Method and system for controlling heat transfer in solid heat storage systems

Abstract

This invention provides a heat storage system and a method for controlling heat transfer from the heat storage system. The heat storage system comprises a solid heat storage module; a heat discharge channel, and a clearance separating the solid heat storage module and the heat discharge channel. The clearance is filled with a first fluid medium and a second fluid medium, the 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 heat storage module, a desired heat flux from the solid heat storage module across the clearance to the heat discharge channel is determined; and a ratio of the volumes of the first fluid medium and the second fluid medium in the clearance is controlled in dependence on the desired heat flux.

Description

[0001] METHOD AND SYSTEM FOR CONTROLLING HEAT TRANSFER IN SOLID HEAT STORAGE SYSTEMS

[0002] Field of the Invention

[0003] This invention relates to a method of controlling heat transfer from a heat storage system, in particular a solid heat storage system, and to a solid heat storage system.

[0004] Background of the invention

[0005] Heat storage devices make use of the ability to store energy at elevated temperatures in materials. This heat ultimately needs to be discharged such that it can be made useful. Heat discharged from such devices can be used to heat fluids. One use of discharged heat is to produce process steam for a variety of end use applications.

[0006] Use can be made of solid storage media such as, but not limited to, bricks, solid concrete, graphite or ceramic materials. The temperature of such solid storage media can go significantly above 500°C to maximize the amount of stored energy. This high temperature provides a challenge for the method of controlling the discharge of the stored heat energy for multiple reasons, chief among them the fact that the high temperature of the storage media can be incompatible with the materials used in the discharge system. An additional challenge is how to use the complete temperature difference between the maximum temperature of the heat storage medium and the temperature of the system used to discharge the stored heat, in order to maximize the utilization of the heat storage media. For example , in systems where the discharged heat i s used to generate proces s steam as di scus sed above , the heat storage medium may operate at a temperature above 500 °C , while the steam system may operate at approximately 250 ° C .

[0007] Another problem with current heat storage systems using solid storage material s and discharging heat to generate proces s steam is the need for high purity boiler feedwater. For duty control these systems require the steam to be superheated in the storage block and subsequently de-superheated to achieve the desired steam conditions during turn down operation . High purity boiler feed water is therefore required to prevent precipitation of the dis solved solids in the superheated boiler tube s .

[0008] It is an aim of the current invention to overcome at least some of the disadvantages of the prior art .

[0009] Summary of the Invention

[0010] According to a f irst aspect of the invention , there is provided a method of controlling heat transfer from a heat storage system, the method comprising : providing a solid heat storage module ; providing a heat dis charge channel , the solid heat storage module and the heat discharge channel being separated by a clearance filled with a first fluid medium and a second fluid medium, the second fluid medium having a thermal conductivity at least fifty time s greater than a thermal conductivity of the first f luid medium; determining a desired heat flux from the solid heat storage module acros s the clearance to heat discharge channel ; and controlling a ratio of the volumes of the first fluid medium and the second fluid medium in the clearance in dependence on the desired heat flux . The second fluid medium may be a liquid metal , and may preferably be liquid tin .

[0011] The thermal conductivity of the first fluid medium may be a maximum of 0 . 1 W m-1K-1. A particularly suitable material for use as the first fluid medium may be air .

[0012] The thermal conductivity of the second f luid medium may be at least one hundred times greater than the thermal conductivity of the f irst fluid medium . Preferably, the thermal conductivity of the second fluid medium may be at least three hundred times greater than the thermal conductivity of the f irst fluid medium . More preferably, thermal conductivity of the second fluid medium may be at least f ive hundred times greater than the thermal conductivity of the f irst fluid medium .

[0013] The method may comprise providing a reservoir for the second fluid medium, and wherein the step of controlling the ratio of the volume s of the f irst fluid medium and the second fluid medium in the clearance comprises changing a pres sure applied to the second fluid medium in the reservoir to change the volume of the second fluid medium in the clearance .

[0014] According to a second a spect of the invention , there is provided a heat storage system compris ing : a solid heat storage module ; a heat discharge channel ; a clearance between the heat storage module and the heat di scharge channel , the clearance being filled with a first fluid medium and a second f luid medium, the second fluid medium having a thermal conductivity at least one thousand times greater than a thermal conductivity of the first fluid medium; a reservoir of the second f luid medium in fluid communication with the clearance ; and a controller configured to determine a de sired heat flux f rom the solid heat storage module acros s the clearance to steam in the heat di scharge channel and to control a ratio of the volumes of the first fluid medium and the second fluid medium in the clearance in dependence on the de sired heat flux .

