or the molten salt return branch pipe 5. The valve stem 11 is vertically arranged, and its bottom end is assembled with the valve flap 9 through the connecting skeleton 10. The upper end of the valve stem 11 extends above the molten salt liquid level control line, and the electric controller 12 is provided at the top. The motor 12a on the electric controller 12 is partially arranged outside the top of the molten salt storage tank 1 and can drive the valve stem 11 to move up and down to realize the opening and closing of the outlet shut-off valve 4 and the inlet shut-off valve 7.

[0033] A guide rod 8a is provided at the center of the bottom of the valve seat 8, and a guide sleeve 9a matching the guide rod is provided at the center of the bottom surface of the valve flap 9. The precise cooperation between the two ensures the opening and closing cooperation between the valve seat 8 and the valve flap 9 when the valve stem 11 moves up and down.

[0034] In this embodiment, the connecting skeleton 10 is a universal coupling.

[0035] Above the set molten salt liquid level line of the molten salt storage tank 1, a horizontal cover plate 13 is further provided. Two through holes 14 for the valve stem to pass through are provided on the cover plate 13. A limiting block 15 with a diameter larger than the valve stem 11 is provided at the top of the valve stem 11. A compression spring 16 is provided on the valve stem between the cover plate 13 and the limiting block 15. A groove 15a is provided in the upper part of the limiting block 15, and a heat insulation pad 17 is provided in the groove 15a.

[0036] In this embodiment, the electric controller 12 is an electric push rod, the top end of its push rod abuts against the heat insulation pad 17. When the push rod of the electric push rod extends, its thrust compresses the compression spring 16, causing the valve stem 11 to move downward to achieve the close fit between the valve flap 9 and the valve seat 8, that is, the outlet shut-off valve 4 or the inlet shut-off valve 7 is closed at this time. When the electric push rod retracts, the elastic force of the compression spring 16 directly pushes the valve stem 11 upward, and at this time the outlet shut-off valve 4 or the inlet shut-off valve 7 opens.

[0037] Embodiment 2: As Figure 4 , Figure 5 and Figure 6 shown, a plurality of electric heating tubes 18 are further arranged in the molten salt storage tank 1, and a plurality of electric heating tube installation ports 19 are further arranged on the cover plate 13. The electric heating tube 18 has a single-head flange structure and is matched with the electric heating tube installation port 19; the rated voltage of the electric heating tube 18 is 6.6 KV or 10 KV.

[0038] Others are the same as those in Embodiment 1 and will not be elaborated here one by one.

[0039] Embodiment 3: As Figure 7 , Figure 8 , Figure 10 , Figure 11 and Figure 12 shown, a molten salt electro-thermal long-term energy storage device includes ten molten salt storage tanks 1, molten salt 20, a molten salt output main pipe 21, a molten salt return main pipe 22, a molten salt pumping station 23, a molten salt heat extractor 24, and an electric control system 25.

