A molten salt cryogenic tank with a liquid level following annular cavity

Abstract

This invention belongs to the field of molten salt energy storage technology, specifically relating to a molten salt cryogenic tank with a liquid level-following annular cavity. The molten salt cryogenic tank includes a tank body and a stirrer. The stirrer includes a motor mounted on the top of the tank body, a drive shaft extending into the tank body, and a flow impeller mounted on the lower end of the drive shaft. An inlet and an outlet are provided on the side wall of the tank body. An annular cavity is provided at the molten salt surface. The annular cavity has a hollow structure, and its overall density is less than that of the molten salt, allowing it to float on the molten salt surface and move up and down with changes in the molten salt level. A central opening in the annular cavity allows the drive shaft and flow impeller to pass through. This invention, through continuous regulation of the molten salt near the liquid surface by the annular cavity, weakens the liquid surface temperature stratification and flow stagnation phenomena, raises the minimum temperature inside the cryogenic tank, reduces the risk of molten salt crystallization in the liquid surface area, and improves the safety and reliability of the cryogenic tank operation.

Description

Technical Field

[0001] This invention belongs to the field of molten salt energy storage technology, specifically relating to a molten salt cryogenic tank with a liquid level-following annular cavity. Background Technology

[0002] With the development of new energy technologies and the increasing demand for peak shaving in power systems, molten salt energy storage technology has been widely applied in fields such as concentrated solar power (CSP), industrial waste heat recovery, and the flexible retrofitting of thermal power units due to its advantages, including large storage capacity, high operating temperature, long heat storage time, and good coupling with thermal power generation systems. Using molten salt as a heat storage medium, thermal energy is stored and released in the form of sensible heat, realizing the time-series transfer of energy and improving the overall utilization efficiency of the energy system. This is of great significance for promoting the consumption of renewable energy and enhancing the stability of the energy system.

[0003] In typical molten salt energy storage systems, a combination of high-temperature and low-temperature tanks is used to achieve energy storage and retrieval. During the heat absorption phase, the low-temperature molten salt is pumped from the low-temperature tank to a heat collector or heat exchanger to absorb heat, and after its temperature rises, it enters the high-temperature tank for storage. During the heat release phase, the high-temperature molten salt flows out of the high-temperature tank and releases heat to the working fluid through a heat exchanger to generate electricity or provide heat. The cooled molten salt then returns to the low-temperature tank to complete the cycle. Therefore, the low-temperature tank not only serves the function of molten salt storage but also directly affects the continuous operation and reliability of the molten salt system.

[0004] However, in practical engineering applications, the temperature of the molten salt inside cryogenic tanks is usually close to the solidification temperature range of molten salt. Furthermore, under conditions such as nighttime, low load, or system shutdown, the temperature of the molten salt inside the tank is prone to fluctuations. If the temperature in a localized area becomes too low, it can easily lead to crystallization or even solidification of the molten salt, resulting in serious problems such as pipeline blockage, equipment damage, and system startup failure. Therefore, cryogenic tanks typically require electric heating or other heating methods to maintain the molten salt within a safe liquid temperature range.

[0005] From the perspective of the thermal and flow characteristics inside the cryogenic tank, the molten salt surface area is often the most prone to problems. On the one hand, the space above the molten salt surface is a gas phase space, where heat is easily dissipated outward through convection and radiation. On the other hand, the area near the tank wall is affected by both heat dissipation from the tank wall and heat dissipation from the liquid surface, easily forming the coldest area. In addition, due to the high density and viscosity of molten salt, its natural convection capacity is weak, and a stagnant flow zone easily forms near the liquid surface, causing the cold molten salt to remain in this area for a long time, thus significantly increasing the risk of local crystallization.

[0006] To improve temperature distribution within cryogenic tanks, existing technologies typically install heating devices on the tank's sidewalls or bottom and equip the tank with agitators to promote molten salt circulation. However, conventional stirring methods have limited effectiveness in cryogenic tanks. Since heating is mainly concentrated on the tank's sidewalls and bottom, and agitators primarily operate with axial or radial stirring in the central area, they are insufficient to effectively reach the molten salt surface and its edges, leading to the formation of cryogenic stagnation zones near the surface. Especially when using downward-flowing axial impellers, if the molten salt in the surface area cannot be promptly guided into the downstream mainstream, it may continue to circulate repeatedly near the surface, failing to receive sufficient heat replenishment from the heating zone.

