A multi-stage phase change heat storage system for light / electricity-heat self-conversion

Abstract

The present invention discloses a multi-stage phase change heat storage system for self-conversion of light / electricity and heat, including a multi-stage heat storage unit, a temperature control system, a pipeline, a heat insulation box, and a transparent vacuum cover; the multi-stage heat storage unit is arranged in the heat insulation box, the vacuum cover is arranged on one side of the heat insulation box, and the vacuum cover covers the multi-stage heat storage unit; the pipeline passes through the multi-stage heat storage unit in sequence; the multi-stage heat storage unit is respectively filled with carbon-based composite phase change materials with different phase change temperatures, and its phase change temperature increases step by step from the inlet layer to the outlet layer; the temperature control system is used to collect the temperature of the multi-stage heat storage unit, and when the collected temperature is lower than the preset temperature, the temperature control system supplies power to the carbon-based composite phase change material in the multi-stage heat storage unit. When there is sufficient sunlight, solar radiation is absorbed to convert and store light energy, and when there is insufficient sunlight, electricity is used to convert electricity into heat and store it, without the need for a water storage tank, and by setting multiple layers of heat storage units with different phase change temperatures, the heat exchange efficiency can be improved.

Description

Technical Field

[0001] The present invention relates to the field of heat storage technology, and in particular to a multi-stage phase change heat storage system for light / electricity-heat self-conversion. Background Art

[0002] The use of clean and renewable energy has covered all industries, with solar energy being the current focus of development. Among the many forms of solar energy utilization, solar thermal technology, as an economical and feasible means, has been effectively applied to various heating-related industries. Due to its high energy efficiency and relatively constant phase change temperature, latent heat of phase change can make up for the intermittent defects of solar energy and has received great attention in solar thermal storage technology. However, in areas with a long rainy season or insufficient sunlight in winter, due to the long-term lack of strong sunlight, the intermittent defects of solar energy cannot be met by the energy storage system with limited heat storage capacity. It is necessary to use power-assisted heating elements to achieve continuous heat output.

[0003] However, the existing phase change heat storage system needs to achieve light-to-heat conversion through photovoltaic panels and then transfer heat to the phase change energy storage module, which has low heat exchange efficiency. In addition, it is necessary to add electric heat conversion equipment, which increases equipment costs and improves the failure rate. Summary of the invention

[0004] The present invention provides a multi-stage phase change heat storage system for light / electricity-heat self-conversion to solve the problems of low heat exchange efficiency and high cost of the existing phase change heat storage system.

[0005] A multi-stage phase-change heat storage system for light / electricity-heat self-conversion, comprising a multi-stage heat storage unit, a temperature control system, a pipeline, a heat insulation box, and a transparent vacuum cover;

[0006] The multi-stage heat storage unit is arranged in the heat insulation box, the vacuum cover plate is arranged on one side of the heat insulation box, and the vacuum cover plate covers the multi-stage heat storage unit;

[0007] The pipeline passes through the multi-stage heat storage unit in sequence; the multi-stage heat storage unit is respectively filled with carbon-based composite phase change materials with different phase change temperatures, and the phase change temperature thereof increases step by step from the inlet layer to the outlet layer;

[0008] The temperature control system is used to collect the temperature of the multi-stage heat storage unit. When the collected temperature is lower than a preset temperature, the temperature control system supplies power to the carbon-based composite phase change material in the multi-stage heat storage unit.

[0009] In the heat storage system provided by the above scheme, a carbon-based composite phase change material is used, which has photothermal effect and electrothermal effect, and has photothermal conversion function and electrothermal conversion function. When there is sufficient sunlight, sunlight directly irradiates the carbon-based composite phase change material in the multi-stage phase change heat storage system through the vacuum cover plate, and the carbon-based composite phase change material can use its photothermal effect to perform photo-thermal self-conversion to store energy; when there is insufficient sunlight, the temperature control system collects the temperature of the multi-stage heat storage unit. When the collected temperature is lower than the preset temperature, the temperature control system can timely supply power to the carbon-based composite phase change material in the multi-stage heat storage unit, and the carbon-based composite phase change material can use its electrothermal effect to perform electro-thermal self-conversion to store energy and ensure continuous heat output. The heat storage system forms a complementary mechanism integrating photo / electro-thermal conversion and thermal energy storage / release. In addition, the phase change temperature of the carbon-based composite phase change material in the multi-stage heat storage unit increases step by step from the inlet layer to the outlet layer, and always forms a certain temperature difference between the fluid and the heat storage unit to improve the heat exchange efficiency. The heat storage system adopts a multi-stage heat storage unit with large energy storage capacity, no need for water tanks, simple and easy-to-obtain materials, reliable safety, and controllable economic costs.

