A coupled heat storage system and operation method

Abstract

The present invention discloses a coupled heat storage system and an operation method, which is suitable for a heat storage system with solid materials as the main heat storage medium, and can effectively solve the problem of the gradual decrease in the outlet temperature of the heat transfer medium during the heat release process of the solid heat storage system. The coupled heat storage system in this embodiment includes a solid heat storage subsystem and a molten salt heat storage subsystem, and different heat source forms are designed respectively; wherein the molten salt heat storage subsystem only uses a single molten salt storage tank, which reduces the floor space and investment cost, and the second heat exchange tube is designed with a heating module partition inside the molten salt storage tank to improve the heat transfer coefficient. During the heat release process of the coupled heat storage system, the heat transfer medium passes through the solid heat storage subsystem and the molten salt heat storage subsystem in turn, which can keep the outlet temperature of the heat transfer medium basically unchanged.

Description

Technical Field

[0001] The present invention belongs to the field of heat storage technology, and in particular relates to a coupled heat storage system and an operation method. Background Art

[0002] At present, renewable energy sources such as wind energy and solar energy are developing rapidly year by year. In addition, the electricity consumption of the whole society is rising year by year, and the peak-to-valley difference of power grid electricity consumption is increasing day by day. The power grid has an increasingly strong demand for low-cost, long-life energy storage technology. According to the quality of energy storage, the current energy storage technology can be simply divided into electrical storage, potential energy storage and heat storage. Among them, electrical storage can be divided into lithium batteries, liquid flow batteries, supercapacitors, etc., potential energy storage can be divided into pumped storage, compressed air, flywheels, etc., and heat storage can be divided into solid heat storage, molten salt heat storage, thermochemical storage, etc.

[0003] Molten salt heat storage systems are currently widely used in solar thermal power stations, clean heating and other scenarios. They have the advantages of high heat storage temperature and long service life, but this technical solution has a large demand for molten salt heat storage media. When the heat storage temperature difference is 200°C and the heat storage capacity is 80MWh, the demand for conventional molten salt is about 1,300 tons. The prices of molten salts such as potassium nitrate, sodium nitrate, and calcium nitrate are greatly affected by the international market and raw materials. When the cost of raw materials such as potassium chloride and nitric acid rises sharply, the price of molten salt will increase significantly, which will directly affect the investment cost and implementation progress of the project. The solid heat storage system is a simple and easy heat storage technology. Its basic principle is to transfer heat to solid heat storage materials such as concrete or refractory bricks for storage. When heat is required, heat is exchanged with the solid heat storage material through the heat transfer medium. The heat storage materials of the solid heat storage system are easy to obtain and the price is stable. The raw materials do not rely on imports and are not sensitive to market price fluctuations. However, the disadvantage of the solid heat storage system is that the charging and discharging process is unstable. As heat is continuously released during the heat release process, the temperature of the solid heat storage material temperature field decreases as a whole, resulting in a continuous decrease in the outlet temperature of the heat transfer medium during the heat release process, which affects the stability of the subsequent heat utilization system. The present invention can effectively solve this problem. Summary of the invention

[0004] The present invention proposes a coupled heat storage system and operation method, which effectively couples solid heat storage with molten salt heat storage. During the heat charging process, a heat transfer medium is used as the heat source of the solid heat storage subsystem, and electric energy is used as the heat source of the molten salt heat storage subsystem. During the heat release process, the heat transfer medium passes through the solid heat storage subsystem and the molten salt heat storage subsystem in sequence. The coupled heat storage system can ensure that the outlet temperature of the heat transfer medium does not change substantially during the heat release process. The present invention is applicable to heat storage systems with solid materials as the main heat storage medium, and can effectively solve the problem of the gradual decrease in the outlet temperature of the heat transfer medium during the heat release process of the solid heat storage system.

[0005] In order to achieve the above object, the present invention adopts the following technical solution.