[0007] Therefore, existing cryogenic tank technology still has shortcomings in terms of temperature uniformity in the liquid surface area, molten salt exchange efficiency, and crystallization risk control. There is an urgent need for a structural solution that can effectively regulate the molten salt liquid surface area to improve the safety and reliability of cryogenic tank operation without significantly increasing system complexity and energy consumption. Summary of the Invention

[0008] To address the above problems, this invention provides a molten salt cryogenic tank with a liquid level-following annular cavity, comprising a tank body and a stirrer. The stirrer includes a motor mounted on the top of the tank body, a drive shaft extending into the tank body, and a flow impeller mounted on the lower end of the drive shaft. The tank body has an inlet and an outlet on its side wall. In particular, an annular cavity is provided at the molten salt surface. The annular cavity has a hollow structure, and its overall density is less than that of the molten salt, allowing it to float on the molten salt surface and move up and down with the molten salt level. The central opening of the annular cavity is used for the drive shaft and the flow impeller to pass through.

[0009] This invention utilizes an annular cavity at the molten salt surface that moves up and down with changes in liquid level to actively regulate the most unstable liquid surface area in a cryogenic tank from both structural and flow mechanism perspectives. Because the overall density of the annular cavity is less than that of the molten salt, it always floats and adheres to the molten salt surface, thus continuously acting on the liquid surface—the location with the strongest heat dissipation and most prone to forming cryogenic stagnation zones—when the liquid level rises or falls, operating conditions change, or heat load fluctuates. Simultaneously, the annular cavity is arranged around the drive shaft and coaxial with the impeller, ensuring that the molten salt near the liquid surface is no longer in a free and unconstrained state during agitator operation. Instead, it is guided and participates in the overall circulating flow field centered on the impeller, prompting the cold molten salt near the liquid surface to promptly enter the downward mainstream and be transported to the lower heating area of ​​the tank. Therefore, this invention, through a liquid surface-following structure and flow field synergy, weakens temperature stratification and flow stagnation in the liquid surface area, raises the lowest temperature near the liquid surface, and reduces the risk of molten salt crystallization in the liquid surface area.

[0010] Furthermore, the tank body is a vertical cylindrical shape, and the annular cavity is a disc-shaped structure with a central opening. With the tank body adopting a vertical cylindrical structure and the annular cavity adopting a disc-shaped structure with a central opening, the annular cavity can achieve a highly compatible and synergistic relationship with the cryogenic tank and agitator in terms of geometry and flow. On the one hand, the vertical cylindrical tank body has axisymmetric characteristics, and the disc-shaped annular cavity is also arranged around the central axis of the tank body, ensuring uniform force and stable posture on the molten salt surface, avoiding tilting or displacement due to liquid level fluctuations or flow field disturbances, thus ensuring the continuity and reliability of liquid level control. On the other hand, the central opening structure provides natural space for the drive shaft and impeller, allowing the annular cavity to always be arranged circumferentially around the impeller without interfering with the normal operation of the agitator. This allows the molten salt near the liquid surface to be uniformly guided into the overall circulation path in an axisymmetric flow field. This not only improves the stability of the annular cavity when moving with the liquid surface but also strengthens its coupling effect with the agitation flow field, which is beneficial for achieving uniform control of temperature and flow state in the liquid surface area.

[0011] Furthermore, the annular cavity is made of carbon steel, which, while meeting the requirements for strength and temperature resistance, also provides a large heat capacity and good structural stability. It can act as a thermal buffer when the temperature of the molten salt surface fluctuates, thereby slowing down the cooling rate of the liquid surface area. At the same time, carbon steel is widely available and its processing and welding technologies are mature, making it easy to manufacture the annular cavity into a sealed hollow structure and ensure its long-term reliable operation. This helps to reduce manufacturing costs and improve the feasibility of engineering applications while ensuring performance.

[0012] Furthermore, the annular cavity is composed of multiple circumferentially connected hollow segments, enabling the overall structure to maintain its annular continuity while possessing better structural flexibility and engineering adaptability. By segmenting the overall cavity, the thermal expansion stress generated during temperature changes can be effectively released; at the same time, the segmented hollow structure facilitates processing, manufacturing, assembly, and maintenance, improving the reliability and safety of the annular cavity under long-term high-temperature operating conditions while ensuring buoyancy and thermal buffering effects.

[0013] Furthermore, the height of the outer side of the upper surface of the annular cavity is greater than the height of the inner side; that is, the upper surface of the annular cavity is designed with a height distribution pattern of higher outer side and lower inner side. This makes it easier for the molten salt flowing back near the liquid surface to converge from the edge of the tank to the center area under the influence of gravity and flow field. This facilitates the entry of the molten salt into the downward mainstream channel centered on the impeller, reducing the residence time at the liquid surface edge. At the same time, this structure of higher outer side and lower inner side can also weaken the impact of liquid surface fluctuations on the stability of the annular cavity, allowing it to maintain a more stable posture as it moves up and down with the liquid surface, thereby improving the reliability of temperature control in the liquid surface area and reducing the risk of local crystallization.

