Cascade phase change heat storage device

Abstract

The invention belongs to the technical field of phase change energy storage, and particularly relates to a cascade phase change heat storage device which comprises an inlet, an outlet and a heat storage device body. The heat storage device body is composed of a shell, a plurality of finned tubes and a phase change material. The finned tube comprises a circular base tube and two-way staggered fins; layering partition plates are arranged in the heat storage device, and the phase change material filling area is divided into an upper area, a middle area and a lower area according to the height. Due to the fact that hot fluid flows in from the upper portion of the heat storage device, exchanges heat with the phase-change materials and then flows out from the lower outlet, the upper area is filled with the high-temperature phase-change materials, the middle area is filled with the medium-temperature phase-change materials, and the lower area is filled with the low-temperature phase-change materials. When flowing through the heat storage device from top to bottom, the hot fluid always keeps relatively stable temperature difference with the phase change materials in different areas, a large amount of heat energy can be stored in a small size, and therefore the heat efficiency of the heat storage system is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of phase change energy storage, and in particular relates to a stepped phase change heat storage device. Background Art

[0002] As the global energy crisis intensifies, energy conservation has become an urgent need. As a major energy consumer, the construction industry accounts for about 40% of the world's total energy consumption. Therefore, building energy conservation has become a key area of ​​research. Phase change heat storage technology, as an efficient and environmentally friendly way of energy storage and utilization, has important application value in building energy conservation. Phase change heat storage technology can store thermal energy in phase change materials, absorb or release heat through the phase change of materials, and achieve long-term storage and regulation of energy. This technology can effectively solve the problem of mismatch between energy supply and demand, improve energy utilization efficiency and system stability, and achieve efficient utilization of building energy and energy conservation and emission reduction. Therefore, conducting research on phase change heat storage technology is of great significance to promoting the development of energy-saving technology and achieving sustainable development of energy.

[0003] Stepped phase change heat storage is one of the effective means to improve the heat exchange rate of phase change heat storage devices. Stepped phase change heat transfer technology follows the principle of "temperature matching, step-by-step utilization" and arranges phase change materials with different phase change temperatures in descending / ascending order of phase change temperature during the heat absorption / release process. Compared with single-stage phase change heat storage, stepped phase change heat storage reduces the temperature of the exothermic fluid along the flow direction, while the melting point temperature of the phase change heat storage material also decreases in steps. The two always maintain a relatively constant temperature difference to improve the overall heat storage / release efficiency.

[0004] Existing stepped phase change heat storage devices include plate heat accumulators, fin-tube heat accumulators, and packed bed heat accumulators. Among them, the manufacturing process of plate heat accumulators and packed bed heat accumulators is complicated and expensive, which is not conducive to widespread promotion. As a common form of phase change heat storage system, fin-tube heat storage has a simple structure and is easy to process, and has considerable application prospects. However, fin-tube heat storage devices conventionally use longitudinal straight fins or annular fins, and the melting process of the phase change material is mainly achieved through fin heat conduction and liquid phase phase change material natural convection. For longitudinal fin tubes, a thicker flow boundary layer will form on the surface of the fins, which seriously inhibits the thermal conductivity and natural convection performance of the fins, and the mutual influence between adjacent fin tubes in the heat storage device is very weak, resulting in low utilization efficiency of the phase change heat storage device. Summary of the invention

[0005] The purpose of the present invention is to provide a stepped phase change heat storage device, which can achieve rapid and uniform melting of phase change materials in various regions of the heat storage device and significantly improve the heat storage efficiency.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: A stepped phase-change heat storage device comprises a main body, an inlet arranged at the upper part of the main body and an outlet arranged at the lower part of the main body, the main body comprises a shell, fin tubes arranged in the shell and a phase-change material located outside the fin tubes inside the shell, the shell is provided with horizontal layered partitions 1, 2, 3 and 4 from top to bottom, each of the layered partitions 1, 2, 3 and 4 is provided with a plurality of evenly distributed openings, and the fin tubes are inserted into the openings; the area between the layered partition 1 and the top of the shell is an inlet header and the inlet is located on the inlet header, an upper fin tube group is provided between the layered partitions 1 and 2, and an upper fin tube group is provided between the layered partitions 2 and 3. A middle fin tube group is arranged between the layered baffles three and the layered baffles four, a lower fin tube group is arranged between the layered baffles four and the shell bottom as an outlet header, and the outlet is located on the outlet header; the upper fin tube group, the middle fin tube group and the lower fin tube group are all composed of a plurality of parallel fin tubes, and the three adjacent fin tubes of the same layer of fin tube group are distributed in a regular triangle, the fin tubes in the same vertical direction in the upper fin tube group, the middle fin tube group and the lower fin tube group are coaxially arranged and interconnected, a high-temperature phase change material is arranged outside the upper fin tube, a medium-temperature phase change material is arranged outside the middle fin tube, and a low-temperature phase change material is arranged outside the lower fin tube, and the fins on the fin tubes are bidirectionally staggered.