[0015] The heat storage module may circumferentially surround the heat dis charge channel such that the clearance is annular in form . The width of the clearance may be between 1 mm and 50 mm . Preferably, the width of the clearance may be between 10 mm and 20 mm .

[0016] The heat storage system may further compri se a pres sure application unit configured to be controlled by the controller to apply a pres sure to the second fluid medium in the reservoir , thus changing the volume of the second fluid medium in the clearance and so controlling the ratio of the volumes of the first fluid medium and the second fluid medium in the clearance .

[0017] The clearance may extend vertically through the heat storage module .

[0018] The heat storage system may further compri se a reservoir of boiler feedwater in fluid communication with the heat discharge channel , such that the heat storage system can be used to generate proces s steam by heating the boiler feedwater .

[0019] Brief Description of the Drawings The invention will now be des cribed with reference to the following drawings , in which :

[0020] Figure 1 schematically illustrates a cross -sectional view of a heat storage system .

[0021] These drawings depict one or more implementations in accordance with the present teachings , by way of example only, not by way of limitation . In the figure s , like reference numerals refer to the same or s imilar element s .

[0022] Detailed Description of the Drawings

[0023] Figure 1 schematically illustrates a heat storage system 10 . The heat storage system compri ses a solid heat storage module 12 and a heat discharge channel 16 . Heat stored in the heat storage module 12 can be transferred to a fluid in the heat discharge channel 16 in order to heat it up . In the following description , the heat storage system 10 will be des cribed with particular reference to the generation of steam in the heat discharge channel 16 by heating boiler feedwater . However , it will be understood that the principles and apparatus de scribed can be applied equally to the heating of other fluids , particularly liquids .

[0024] The heat storage module 12 and the heat di scharge channel 16 are separated by a clearance 14 . The width of the clearance 14 is relatively narrow, for example being anywhere f rom 1 mm to 50 mm, or preferably between 10 mm and 20 mm . In other embodiments , the width of the clearance 14 may be between 5mm and 25 mm, between 5 mm and 40 mm, or between 15 mm and 30 mm . In general , it i s desirable for the clearance 14 to be small to reduce the overall volume thereof, while taking into account manufacturing tolerances and ensuring that the clearance 14 is continuous through the heat storage system 10. Reducing the volume of the clearance 14 minimises the volume of a heat control medium that must be stored in a reservoir for use in the heat storage system 10, as will be explained below.

[0025] The clearance 14 and heat storage module 12 are shown in Figure 1 to be annular in form, as this enables the most efficient transfer of heat into the fluid in the heat discharge channel 16. However, it will be understood that, while preferable, the annular form of the clearance 14 and heat storage module 12 is not strictly necessary and the heat storage module 12 and the clearance 14 may merely be adjacent one side of the heat discharge channel 16.

[0026] The clearance 14 is filled with a first fluid medium 18 and a second fluid medium 20. A ratio can be defined between the volumes of the first fluid medium 18 and the second fluid medium 20 present in the clearance 14 at any one time. It is, however, possible for the clearance to, at any given point, be completely filled with the first fluid medium 18 or the second fluid medium 20, such that the ratio between the volumes of the first fluid medium 18 and the second fluid medium 20 is 1:0 or 0:1, as the case may be.

[0027] The first and second fluid media 18, 20 in the clearance 14 are in thermal connection with the heat storage module 12 and the heat discharge channel 16 so that heat stored in the heat storage module 12 is transferred to the heat discharge channel 16 via the first and second fluid media 18, 20. The transfer of the heat from the heat storage module 12 to the heat discharge channel 16 allows the generation of process steam from boiler feedwater. The process steam may be heated to a temperature of approximately 250 °C.

[0028] Beneficially, the thermal conductivity of the first fluid medium may be very low, in order to minimise heat transfer across the clearance 14 when completely filled with the first fluid medium 18. For example, the thermal conductivity of the first fluid medium may be a maximum of 0.1 W m-1K-1. Completely filling the clearance 14 with the first fluid medium 18 therefore defines an effective 'off' state for the heat storage system 10, where transfer of heat from the heat storage module 12 is at a minimum and mostly defined by radiative heat transfer.

