heat exchanger

Abstract

A thermosiphon system (1) includes: a geothermal wellbore (2); a pipe (3) extending downward along the wellbore (2); and a heat exchanger (5) connected to the lower end of the pipe (3) disposed in the geothermal wellbore (2). The heat exchanger (5) includes one or more blades (19) arranged to return working fluid in the form of supercritical carbon dioxide exiting the lower end of the pipe (3) upward along the geothermal wellbore (2) via a helical path (22) between the outer diameter of the pipe (3) and the inner diameter of the geothermal wellbore (2). The helical path (22) has a pitch length (P) and a total length (H) parallel to the geothermal wellbore (2). The thermosiphon system (1) also includes a turbine (9) driven by a mass flow of the working fluid.

Description

Technical Field

[0001] This specification relates to a heat exchanger used in a geothermal thermosiphon system. Background Technology

[0002] The thermodynamic properties of supercritical carbon dioxide are well understood and used in a variety of applications. Recently, there has been interest in using supercritical carbon dioxide as an auxiliary working fluid in surface-based power generation systems, relying on fluid heat transfer processes to generate electricity. This is used in heat pumps and geothermal power generation.

[0003] Geothermal systems that generate power output in the range of hundreds of kilowatt-hours to megawatt-hours depend directly on the temperature of the working fluid (such as water or brine) delivered to the surface. The well depth of these systems depends on the local geothermal gradient (temperature increase per kilometer of depth) and is driven by the need to return sufficiently high temperatures to the surface to drive turbines via either a direct circulation or binary circulation system. In areas with sufficiently high geothermal gradients, direct circulation systems can be used, where steam extracted from the reservoir drives a reaction turbine. In areas with lower geothermal gradients, such systems become less attractive due to the greater depth required to generate the desired temperatures and surface mass flow to drive the turbines, coupled with the geological challenges associated with large well depths.

[0004] In this context, unconventional enhanced geothermal systems can be considered. These systems involve stimulating high-pressure, high-temperature reservoir rocks at depths typically 4 to 6 kilometers and extracting heat from the rocks through reservoir injection / circulation of working fluids in open-loop injection / production wells / subsurface systems. However, such systems inevitably introduce greater risks and costs with increasing depth due to geological and engineering reasons.

[0005] Progress has been made in developing unconventional geothermal resources using closed-loop systems. However, such systems still rely on heat brought to the surface via conventional turbines to generate electricity, thus requiring considerable depth to achieve meaningful power output across large land areas.

[0006] US 2021 / 140413 A1 describes a surface turbine system with a “Catherine wheel” design that rotates by the reaction force of working fluid jets from a nozzle, similar to a rotating garden sprinkler system.

[0007] US 2022 / 0282640 A1 describes the delivery of regulated working fluid to drive a steam reaction turbine.

[0008] US 2021 / 062682 A1 describes a downhole heat exchanger design that is retrofitted into an existing geothermal well to heat the working fluid used for power generation.

[0009] US 2017 / 130703 A1 describes optimizing multiple well locations in layered subsurface formations with different thermal conductivity characteristics to increase the transfer of heat energy to the surface.

[0010] Alvaro Amaya, Joseph Scherer, John Muir, Mehul Patel, and Brian Higgins: "GreenFire Energy Closed-Loop Geothermal Demonstration using Supercritical Carbon Dioxide as Working Fluid" ） The “Thermosiphon using supercritical carbon dioxide as the working fluid” was described in the Proceedings of the 45th Geothermal Reservoir Engineering Symposium held in Stanford, California, USA (February 10-12, 2020) (hereinafter referred to as “Amaya2020”). Summary of the Invention

[0011] According to a first aspect of the invention, a heat exchanger is provided for the lower end of a pipe disposed in a geothermal well. The heat exchanger includes one or more blades arranged to return working fluid exiting the lower end of the pipe upwards into the geothermal well via a helical path between the outer diameter of the pipe and the inner diameter of the geothermal well. The helical path has a pitch length and a total length parallel to the geothermal well.