[0014] Furthermore, the lower surface of the annular cavity is planar, which forms a stable and continuous interface near the molten salt surface. This facilitates the molten salt near the surface to participate in the overall circulation along a regular path under stirring, avoiding dead zones or uneven temperature distribution caused by local undulations. At the same time, the planar shape of the lower surface helps to evenly distribute the heat and flow from the molten salt, enabling the annular cavity to exert a stable guiding and thermal buffering effect on the surface area during its movement with the liquid surface, thereby improving the controllability and reliability of the temperature regulation of the cryogenic tank.

[0015] Furthermore, the lower surface of the annular cavity is provided with multiple radially extending grooves, which allow the molten salt that has been heated and has flowed back along the side wall of the tank to be guided in an orderly manner to the inner side of the annular cavity and near the impeller after reaching the liquid surface area. This allows the heat input from the side wall and bottom heating to be more effectively transferred to the liquid surface and the central downward mainstream area. This strengthens the layer exchange process between the molten salt near the liquid surface and the lower molten salt, weakens the low-temperature retention layer formed in the liquid surface area due to heat dissipation to the gas phase, raises the minimum temperature in the low-temperature tank, and reduces the risk of crystallization of molten salt in the liquid surface area.

[0016] Furthermore, multiple radially separated guide ribs are arranged radially inward at the inner diameter of the annular cavity, dividing the molten salt guided through the annular cavity to the inner diameter region into multiple radial flow channels. This weakens the circumferential flow phenomenon that easily forms at the inner diameter, allowing the molten salt to enter the impeller's suction area in a more stable and uniform manner. By rectifying and splitting the flow entering the central region, not only are the adverse effects of local high-speed jets on the liquid surface and stirring stability avoided, but continuous layer exchange between the liquid surface and the downstream mainstream is also promoted. This helps to raise the minimum temperature of the liquid surface region and improve the safety and reliability of the cryogenic tank operation.

[0017] Furthermore, the radial guide ribs are formed by the radial extension of the lower surface of the annular cavity, allowing them to directly act on the flow region below the molten salt surface, thereby forming a closer flow field synergy with the push-type impeller. This structure can perform three-dimensional guidance and rectification of the flow near the liquid surface on the molten salt side, so that the molten salt guided by the annular cavity can more stably flow into the downward mainstream channel after entering the inner diameter region, reducing flow stagnation and temperature stratification in the liquid surface region, improving the liquid surface layer exchange efficiency and reducing the risk of local crystallization.

[0018] Furthermore, the radial guide ribs and the annular cavity are integrally molded, avoiding assembly errors and weak points caused by separate connections. This ensures that the guide ribs maintain a stable relative position as they float and move up and down with the annular cavity, thereby guaranteeing the continuity and consistency of liquid surface guidance and rectification. At the same time, the integrally molded structure helps to improve the overall structural strength and heat cycle resistance, reducing the risk of loosening or failure caused by thermal expansion and contraction in high-temperature molten salt environments, and improving the reliability and service life of the annular cavity under long-term operating conditions.

[0019] The beneficial effects of this invention are: (1) The present invention provides an annular cavity at the surface of the molten salt that can move up and down with the change of liquid level, so that it always acts on the liquid surface area in the cryogenic tank where heat dissipation is strongest and where low temperature stagnation is most likely to occur; through the continuous regulation of the molten salt near the liquid surface by the annular cavity, the liquid surface area is no longer in a free and unconstrained state, but is stably incorporated into the overall circulating flow field, thereby weakening the phenomenon of liquid surface temperature stratification and flow stagnation, raising the minimum temperature in the cryogenic tank, reducing the risk of molten salt crystallization in the liquid surface area, and improving the safety and reliability of cryogenic tank operation.

[0020] (2) The present invention sets the upper surface of the annular cavity in a height distribution pattern with the outer side higher and the inner side lower, which can form a guiding trend towards the center of the tank in terms of geometry. This makes it easier for the molten salt near the liquid surface to gather from the edge of the tank to the central area and enter the downward mainstream channel under the combined action of gravity and flow field. This structure not only helps to shorten the residence time of the molten salt on the liquid surface and strengthen the layer exchange process between the liquid surface and the lower molten salt, but also improves the attitude stability of the annular cavity when it moves up and down with the liquid surface, thereby further reducing the crystallization risk in the liquid surface area.