[0040] In this embodiment, the total number of molten salt storage tanks 1 is ten, which are evenly divided into two columns, with 5 in each column and arranged in a matrix at equal intervals; the main molten salt output pipe 21 is arranged near the bottom in the middle of the two columns of molten salt storage tanks 1, and the main molten salt return pipe 22 is arranged near the top in the middle of the two columns of molten salt storage tanks 1. Both are welded into one body by a number of three-way or four-way welded pipe joints of the same specification and segmented molten salt pipe short joints with U-shaped joints. The first molten salt outlet 3 on each molten salt storage tank 1 is respectively connected to the corresponding three-way or four-way welded pipe joint on the main molten salt output pipe 21 by welding; the first molten salt inlet 6 on each molten salt storage tank 1 is respectively connected to the corresponding three-way or four-way welded pipe joint on the main molten salt return pipe 22 by welding; each molten salt storage tank 1 leaves a certain space above the set molten salt liquid level line, and this space is filled with inert gas. The spaces of two adjacent ones are communicated through a short connecting pipe 26, and the short connecting pipe 26 penetrates the space at the top of the molten salt storage tank 1 to facilitate the balancing of the air pressure in each molten salt storage tank; two adjacent molten salt storage tanks are communicated through a short connecting pipe 26 near the set molten salt liquid level line, and the short connecting pipe 26 penetrates the uppermost part inside the molten salt storage tank to facilitate the balancing of the molten salt liquid level height in each molten salt storage tank; a corrugated expansion joint is arranged in the middle section of the short connecting pipe 26; all the molten salt storage tanks 1, the main molten salt output pipe 21, the main molten salt return pipe 22 and the molten salt pumping station 23 connected into one body by welding are integrally arranged in a rectangular trough-shaped heat preservation shell 27. The top of the heat preservation shell 27 is provided with a heat preservation cover 28, and only the motors on the electric controller 12 and the top of the molten salt pump 23b are exposed outside the heat preservation cover 28. At the same time, the main molten salt outlet 23d on the molten salt pump 23b and the external interface of the main molten salt return pipe 22 both extend out of one side wall of the heat preservation shell 27 and are connected to the molten salt heat extractor 24.

[0041] In this embodiment, the outer layer 27a of the heat preservation shell 27 is a rectangular trough-shaped reinforced concrete structure, and heat preservation and heat insulation layers 27b are arranged on its inner bottom and four inner vertical surfaces. High-temperature resistant heat insulation bricks 27c are also laid on the heat preservation and heat insulation layer 27b at the bottom, and the molten salt storage tanks 1 and the molten salt pumping station 23 are placed on the heat insulation bricks 27c; a high-temperature resistant heat insulation coating layer 29 is coated on the outer surfaces of all metal tanks and pipelines in the heat preservation shell 27, and high-temperature resistant solid particle heat storage materials 30 are also filled in the gaps.

[0042] Among them, one of the ten molten salt storage tanks 1 is guaranteed to be empty, and the outlet shut-off valve 4 in the empty molten salt storage tank is in the closed state while the inlet shut-off valve 7 is in the open state. The remaining nine molten salt storage tanks are filled with the molten salt 20. The first molten salt outlet 3 on each molten salt storage tank 1 is connected to the main molten salt output pipe 21, and the first molten salt inlet 6 on each molten salt storage tank 1 is connected to the main molten salt return pipe 22.

[0043] The molten salt pump station 23 includes a storage tank 23a and a molten salt pump 23b disposed therein. The overall height of the storage tank 23a is not lower than that of the molten salt storage tank 1. A molten salt main inlet 23c is provided at the bottom of the storage tank 23a, and the molten salt main inlet 23c is connected to the molten salt output main pipe 21. The molten salt pump 23b is provided with a molten salt main outlet 23d.

[0044] The molten salt heat exchanger 24 is provided with a second molten salt inlet 24a, a second molten salt outlet 24b, a heat extraction medium inlet 24c, and a heat extraction medium outlet 24d. Among them, the second molten salt inlet 24a is connected to the molten salt main outlet 23d, and the second molten salt outlet 24b is connected to the molten salt return main pipe 22.

[0045] The electric control system 25 has a linkage control relationship with the electric heating tubes 18 in each molten salt storage tank 1, the motor on the molten salt pump 23b, the motor 12a on the outlet shut-off valve 4, and the motor 12a on the inlet shut-off valve 7. This control relationship includes molten salt temperature control, switching of various motors, start and stop of electric heating, opening and closing of shut-off valves, etc.

[0046] This molten salt electric heat storage long-term energy storage device is provided with two working states, namely, the electric heat storage state and the heat release state. In daily operation, these two states cycle repeatedly. The following briefly describes these two working states respectively.