[0012] In this way, the residence time of the working fluid (and thus heat transfer) can be controlled by varying the pitch length of one or more blades, the number of blades, and / or the total length of the helical path parallel to the wellbore. Variations in the working fluid's properties can be controlled to occur primarily within the total length of the helical path, thus providing greater control over thermosiphon operations using the working fluid.

[0013] The pipeline can be positioned parallel and concentric with the well shaft.

[0014] A heat exchanger may include two or more blades. A heat exchanger may include three blades. Two or more blades may be staggered / crossed.

[0015] At least one of one or more blades can span the total length of the spiral path. Each (i.e., all) blades can span the total length of the spiral path.

[0016] One or more blades can form an Archimedes screw structure between the outer diameter of the pipe and the inner diameter of the geothermal well. The Archimedes screw structure can define a helical path.

[0017] Heat exchangers can be used at the lower end of insulated pipes. Insulated pipes can be vacuum insulated pipes.

[0018] The heat exchanger may also include one or more flow splitting structures configured to divide the working fluid flow received from the pipe into two or more streams.

[0019] The heat exchanger may also include one or more flow redirection structures configured to redirect the working fluid flow received from the conduit into a helical path. The flow redirection structure may have a "bull-nose" shape. The flow redirection structure may take the form of a curved section configured to cause the working fluid flow to undergo a 180° change of direction within the radius of curvature.

[0020] The heat exchanger is configured to be attached to the lower end of the pipe. The heat exchanger can be configured to be detachably attached to the lower end of the pipe, for example, using a flange connection secured by bolts. The heat exchanger can be configured to be permanently attached to the lower end of the pipe, for example, by welding. The heat exchanger can be configured to be attached to the lower end of the pipe using a single pipe fitting.

[0021] Alternatively, the heat exchanger can be integrally formed with the lower end of the pipe.

[0022] Heat exchangers can be used with supercritical carbon dioxide as the working fluid. Supercritical carbon dioxide can be "dry," meaning it does not contain water (from a practical standpoint). Supercritical carbon dioxide can also be "wet," meaning it contains a non-negligible amount of water. Heat exchangers can be made of corrosion-resistant alloys, such as chromium-containing stainless steel. Known grades of steel used for carbon capture and storage will be suitable for forming heat exchangers.

[0023] Alternatively, the working fluid can be water or brine. When the working fluid contains water (aqueous), the temperature of the heat exchanger may be higher. Suitable materials may include, for example, steel known for its use as steam ducts in conventional non-geothermal power plants.

[0024] Alternatively, the working fluid can be a substituted hydrocarbon or an unsubstituted hydrocarbon, such as propane or butane.

[0025] Each of one or more blades can be welded to the annular portion of the heat exchanger.

[0026] The helical path can have a pitch length between 10 cm and 40 cm. Unless otherwise explicitly stated, all ranges defined herein include the defined endpoints. The pitch length can preferably be between 20 cm and 35 cm. The most preferred pitch length is 30 cm, accurate to two significant figures.

[0027] Each blade can extend into the space between the outer diameter of the pipe and the inner diameter of the geothermal well, with a radial extension distance between 5 cm and 50 cm. Typically, the relative dimensions of the pipe should be chosen such that the ratio of the cross-sectional area of ​​the pipe to the cross-sectional area of ​​the space between the outer diameter of the pipe and the inner diameter of the geothermal well is approximately equal to the ratio of the density of the cooler working fluid flowing in from the lower end of the pipe to the density of the heated working fluid along the length of the heat exchanger. In other words, the heat exchanger should maximize the flow rate of the working fluid without creating flow restriction.