[0010] Furthermore, each stage of the heat storage unit includes a container and a carbon-based composite phase change material arranged in the container; the container can be made of polyvinyl chloride; the carbon-based composite phase change material is composed of a carbon-based supporting matrix: a phase change material in a mass ratio of 40-70:30-60 (i.e., the mass of the phase change material accounts for 30-60%); wherein the carbon-based supporting matrix is ​​a blend of graphite, carbon fiber, and conductive carbon black, the graphite is at least one of flake graphite, microcrystalline graphite, nanographite, expanded graphite, and graphene, and the carbon fiber is at least one of activated carbon fiber and asphalt carbon fiber; wherein the phase change material is at least one of paraffin, fatty acid, and alcohol polymers with a phase change temperature of 30-90°C.

[0011] Graphite, carbon fiber, and conductive carbon black can produce photothermal effects, have high solar radiation absorption rate, relatively low emissivity, and excellent thermal conductivity; and they also have high conductivity. The carbon composite phase change material prepared with the above materials as the carbon-based supporting matrix has photothermal effect and electrothermal effect, and can realize light-heat self-conversion and electro-heat self-conversion.

[0012] Furthermore, the manufacturing process of each level of heat storage unit includes:

[0013] Three types of carbon materials, graphite, carbon fiber and conductive carbon black, are selected and blended to obtain a carbon-based support matrix;

[0014] The carbon-based support matrix and the phase change material are placed in a reaction container according to a preset mass ratio, stirred at 90-95° C., and maintained in a vacuum state of -0.1 MPa, and after sufficient impregnation, the carbon-based composite phase change material is obtained;

[0015] The carbon-based composite phase change material is weighed in a certain amount according to a preset filling density, filled into the container to which the pipeline has been fixed, and compacted according to the filling density requirement to obtain the heat storage unit.

[0016] Furthermore, the filling density of the carbon-based composite phase change material in the heat storage unit is 1.5-2.5 g / cm 3 By designing the filling density of the heat storage unit within this range, the carbon-based composite phase change material can undergo electric and thermal self-conversion within a low operating voltage range without the need for external heating elements.

[0017] Further, the temperature control system includes a plurality of temperature sensors, a low voltage transformer, a temperature controller, and a plurality of electrode pairs;

[0018] The multiple temperature sensors, multiple electrode pairs, and low-voltage transformer are all connected to the temperature controller;

[0019] The multiple electrode pairs are arranged in the multi-stage heat storage unit, and the multiple temperature sensors are used to collect the temperature of each stage of the heat storage unit and transmit it to the thermostat. When the collected temperature is lower than the preset temperature, the switch of the thermostat is closed.

[0020] The low-voltage transformer is connected to an external power supply, and the thermostat is connected in series between the low-voltage transformer and multiple electrode pairs. Multiple temperature sensors collect the temperature in each level of the heat storage unit in real time and transmit it to the thermostat. When the collected temperature is lower than the preset temperature, the thermostat closes, and multiple electrode pairs are powered and supply power to the carbon-based composite phase change material in the heat storage unit. After the carbon-based composite phase change material is powered, it uses its own electrothermal effect to perform electric-thermal self-conversion to store energy. The temperature control system can determine the situation of insufficient input energy due to photothermal conversion, so as to promptly start the electrothermal conversion function to ensure continuous heat output.

[0021] Furthermore, each of the electrode pairs includes two copper foil electrodes, and the two copper foil electrodes are arranged on both sides of the non-pipe inlet and outlet of the heat storage unit.