[0047] 1. When the long-duration energy storage device is in the electric heat storage working state: The inlet shut-off valve 7 and the outlet shut-off valve 4 on one empty molten salt storage tank 1 and nine molten salt storage tanks 1 filled with molten salt 20 are all in the closed state. At this time, the electric heating tube 18 in the empty molten salt storage tank 1 does not generate heat, and the molten salt in the nine molten salt storage tanks 1 filled with molten salt 20 is statically heated by the electric heating tube 18 to the set upper temperature limit, completing a complete molten salt electric heat storage process. 2. When the long-duration energy storage device is in the heat release working state: The outlet shut-off valve 4 on one empty molten salt storage tank 1 is in the closed state while the inlet shut-off valve 7 is in the open state. The inlet shut-off valves 7 on the remaining nine molten salt storage tanks 1 filled with molten salt 20 are all in the closed state, and only the outlet shut-off valve 4 on one molten salt storage tank 1 waiting to be emptied by the molten salt pump 23b is in the open state. At this time, the heated molten salt 20 is pumped into the molten salt heat exchanger 24 by the molten salt pump 23 to release heat, and the molten salt 20 after heat release flows back into the empty molten salt storage tank 1 through the molten salt return main pipe 22 until all the molten salt in this molten salt storage tank 1 is emptied into the empty molten salt storage tank, and this molten salt storage tank 1 then becomes a new empty molten salt storage tank 1. In this way, the nine molten salt storage tanks 1 filled with molten salt 20 will be emptied in turn, completing a complete molten salt heat release cycle, and then entering the next round of electric heat storage process. In each round of molten salt heat release process, there is no fixed molten salt storage tank 1 that is empty. Each molten salt storage tank 1 filled with molten salt 20 will experience being empty once, and the originally empty molten salt storage tank will also be filled with molten salt pumped from other storage tanks. Then, after one round of heat release, it enters the next round of electric heat storage process, and thus the heat storage - heat release - heat storage cycle continues continuously.

[0048] The installation height of the molten salt heat exchanger 24 and its second molten salt outlet 24b and second molten salt inlet 24a are all higher than the height of the molten salt total outlet 23d on the molten salt pump 23b. The installation heights of the molten salt return main pipe 22 and the first molten salt inlet 6 are all higher than the liquid level height of the molten salt in the molten salt storage tank 1, and the molten salt return main pipe 22 is installed inclined downward towards the first molten salt inlet 6. Usually, such a design ensures that when the molten salt pump 23b stops working, the molten salt 20 in the molten salt heat exchanger 24 and the molten salt in the molten salt return main pipe 22 automatically flow back into the molten salt storage tank 1 or the molten salt pump receiving tank 23a, eliminating the blockage caused by the freezing of the molten salt 20 staying in the molten salt heat exchanger 24 or the molten salt return main pipe 22 due to temperature reduction when the system shuts down.

[0049] Example 4: As Figure 13As shown in the figure, the molten salt electro-thermal long-term energy storage device further includes an independent molten salt electro-heater 31, which is arranged between the molten salt pump station 23 and the molten salt heat extractor 24. There is a molten salt branch pipe 32 connecting the molten salt output main pipe 21 and the molten salt return main pipe 22, and a stop valve 33 is arranged on the molten salt branch pipe 32. In this way, a small circulation loop is formed by the molten salt pump station 23, the independent molten salt electro-heater 31, the molten salt heat extractor 24, the molten salt branch pipe 32 and the stop valve 33.

[0050] Others are the same as those in Embodiment 3 and will not be described in detail here.

[0051] Embodiment 5: As Figure 14 shown in the figure, the molten salt electro-thermal long-term energy storage device further includes a supercritical carbon dioxide power generation unit 34, which includes a generator 34a, a turbine 34b, a secondary compressor 34c, a primary compressor 34d, a precooler or condenser 34e, a low-temperature recuperator 34f, a high-temperature recuperator 34g and a supercritical carbon dioxide transmission pipeline 34h. A complete supercritical carbon dioxide Brayton cycle power generation system is formed by connecting through the supercritical carbon dioxide transmission pipeline 34h and the heat extraction medium outlet 24d and the heat extraction medium inlet 24c on the molten salt heat extractor 24.