[0028] The total length of the heat exchanger parallel to the geothermal well shaft can be between 5 m and 120 m. The total length is preferably 100 m, accurate to two significant figures. The total length can be formed by two or more modular sections. For example, each modular section can have a length of 20 m, accurate to two significant figures.

[0029] The heat exchanger may include or take the form of a cylindrical body, the inner diameter of which is equal to the inner diameter of the lower end of the pipe. One or more blades may extend from the outer diameter of the cylindrical body. At the connection point with the cylindrical body, each blade may form a 90° angle with the cylindrical body.

[0030] A thermosiphon system may include a geothermal well, pipes extending downwards along the well, and a heat exchanger. The heat exchanger may be connected to the lower end of the pipes. A turbine may be driven by a working fluid. The turbine may be driven by a mass flow stream of the working fluid. The turbine may be on the ground or below the ground, closer to the surface of the geothermal well.

[0031] The working fluid of a thermosiphon system can be supercritical carbon dioxide.

[0032] According to a second aspect of the invention, a method is provided that includes connecting a heat exchanger of the first aspect to a conduit to form an assembly. The method further includes lowering the assembly into a geothermal wellbore such that the heat exchanger is located at the lowermost end of the conduit.

[0033] Pipes can be inserted into geothermal wells to create a temperature difference of at least 40 K, at least 50 K, at least 60 K, at least 70 K, or at least 80 K between the surface and the heat exchanger.

[0034] The method of the second aspect may include features corresponding to any feature of the heat exchanger and / or thermosiphon system of the first aspect. The definitions applicable to the heat exchanger and / or thermosiphon system (or its features) of the first aspect may also be applied to the method (or its features) of the second aspect.

[0035] According to a third aspect of the invention, a heat exchanger of the first aspect is provided, and the thermosiphon system comprising the heat exchanger of the first aspect is used for power generation.

[0036] The method of the third aspect may include features corresponding to any feature of the heat exchanger and / or thermosiphon system of the first aspect. The definitions applicable to the heat exchanger and / or thermosiphon system (or its features) of the first aspect may also be applied to the method (or its features) of the third aspect. Attached Figure Description

[0037] Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, wherein: Figure 1 A thermosiphon system is schematically illustrated; Figure 2A This is a schematic cross-sectional view of a heat exchanger; Figure 2B It is by Figure 2A The dashed line A-A' in the figure represents a schematic cross-sectional view of the heat exchanger taken in the plane; Figure 3 The diagram schematically illustrates the flow of working fluid through the blades of a heat exchanger; Figure 4A It is by Figure 4B The dashed line B-B' in the figure represents a schematic cross-sectional view of the heat exchanger taken in the plane; Figure 4B This is a schematic side view of a heat exchanger with the outer tubes removed; and Figure 5 The calculated variation of blade pitch relative to the first vertical axis as a function of thermal conductivity is shown, as well as the calculated variation of the ratio of total travel distance to vertical travel distance relative to the second vertical axis. Detailed Implementation

[0038] In the following description, the same parts are indicated by the same reference numerals.

[0039] A thermosiphon (or simply "thermosyphon") works by the fact that heated fluids rise while cooler fluids sink. This effect is caused by density differences resulting from temperature changes. For example, when a fluid is heated, its density decreases, causing it to rise. Conversely, cooler fluids have a higher density and therefore sink. This phenomenon is called convection.

[0040] A thermosiphon consists of two parts: a heating section and a cooling section. The heating section contains the working fluid to be heated, which is heated by a heat source (such as a burner, solar panel, or electronic components). The cooling section contains a cooling medium (such as air or water) that absorbs heat from the working fluid and carries it away.

[0041] In this specification, the heat source is a geothermal source at the bottom of the well, and CO2 is located at or near the critical point of the working fluid. The less dense, heated working fluid rises from the bottom of the well to the wellhead along the well annulus, while the denser, cooler working fluid returns from the wellhead to the bottom of the well along the central conduit. The depth of the well acts as a compressor: the fluid pressure increases due to the weight of the denser, cooler working fluid column.