[0006] A coupled heat storage system comprises a first interface, a first heat exchange tube, a solid heat storage material, a second interface, a valve, a second heat exchange tube, a third interface, an electric heater, molten salt, a tank, a first heating module, a second heating module, a third heating module and a connecting pipe;

[0007] The first interface (1) and the second interface (4) are respectively located at two ends of the first heat exchange tube (2); the first heat exchange tube (2) is arranged horizontally inside the solid heat storage material (3); the first interface (1) is located at the upper part of the solid heat storage material (3); and the second interface (4) is located at the lower part of the solid heat storage material (3); the first interface (1) is connected to the inlet of the second heat exchange tube (6) through the valve (5); and the outlet of the second heat exchange tube (6) is connected to the third interface (7);

[0008] The second heat exchange tube (6) and the electric heater (8) are both arranged inside the tank body (10); the second heat exchange tube (6) is arranged upward in a spiral from the bottom of the tank body (10) and is connected to a third interface (7) at the top of the tank body (10); the electric heater (8) is located at the bottom of the tank body (10).

[0009] In some embodiments, the second heat exchange tube (6) is divided into at least three heating modules along the vertical direction inside the tank body (10), and adjacent heating modules are connected by connecting tubes (14); the distance between the connecting tubes in the vertical direction is not less than 10 times the distance between the upper and lower adjacent second heat exchange tubes (6) in the heating module, allowing the molten salt (9) between the adjacent heating modules to flow upward, thereby improving the heat transfer coefficient of the surface of the second heat exchange tube (6).

[0010] In some embodiments, the second heat exchange tube (6) is divided into a first heating module (11), a second heating module (12) and a third heating module (13) along the vertical direction inside the tank body (10); the first heating module (11) and the second heating module (12) are connected by a connecting tube (14); the second heating module (12) and the third heating module (13) are connected by another connecting tube (14).

[0011] In some embodiments, the distances between the upper and lower adjacent second heat exchange tubes (6) in the first heating module (11), the second heating module (12) and the third heating module (13) are the same.

[0012] In some embodiments, a method for operating a coupled heat storage system is proposed, wherein the system described in any of the above embodiments is operated according to the following method, specifically including two modes: a heat charging mode and a heat releasing mode:

[0013] Heat charging mode: the valve (5) is closed; a high-temperature heat transfer medium flows in from the first interface (1), passes through the first heat exchange tube (2), and flows out through the second interface (4); the high-temperature heat transfer medium transfers heat to the solid heat storage material (3) in the first heat exchange tube (2) and stores the heat in the solid heat storage material (3); at the same time, the electric heater (8) heats the molten salt (9) until the temperatures of the first heating module (11), the second heating module (12), and the third heating module (13) substantially reach the design values;

[0014] Heat release mode: the valve (5) is opened; the low-temperature heat transfer medium passes through the second interface (4), the first heat exchange tube (2), the valve (5), the second heat exchange tube (6) in sequence and flows out through the third interface (7); after the low-temperature heat transfer medium absorbs the heat released by the solid heat storage material (3) in the first heat exchange tube (2), it is determined whether the temperature of the low-temperature heat transfer medium at the valve (5) is greater than the temperature of the molten salt (9); if not, the low-temperature heat transfer medium absorbs the heat released by the molten salt (9) in the second heat exchange tube (6); if yes, the low-temperature heat transfer medium releases heat to the molten salt (9) in the second heat exchange tube (6).

[0015] In some embodiments, the molten salt (9) in the tank body (10) has the same temperature at the top and bottom at the end of the heat charging process, and its average temperature is no more than 10°C higher than the target outlet temperature of the low-temperature heat transfer medium during the heat release process; at the end of the heat release process, the temperature of the molten salt (9) at the top of the tank body (10) remains unchanged, and the temperature difference between the molten salt (9) at the top and bottom of the tank body (10) does not exceed 50°C.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] The coupled heat storage system and operation method of the present invention are suitable for heat storage systems with solid materials as the main heat storage medium, and can effectively solve the problem of the gradual decrease in the outlet temperature of the heat transfer medium during the heat release process of the solid heat storage system. The coupled heat storage system in this embodiment includes a solid heat storage subsystem and a molten salt heat storage subsystem, and different heat source forms are designed respectively; the molten salt heat storage subsystem uses a single molten salt storage tank, which reduces the floor space and investment cost, and the second heat exchange tube is designed with a heating module partition inside the molten salt storage tank to improve the heat transfer coefficient. During the heat release process, the heat transfer medium passes through the solid heat storage subsystem and the molten salt heat storage subsystem in turn, which can make the outlet temperature of the heat transfer medium basically unchanged during the heat release process. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a schematic structural diagram of a coupled heat storage system provided by an embodiment of the present invention;