[0007] Furthermore, each fin tube includes a circular base tube located in the center and a plurality of fins welded to the surface of the base tube, each fin includes a main fin connected to the outer surface of the base tube and extending along the length direction, and a branch fin group connected to the top of the main fin, the branch fin group includes a branch fin group one and a branch fin group two, the branch fin group one includes a plurality of branch fins one, the branch fin group two includes a plurality of branch fins two, the branch fins one and the branch fins two have the same height and width, each branch fin one and branch fin two are adjacent and have opposite bending directions, and the branch fins one and branch fins two at the top of each main fin appear alternately in sequence; the plurality of fins are evenly arranged radially around the periphery of the base tube in the heat storage chamber.

[0008] Furthermore, adjacent fins on the outer periphery of the fin tube base tube are symmetrically arranged.

[0009] Furthermore, the number of fins on the outer circumference of the fin tube is not less than 4.

[0010] Furthermore, the number of fins of the fin tube in the circumferential direction is adjustable in the upper, middle and lower areas of the shell, and specifically the number of outer fins of the upper fin tube is smaller than the number of outer fins of the middle fin tube, and the number of outer fins of the middle fin tube is smaller than the number of outer fins of the lower fin tube.

[0011] Furthermore, the fin tube material is steel, aluminum alloy or copper; the phase change material is a hydrated salt, paraffin or fatty acid phase change material.

[0012] The present invention has the advantages that: 1. The bidirectional staggered fins of the present invention inhibit the growth of the flow boundary layer on the fin surface, strengthen the natural convection process of the phase change material in each discrete area, and improve the heat transfer efficiency and heat storage rate of the fins. Compared with the rectangular straight fin phase change heat storage device, the complete melting and solidification time of the phase change material of the present invention are shortened by 56.02% and 72.85% respectively; 2. The combination of the bidirectional staggered fin tube and the honeycomb arrangement of the present invention significantly improves the uniformity of the temperature in the phase change heat storage device. Compared with the straight fin phase change heat storage device with a rectangular arrangement, the uniformity index of the liquid phase fraction in the melting process of the present invention is increased from 0.2858 to 0.898, and the uniformity index of the liquid phase fraction in the solidification process is increased from 0.5679 to 0.9879, and the uniformity is significantly improved; 3. The cascade phase change heat storage device of the present invention adopts a new type of fin tube and a three-stage phase change material, which can store a large amount of heat energy in a small volume, and can also adjust the number of fins in different areas to further improve the heat storage efficiency; 4. The bidirectional staggered finned tube and the stepped phase change heat storage device of the present invention can be quickly obtained through cutting and bending processes, are easy to realize automated production, easy to realize batch production, are closely related to industrial production, have extremely strong industrial application value, and further broaden its application in wastewater waste heat recovery in the chemical, construction and other industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic structural diagram of the stepped phase change thermal storage device of the present invention.

[0014] Figure 2 The present invention Figure 1 Middle AA section view.

[0015] Figure 3 It is a structural schematic diagram of the bidirectional staggered fin tube in the present invention.

[0016] Figure 4 It is a schematic diagram of the structure of the fin tube in Example 1 of the present invention.

[0017] Figure 5 It is a schematic diagram of the structure of the fin tube in Example 2 of the present invention.

[0018] Figure 6 Schematic diagrams of the structures of phase change heat storage devices under different arrangements, wherein Figure a is a rectangular longitudinal arrangement of straight finned tubes; Figure b is a honeycomb longitudinal arrangement of straight finned tubes; and Figure c is a bidirectional staggered fin honeycomb arrangement of the present invention.