[0042] When the working fluid in the heating section is heated, its density decreases, causing it to rise. Then, the cooler working fluid in the cooling section sinks, forming a natural circulation loop. The mass flow of the working fluid can be used to drive a turbine to extract work from a thermosiphon.

[0043] As demonstrated at Amaya 2020, thermosiphon systems using supercritical carbon dioxide as the working fluid have the potential for use in a wider range of locations, such as areas with low geothermal gradients, and / or wells drilled to relatively shallow depths compared to conventional geothermal power systems. Such thermosiphon systems can also be installed in existing wells originally drilled for other purposes, such as oil / gas extraction, geological exploration, etc., if a temperature difference of more than approximately 40 K–60 K is achieved between the well bottom and the surface. The mass flow rate of carbon dioxide can be used to drive turbines at (or closer to) the surface to generate electricity.

[0044] To fully realize the potential of this thermosiphon system, it is necessary to control and optimize the residence time of the working fluid at the bottom of the well in order to control the changes in characteristics (pressure, density, etc.) between the cold, dense working fluid descending to the bottom of the well and the heated, less dense working fluid ascending to the wellhead.

[0045] This specification relates to heat exchangers that can be installed at the bottom of a well to allow for control and optimization of residence time and heat transfer to the working fluid in a thermosiphon. The heat exchangers according to this specification can provide the benefits of using supercritical carbon dioxide as the working fluid to thermosiphons (such as those described in Amaya2020), and example systems using supercritical carbon dioxide as the working fluid will be described. However, it will become apparent from the discussion below that the design and operating principles of the heat exchangers according to this specification are applicable to any thermosiphon system, regardless of the substance used as the working fluid. Supercritical carbon dioxide is indeed a preferred example because relatively small temperature changes (e.g., tens of K) can lead to significant changes in properties such as density.

[0046] The unconventional geothermal applications described in Amaya 2020 may be particularly suitable for remote areas where grid supply is limited / unreliable and local geothermal gradients / subsurface conditions do not make conventional geothermal systems economically viable (e.g., replacing traditional gas stations with electric vehicle (EV) charging stations in remote areas), or where existing power infrastructure is difficult to access. However, because local geothermal gradients and other geological factors can vary considerably, it is important to be able to control residence time and heat transfer rates at the wellbore, for example, to enable the system to have reduced-range surface pressure, temperature, and mass flow across a wide range of locations, thus allowing the use of standardized surface turbines (or at least one of several standardized arrangements), rather than requiring each unit to have turbines tailored to the characteristics of the working fluid (as might be expected with the simple terminals described in Amaya 2020). These considerations will reduce installation and maintenance costs, which will be crucial if unconventional geothermal systems are to realize their potential in practice.

[0047] refer to Figure 1 A schematic diagram of thermosiphon system 1 is shown.

[0048] In the thermosiphon system 1, the working fluid circulates around a closed-loop flow path. The relatively cold and dense working fluid descends along the wellbore 2 in the pipe 3 to the bottom 4 under gravity. At the bottom 4, a heat exchanger 5 facilitates heat transfer from the surrounding rock to the working fluid, causing a decrease in the working fluid's density and generating natural convection that carries the surface mass flow rate of the working fluid back up along the wellbore 2 in the wellbore annulus 6 surrounding the pipe 3. The heat exchanger 5 extends the return path portion in the region with the highest temperature, i.e., around the lower end of the pipe 3 at the bottom 4, thereby increasing the time available for heat transfer to the working fluid (hereinafter referred to as "residence time"). Furthermore, the heat exchanger 5 helps generate turbulence in this region, increasing the uniformity of the working fluid temperature. The precise design of the heat exchanger 5 can vary; one example will be referenced... Figure 2A , Figure 2B , Figure 3 , Figure 4A and Figure 4B Further description. However, in all cases, the heat exchanger 5 is designed to allow sufficient residence time and thermal conduction of the working fluid near the bottom of the well to undergo a controlled characteristic change towards a lower density state, thereby transporting the mass flow rate upwards along the wellbore 2 back to the surface. While almost any fluid can be used to achieve thermosiphon, a working fluid exhibiting a large density change with respect to a temperature difference of approximately tens of degrees K is preferred in order to generate a sufficiently large mass flow rate to allow for the extraction of useful work. A particularly preferred example of a working fluid is supercritical carbon dioxide. Other supercritical fluids can also be used to provide the working fluid, such as water / brine or hydrocarbons (e.g., butane and propane (although non-flammable working fluids are preferred)).