[0021] (3) The lower surface of the annular cavity of the present invention is set as a plane and forms multiple radially extending grooves, so that the molten salt that has been heated and has returned along the side wall and bottom of the tank can be orderly guided to the inner side of the annular cavity and the vicinity of the impeller after reaching the liquid surface area, thereby more effectively transporting the heat input from the heating area to the liquid surface and the central downward mainstream area; this structure enhances the layer exchange efficiency of the liquid surface area, weakens the low temperature retention layer formed near the liquid surface due to heat dissipation to the gas phase, and is conducive to raising the minimum temperature of the system.

[0022] (4) The present invention sets multiple radial guide ribs that are separated from each other at the inner diameter of the annular cavity, which divides the molten salt guided to the inner diameter area through the annular cavity into multiple radial flow channels, weakens the circumferential flow phenomenon that is easy to occur at the inner diameter, and makes the molten salt enter the suction area of ​​the impeller more stably and uniformly; by rectifying and splitting the flow entering the central area, the unstable flow caused by local high-speed jet is avoided, and the continuous layer replacement between the liquid surface and the downward mainstream is promoted, which improves the stability and anti-crystallization ability of the cryogenic tank.

[0023] Based on the above beneficial effects, this invention has good application prospects in the field of molten salt energy storage technology. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a molten salt cryogenic tank with a liquid level-following annular cavity.

[0025] Figure 2 This is a schematic diagram of a top view of an annular cavity.

[0026] Figure 3 This is a schematic diagram of a cross-section of another type of annular cavity.

[0027] In the diagram: 1. Tank body; 2. Motor; 3. Drive shaft; 4. Flow impeller; 5. Inlet; 6. Outlet; 7. Annular cavity. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments.

[0029] This embodiment provides a molten salt cryogenic tank with a liquid level-following annular cavity, such as... Figure 1 and Figure 2 As shown, the system includes a tank 1 and a stirrer. The tank 1 is a vertical cylindrical structure with an inner diameter of 5m to 15m and a height of 6m to 12m. It is constructed from carbon steel or low-alloy steel and welded together. An inlet 5 and an outlet 6 are provided on the side wall of the tank 1. The inlet 5 is used to replenish the cryogenic tank with reflux molten salt, and the outlet 6 is used to transport the molten salt to an external pipeline system. The inlet 5 and outlet 6 are located at different heights on the side wall of the tank. Preferably, the inlet 5 is located on the upper middle side wall to facilitate the diffusion of the reflux molten salt within the tank, and the outlet 6 is located on the lower middle side wall to facilitate stable salt supply and reduce liquid surface disturbance. An insulation layer and an electric heating layer (not shown) may be installed on the outside of the tank 1 to reduce heat loss and maintain the molten salt within its liquid temperature range. For example, the sidewall and bottom of tank 1 can adopt a composite structure of "corrosion-resistant inner lining / anti-corrosion layer - heating layer - insulation layer - outer protective layer": a protective layer resistant to high temperature and nitrate corrosion (such as stainless steel / nickel-based alloy lining, or high temperature corrosion-resistant coating such as aluminized / ceramic corrosion-resistant coating) is preferably set on the side close to the molten salt to reduce the corrosion and penetration of molten salt on the carbon steel substrate; a heating layer is arranged on the outside of it, preferably using electric heating, but jacket or coil heating can also be used to supplement heat during shutdown, low load or start-up phases to prevent crystallization; an insulation layer is set on the outside of the heating layer, preferably using high temperature resistant insulation materials such as rock wool, aluminum silicate fiber felt, aerogel felt, etc., to significantly reduce heat loss to the environment; the outermost layer is a metal outer protective layer (such as galvanized plate / stainless steel plate) for protection against wind and rain, mechanical damage and fixing the insulation structure, thereby achieving corrosion resistance, temperature stability and energy saving effect during long-term operation of the tank.

[0030] The agitator is installed on the top of the tank 1 and includes a motor 2, a drive shaft 3, and an impeller 4 located at the lower end of the drive shaft 3. The motor 2 is fixed to a support on the top of the tank 1. The drive shaft 3 extends axially downward into the tank 1 and is made of heat-resistant steel, which can be used with a mechanical seal or a packing seal to achieve a tank top seal. The impeller 4 is an axial flow impeller, installed at the lower end of the drive shaft 3. The impeller diameter is 0.3 to 0.4 times the inner diameter of the tank 1; the installation depth of the impeller below the molten salt surface is 0.2 to 0.4 m to ensure a stable axial circulating flow field during operation. The motor 2 drives the drive shaft 3 to rotate, which in turn drives the impeller 4 to rotate. Using a downward pushing flow method, the molten salt forms a circulating flow within the tank, flowing downward from the center and upward back up against the wall, thereby promoting uniform molten salt temperature.

