Method and system for controlling heat transfer in a solid thermal storage system

Abstract

A thermal storage system and a method for controlling heat transfer from the thermal storage system are provided. The thermal storage system includes a solid thermal storage module; a heat release channel; and a gap separating the solid thermal storage module from the heat release channel. The gap is filled with a first fluid medium and a second fluid medium having a thermal conductivity at least fifty times greater than a thermal conductivity of the first fluid medium. To control heat transfer from the solid thermal storage module, a desired heat flux from the solid thermal storage module through the gap to the heat release channel is determined; and a volume ratio of the first fluid medium to the second fluid medium in the gap is controlled in accordance with the desired heat flux.

Description

Technical Field

[0001] This invention relates to a method for controlling heat transfer from a thermal storage system, particularly a solid thermal storage system, and to a solid thermal storage system. Background Technology

[0002] Thermal storage devices utilize the ability to store energy within materials at elevated temperatures. This heat must eventually be released to be utilized. The heat released from such devices can be used to heat fluids. One application of the released heat is to generate process steam for a variety of end-use applications.

[0003] Solid storage media, such as but not limited to brick, solid concrete, graphite, or ceramic materials, can be used. These solid storage media can operate at temperatures significantly above 500°C to maximize the amount of stored energy. This high temperature presents challenges to methods for controlling the release of stored heat energy for several reasons, the most significant being the potential incompatibility of the storage medium's temperature with the materials used in the release system. Another challenge is utilizing the full temperature difference between the maximum temperature of the storage medium and the temperature of the system used to release the stored heat to maximize the utilization of the storage medium. For example, in a system that uses the released heat to generate process steam, as described above, the storage medium can operate at temperatures above 500°C, while the steam system operates at approximately 250°C.

[0004] Another problem with current thermal storage systems that use solid storage materials and release heat to generate process steam is the need for high-purity boiler feedwater. For load control, these systems require superheating the steam in the storage blocks and then desuperheating it to achieve desired steam conditions during de-load operation. Therefore, high-purity boiler feedwater is needed to prevent dissolved solids from settling in the superheated boiler tubes.

[0005] One object of the present invention is to overcome at least some of the disadvantages of the prior art. Summary of the Invention

[0006] According to a first aspect of the present invention, a method for controlling heat transfer from a thermal storage system is provided, the method comprising: providing a solid thermal storage module; providing a heat release channel, the solid thermal storage module and the heat release channel being separated by a gap, the gap being filled with a first fluid medium and a second fluid medium, the second fluid medium having a thermal conductivity at least fifty times that of the first fluid medium; determining a desired heat flux from the solid thermal storage module through the gap to the heat release channel; and controlling the volume ratio of the first fluid medium to the second fluid medium in the gap according to the desired heat flux.

[0007] The second fluid medium can be liquid metal, and preferably liquid tin.

[0008] The thermal conductivity of the first fluid medium can be up to 0.1 W / m. -1 K -1 Air can be a particularly suitable material for use as the primary fluid medium.

[0009] The thermal conductivity of the second fluid medium can be at least one hundred times that of the first fluid medium. Preferably, the thermal conductivity of the second fluid medium can be at least three hundred times that of the first fluid medium. More preferably, the thermal conductivity of the second fluid medium can be at least five hundred times that of the first fluid medium.

[0010] The method may include providing a reservoir for a second fluid medium, and wherein the step of controlling the volume ratio of a first fluid medium to a second fluid medium in the gap includes changing the pressure of the second fluid medium applied to the reservoir to change the volume of the second fluid medium in the gap.

[0011] According to a second aspect of the present invention, a thermal storage system is provided, the thermal storage system comprising: a solid thermal storage module; a heat release channel; a gap between the thermal storage module and the heat release channel, the gap being filled with a first fluid medium and a second fluid medium, the second fluid medium having a thermal conductivity at least one thousand times that of the first fluid medium; a reservoir for the second fluid medium in fluid communication with the gap; and a controller configured to determine a desired heat flux of steam from the solid thermal storage module through the gap to the heat release channel, and to control the volume ratio of the first fluid medium to the second fluid medium in the gap according to the desired heat flux.

[0012] The thermal storage module can circumferentially surround the heat release channel, making the gap annular. The width of the gap can be between 1 mm and 50 mm. Preferably, the width of the gap can be between 10 mm and 20 mm.