[0049] The mass flow rate of the working fluid returning to the surface is used to drive turbine 9 before being recirculated through system 1 (i.e., guided downwards along wellbore 2 in pipe 15). For example, when the working fluid is supercritical carbon dioxide, the mass flow rate at the surface can be 10 to 12 kg·s⁻¹. -1 The pressure ranges from 1000 to 1500 psi (6.9 MPa to 10 MPa).

[0050] Upstream of turbine 9, an upstream turbine selector valve 10 is provided in the flow path. Similarly, downstream of turbine 9, a downstream turbine selector valve 11 is provided in the flow path. The upstream selector valve 10 and the downstream turbine selector valve 11 are connected via turbine bypass 12 and can be used to guide all or part of the mass flow through turbine bypass 12.

[0051] A flow measurement station 13 is positioned downstream of turbine bypass 12 in the flow path to monitor the characteristics of the working fluid before it re-enters pipe 3 and descends along well 2. The measurement station 13 may include any suitable measuring instrument, including but not limited to: a Pitot tube (not shown) connected to a pressure sensor (not shown) for measuring total fluid pressure; a total fluid pressure thermocouple (not shown) for measuring total flow temperature; and a single-phase Coriolis flow meter (not shown) for measuring fluid velocity, etc. The pressure sensor and thermocouple can be used to determine the working fluid density from thermodynamic charts. The Coriolis flow meter can be used to measure both mass flow rate and working fluid density.

[0052] To start system 1, turbine bypass 12 is used to bypass turbine 9 until a sufficiently strong thermosiphon is generated. This is achieved by using upstream selector valve 10 and downstream selector valve 11. Specifically, once the thermosiphon has been established and has a sufficient mass flow rate (measured by measuring station 13), for example, approximately 10 kg / s for a working fluid in the form of supercritical carbon dioxide. -1 The downstream selector valve 11 can then be fully opened. The upstream turbine selector valve 10 can then be adjusted to deliver an increased amount of carbon dioxide flow to the inlet 14 of the turbine 9 until the upstream turbine selector valve 10 is fully open. The amount of working fluid (e.g., carbon dioxide) in the thermosiphon system 1 should be increased until the desired mass flow rate is reached as measured by the measuring station 13. Adding too much working fluid may result in a decrease in the mass flow rate (e.g., due to increased friction), although the desired flow rate can be achieved by optimizing the total amount of working fluid.

[0053] The turbine 9 is connected to the generator 15 via a drive shaft 16 and a coupling 17. Optionally, the generator 15 can be disconnected from the turbine 9 when power generation is not required.

[0054] If system 1 is capable of delivering a mass flow rate greater than required to drive a single turbine 9, it is possible to install turbine 9 and generator 15 in a parallel configuration to increase power output. In this case, doubling the mass flow rate can double the generated electrical energy.

[0055] Turbine 9 does not need to be located on the surface. In some examples, turbine 9 and generator 15 can be located underground, for example, just below the surface, where the temperature tends to be more stable than the temperature above the surface.