[0022] Furthermore, the output voltage range of the low-voltage transformer is 2-15V. When a voltage within this range is applied to the carbon-based composite phase change material, the electrothermal conversion efficiency of the carbon-based composite phase change material can be maximized.

[0023] Furthermore, the vacuum cover is a vacuum double-layer transparent cover, and is made of at least one of polyvinyl chloride, acrylic plate, and polycarbonate. The vacuum double-layer transparent cover can ensure the transmission of sunlight while preventing heat loss.

[0024] Furthermore, the pipeline is a copper tube coated with thermally conductive insulating silicone, has a spiral shape, and a thread density of diameter: center-diameter distance of 1:2-4; a pipeline of this structure can improve heat conversion efficiency.

[0025] Furthermore, the number of stages of the multi-stage heat storage unit is three or more, and each heat storage unit is separated by a heat insulation material; the overall phase change temperature range of the carbon-based composite phase change material in the multi-stage heat storage unit is 35-75°C. The water inlet of the heat storage system is located at the entrance of the heat storage unit with the lowest phase change temperature, and the water outlet is located at the exit of the heat storage unit with the highest phase change temperature.

[0026] Beneficial Effects

[0027] The present invention proposes a multi-stage phase change heat storage system for light / electricity-heat self-conversion, which has the following advantages:

[0028] 1. The present invention uses a carbon-based composite phase change material, which has a photothermal effect and an electrothermal effect, and can perform photo / electric-thermal self-conversion while storing heat; the carbon-based composite phase change material can undergo electrothermal self-conversion without the need for an external heating element.

[0029] 2. The present invention explores the application of carbon-based composite phase change materials in photothermal and electrothermal conversion, and establishes a complementary system between the two. When there is sufficient sunlight, photothermal conversion provides heat, and when there is insufficient sunlight, electricity is used to store heat, reducing costs and allocating resources rationally.

[0030] 3. The present invention sets a gradient change in phase change temperature between heat storage units at different levels to always form a certain temperature difference between the fluid and the heat storage unit to improve the heat exchange efficiency.

[0031] 4. The temperature control system of the present invention can determine the situation where the system has insufficient input energy due to light-heat conversion, so as to start the electric-heat conversion function in time to ensure continuous heat output.

[0032] 5. The system of the present invention has a large energy storage capacity, does not require a water tank, uses simple and readily available materials, is safe and reliable, and has controllable economic costs, and is expected to be widely promoted and applied. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0034] Figure 1 It is an overall schematic diagram of a multi-stage phase change heat storage system for light / electricity-heat self-conversion provided by an embodiment of the present invention;

[0035] Figure 2 is a top cross-sectional view of a multi-stage phase change heat storage system for light / electricity-heat self-conversion provided by an embodiment of the present invention;

[0036] Figure 3 is a side cross-sectional view of a multi-stage phase change heat storage system for light / electricity-heat self-conversion provided by an embodiment of the present invention;

[0037] Figure 4 It is a schematic diagram of a temperature control system of a multi-stage phase change heat storage system for light / electricity-heat self-conversion provided in an embodiment of the present invention.

[0038] In the figure: 1. vacuum cover plate; 2. carbon-based composite phase change material; 3. copper foil electrode; 4. water outlet; 5. water inlet; 6. pipeline; 7. thermal insulation box; 8. container; 9. temperature control system; 901. temperature sensor; 902. temperature controller; 903. low voltage transformer; 904. electrode pair. DETAILED DESCRIPTION

[0039] To make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described in detail below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention.

[0040] Example 1

[0041] This embodiment provides a multi-stage phase change heat storage system for light / electricity-heat self-conversion, such as Figure 1 , 2 , 3, and 4, it includes a multi-stage heat storage unit, a temperature control system 9, a pipeline 6, an insulation box 7, and a transparent vacuum cover 1; the multi-stage heat storage unit is arranged in the insulation box 7, the vacuum cover 1 is arranged on one side of the insulation box 7, and the vacuum cover 1 covers the multi-stage heat storage unit; the pipeline 6 passes through the multi-stage heat storage unit in sequence; the multi-stage heat storage unit is filled with carbon-based composite phase change materials 2 with different phase change temperatures, and its phase change temperature increases step by step from the inlet layer to the outlet layer; the temperature control system 9 is used to collect the temperature of the multi-stage heat storage unit, and when the collected temperature is lower than the preset temperature, the temperature control system supplies power to the carbon-based composite phase change material 2 in the multi-stage heat storage unit. The water inlet 4 of the heat storage system is located at the inlet of the heat storage unit with the lowest phase change temperature, and the water outlet 5 is located at the outlet of the heat storage unit with the highest phase change temperature.