[0019] Figure 7 for Figure 6Complete melting / solidification time diagram of the three phase change energy storage devices.

[0020] Figure 8 for Figure 6 Comparison chart of uniformity index of melting / solidification process of three phase change energy storage devices. DETAILED DESCRIPTION

[0021] Example 1 like Figure 1-4As shown, a stepped phase change heat storage device includes a main body, an inlet 1 arranged at the upper part of the main body and an outlet 14 arranged at the lower part of the main body, the main body includes a shell 2, a fin tube 5 arranged in the shell and a phase change material located outside the fin tube inside the shell, the shell is provided with horizontal layered partition 1 4, layered partition 2 8, layered partition 3 10 and layered partition 4 12 from top to bottom, and the layered partition 1, layered partition 2, layered partition 3 and layered partition 4 are each provided with a plurality of evenly distributed openings, and the fin tubes are inserted into the openings; the area between the layered partition 1 and the top of the shell is an inlet header 3 and the inlet is located on the inlet header, and the layered partition 1 and the layered partition 2 are ... An upper fin tube group is arranged between the layered baffles 2 and 3, a middle fin tube group is arranged between the layered baffles 3 and 4, a lower fin tube group is arranged between the layered baffles 4 and 4, the area between the layered baffles 4 and the bottom of the shell is the outlet header 13 and the outlet is located on the outlet header; the upper fin tube group, the middle fin tube group and the lower fin tube group are all composed of a plurality of parallel fin tubes, and the three adjacent fin tubes of the same layer of fin tube group are distributed in an equilateral triangle, and near the center of gravity of any equilateral triangle, there are 6 branch fins 17 or 6 branch fins 2 18; between the fin tubes in the same vertical direction in the upper fin tube group, the middle fin tube group and the lower fin tube group The finned tubes are coaxially arranged and interconnected, a high-temperature phase change material 7 is arranged outside the upper finned tube, a medium-temperature phase change material 9 is arranged outside the middle finned tube, and a low-temperature phase change material 11 is arranged outside the lower finned tube. The fins on the finned tubes are arranged in a bidirectional staggered manner; each finned tube includes a circular base tube 15 located in the center and a plurality of fins 6 welded to the surface of the base tube, each fin includes a main fin 16 connected to the outer surface of the base tube and extending along the length direction and a branch fin group connected to the top of the main fin, the branch fin group includes a branch fin group 1 and a branch fin group 2, the branch fin group 1 includes a plurality of branch fins 17, the branch fin group 2 includes a plurality of branch fins 18, the branch fins 1 and 1 are connected to the main fin group. The height and width of branch fin 2 are the same, each branch fin 1 is adjacent to branch fin 2 and has opposite bending directions, and branch fin 1 and branch fin 2 on the top of each main fin appear alternately in sequence; multiple fins are evenly arranged radially on the periphery of the base tube in the heat storage chamber; adjacent fins on the periphery of the fin tube base tube are symmetrically arranged; in this embodiment, the number of fins on the circumferential direction of the outer periphery of the fin tube is 6 in the upper, middle and lower areas; in actual use, the number of fins of the fin tube in the circumferential direction can be adjusted in the upper, middle and lower areas of the shell; the fin tube material is steel or aluminum alloy or copper; the phase change material is a hydrated salt or paraffin or fatty acid phase change material.

[0022] When in use, the high-temperature hot fluid enters the inlet header 2 through the inlet 1, enters the interior of the finned tube through the opening of the layered partition 4, exchanges heat with the phase change materials in the upper, middle and bottom regions in sequence, and then enters the outlet header 13 through the opening region of the sealing layer partition 12, and after merging, flows out of the heat storage device through the outlet 14; Figure 2 A schematic diagram of the fin arrangement of the stepped phase change heat storage device provided by the present invention (AA cross-sectional view). The fin tubes in the heat storage device are arranged in an equilateral triangle. On the AA cross-sectional view, the shaded area in the fin tube is the base tube 15, the main fin 16 and the branch fin 17. Near the center of gravity of any equilateral triangle, there are 6 branch fins 17 or 6 branch fins 2 18. Two adjacent branch fins 17 of a single fin tube can significantly enhance the heat conduction and natural convection in the area, and the melting performance is increased by more than 20%. In the heat storage device of the present invention, the synergistic effect of the 6 branch fins 17 or the 6 branch fins 2 18 will further enhance the heat storage performance; for example: hot water with a temperature of 80°C enters the inlet header from the inlet. Phase change materials with melting points of 65°C, 55°C and 45°C are filled in the upper, middle and bottom areas respectively, and the melting points of the three phase change materials vary significantly. By adjusting the length of the finned tubes, the temperature of the hot water is reduced to 70°C and 60°C when it enters the middle and bottom areas respectively.