[0056] The density of the working fluid will increase as it flows through turbine 9. In some examples, additional expansion and cooling of the working fluid can be provided by further introducing an expander (not shown) and / or a condenser (not shown) into the flow path. This can help improve the performance of turbine 9. For example, in cases where the turbine outlet does not meet desired conditions, such as if the working fluid does not reach a sufficient fluid density to sustain thermosiphon transport of the surface mass flow rate, a condenser can be used to give the working fluid (e.g., supercritical carbon dioxide) a higher density.

[0057] To reduce (or even prevent) the gradual heating of relatively high-density working fluids (such as supercritical carbon dioxide) before reaching the bottom of the well 4, pipe 3 can be insulated. When insulated, the thermal conductivity of pipe 3 is preferably less than 0.1 W·mK. -1This thermal conductivity can be provided in a variety of ways and with a variety of materials, such as using vacuum insulated pipes (VITs) already used in the oil and gas industry.

[0058] The depth of wellbore 2 is based on the local geothermal gradient and should be deep enough to experience a temperature difference at the bottom of the well 4 (relative to the well outlet 7) sufficient to generate a surface mass flow. In addition to the amount of working fluid in the thermosiphon system 1, the mass flow rate can also be adjusted by changing the design parameters of the components of system 1 (e.g., the cross-sectional areas of pipe 3 and annulus 6).

[0059] The design parameters of the thermosiphon system 1 (e.g., the diameters of pipe 3 and annulus 6) can be evaluated using a digital model through thermofluid simulation. A digital model can also be used to determine parameters of the heat exchanger 5, such as its dimensions and length (described further below). The digital model should be based on the thermodynamic cycling characteristics, pressure, temperature, and associated density of the working fluid (e.g., supercritical carbon dioxide), as well as an analytical model of the system flow process. The digital model can take any suitable form, including but not limited to finite element models.

[0060] Although the working fluid does not necessarily have to undergo a phase change (e.g., from liquid to gas) at any point around the closed loop of the thermosiphon system 1, in some examples the working fluid may undergo a phase change.

[0061] refer to Figure 2A The diagram shows a schematic cross-section of an example of heat exchanger 5.

[0062] The working fluid (e.g., supercritical carbon dioxide) descends through the lower part of pipe 3 before being redirected back towards the surface by the redirection element 18 positioned below the outlet of pipe 3. In the example shown, the redirection element 18 is part of the heat exchanger 5 and takes the form of a hemispherical bend 18 (sometimes referred to as a "bullneck" shape) connected to an outer tube 21 (or "shroud") that extends upwards along pipe 3 within the well annulus 6 and bends back a portion thereafter. The redirection element 18 is not mandatory, but it helps improve the circulation of the working fluid compared to relying solely on the bottom of the well 4 to redirect the downward momentum of the working fluid.

[0063] Also refer to Figure 2B This shows that in the case of Figure 2A The dashed line A-A' represents a schematic cross-section of the heat exchanger 5 taken from the plane.

[0064] The heat exchanger 5 has blades 19 arranged to allow the working fluid leaving the lower end of the pipe 3 to return upwards along the geothermal wellbore 2 via a helical path between the outer diameter of the pipe 3 and the inner diameter of the wellbore 2 (or, when drilling into porous rock, the casing of the wellbore 2). In the example shown, the blades 19 are connected between the outer pipe 21 (or “shroud”) and the outer cylindrical surface of the pipe 3. The working fluid follows the helical path defined by the blades, thus following an extended path (compared to moving vertically upwards along the well annulus 6). In this way, the time available for heat transfer from the surrounding rock to the working fluid in the wellbore 2 is increased. Furthermore, the blades 19 are formed of the same metallic material as the other components of the heat exchanger 5 and the pipe 3, which allows for a larger contact area for heat conduction with the working fluid. Although... Figure 2B The exemplary heat exchanger 5 shown includes three blades 19, but the number of blades 19 is not limited to three. For example, there may be one, two, or more than three blades 19.