[0019] In the figure: 1. first interface; 2. first heat exchange tube; 3. solid heat storage material; 4. second interface; 5. valve; 6. second heat exchange tube; 7. third interface; 8. electric heater; 9. molten salt; 10. tank body; 11. first heating module; 12. second heating module; 13. third heating module; 14. connecting pipe. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0021] like Figure 1 As shown, a coupled heat storage system includes a first interface 1, a first heat exchange tube 2, a solid heat storage material 3, a second interface 4, a valve 5, a second heat exchange tube 6, a third interface 7, an electric heater 8, molten salt 9, a tank 10, a first heating module 11, a second heating module 12, a third heating module 13 and a connecting pipe 14;

[0022] The first interface 1 and the second interface 4 are respectively located at both ends of the first heat exchange tube 2, the first heat exchange tube 2 is inside the solid heat storage material 3 and is arranged horizontally, the first interface 1 is located at the upper part of the solid heat storage material 3, and the second interface 4 is located at the lower part of the solid heat storage material 3; the first interface 1 is connected to the inlet of the second heat exchange tube 6 through a valve 5, and the outlet of the second heat exchange tube 6 is connected to the third interface 7; the second heat exchange tube 6, the molten salt 9 and the electric heater 8 are all arranged inside the tank body 10, the second heat exchange tube 6 enters the tank body 10 from the lower part of the tank body 10, and the overall structure is a spiral upward arrangement, the third interface 7 is located at the upper part of the tank body 10, the electric heater 8 is located at the bottom of the tank body 10, the tank body 10 is filled with molten salt 9, and the second heat exchange tube 6 and the electric heater 8 are both immersed in the molten salt 9; the second heat exchange tube 6 is divided into a plurality of heating modules along the vertical direction inside the tank body 10, as exemplarily shown Figure 1 As shown, the heating module includes a first heating module 11, a second heating module 12 and a third heating module 13. There is no second heat exchange tube 6 between adjacent heating modules, but they are connected by a connecting tube 14. The longitudinal distance of the connecting tube 14 between adjacent heating modules is not less than 10 times the longitudinal distance between the upper and lower adjacent heat exchange tubes inside the heating module.

[0023] A coupled heat storage system of the present invention operates according to the following method:

[0024] Heat charging mode: close valve 5. Take heat transfer oil as the heat transfer medium for illustration. In this embodiment, the outlet temperature of the heat transfer oil is required to be stable at about 300°C during the heat release process. 350°C high-temperature heat transfer oil flows in from the first interface 1, passes through the first heat exchange tube 2 and then flows out through the second interface 4. The temperature of the high-temperature heat transfer oil drops to 120°C after transferring heat to the solid heat storage material 3 in the first heat exchange tube 2, and the heat is stored in the solid heat storage material 3. The electric heater 8 is turned on, and the molten salt 9 around the electric heater 8 is heated first. When the temperature of the molten salt 9 rises, it will automatically flow upward and continue to heat the upper molten salt 9 in the tank body 10 until the temperature of the first heating module 11, the second heating module 12 and the third heating module 13 basically reaches about 310°C; the deviation value between the average temperature of the molten salt 9 and the target outlet temperature of the heat transfer oil is optimized according to the actual situation, generally not exceeding 10°C.

[0025] Heat release mode: open valve 5. Low-temperature heat transfer oil at 100°C flows in from the second interface 4, passes through the first heat exchange tube 2, valve 5, second heat exchange tube 6 in sequence, and finally flows out through the third interface 7; the low-temperature heat transfer medium absorbs the heat released by the solid heat storage material 3 in the first heat exchange tube 2 and the temperature rises. If the temperature of the heat transfer oil at the valve 5 exceeds 310°C, the heat transfer oil releases heat to the molten salt 9 in the second heat exchange tube 6 until the temperature drops to 310°C. If the temperature of the heat transfer oil at the valve 5 is lower than 300°C, the heat transfer oil absorbs heat from the molten salt 9 in the second heat exchange tube 6 until the temperature is not lower than 300°C; in the process of the heat transfer oil absorbing heat in the second heat exchange tube 6, the temperatures of the third heating module 13, the second heating module 12 and the first heating module 11 decrease in sequence. There is no second heat exchange tube 6 between adjacent heating modules and the longitudinal distance is set relatively large. The molten salt 9 between adjacent heating modules will flow upward, which can improve the heat transfer coefficient on the surface of the second heat exchange tube 6 and reduce the temperature drop rate of the molten salt 9 in the heating module.