[0029] In contrast, the second fluid medium 20 has a thermal conductivity that is significantly higher than that of the first fluid medium 18. For example, the second fluid medium 20 may have a thermal conductivity that is at least fifty times greater, preferably at least one hundred times greater, more preferably at least three hundred times greater and most preferably at least five hundred times greater than that of the first fluid medium 18.

[0030] Since the second fluid medium 20 has a higher conductivity than the first fluid medium 18, the greater the proportion of the second fluid medium 20 inside the clearance 14 to the first fluid medium 18, the higher the heat flux (i.e. the rate of transfer of heat) between the heat storage module 12 and the heat discharge channel 16 across the clearance 14. Equally, the lower the proportion 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 to the heat discharge channel 16 across the clearance 14. Filling the clearance 14 completely with the second fluid medium 20 defines a maximum heat flux from the heat storage module to the heat discharge channel 16 across the clearance 14.

[0031] As the heat storage clearance 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 ratios of the heat-exchange areas across the first and second fluid media 18, 20 in the clearance 14. Control of the ratio between the volumes of the first fluid medium 18 and the second fluid medium 20 therefore allows linear control of the heat flux between the heat storage module 12 and the heat discharge channel 16 across the clearance 14 as a whole .

[0032] The heat storage system 10 may comprise a controller (not shown) which determines the desired heat flux across the clearance 14 and controls the respective volumes of the first and second fluid media 18, 20 in the clearance 14 accordingly. To determine the desired heat flux, the controller may receive information from various sensors, such as temperature sensors in the heat storage module 12 and the heat discharge channel 16 so that the respective temperatures of the heat storage module 12 and the steam inside the heat discharge channel 16 may be known. The controller may also be configured to receive an input from an operator that indicates whether or not it is desired for the heat storage system 10 to discharge heat or not, or that specifically indicates the desired heat flux across the clearance 14. The skilled person will be aware of other characteristics that may be monitored and relayed to the controller , or operator inputs that may be given directly to the controller that may enable the controller to determine the desired heat flux acros s the clearance 14 .

[0033] To enable control of the ratio between the volumes of the first fluid medium 18 and the second f luid medium 20 in the clearance 14 , the heat storage system 10 may comprise a reservoir 22 of the second fluid medium 20 , as seen in Figure 1 , so that the amount of the second f luid medium 20 in the clearance 14 can be varied . The reservoir 22 is in f luid communication with the clearance 14 , but the second fluid medium 20 inside the reservoir 22 i s not in deliberate operative thermal connection with the heat storage module 12 , except for unavoidable conduction of heat through the second fluid medium 20 inside the clearance 14 , if any . As such , the second fluid medium 20 can be forced out of the reservoir 22 and into the clearance 14 to increase the heat f lux acros s the clearance 14 . Conversely, the second fluid medium 20 can be allowed to f low back into the re servoir 22 f rom the clearance 14 , thus decreasing the heat flux acros s the clearance 14 .

[0034] In Figure 1 , the control of the amount of the second fluid medium 20 in the reservoir 22 and the clearance 14 is achieved through the application of a pres sure P to the second fluid medium 20 in the reservoir 22 by a pres sure application unit under the control of the controller . By applying a greater pres sure , more of the second fluid medium is forced out of the reservoir 22 and into the clearance 14 . S imilarly, applying a lower pre ssure forces les s of the second fluid medium 20 out of the reservoir 22, or enables some of the second fluid medium 20 to return to the reservoir 22. The skilled person will be aware of alternative methods to control the flow of the second fluid medium 20 into and out of the reservoir 22.

[0035] The first fluid medium 18 may preferably be a gas. A particularly suitable material to act as the first fluid medium 18 is air. Not only does air have a very low thermal conductivity (0.05 W m-1K-1at 400°C) , but use of air means that the surrounding atmosphere can effectively form a reservoir of air, acting in a similar way to the reservoir 22 containing the second fluid medium 20. When the pressure applied to the second fluid medium 20 inside the reservoir 22 is increased so as to force more of the second fluid medium 20 into the clearance 14, the air 18 is forced out of the clearance 14. Similarly, when the pressure applied to the second fluid medium 20 inside the reservoir 22 is reduced, air 18 flows into the clearance 14 from the surrounding atmosphere to fill the space left by the retreating second fluid medium 20. In this case, the heat storage system 10 should preferably include a filtering means (not shown) that prevents airborne contaminants, such as dust, from entering the clearance 14.