[0065] The heat exchanger 5 also has an internal diverter 20, which is configured to split the working fluid flow received from the pipe into two or more streams. This helps the redirection element 18 direct the working fluid into the annulus 6. Although in Figure 2B The example shows three splitters 20, but the number of splitters 20 does not have to be three.

[0066] exist Figure 2A and Figure 2B In the example shown, the blades 19 of the heat exchanger 5 are housed between the outside of the pipe 3 and the outer pipe (shroud) 21 to ensure that all working fluid flows through a spiral path.

[0067] Despite Figure 2B The diagram shows blade 19 extending between pipe 3 and outer tube 22, but this is not mandatory. For example, blade 19 may alternatively only span a portion of this distance and protrude from pipe 3 and / or outer tube 22. Furthermore, blade 19 may be segmented (consisting of independent sections connected to the support but not interconnected), staggered, and / or crossed. In other examples, outer tube 21 may be omitted, and the size of blade 19 may be just appropriate for the diameter of wellbore 2 (or its casing, e.g., when drilling into porous rock). This allows some working fluid to bypass the helical path, but it is acceptable as long as a sufficient proportion of the working fluid follows the helical path and the desired variation in the properties (primarily density) of the working fluid is achieved between the outlet of pipe 3 and the end of heat exchanger 5.

[0068] Heat exchanger 5 is made of a corrosion-resistant alloy, such as chromium-containing stainless steel. Alternatively, known steel grades used for carbon capture and storage would be suitable for forming the heat exchanger.

[0069] refer to Figure 3 A schematic diagram of the working fluid flowing through the blades 19 of an exemplary heat exchanger 5 is shown.

[0070] Figure 3 The blades 19 of the heat exchanger 5 shown are configured such that the working fluid follows a helical path 22 between the outer diameter of the pipe 3 and the outer tube (shroud) 21, which in turn is within the inner diameter of the wellbore 2. By guiding the working fluid (e.g., supercritical carbon dioxide) within the heat exchanger 5 along the helical path 22, rather than, for example, along a path parallel to the axis of the wellbore 2, the time available for heat transfer to the working fluid is increased. For example, if the blades 19 form a 67 m long helical path 22 within a straight length of 20 m parallel to the axis of the wellbore 2, the path length within the heat exchanger 5 can be increased by a factor of 67 / 20 = 3.35, thereby increasing the residence time. In addition to defining the helical path, the blades 19 also serve to provide improved heat transfer by increasing the surface area of ​​the working fluid, which comes into contact with a surface heated by conduction from the surrounding rock.

[0071] As the working fluid moves along the helical path, the rotation of the flow along the helical path 22 can also cause heavier fluids to move toward the outside of the heat exchanger 5. Due to this region of the wellbore 2, the annulus 6 is inherently hotter because it is closer to the heat source (hot rock), which can further improve heat transfer in the heat exchanger 5.

[0072] Also refer to Figure 4A and Figure 4B An example of heat exchanger 5 is shown, which spans a spiral path 22 with a greater number of rotations. Figure 4B This is a schematic side view of an exemplary heat exchanger 5, in which the outer tube 21 is cut off. Figure 4A Is Figure 4B A schematic cross-sectional view taken from the plane represented by the dashed line B-B'.

[0073] In the exemplary heat exchanger 5 shown, blades 19 define an Archimedean spiral structure 23 for returning the working fluid (e.g., supercritical carbon dioxide) upward through the annulus 6 of the wellbore 2 via a spiral path 22. This can significantly increase the heat transferred to the working fluid.

[0074] Figure 4A An Archimedean spiral structure 23 with three blades 25 is shown, but the number of blades does not have to be three. For example, the number of blades can be one, two, or more than three.

[0075] The pitch P of the blades 19 and the total length H of the heat exchanger 5 parallel to the axis of the wellbore 2 can be selected to provide the desired improvement in heat transfer to the working fluid. This will also increase the residence time due to the increased length along the helical path 22 (compared to the straight length H).