[0023] Example 2 The difference between Example 2 and Example 1 is that the number of fins of the finned tube can be adjusted according to the temperature difference between the thermal fluid and the phase change material. Specifically, the number of fins on the outer side of the upper finned tube is less than the number of fins on the outer side of the middle finned tube, the number of fins on the outer side of the middle finned tube is less than the number of fins on the outer side of the lower finned tube, the number of fins on the circumference of the base tube in the high temperature area is N=4, the number of fins in the medium temperature area is N=6, and the number of fins in the low temperature area is N=8; hot water with a temperature of 80°C enters the inlet header from the inlet. Phase change materials with melting points of 55°C, 50°C and 45°C are filled in the upper, middle and bottom areas respectively, and the melting points of the three phase change materials do not change much. If the length of the finned tube is large, when the hot water flows from top to bottom through the heat storage device, the temperature difference between the thermal fluid and the high-temperature phase change material in the upper area is the largest, and the temperature difference between the thermal fluid and the low-temperature phase change material in the bottom area is the smallest. Therefore, the number of fins on the finned tubes in the low temperature difference area can be increased to increase the heat transfer area and improve the heat storage performance. Figure 5 Schematic diagram of a finned tube with adjustable fin number.

[0024] Comparative test The melting / solidification numerical simulation of three single-stage phase change thermal storage devices is carried out. Figure 6As shown. The physical model of the straight fin phase change thermal storage device with rectangular arrangement is as follows: the inner diameter of the inner tube is 20 mm, the height of the thermal storage device is 60 mm, the minimum spacing between adjacent fin units is 6 mm, the longitudinal fin height is 35 mm, the fin thickness is 2 mm, and the number of fins along the circumference is 6. The physical model of the straight fin phase change thermal storage device with honeycomb arrangement has basically the same fin geometric parameters as the former, only the arrangement method is different, and its fin tube arrangement method is honeycomb-shaped, and the minimum spacing between adjacent units is 6 mm. The physical model of the bidirectional staggered fin phase change thermal storage device with honeycomb arrangement is as follows: the inner diameter of the inner tube is 20 mm, the height of the thermal storage device is 60 mm, the minimum spacing between adjacent fin units is 6 mm, the height of the straight fin is 14.5 mm, the height of the bifurcated fin is 25.5 mm, the thickness of the fin is 2 mm, the deflection angle of the bifurcated fin is 67°, the length of the single-cycle fin is 20 mm, the number of axial cycles is 3, there are a total of 6 rows of fins along the circumference and they are evenly distributed, and the angle between two adjacent rows of fins is 60°. The liquid PCM is regarded as a laminar, unsteady, and incompressible fluid. The thermal properties of PCM are considered to be constant and independent of temperature. By considering the density change of the buoyancy term, the Boussinesq approximation is used to solve the natural convection problem. For the melting process, the inner tube is set to a constant wall temperature of 343.15 K, the upper and lower walls are set to adiabatic boundary conditions, the annular wall is set to symmetric boundary conditions, and the initial temperature is constant at 313.15 K. For the solidification process, the inner tube is set to a constant wall temperature of 313.15 K, the upper and lower walls are set to adiabatic boundary conditions, the annular wall is set to symmetric boundary conditions, and the initial temperature is constant at 343.15 K. The heat storage chamber composed of the inner tube and the tube shell is filled with RT54HC paraffin, and the phase change temperature of RT54HC paraffin is 327K. The heat storage characteristics of the three phase change thermal storage devices mentioned above are numerically calculated, and their complete melting / solidification times are shown in Figure 7 As shown, the liquid fraction uniformity index at Z = 20 mm is as follows Figure 8Compared with the straight fin phase change heat storage device with rectangular arrangement, the straight fin phase change heat storage device with honeycomb arrangement has its complete melting and complete solidification time reduced by about 29.44% and 56.02%, respectively, while the bidirectional staggered fin phase change heat storage device with honeycomb arrangement has its complete melting and complete solidification time reduced by about 35.79% and 72.85%, respectively. For the melting process at Z=20mm and the melting time of 1200s, the liquid fraction uniformity indexes of the three phase change thermal storage devices are 0.2858, 0.5953 and 0.898 respectively (the closer to 1, the better the uniformity); for the solidification process at Z=20mm and the melting time of 3600s, the liquid fraction uniformity indexes of the three phase change thermal storage devices are 0.5679, 0.7552 and 0.9879 respectively. Among them, the energy storage device with honeycomb arrangement and bidirectional staggered fins in the fin tube structure has the best uniformity regardless of the melting process or the solidification process. From the above numerical research results, it can be seen that the new honeycomb arrangement method not only effectively improves the thermal storage performance of the thermal storage device, but also further improves its uniformity in the energy storage / release process.