[0013] The thermal storage system may also include a pressure application unit configured to be controlled by a controller to apply pressure to a second fluid medium in the reservoir, thereby changing the volume of the second fluid medium in the gap and thus controlling the volume ratio of the first fluid medium to the second fluid medium in the gap.

[0014] The gap can extend vertically through the thermal storage module.

[0015] The thermal storage system may also include a boiler feedwater storage vessel in fluid communication with the heat release channel, so that the thermal storage system can be used to generate process steam by heating the boiler feedwater. Attached Figure Description

[0016] The invention will now be described with reference to the following figures, in which:

[0017] Figure 1 A cross-sectional view of a thermal storage system is schematically illustrated.

[0018] These accompanying drawings depict one or more specific embodiments of the teachings by way of example only and not limitation. In the drawings, similar reference numerals denote the same or similar elements.

[0019] Detailed description of the attached figures

[0020] Figure 1 A thermal storage system 10 is schematically illustrated. This system includes a solid thermal storage module 12 and a heat release channel 16. The heat stored in the thermal storage module 12 can be transferred to a fluid in the heat release channel 16 to heat the fluid. In the following description, the thermal storage system 10 will be described with particular reference to the generation of steam in the heat release channel 16 by heating boiler feedwater. However, it should be understood that the described principles and apparatus can be equally applied to heating other fluids, particularly liquids.

[0021] The thermal storage module 12 is separated from the heat release channel 16 by a gap 14. The width of the gap 14 is relatively narrow, for example, any value between 1 mm and 50 mm, or preferably between 10 mm and 20 mm. In other embodiments, the width of the gap 14 may be between 5 mm and 25 mm, between 5 mm and 40 mm, or between 15 mm and 30 mm. Generally, it is desirable for the gap 14 to be small in order to reduce its overall volume, while taking into account manufacturing tolerances and ensuring that the gap 14 is continuous throughout the thermal storage system 10. As will be explained below, reducing the volume of the gap 14 minimizes the volume of the thermal control medium that must be stored in the reservoir used in the thermal storage system 10.

[0022] Gap 14 and thermal storage module 12 in Figure 1 The gap 14 is shown in an annular form because this allows heat to be transferred most efficiently to the fluid in the heat release channel 16. However, it should be understood that, although preferred, the annular form of the gap 14 and the heat storage module 12 is not strictly necessary, and the heat storage module 12 and the gap 14 may be adjacent to only one side of the heat release channel 16.

[0023] The gap 14 is filled with a first fluid medium 18 and a second fluid medium 20. The volume ratio of the first fluid medium 18 to the second fluid medium 20 present in the gap 14 can be defined at any given time. However, the gap can be completely filled with either the first fluid medium 18 or the second fluid medium 20 at any given time, such that the volume ratio of the first fluid medium 18 to the second fluid medium 20 is 1:0 or 0:1, as appropriate.

[0024] The first fluid medium 18 and the second fluid medium 20 in the gap 14 are thermally connected to the thermal storage module 12 and the heat release channel 16, so that the heat stored in the thermal storage module 12 is transferred to the heat release channel 16 via the first fluid medium 18 and the second fluid medium 20. This heat transfer from the thermal storage module 12 to the heat release channel 16 enables the generation of process steam from the boiler feedwater. The process steam can be heated to a temperature of approximately 250°C.

[0025] Advantageously, the thermal conductivity of the first fluid medium can be very low in order to minimize heat transfer through the gap 14 when it is completely filled with the first fluid medium 18. For example, the thermal conductivity of the first fluid medium can be at most 0.1 W / m. -1 K -1 Therefore, the effective “closed” state of the thermal storage system 10 is defined by completely filling the gap 14 with the first fluid medium 18, where heat transfer from the thermal storage module 12 is minimal and is mainly defined by radiative heat transfer.

[0026] In contrast, the thermal conductivity of the second fluid medium 20 is significantly higher than that of the first fluid medium 18. For example, the thermal conductivity of the second fluid medium 20 may be at least fifty times that of the first fluid medium 18, preferably at least one hundred times, more preferably at least three hundred times, and most preferably at least five hundred times.

[0027] Since the thermal conductivity of the second fluid medium 20 is higher than that of the first fluid medium 18, the larger the ratio of the second fluid medium 20 to the first fluid medium 18 within the gap 14, the higher the heat flux (i.e., heat transfer rate) through the gap 14 between the heat storage module 12 and the heat release channel 16. Similarly, the lower the ratio of the second fluid medium 20 to the first fluid medium 18, the lower the heat flux, and the slower the rate of heat transfer from the heat storage module 12 through the gap 14 to the heat release channel 16. Completely filling the gap 14 with the second fluid medium 20 limits the maximum heat flux from the heat storage module through the gap 14 to the heat release channel 16.