[0036] Conveniently, the second fluid medium 20 may be a liquid. When used with a gaseous first fluid medium 18, such as air, this enables easy separation of the first and second fluid media 18, 20 within the clearance 14. As can be seen in Figure 1, the second fluid medium 20 fills the clearance 14 up to a filling level 24. Therefore, changing the proportion of the first fluid medium 18 to the second fluid medium 20 in the clearance is equivalent in this scenario to a change in the filling level 24 of the clearance 14 : raising the filling level 24 occurs when the proportion of the second fluid medium 20 increa ses , while decreas ing the proportion of the second f luid medium 20 lowers the filling level 24 .

[0037] It will be apparent that beneath the filling level 24 , the clearance 14 is filled entirely with the second fluid medium 20 ; above the f illing level 24 , the clearance 14 is f illed entirely with the first fluid medium 18 . Since the second fluid medium 20 ha s a much higher thermal conductivity than the first fluid medium 18 , this effectively splits the heat storage system into a low heat flux region 26 and a high heat flux region 28 . It will be understood that while the heat flux acros s each of the low heat flux region 26 and the high heat flux region 28 will be determined by the thermal conductivities of the f irst fluid medium 18 and the second fluid medium 20 , the heat flux acros s the clearance 14 as a whole will be an average value based on the ratio of the volumes of the first fluid medium 18 and the second fluid medium 20 .

[0038] The natural separation between the gaseous first fluid medium 18 and the liquid second fluid medium 20 make s control of the ratio of their respective volume s much more straightforward . In preferred embodiments where the second fluid medium 20 is a liquid and pre s sure is used to control the filling level 24 and the proportion of the first and second fluid media 18 , 20 in the clearance 14 , the combination of these two aspect s of the heat storage system 10 enables fine and fast control of the rate of heat trans fer between the heat storage module 12 and the heat di scharge channel 16 . An additional benefit of the use of a liquid as the second fluid medium 20 is that liquids generally have significantly higher thermal conductivities than gases, which increases the maximum available heat flux that can be achieved between the heat storage module 12 and the heat discharge channel 16. In particular, the second fluid medium may be a liquid metal. Liquid metals have very high thermal conductivity, which increases the heat flux range that the heat storage system 10 can provide. In addition, metals tend to have a wide temperature range in which they are liquid, enabling the use of a heat storage modules with higher maximum operating temperatures without any degradation in the thermal properties of the liquid metal. The higher boiling points associated with metals results in a lower vapour pressure at the operating temperatures of the heat storage system 10, causing less evaporative loss of the second fluid medium 20 compared to materials with lower boiling points.

[0039] For example, one particularly suitable material to 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 approximately 232 °C, which allows it to be liquid at the operating temperatures of the heat storage system 10. The thermal conductivity of liquid tin from its melting point to approximately 500°C is between approximately 30 W m-1K-1and 40 W m-1K-1. The density of liquid tin is approximately 7000 kg m-3, and so requires only moderate pressures to overcome the sort of static heights required in heat storage systems, such as that shown in Figure 1, with a pressure differential of approximately 7 bar (0.7 MPa) being required to raise liquid tin by a height of 10 m. This corresponds to a relatively low power requirement to control the filling level 24, especially compared to the use of a gas as the second fluid medium 20.

[0040] By controlling the ratio between the volumes of the first and second fluid media 18, 20 to control the heat flux from the heat storage module 12 as a whole, it is possible for the heat storage system 10 to discharge heat to steam in the heat discharge channel 16 without superheating the steam. In other words, by carefully controlling the heat flux across the clearance 14, the heat energy delivered to the steam can be controlled such that the steam is heated to the desired temperature without superheating. This allows the use of lower purity steam in the heat discharge channel 16, as avoiding superheating the steam means that any dissolved solids in the steam do not precipitate out and adhere to the heat discharge channel 16. This is of great benefit to consumers in the process industry, who may not have equipment that is capable of producing high purity boiler feedwater (BFW) . However, it will be understood that it is still possible to use high purity BFW within the apparatus and method of the invention.

[0041] In addition, by controlling the ratio of the volumes of the first fluid medium 18 to the second fluid medium 20 in the manner outlined above, the heat storage system 10 requires minimal mechanical components, making it simple, reliable and relatively low cost.