[0031] To address the issues of easy heat dissipation, formation of a low-temperature stagnant layer, and crystallization at the surface of the molten salt in the cryogenic tank, this embodiment incorporates an annular cavity 7 at the molten salt surface. The annular cavity 7 is a disc-shaped structure with a central opening, forming an annular disc. The central opening is coaxially fitted onto the outside of the drive shaft 3, allowing the drive shaft 3 and impeller 4 to pass through, ensuring normal operation of the agitator and that the annular cavity 7 is always arranged around the agitator axis. The annular cavity 7 is made of carbon steel, using Q235 or Q345 carbon steel plates rolled and welded to form a sealed cavity structure. Its interior is hollow and sealed, either as a vacuum or filled with inert gas to ensure long-term buoyancy stability and prevent the introduction of impurities. Because the annular cavity 7 is hollow, its overall average density is less than that of the molten salt, allowing it to float on the molten salt surface and automatically move up and down with the molten salt level, always remaining near the surface to function effectively.

[0032] The outer diameter of the annular cavity 7 is 0.7 to 0.9 times the inner diameter of the tank body to cover the high heat dissipation area at the edge of the liquid surface while avoiding interference with the tank wall. The inner diameter of the annular cavity 7 is larger than the outer diameter of the impeller 4, with an installation clearance. The radial width of the annular cavity 7 is 0.1 to 0.2 times the inner diameter of the tank body 1. The thickness (height) of the annular cavity 7 is 0.08 to 0.20 m. The buoyancy of the annular cavity 7 can be matched by the cavity volume to ensure that it has a preset salt depth at the liquid surface, such as an immersion depth of 10 to 30 mm, to ensure stable floating and sufficient heat exchange with the molten salt near the liquid surface.

[0033] In this embodiment, the cryogenic tank maintains and replenishes the molten salt under the influence of the sidewall and bottom heating layers. However, due to the strong heat dissipation from the liquid surface to the gas phase, the liquid surface and its edge areas may still become the lowest temperature zone and form a stagnant flow layer. The annular cavity 7 floats and adheres to the liquid surface, ensuring it always covers the key areas of the liquid surface. Simultaneously, the annular cavity 7, surrounding the stirring axis, constrains and guides the flow near the liquid surface, making it easier for the molten salt near the liquid surface to be entrained into the overall circulating flow field generated by the impeller 4, promoting continuous layer exchange between the liquid surface area and the lower molten salt in the tank. Because the annular cavity 7 is made of carbon steel, it has a large heat capacity and good structural stability, and can play a certain thermal buffering role when the liquid surface heat load fluctuates, thereby slowing down the temperature drop trend in the liquid surface area. Through the coordinated use of the liquid surface-following floating structure and the stirring circulating flow field, the temperature stratification and stagnant flow phenomenon in the liquid surface area of ​​the cryogenic tank are weakened, the lowest temperature near the liquid surface is raised, the risk of crystallization of molten salt in the liquid surface area of ​​the cryogenic tank is reduced, and the safety and reliability of the cryogenic tank operation are improved.

[0034] Example 2 Based on Embodiment 1, the annular cavity 7 is composed of multiple hollow segments connected sequentially along the circumference. Each hollow segment is connected end to end in the circumference to form a closed annular cavity structure.

[0035] Each hollow section is an arc-shaped hollow cavity with an arc length of 45. o ~90 o The corresponding circumferential angles are designed to ensure structural continuity while facilitating manufacturing and assembly. Each hollow section is formed by bending and welding carbon steel plates, with an internal cavity structure sealed by a full-penetration weld to ensure buoyancy stability and reliability for long-term high-temperature operation.

[0036] Adjacent hollow sections can be circumferentially connected via butt welding, flange connections, or plug-in connection structures. Butt welding is preferred, as it allows multiple hollow sections to form a continuous, stress-bearing annular structure after connection. In applications requiring thermal expansion absorption or easy disassembly and maintenance, compensating gaps or flexible connection structures can be incorporated at the joints. After connection, the upper and lower surfaces of each hollow section remain substantially flush circumferentially to ensure uniform stress and stability of the annular cavity when floating on the molten salt surface.

[0037] This embodiment decomposes the annular cavity 7 into multiple hollow segments connected circumferentially, which not only facilitates segmented manufacturing, transportation and on-site assembly, but also effectively releases the circumferential stress generated by thermal expansion and contraction of the overall annular structure during temperature changes, reducing the risk of structural deformation or weld failure, and improving the structural reliability and safety of the annular cavity 7 under long-term operation conditions in the molten salt cryogenic tank.