[0026] Although the present invention is described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments, which are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in the field can make many modifications without departing from the purpose of the present invention, all of which are within the protection of the present invention. Any non-substantial changes to the present invention using this concept shall be deemed as an act of infringing the protection scope of the present invention.

Claims

1. A method for operating a coupled heat storage system, characterized in that: The coupled heat storage system comprises a first interface (1), a first heat exchange tube (2), a solid heat storage material (3), a second interface (4), a valve (5), a second heat exchange tube (6), a third interface (7), an electric heater (8), a tank (10) filled with molten salt (9), a first heating module (11), a second heating module (12), a third heating module (13) and a connecting pipe (14); The first interface (1) and the second interface (4) are respectively located at two ends of the first heat exchange tube (2); the first heat exchange tube (2) is arranged horizontally inside the solid heat storage material (3); the first interface (1) is located at the upper part of the solid heat storage material (3); and the second interface (4) is located at the lower part of the solid heat storage material (3); the first interface (1) is connected to the inlet of the second heat exchange tube (6) through the valve (5); and the outlet of the second heat exchange tube (6) is connected to the third interface (7); The second heat exchange tube (6) and the electric heater (8) are both arranged inside the tank body (10); the second heat exchange tube (6) is arranged upward in a spiral shape from the bottom of the tank body (10) and is connected to a third interface (7) at the top of the tank body (10); the electric heater (8) is located at the bottom of the tank body (10); the second heat exchange tube (6) is divided into at least three heating modules along the vertical direction inside the tank body (10), and adjacent heating modules are connected by connecting tubes (14); The operation method specifically includes two modes: heat charging mode and heat releasing mode: Heat charging mode: the valve (5) is closed; a high-temperature heat transfer medium flows in from the first interface (1), passes through the first heat exchange tube (2), and flows out through the second interface (4); the high-temperature heat transfer medium transfers heat to the solid heat storage material (3) in the first heat exchange tube (2) and stores the heat in the solid heat storage material (3); at the same time, the electric heater (8) heats the molten salt (9) until the temperatures of the first heating module (11), the second heating module (12), and the third heating module (13) reach the designed values; Heat release mode: the valve (5) is opened; the low-temperature heat transfer medium passes through the second interface (4), the first heat exchange tube (2), the valve (5), the second heat exchange tube (6) in sequence and flows out through the third interface (7); after the low-temperature heat transfer medium absorbs the heat released by the solid heat storage material (3) in the first heat exchange tube (2), it is determined whether the temperature of the low-temperature heat transfer medium at the valve (5) is greater than the temperature of the molten salt (9); if not, the low-temperature heat transfer medium absorbs the heat released by the molten salt (9) in the second heat exchange tube (6); if yes, the low-temperature heat transfer medium releases heat to the molten salt (9) in the second heat exchange tube (6).

2. The method according to claim 1, characterized in that The distance between the connecting pipes in the vertical direction is not less than 10 times the distance between the upper and lower adjacent second heat exchange tubes (6) in the heating module, allowing the molten salt (9) between the adjacent heating modules to flow upward, thereby improving the heat transfer coefficient on the surface of the second heat exchange tube (6).

3. The method according to claim 2, characterized in that The second heat exchange tube (6) is divided into a first heating module (11), a second heating module (12) and a third heating module (13) along the vertical direction inside the tank body (10); the first heating module (11) and the second heating module (12) are connected by a connecting tube (14); and the second heating module (12) and the third heating module (13) are connected by another connecting tube (14).

4. The method according to claim 3, characterized in that The distances between the upper and lower adjacent second heat exchange tubes (6) in the first heating module (11), the second heating module (12) and the third heating module (13) are the same.

5. The method according to claim 1, characterized in that: The molten salt (9) in the tank body (10) has the same temperature at the top and bottom at the end of the heat charging process, and its average temperature is not higher than 10°C of the target outlet temperature of the low-temperature heat transfer medium during the heat release process; at the end of the heat release process, the temperature of the molten salt (9) at the top of the tank body (10) remains unchanged, and the temperature difference between the molten salt (9) at the top and bottom of the tank body (10) does not exceed 50°C.

Images