[0052] Others are the same as those in Embodiment 3 and will not be described in detail here.

[0053] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Those skilled in the art to which the present invention pertains can make various modifications or supplements to the described specific embodiments or use similar ways to replace them, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

[0054] Although terms such as molten salt storage tank, molten salt output branch pipe, first molten salt outlet, outlet shut-off valve, molten salt return branch pipe, first molten salt inlet, inlet shut-off valve, valve seat, guide rod, valve flap, guide sleeve, connecting skeleton, valve stem, electric controller, motor, cover plate, through hole, limit block, groove, compression spring, heat insulation pad, electric heating tube, electric heating tube installation opening, molten salt, molten salt output main pipe, molten salt return main pipe, molten salt pumping station, storage tank, molten salt pump, molten salt total inlet, molten salt total outlet, molten salt heat extractor, second molten salt inlet, second molten salt outlet, heat extraction medium inlet, heat extraction medium outlet, electric control system, short connecting pipe, thermal insulation housing, outer layer, thermal insulation layer, thermal insulation cover, heat insulation coating layer, solid particle heat storage material, independent molten salt electric heater, molten salt branch pipe, stop valve, supercritical carbon dioxide power generation unit, generator, turbine, second-stage compressor, first-stage compressor, pre-cooler or condenser, low-temperature recuperator, high-temperature recuperator, supercritical carbon dioxide transmission pipeline, etc. are used more frequently in this text, it does not exclude the possibility of using other terms. These terms are only used to more conveniently describe and explain the essence of the present invention; interpreting them as any additional limitation is contrary to the spirit of the present invention.

Claims

1. A molten salt storage tank, characterized in that, a molten salt output branch pipe is arranged near the bottom of the molten salt storage tank, and the outer port of the molten salt output branch pipe is the first molten salt outlet, and an outlet shut-off valve is arranged at its inner port. The molten salt storage tank is provided with a molten salt return branch pipe extending from the upper part to the bottom of the storage tank, and the outer end of the molten salt return branch pipe is the first molten salt inlet. The first molten salt inlet is above the molten salt liquid level line, and an inlet shut-off valve is arranged at the inner port of the molten salt return branch pipe; the outlet shut-off valve and the inlet shut-off valve have the same structure, which are, from bottom to top, a valve seat, a valve flap, a connecting skeleton, a valve rod, and an electric controller. Among them, the valve seat is communicated with the inner port of the molten salt output pipe or the molten salt return pipe. The valve rod is arranged vertically, and its bottom end is assembled with the valve flap through the connecting skeleton. The upper end of the valve rod extends above the molten salt liquid level control line, and the electric controller is arranged at the top. The motor part on the electric controller is arranged outside the top of the molten salt storage tank and can drive the valve rod to move up and down to realize the opening and closing of the outlet shut-off valve and the inlet shut-off valve. A guide rod is arranged at the center of the bottom of the valve seat, and a guide sleeve matching the guide rod is arranged at the center of the bottom surface of the valve flap. The precise cooperation between the two ensures the opening and closing cooperation between the valve seat and the valve flap when the valve rod moves up and down; the connecting skeleton is a universal coupling. A horizontal cover plate is arranged above the set molten salt liquid level line of the molten salt storage tank. Two through holes for the valve rod to pass through are arranged on the cover plate. A limiting block with a diameter larger than the valve rod is arranged at the top of the valve rod. A compression spring is arranged on the valve rod between the cover plate and the limiting block. A groove is arranged at the upper part of the limiting block, and a heat insulation pad is arranged in the groove. The electric controller is an electric push rod, and the top of its push rod abuts against the heat insulation pad. When the push rod of the electric push rod extends, its thrust compresses the compression spring and makes the valve rod move downwards to realize the close fit between the valve flap and the valve seat, that is, the shut-off valve is closed at this time. When the electric push rod retracts, the elastic force of the compression spring directly pushes the valve rod upwards, and the shut-off valve is opened at this time.

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