[0038] Example 3 Based on Embodiment 1 or 2, the upper surface of the annular cavity 7 is configured with a radially higher outer side and a lower inner side height distribution. Using the height of the upper surface at the inner edge of the annular cavity 7 as a reference, the height of the upper surface at the outer edge of the annular cavity 7 is 20-50 mm higher than the height of the upper surface at the inner edge, thus forming a radially inclined slope structure on the upper surface of the annular cavity. The inclination angle (relative to the horizontal plane) of the slope structure is 3°. o ~6 o The aforementioned upper surface structure, with its higher outer surface and lower inner surface, allows molten salt near the liquid surface to more easily converge from the tank edge to the central region under the influence of gravity and flow field, and enter the vicinity of the impeller 4. This enhances the layer exchange process in the liquid surface area and reduces the risk of local crystallization.

[0039] Example 4 Based on Example 3, the lower surface of the annular cavity 7 is configured as an integral planar structure, and the lower surface of the annular cavity 7 is located within the same horizontal reference plane. When the annular cavity 7 is formed by connecting adjacent hollow sections, the height deviation of the lower surface at the connection point is controlled to be ≤1mm, so as to ensure that the contact state between the lower surface of the annular cavity 7 and the molten salt is continuous and stable when the annular cavity 7 floats on the surface of the molten salt.

[0040] In this embodiment, by making the lower surface of the annular cavity 7 a plane, the molten salt near the liquid surface can participate in the overall circulation flow along a regular and continuous interface during the operation of the agitator, avoiding the formation of stagnant dead zones or uneven flow in the liquid surface area due to local undulations or abrupt structural changes. At the same time, the flat lower surface helps the annular cavity 7 maintain uniform force and stable posture as it moves up and down with the liquid surface.

[0041] Example 5 Based on Example 4, a plurality of radially extending grooves are provided on the lower surface of the annular cavity 7. The grooves extend from the outer edge of the annular cavity 7 toward the central opening and are evenly distributed in the circumferential direction to form a clear radial flow path near the molten salt surface. The number of grooves is 12 to 24, so as to ensure the flow guiding effect while taking into account the overall strength of the annular cavity and the manufacturing feasibility.

[0042] The cross-sectional shape of each groove is circular or trapezoidal to reduce stress concentration and lower the risk of structural fatigue under high-temperature conditions. The geometric dimensions of the grooves are as follows: groove depth is 3–10 mm; groove width is 10–40 mm; groove length is 0.6–0.9 times the radial width of the annular cavity. The circumferential spacing between adjacent grooves is 2–5 times the groove width to avoid the grooves being too dense, which would weaken the structure, or too sparse, which would affect the flow guiding effect.

[0043] In a further preferred embodiment, the depth of the groove is kept constant radially, or it can gradually increase from the outer edge to the inner edge. Preferably, the groove depth on the side near the central opening is increased by 0-5 mm compared to the outer edge side to enhance the guiding ability of molten salt to converge into the inner diameter region. For annular cavities composed of multiple circumferentially connected hollow sections, grooves are respectively disposed on the lower surface of each hollow section, and the misalignment of the grooves at the connection of adjacent hollow sections is preferably controlled within 2 mm to ensure the continuity of liquid surface guidance and avoid local salt accumulation.

[0044] In this embodiment, by setting the aforementioned radial groove on the lower surface of the annular cavity 7, the molten salt that flows back to the liquid surface area after being heated on the side wall and bottom of the tank 1 can be transported in an orderly manner to the central area along the low-resistance channel formed by the groove, and is promptly drawn into and sinks near the downward mainstream formed by the impeller 4. This strengthens the layer exchange process between the molten salt near the liquid surface and the lower molten salt, weakens the low-temperature retention layer formed in the liquid surface area due to heat dissipation to the gas phase, further raises the minimum temperature in the low-temperature tank and reduces the risk of crystallization.

[0045] Example 6 Based on Embodiment 5, multiple radial guide ribs are provided at the inner diameter of the annular cavity 7, extending horizontally inward toward the drive shaft 3 and separated from each other. The radial guide ribs are located in the plane of the lower surface of the annular cavity 7 or are flush with the lower surface, and their extension direction is consistent with the radial direction of the annular cavity 7, which is used to guide and rectify the molten salt entering the inner diameter region in the plane.

[0046] Radial guide ribs are arranged at intervals in the circumferential direction, with a number ranging from 12 to 24, to weaken the circumferential flow in the inner diameter region while avoiding excessive blockage of the central flow field. The radial length of each radial guide rib is 0.1 to 0.2 times the inner diameter of the annular cavity 7, and its inner end maintains a safe clearance from the outer side of the impeller 4 to avoid collision with the impeller 4. The thickness of the radial guide ribs is 8 to 20 mm to balance the guiding effect, structural strength, and thermal expansion adaptability under high-temperature conditions.