[0042] Each level of heat storage unit includes a container 8 and a carbon-based composite phase change material 2 disposed in the container 8. The temperature control system 9 includes a plurality of temperature sensors 901, a low-voltage transformer 903, a temperature controller 902, and a plurality of electrode pairs 904; the plurality of temperature sensors 901, the low-voltage transformer 903, and the plurality of electrode pairs 904 are all connected to the temperature controller 902; the plurality of electrode pairs 904 are separately disposed in the multi-level heat storage unit, and the plurality of temperature sensors 901 are used to collect the temperature of each level of heat storage unit and transmit it to the temperature controller 902, and when the collected temperature is lower than the preset temperature, the switch of the temperature controller 902 is closed. More specifically, the temperature controller 902 is equipped with a plurality of switches, the number of which corresponds to the number of levels of the multi-level heat storage unit, and the plurality of switches respectively control the on and off of the power supply of the plurality of electrode pairs 904, so as to realize the independent temperature control of each level of heat storage unit. Each of the electrode pairs 904 includes two copper foil electrodes 3, and the two copper foil electrodes 3 are separately disposed on both sides of the non-pipeline inlet and outlet of the heat storage unit. The output voltage range of the low voltage transformer is 2-15V. The vacuum cover plate 1 is a vacuum double-layer transparent cover plate, and is made of at least one material selected from polyvinyl chloride, acrylic plate, and polycarbonate. The use of a vacuum double-layer transparent cover plate can ensure the transmission of sunlight while preventing heat loss. In this embodiment, polyvinyl chloride is used. The pipe 6 is a copper pipe coated with thermally conductive insulating silicone.

[0043] In this embodiment, preferred method 1: the number of heat storage unit layers is 3; the phase change materials for preparing the carbon-based composite phase change material are three types of paraffin with theoretical phase change temperatures in the range of 30-40°C, 45-55°C, and 60-70°C; the filling density of the heat storage unit is 1.5g / cm 3 ; The output voltage of the low voltage transformer is 10V; The design mass ratio of the carbon support matrix and the phase change material in the carbon-based composite phase change material is 40:60.

[0044] In this embodiment, the container specifications and materials are selected as follows: the design size of the PVC container of each level of the heat storage unit is 50cm×10cm×10cm; the vacuum cover plate is made of transparent PVC material and bonded into a double-layer vacuum structure; the diameter of the pipe is 5cm, the thread density is diameter: center-diameter distance 1:2, and it is installed in the center of the heat storage unit.

[0045] In this embodiment, the manufacturing process of each stage of the heat storage unit is as follows: expandable graphite, micron-sized pitch-based carbon fiber, and conductive carbon black are weighed and blended in a mass ratio of 1:1:1 to obtain a carbon support matrix; the carbon support matrix and the phase change material are weighed and mixed in a mass ratio of 40:60, and the mixture is placed in a reaction container, stirred at 90-95°C, and maintained in a vacuum state of -0.1MPa, and the carbon-based composite phase change material is obtained after full impregnation; according to the designed filling density of 1.5g / cm 3A quantitative amount of the carbon-based composite phase change material is weighed, filled into a PVC container with a fixed pipe, and compacted according to the filling density requirement to obtain the heat storage unit.