[0028] Since the heat storage gap 14 has a substantially constant cross-section along its length, the ratio of the volume of the first fluid medium 18 to the volume of the second fluid medium 20 is the same as the ratio of the heat exchange area of ​​the first fluid medium 18 to the second fluid medium 20 in the gap 14. Therefore, controlling the volume ratio of the first fluid medium 18 to the second fluid medium 20 allows for linear control of the heat flux passing through the entire gap 14 between the heat storage module 12 and the heat release channel 16.

[0029] The thermal storage system 10 may include a controller (not shown) that determines the desired heat flux through the gap 14 and accordingly controls the corresponding volumes of the first fluid medium 18 and the second fluid medium 20 in the gap 14. To determine the desired heat flux, the controller may receive information from various sensors, such as temperature sensors in the thermal storage module 12 and the heat release channel 16, so that the corresponding temperatures of the steam within the thermal storage module 12 and the heat release channel 16 can be known. The controller may also be configured to receive input from an operator indicating whether heat release from the thermal storage system 10 is desired, or specifically indicating the desired heat flux through the gap 14. Those skilled in the art will recognize other features that can be monitored and forwarded to the controller, or operator input that can be directly provided to the controller to enable the controller to determine the desired heat flux through the gap 14.

[0030] In order to control the volume ratio of the first fluid medium 18 to the second fluid medium 20 in the gap 14, the thermal storage system 10 may include, for example: Figure 1 The reservoir 22 for the second fluid medium 20 shown allows for variation in the amount of the second fluid medium 20 in the gap 14. The reservoir 22 is in fluid communication with the gap 14, but there is no intentional effective thermal connection between the second fluid medium 20 within the reservoir 22 and the heat storage module 12, except for unavoidable heat conduction (if any) through the second fluid medium 20 in the gap 14. Therefore, the second fluid medium 20 can be discharged from the reservoir 22 and enter the gap 14 to increase the heat flux through the gap 14. Conversely, the second fluid medium 20 can be allowed to flow from the gap 14 back into the reservoir 22, thereby reducing the heat flux through the gap 14.

[0031] exist Figure 1 In this system, the amount of the second fluid medium 20 in the reservoir 22 and the gap 14 is controlled by applying pressure P to the second fluid medium 20 in the reservoir 22 by a pressure application unit under the control of the controller. By applying greater pressure, more of the second fluid medium is discharged from the reservoir 22 and enters the gap 14. Similarly, applying lower pressure forces less of the second fluid medium 20 out of the reservoir 22, or allows some of the second fluid medium 20 to return to the reservoir 22. Alternative methods for controlling the inflow and outflow of the second fluid medium 20 from the reservoir 22 will be appreciated by those skilled in the art.

[0032] The first fluid medium 18 is preferably a gas. Air is a particularly suitable material for serving as the first fluid medium 18. Air has a very low thermal conductivity (0.05 W / m² at 400°C). -1 K -1Furthermore, the use of air means that the surrounding atmosphere can effectively form an air reservoir, which functions in a similar manner to the reservoir 22 containing the second fluid medium 20. When the pressure of the second fluid medium 20 applied to the reservoir 22 increases to force more of the second fluid medium 20 into the gap 14, air 18 is expelled from the gap 14. Similarly, when the pressure of the second fluid medium 20 applied to the reservoir 22 decreases, air 18 flows from the surrounding atmosphere into the gap 14 to fill the space left by the returning second fluid medium 20. In this case, the thermal storage system 10 should preferably include a filter element (not shown) to prevent airborne contaminants such as dust from entering the gap 14.

[0033] Conveniently, the second fluid medium 20 can be a liquid. When used with a gaseous first fluid medium 18, such as air, this allows for easy separation of the first fluid medium 18 and the second fluid medium 20 within the gap 14. Figure 1 As shown, the second fluid medium 20 fills the gap 14 to the filling level 24. Therefore, in this scenario, changing the ratio of the first fluid medium 18 to the second fluid medium 20 in the gap is equivalent to changing the filling level 24 of the gap 14: when the proportion of the second fluid medium 20 increases, the filling level 24 will rise, while decreasing the proportion of the second fluid medium 20 will lower the filling level 24.