[0047] Furthermore, the guide ribs are integrally formed by extending radially from the lower surface of the annular cavity 7. That is, the radial guide ribs and the annular cavity 7 are located in the same horizontal reference plane and are manufactured using an integral molding structure. The integral molding method avoids structural weaknesses caused by subsequent welding or assembly connections, ensuring that the radial guide ribs maintain a stable relative positional relationship as the annular cavity 7 moves up and down with the liquid surface, thereby guaranteeing the long-term consistency and reliability of the guiding effect.

[0048] In this embodiment, the radial guide ribs and the radial grooves in Embodiment 5 can be aligned in the circumferential direction or staggered, preferably staggered, so that the molten salt that gathers in the inner diameter region along the radial grooves is first divided and redistributed in the plane by the radial guide ribs before entering the central region, thereby weakening the circumferential swirling trend of the inner diameter region, so that the molten salt enters the suction area of ​​the impeller in a more uniform and stable state and participates in the downward mainstream circulation.

[0049] This embodiment achieves precise flow guidance and rectification control of molten salt near the liquid surface in a plane by setting the aforementioned horizontally extending radial guide ribs at the inner diameter of the annular cavity 7. This allows the molten salt in the liquid surface area guided by the radial grooves to enter the central circulation channel in an orderly manner, strengthening the layer exchange process between the liquid surface and the lower molten salt, improving the operational stability of the cryogenic tank and reducing the risk of crystallization in the liquid surface area.

[0050] Example 7 Based on Example 6, the bottom surface of tank 1 is designed as a concave structure. This allows the downward molten salt flow generated by the agitator to be naturally guided towards the center of tank 1 as it approaches the bottom, forming a continuous and smooth upward reflux channel along the side wall of tank 1. This reduces the stagnant flow area and low-speed swirling area that are prone to occur in flat-bottomed structures. The concave bottom surface, in conjunction with the axial circulating flow field formed by the impeller 4, enhances the integrity and stability of the overall molten salt circulation within the cryogenic tank. This allows the heat input from the bottom and side wall heating to participate in the circulation more efficiently and be transferred to the liquid surface area, thereby improving the effect of the agitation flow field and increasing the liquid surface layer exchange efficiency.

[0051] Example 8 Based on Example 7, multiple trapezoidal grooves are provided vertically along the inner wall of tank 1. Preferably, the trapezoidal grooves extend continuously along the axial direction (vertical direction) of tank 1 and are evenly distributed circumferentially, with a number of 24 to 72 grooves. The circumferential spacing between adjacent trapezoidal grooves (measured from the centerline of the groove) is 100 to 250 mm to enhance near-wall disturbance while avoiding excessive density that weakens the tank wall strength. The vertical extension length of each trapezoidal groove is 0.7 to 0.95 times the effective liquid level height of the tank. In a further embodiment, the trapezoidal grooves extend upwards from 0.2 to 1.0 m above the bottom of the tank to 0.1 to 0.6 m from the highest liquid level to avoid stress concentration at the bottom structural welds and to accommodate flow control in the liquid surface area.

[0052] The trapezoidal trough has a trapezoidal groove structure in its radial cross-section, with a trough depth (radial recess depth) of 5–15 mm; a trough opening width (opening width near the molten salt side) of 30–100 mm; a trough bottom width of 10–60 mm; and an angle of 15°–30° between the two trough walls and the normal to the inner wall, to reduce stress concentration and form a stable near-wall secondary flow while considering processing feasibility. To avoid sharp corners and salt accumulation at the trough opening, rounded corners are provided at the trough opening and bottom corners, with a rounded corner radius of 3–8 mm. In a further embodiment, the trapezoidal trough is segmented, that is, the vertical direction is divided into several height segments, with the trough depth of each segment remaining constant or varying gradually; for example, the trough depth is 10–15 mm in the area near the bottom where heating is stronger, and 5–10 mm in the area near the liquid surface, to adapt to the heat flux and flow characteristics at different heights.

[0053] The trapezoidal channel is formed by rolling / pressing during the inner wall plate forming process, or it can be formed by milling, planing, or welding followed by machining after the tank body is formed. When the inner wall of the tank is equipped with an anti-corrosion layer or a corrosion-resistant lining, the trapezoidal channel can be formed simultaneously on the surface of the lining, or its surface can be further processed after the lining is installed to ensure that the channel shape is continuous with the protective layer. Through the above-mentioned vertical trapezoidal channel structure, the stagnant layer attached to the wall can be weakened and circumferential swirl can be suppressed without increasing the stirring power, so that the molten salt flowing back from the side wall forms a more stable vertical upward channel along the channel. At the same time, it enhances the near-wall disturbance and convective heat transfer coefficient, so that the heat input from the side wall / bottom heating participates in the overall circulation more quickly and is transported upward to the liquid surface area, thereby further improving the temperature uniformity inside the tank and reducing the risk of crystallization in the liquid surface and near-wall areas.