[0046] The performance test conditions in this embodiment are as follows: the test is divided into two parts: photothermal conversion energy storage test and electrothermal conversion energy storage test. In the first part, the photothermal conversion energy storage test is carried out at room temperature of 25°C. Since the system is in a low temperature state as a whole, in order to avoid turning on the electrothermal conversion function, the electrothermal conversion function is turned off before the test, and then the light source intensity is 1000W / m 2 After 8 hours of irradiation with a simulated light source, the light source is turned off, and cold water with a temperature of 20°C is passed from the water inlet at a rate of 0.004kg / s, and the outlet water temperature is recorded. In the second part, the electrothermal conversion energy storage is carried out at room temperature of 25°C, and the overall temperature of the system is restored to the initial state. The light source is not turned on. Since the temperature sensor detects that the temperature of the heat storage unit is lower than the set temperature, the system automatically triggers the electrothermal energy storage function. After the heat storage is completed, cold water with a temperature of 20°C is passed from the cold end water inlet at a rate of 0.004kg / s, and the outlet water temperature is recorded.

[0047] The test results in this embodiment are as follows: after the illumination stage is completed, the outlet water temperature is cooled to 25°C as the time node, and the hot water outlet temperature is integrated and calculated within the time node, and the result is: the cold water heat exchange efficiency in the photothermal stage = the total heat stored in the water during the water flow process / the total energy input of the light source * 100% = 61%; after the electric heating stage is completed, the outlet water temperature is cooled to 25°C as the time node, and the hot water outlet temperature is integrated and calculated within the time node, and the result is: the cold water heat exchange efficiency in the electric heating stage = the total heat absorbed by the water during the water flow process / the total energy input of the power supply * 100% = 52%;

[0048] Example 2

[0049] Example 2 In the same manner as Example 1, another preferred multi-stage phase change heat storage system for light / electricity-heat self-conversion is provided (eg Figure 1 , 2 , 3, and 4).

[0050] In this embodiment, preferred mode 2: the number of heat storage unit layers is 4; the phase change materials for preparing the carbon-based composite phase change material are four kinds of paraffin waxes with theoretical phase change temperatures in the range of 30-40°C, 41-50°C, 51-60°C, and 61-70°C; the filling density of the heat storage unit is 1.5g / cm 3 ; The output voltage of the low voltage transformer is 10V; The design mass ratio of the carbon support matrix and the phase change material in the carbon-based composite phase change material is 40:60.

[0051] The test results in this embodiment are that the cold water heat exchange efficiency in the photothermal stage is 63%; the cold water heat exchange efficiency in the electric heating stage is 56%. Compared with the results of Example 1, it is shown that, to a certain extent, the increase in the number of heat storage units can achieve a more efficient heat exchange efficiency.

[0052] Comparative Example 1

[0053] Comparative Example 1 uses the same structure and testing method as Example 1 (such as Figure 1 , 2 , 3, and 4). With respect to the conditions in the preferred embodiment 1, the difference is that: in comparative example 1, only one carbon-based composite phase change material is prepared, paraffin with a phase change temperature in the range of 61-70° C. is selected as the phase change material, and all heat storage units are filled with the carbon-based composite phase change material.

[0054] The test results in this comparative example are: in the illumination stage, the heat exchange efficiency of cold water in the photothermal stage is 58%; in the electric heating stage, the heat exchange efficiency of cold water in the electric heating stage is 48%. The results compared with Example 1 show that by setting the phase change temperature gradient of the phase change material, a certain temperature difference can be always formed between the fluid and the heat storage unit, thereby ensuring the smooth and efficient heat exchange process.

[0055] Comparative Example 2

[0056] Comparative Example 2 uses the same structure and testing method as Example 2 (such as Figure 1 , 2 , 3, and 4). With respect to the conditions in the preferred embodiment 2, the difference is that: the filling density of the heat storage unit in comparative example 2 is 0.5 g / cm 3 ; The output voltage of the low voltage transformer is 5V; The design mass ratio of the carbon support matrix and the phase change material in the carbon-based composite phase change material is 30:70.

[0057] The test results in this comparative example show that the electrothermal conversion function cannot be achieved and the efficiency is almost 0. The comparison results with Example 2 show that when the heat storage unit cannot control multiple factors (filling density, working voltage, thermal properties of carbon-based composite phase change material conductivity and heat conductivity) within a reasonable range, the electrothermal conversion process cannot occur, and the system cannot form a complementary mechanism of photothermal, electrothermal conversion and heat storage.

[0058] It can be understood that the same or similar parts of the above embodiments can be referenced to each other, and the contents not described in detail in some embodiments can refer to the same or similar contents in other embodiments.