[0034] It will be apparent that below the filling level 24, the gap 14 is completely filled with the second fluid medium 20; above the filling level 24, the gap 14 is completely filled with the first fluid medium 18. Since the thermal conductivity of the second fluid medium 20 is much higher than that of the first fluid medium 18, the thermal storage system can be effectively divided into a low heat flux region 26 and a high heat flux region 28. It should be understood that although the heat flux through each of the low heat flux region 26 and the high heat flux region 28 will be determined by the thermal conductivity of the first fluid medium 18 and the second fluid medium 20, the heat flux through the entire gap 14 will be the average of the volume ratio of the first fluid medium 18 to the second fluid medium 20.

[0035] The natural separation between the gaseous first fluid medium 18 and the liquid second fluid medium 20 allows for more direct control over the ratio of their respective volumes. In a preferred embodiment where the second fluid medium 20 is liquid and pressure is used to control the filling level 24 and the ratio of the first fluid medium 18 to the second fluid medium 20 in the gap 14, this combination of the two aspects of the thermal storage system 10 enables precise and rapid control of the rate of heat transfer between the thermal storage module 12 and the heat release channel 16.

[0036] An additional benefit of using a liquid as the second fluid medium 20 is that the thermal conductivity of liquids is generally much higher than that of gases, which increases the maximum available heat flux that can be achieved between the thermal storage module 12 and the heat release channel 16. In particular, the second fluid medium can be a liquid metal. Liquid metals have very high thermal conductivity, which increases the range of heat flux that the thermal storage system 10 can provide. Furthermore, metals tend to have a wide temperature range within which they are liquids, allowing the use of thermal storage modules with higher maximum operating temperatures without degrading the thermal performance of the liquid metal. The higher boiling point associated with metals results in lower vapor pressures at the operating temperature of the thermal storage system 10, which leads to less evaporation loss of the second fluid medium 20 compared to materials with lower boiling points.

[0037] For example, a particularly suitable material for use as the second fluid medium 20 is liquid tin. Tin has a very high boiling point of 2600°C, but a relatively low melting temperature of about 232°C, which makes it liquid at the operating temperature of the heat storage system 10. Liquid tin has a thermal conductivity of about 30 W / m² from its melting point to about 500°C. -1 K -1 and 40W m -1 K -1 Between. The density of liquid tin is approximately 7000 kg m³. -3 Therefore, only moderate pressure is needed to overcome the requirements of thermal storage systems, such as... Figure 1 The static height shown requires a pressure difference of approximately 7 bar (0.7 MPa) to raise the liquid tin to a height of 10 m. This corresponds to relatively low power requirements for controlling the filling level 24, especially compared to using gas as the second fluid medium 20.

[0038] By controlling the volume ratio of the first fluid medium 18 to the second fluid medium 20 to control the heat flux from the entire thermal storage module 12, the thermal storage system 10 can release heat to the steam in the heat release channel 16 without overheating the steam. In other words, by carefully controlling the heat flux through the gap 14, the heat energy transferred to the steam can be controlled so that the steam is heated to the desired temperature without overheating. This allows the use of lower purity steam in the heat release channel 16, because avoiding overheating the steam means that any dissolved solids in the steam will not precipitate and adhere to the heat release channel 16. This is highly beneficial for consumers in the processing industry who may not have equipment capable of producing high-purity boiler feedwater (BFW). However, it should be understood that high-purity BFW can still be used in the apparatus and method of the present invention.

[0039] Furthermore, by controlling the volume ratio of the first fluid medium 18 to the second fluid medium 20 in the manner described above, the thermal storage system 10 requires the fewest mechanical components, thus making the thermal storage system simple, reliable, and relatively low in cost.

[0040] In embodiments where the thermal storage system 10 is used to generate process steam, the thermal storage system 10 may include an integrated feedwater reservoir (not shown) to supply boiler feedwater to the heat release channel 16. To protect the feedwater reservoir from the heat of the thermal storage module 12, the thermal storage module 12 may be physically separated from the feedwater reservoir, for example, by arranging the thermal storage module 12 spaced apart from the feedwater reservoir and vertically above the feedwater reservoir, such that the heat release channel 16 extends vertically from the feedwater reservoir and through the thermal storage module 12. This allows the thermal storage module 12 to "operate" at a higher maximum temperature, thereby storing more energy. Alternatively, the thermal storage system 10 may not include an integrated feedwater reservoir used with existing reservoir systems.