Claims

1. A stepped phase change heat storage device, comprising a main body, an inlet arranged at the upper part of the main body and an outlet arranged at the lower part of the main body, wherein the main body comprises a shell, fin tubes arranged in the shell and a phase change material located outside the fin tubes in the shell, characterized in that: The shell is provided with horizontal layered partitions 1, 2, 3 and 4 from top to bottom, and each of the layered partitions 1, 2, 3 and 4 is provided with a plurality of evenly distributed openings, and fin tubes are inserted into the openings; the area between the layered partition 1 and the top of the shell is an inlet header, and the inlet is located on the inlet header; an upper fin tube group is provided between the layered partition 1 and the 2, a middle fin tube group is provided between the layered partition 2 and the 3, a lower fin tube group is provided between the layered partition 3 and the 4, and a lower fin tube group is provided between the layered partition 4 and the 5th layer. The area between the bottom of the shell is the outlet header and the outlet is located on the outlet header; the upper fin tube group, the middle fin tube group and the lower fin tube group are all composed of a plurality of parallel fin tubes, and the three adjacent fin tubes of the same fin tube group are distributed in a regular triangle, the fin tubes in the same vertical direction in the upper fin tube group, the middle fin tube group and the lower fin tube group are coaxially arranged and interconnected, the upper fin tube is provided with a high-temperature phase change material, the middle fin tube is provided with a medium-temperature phase change material, and the lower fin tube is provided with a low-temperature phase change material, and the fins on the fin tubes are bidirectionally staggered.

2. The cascade phase change thermal storage device according to claim 1, characterized in that: Each fin tube includes a circular base tube located in the center and a plurality of fins welded to the surface of the base tube, each fin includes a main fin connected to the outer surface of the base tube and extending along the length direction, and a branch fin group connected to the top of the main fin, the branch fin group includes a branch fin group one and a branch fin group two, the branch fin group one includes a plurality of branch fins one, the branch fin group two includes a plurality of branch fins two, the branch fins one and the branch fins two have the same height and width, each branch fin one and the branch fin two are adjacent and have opposite bending directions, and the branch fins one and the branch fins two on the top of each main fin appear alternately in sequence; the plurality of fins are evenly arranged radially around the base tube in the heat storage chamber.

3. The cascade phase change thermal storage device according to claim 2, characterized in that: Adjacent fins on the outer periphery of the fin tube base tube are symmetrically arranged.

4. The step-by-step phase-change thermal storage device according to claim 3, characterized in that: The number of fins on the outer circumference of the fin tube is not less than 4.

5. The step-by-step phase-change thermal storage device according to claim 4, characterized in that: The number of outer fins of the upper fin tube is less than the number of outer fins of the middle fin tube, and the number of outer fins of the middle fin tube is less than the number of outer fins of the lower fin tube.

6. The step-by-step phase-change thermal storage device according to claim 5, characterized in that: The fin tube material is steel, aluminum alloy or copper; the phase change material is hydrated salt, paraffin or fatty acid phase change material.

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