[0059] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A multi-stage phase change heat storage system for light / electricity-heat self-conversion, characterized in that: It includes a multi-stage heat storage unit, a temperature control system, pipelines, a heat insulation box, and a transparent vacuum cover; The multi-stage heat storage unit is arranged in the heat insulation box, the vacuum cover plate is arranged on one side of the heat insulation box, and the vacuum cover plate covers the multi-stage heat storage unit; The pipeline passes through the multi-stage heat storage unit in sequence; the multi-stage heat storage unit is respectively filled with carbon-based composite phase change materials with different phase change temperatures, and the phase change temperature thereof increases step by step from the inlet layer to the outlet layer; The temperature control system is used to collect the temperature of the multi-stage heat storage unit. When the collected temperature is lower than a preset temperature, the temperature control system supplies power to the carbon-based composite phase change material in the multi-stage heat storage unit. Each level of heat storage unit includes a container and a carbon-based composite phase change material arranged in the container; the carbon-based composite phase change material is composed of a carbon-based support matrix: a phase change material in a mass ratio of 40-70:30-60; wherein the carbon-based support matrix is ​​a blend of graphite, carbon fiber, and conductive carbon black, the graphite is at least one of flake graphite, microcrystalline graphite, nanographite, expanded graphite, and graphene, and the carbon fiber is at least one of activated carbon fiber and pitch carbon fiber; wherein the phase change material is at least one of paraffin, fatty acid, and alcohol polymers with a phase change temperature of 30-90°C; The temperature control system includes a plurality of temperature sensors, a low voltage transformer, a temperature controller, and a plurality of electrode pairs; The multiple temperature sensors, multiple electrode pairs, and low-voltage transformer are all connected to the temperature controller; The multiple electrode pairs are arranged in the multi-stage heat storage unit, and the multiple temperature sensors are used to collect the temperature of each stage of the heat storage unit and transmit it to the thermostat. When the collected temperature is lower than the preset temperature, the switch of the thermostat is closed.

2. The multi-stage phase change heat storage system for light / electricity-heat self-conversion according to claim 1 is characterized in that: The manufacturing process of each level of thermal storage unit includes: Three types of carbon materials, graphite, carbon fiber and conductive carbon black, are selected and blended to obtain a carbon-based support matrix; The carbon-based support matrix and the phase change material are placed in a reaction container according to a preset mass ratio, stirred at 90-95° C., and maintained in a vacuum state of -0.1 MPa, and after sufficient impregnation, the carbon-based composite phase change material is obtained; A quantitative amount of the carbon-based composite phase change material is weighed according to a preset filling density, filled into the container to which the pipeline has been fixed, and compacted according to the filling density requirement to obtain a heat storage unit.

3. The multi-stage phase change heat storage system for light / electricity-heat self-conversion according to any one of claims 1 to 2, characterized in that: The filling density of the carbon-based composite phase change material in the heat storage unit is 1.5-2.5g / cm 3 .

4. The multi-stage phase change heat storage system for light / electricity-heat self-conversion according to claim 1 is characterized in that: Each electrode pair includes two copper foil electrodes, and the two copper foil electrodes are arranged on both sides of the non-pipe inlet and outlet of the heat storage unit.

5. The multi-stage phase change heat storage system for light / electricity-heat self-conversion according to claim 1 or 4, characterized in that: The output voltage range of the low voltage transformer is 2-15V.

6. The multi-stage phase change heat storage system for light / electricity-heat self-conversion according to claim 1, characterized in that: The vacuum cover plate is a vacuum double-layer transparent cover plate, and is made of at least one material selected from polyvinyl chloride, acrylic plate, and polycarbonate.

7. The multi-stage phase change heat storage system for light / electricity-heat self-conversion according to claim 1, characterized in that: The pipeline is a copper pipe coated with thermally conductive insulating silicone, has a spiral shape, and a thread density of diameter: center-diameter distance of 1:2-4.

8. The multi-stage phase change heat storage system for light / electricity-heat self-conversion according to claim 1, characterized in that: The number of stages of the multi-stage heat storage unit is three or more, and each heat storage unit is separated by a heat insulation material.

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