[0041] While many possible variations of the methods for controlling heat transfer and associated devices have been described above, it will be apparent to those skilled in the art that additional modifications and variations can be made without departing from the scope of the invention as claimed in the appended claims. For example, the heat stored in the thermal storage module 12 can be used to maintain the liquid metal used as the second fluid medium 20 at a temperature above its melting point. The thermal storage system 10 may also include a plurality of heat release channels 16 with associated gaps 14. Furthermore, although the thermal storage system 10 has been described primarily with reference to an embodiment that uses the heat stored in the thermal storage module 12 to generate process steam, those skilled in the art will understand that the same systems and methods can be used to control the release of heat from the thermal storage system 10 to other fluids in the heat release channels 16.

Claims

1. A method for controlling heat transfer from a thermal storage system (10), the method comprising: Provide solid thermal storage modules (12); A heat release channel (16) is provided, the solid heat storage module (12) is separated from the heat release channel (16) by a gap (14), the gap is filled with a first fluid medium (18) and a second fluid medium (20), the thermal conductivity of the second fluid medium (20) is at least fifty times that of the thermal conductivity of the first fluid medium (18); Determine the desired heat flux from the solid thermal storage module (12) through the gap (14) to the heat release channel (16); as well as The volume ratio of the first fluid medium (18) to the second fluid medium (20) in the gap (14) is controlled according to the desired heat flux.

2. The method according to claim 1, wherein the second fluid medium (20) is liquid metal.

3. The method according to claim 2, wherein the second fluid medium (20) is liquid tin.

4. The method according to any of the preceding claims, wherein the thermal conductivity of the first fluid medium (18) is at most 0.1 W / m. -1 K -1 .

5. The method according to any of the preceding claims, wherein the first fluid medium (18) is air.

6. The method according to any of the preceding claims, wherein the thermal conductivity of the second fluid medium (20) is at least one hundred times that of the thermal conductivity of the first fluid medium (18), preferably at least three hundred times that of the thermal conductivity of the first fluid medium (18), and more preferably at least five hundred times that of the thermal conductivity of the first fluid medium (18).

7. The method according to any of the preceding claims, the method comprising providing a reservoir (22) for the second fluid medium (20), and wherein the step of controlling the volume ratio of the first fluid medium (18) to the second fluid medium (20) in the gap (14) comprises changing the pressure of the second fluid medium (20) applied to the reservoir (22) to change the volume of the second fluid medium (20) in the gap (14).

8. A thermal storage system (10), the thermal storage system comprising: Solid thermal storage module (12); Heat release channel (16); The gap (14) between the heat storage module (12) and the heat release channel (16) is filled with a first fluid medium (18) and a second fluid medium (20), wherein the thermal conductivity of the second fluid medium (20) is at least one thousand times that of the thermal conductivity of the first fluid medium (18); A reservoir (22) for the second fluid medium (22), the reservoir being in fluid communication with the gap (14); and A controller configured to determine the desired heat flux of steam from the solid thermal storage module (12) through the gap (14) into the heat release channel (16), and to control the volume ratio of the first fluid medium (18) to the second fluid medium (20) in the gap (14) according to the desired heat flux.

9. The thermal storage system (10) according to claim 8, wherein the thermal storage module (12) surrounds the heat release channel (16) circumferentially, such that the gap (14) is annular.

10. The thermal storage system (10) according to claim 8 or claim 9, wherein the width of the gap (14) is between 1 mm and 50 mm.

11. The thermal storage system (10) according to claim 10, wherein the width of the gap (14) is between 10 mm and 20 mm.

12. The thermal storage system (10) according to any one of claims 8 to 11, the thermal storage system further comprising a pressure application unit configured to be controlled by the controller to apply pressure to the second fluid medium (20) in the reservoir (22), thereby changing the volume of the second fluid medium (20) in the gap (14), and thus controlling the volume ratio of the first fluid medium (18) to the second fluid medium (20) in the gap (14).

13. The thermal storage system (10) according to any one of claims 8 to 12, wherein the gap (14) extends vertically through the thermal storage module (12).

14. The thermal storage system (10) according to any one of claims 8 to 13, the thermal storage system further comprising a boiler feedwater reservoir, the reservoir being in fluid communication with the heat release channel (16).

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