[0042] In embodiments where the heat storage system 10 is used to generate process steam, the heat storage system 10 may comprise an integrated feedwater reservoir (not shown) to provide the boiler feedwater to the heat discharge channel 16. To protect the feedwater reservoir from the heat of the heat storage module 12 , the heat storage module 12 can be phys ically separated from the feedwater reservoir , for example by arranging the heat storage module 12 spaced apart from and vertically above the feedwater reservoir , such that the heat discharge channel 16 extends vertically from the feedwater reservoir and through the heat storage module 12 . This can enable the heat storage module 12 to be ' run' at a higher maximum temperature and so store more energy . Alternatively, the heat storage system 10 can be provided without an integrated feedwater reservoir for use with exi sting reservoir systems .

[0043] While many pos sible variations of the method of controlling heat transfer and the a s sociated apparatus have been described above , it will be clear to the s killed person that additional variations and modifications can be made without departing from the scope of the invention as claimed in the appended claims . For example , it may be pos sible to use the heat stored in the heat storage module 12 to keep the liquid metal used as the second fluid medium 20 at a temperature above it s melting point . The heat storage system 10 may also incorporate a plurality of heat di scharge channels 16 , with as sociated clearances 14 . In addition , while the heat storage system 10 has been described primarily with reference to an embodiment in which heat stored in the heat storage module 12 is used to generate proces s steam, the s killed person will understand that the same system and methods may be used in order to control the dis charge of heat from the heat storage system 10 to other fluids in the heat discharge channel 16 .

Claims

C L A I M S1. A method of controlling heat transfer from a heat storage system (10) , the method comprising: providing a solid heat storage module (12) ; providing a heat discharge channel (16) , the solid heat storage module (12) and the heat discharge channel (16) being separated by a clearance (14) filled with a first fluid medium (18) and a second fluid medium (20) , the second fluid medium (20) having a thermal conductivity at least fifty times greater than a thermal conductivity of the first fluid medium (18) ; determining a desired heat flux from the solid heat storage module (12) across the clearance (14) to the heat discharge channel (16) ; and controlling a ratio of the volumes of the first fluid medium (18) and the second fluid medium (20) in the clearance (14) in dependence on the desired heat flux.

2. The method of Claim 1, wherein the second fluid medium (20) is a liquid metal.

3. The method of Claim 2, wherein the second fluid medium (20) is liquid tin.

4. The method of any preceding claim, wherein the thermal conductivity of the first fluid medium (18) is a maximum of 0.1 W nr1K-1.

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

6. The method of any preceding claim, wherein the thermal conductivity of the second fluid medium (20) is at least one hundred times greater than the thermal conductivity of the first fluid medium (18) , preferably at least three hundred times greater than the thermal conductivity of the first fluid medium (18) , more preferably at least five hundred times greater than the thermal conductivity of the first fluid medium (18) .

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

8. A heat storage system ( 10 ) comprising : a solid heat storage module (12) ; a heat discharge channel (16) ; a clearance (14) between the heat storage module (12) and the heat discharge channel (16) , the clearance (14) being filled with a first fluid medium (18) and a second fluid medium (20) , the second fluid medium (20) having a thermal conductivity at least one thousand times greater than a thermal conductivity of the first fluid medium (18) ;a reservoir (22) of the second fluid medium (22) in fluid communication with the clearance (14) ; and a controller configured to determine a desired heat flux from the solid heat storage module (12) across the clearance (14) to steam in the heat discharge channel (16) and to control a ratio of the volumes of the first fluid medium (18) and the second fluid medium (20) in the clearance (14) in dependence on the desired heat flux.

9. The heat storage system (10) of Claim 8, wherein the heat storage module (12) circumferentially surrounds the heat discharge channel (16) such that the clearance (14) is annular in form.

10. The heat storage system (10) of Claim 8 or Claim 9, wherein the width of the clearance (14) is between 1 mm and 50 mm.

11. The heat storage system (10) of Claim 10, wherein the width of the clearance (14) is between 10 mm and 20 mm.

12. The heat storage system (10) of any of Claims 8 to 11, further comprising a pressure application unit configured to be controlled by the controller to apply a pressure to the second fluid medium (20) in the reservoir (22) , thus changing the volume of the second fluid medium (20) in the clearance (14) and so controlling the ratio of the volumes of the first fluid medium (18) and the second fluid medium (20) in the clearance ( 14 ) .

13. The heat storage system (10) of any of Claims 8 to 12, wherein the clearance (14) extends vertically through the heat storage module (12) .

14. The heat storage system (10) of any of Claims 8 to 13, further comprising a reservoir of boiler feedwater in fluid communication with the heat discharge channel (16) .

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