[0054] It should be noted that, compared to the traditional V-shaped groove structure, the trapezoidal groove has a wider bottom and gentler corners, which is more conducive to the formation of a stable upward flow of molten salt along the wall within the groove, reducing stagnation and local swirling in the sharp corner areas, thereby reducing the risk of molten salt crystals adhering and growing within the groove. At the same time, the trapezoidal groove structure has less stress concentration under high-temperature thermal cycling conditions, making it more suitable for continuous coverage of anti-corrosion layers or corrosion-resistant linings, and has better structural reliability and engineering feasibility. Therefore, it is more suitable for application in the molten salt cryogenic tank described in this invention.

[0055] Example 9 Based on Example 8, multiple impellers 4 are arranged along the height direction on the drive shaft 3, and their diameters gradually increase from top to bottom. This gives the lower molten salt a stronger circulation driving capability while suppressing the disturbance of the upper liquid surface, thereby forming a stable axial circulation flow field that runs through the entire tank. This improves the heat transport efficiency of the bottom and sidewall heating, weakens temperature stratification, and reduces the risk of crystallization in the liquid surface area.

[0056] In summary, this invention addresses the engineering challenges of high heat dissipation, slow flow, and susceptibility to temperature stratification and crystallization in the surface area of ​​molten salt energy storage systems. It proposes a molten salt cryogenic tank structure with a moving annular cavity at the liquid surface. By incorporating an annular cavity 7 that moves vertically with the liquid level at the molten salt surface, and in conjunction with a downward-pushing impeller 4, a concave tank bottom, sidewall and bottom heating structures, and vertical trapezoidal grooves on the inner wall, the surface area actively participates in the overall circulation flow, solving the problem of insufficient stratification at the surface. This invention improves the efficiency of heat transfer from the bottom and sidewall heating to the surface area through passive flow guidance and flow field shaping, raising the minimum operating temperature of the system, reducing temperature stratification, and minimizing the risk of molten salt crystallization and salt buildup. In terms of engineering implementation, all structures are conventional metal components or have formed features, making manufacturing and maintenance easy. It is compatible with existing molten salt storage tank technologies and features high reliability and adaptability. This technology is applicable to molten salt energy storage systems for solar thermal power generation and can be further extended to industrial high-temperature thermal storage and energy peak shaving, demonstrating significant engineering value and application prospects.

[0057] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A molten salt cryogenic tank with a liquid level following annular cavity, comprising a tank body and a stirrer, the stirrer comprising a motor arranged at the top of the tank body, a drive shaft extending into the interior of the tank body and an impeller arranged at the lower end of the drive shaft, an inlet and an outlet being arranged in the side wall of the tank body; characterized in that: An annular cavity is provided at the surface of the molten salt. The annular cavity is a hollow structure, and its overall density is less than that of the molten salt, allowing it to float on the surface of the molten salt and move up and down with changes in the molten salt level. The central opening of the annular cavity is used for the transmission shaft and the impeller to pass through. The tank body is a vertical cylindrical shape, and the annular cavity is a disc-shaped structure with a central opening. The lower surface of the annular cavity has multiple radially extending grooves, the depth of which gradually increases from the outer edge to the inner edge. The inner wall of the tank body has multiple trapezoidal grooves arranged vertically.

2. The molten salt cryogenic tank with a liquid level-following annular cavity as described in claim 1, characterized in that: The annular cavity is made of carbon steel.

3. The molten salt cryogenic tank with a liquid level-following annular cavity as described in claim 1, characterized in that: The annular cavity is composed of multiple hollow segments connected circumferentially.

4. The molten salt cryogenic tank with a liquid level-following annular cavity as described in claim 1, characterized in that: The height of the outer side of the upper surface of the annular cavity is greater than the height of the inner side of the upper surface of the annular cavity.

5. The molten salt cryogenic tank with a liquid level-following annular cavity as described in claim 1, characterized in that: The lower surface of the annular cavity is a plane.

6. The molten salt cryogenic tank with a liquid level-following annular cavity as described in any one of claims 1-5, characterized in that: The annular cavity has multiple radially separated guide ribs arranged radially inward at its inner diameter.

7. The molten salt cryogenic tank with a liquid level-following annular cavity as described in claim 6, characterized in that: The radial guide ribs are formed by radially extending the lower surface of the annular cavity.

8. The molten salt cryogenic tank with a liquid level-following annular cavity as described in claim 7, characterized in that: The radial guide rib and the annular cavity are integrally formed.

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