[0076] Figure 4A and Figure 4B The exemplary heat exchanger 5 shown includes three blades 19 with a blade depth of approximately 10 cm, arranged at right angles to the conduit 5 (the space between the conduit 3 and the outer tube (shroud) 21), with a pitch P of approximately 30 cm and a heat transfer coefficient of approximately 5 W·mK. -1 and 10 W·mK -1 between.

[0077] Also refer to Figure 5 This paper describes a method for determining the blade pitch of a heat exchanger.

[0078] In the example where 200 kW of electrical energy is expected at the surface, if we assume an efficiency of 50% and the working fluid is supercritical carbon dioxide, then 400 kW of heat transfer is required in wellbore 2 to provide the required mass flow rate of carbon dioxide to turbine 9.

[0079] Heat transfer to supercritical carbon dioxide is achieved through surface heat conduction along the surface of the blades 19 of the heat exchanger 5. Heat conduction can also occur from the outer tube 21, but preferably the pitch P, length H, and number of blades 19 should such that heat conduction via the blades 19 dominates. Therefore, heat conduction from sources other than the blades 19 will be omitted from the calculations below.

[0080] For a heat exchanger 5 with a length of H and a spiral blade 19 wound around the lower part of a pipe 3 with a diameter of D, the length L of the spiral blade 19 is: (1) Rearrange the blades to obtain the blade pitch P: (2) The blade conduction area required for this example to transfer 400 kW of heat depends on the surface thermal conductivity of blade 19, the temperature difference between the surface of blade 19 and supercritical carbon dioxide due to the geothermal gradient, and the surface area of ​​blade 19. Typically, the heat conduction of a flat plate near a heat source is around 2 W·m². -2 ΔT up to 20 W·m -2 The variation is between ΔT and ΔT, where ΔT is the temperature difference. For 10 W·m -2 The thermal conductivity ΔT and the temperature difference ΔT = 50K will require 400,000 / 10 / 50 = 800 m 2The surface area of ​​blade 19. For blade 19 with a width W (radial distance between pipe 3 and outer pipe 21) equal to 0.2 m, for a three-bladed heat exchanger 5 (each blade 19 has two heat transfer surfaces), this is equivalent to a blade length L = 800 / 0.2 / 3 / 2 = 667 m. For heat exchanger 5 with a length H, the blade pitch 24 can be calculated by equation (2).

[0081] Figure 5 The diagram shows calculated values ​​for the blade pitch P of a three-bladed heat exchanger 5 with respect to a range of conductivity values, for a series of heat transfer values. For higher geothermal gradients and higher conductivity values, the blade pitch P can be increased. The blade pitch P can be approximately 0.2 m. The residence ratio is the ratio L / H.

[0082] improve

[0083] It should be understood that various modifications can be made to the embodiments described above. Such modifications may involve equivalents and other features known in the design and use of methods and apparatus for geothermal energy generation and / or thermosiphoning, and may be used in place of or in addition to the features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

[0084] Although specific combinations of features have been described in the claims of this application, it should be understood that the scope of this invention also includes any novel feature or any novel combination of features or any generalization thereof explicitly or implicitly disclosed herein, whether or not it relates to the same invention as currently claimed in any claim, and whether or not it alleviates any or all the same technical problems as those of this invention. The applicant hereby declares that new claims may be conceived for these features and / or combinations of these features during the examination of this application or any further applications derived therefrom.

Claims

1. A thermosiphon system, comprising: Geothermal wellbore; The pipe extends downward along the well shaft; A heat exchanger, connected to the lower end of the pipe disposed within the geothermal wellbore, the heat exchanger comprising one or more blades arranged such that working fluid in the form of supercritical carbon dioxide exiting the lower end of the pipe returns upward along the geothermal wellbore via a helical path between the outer diameter of the pipe and the inner diameter of the geothermal wellbore, the helical path having a pitch length and a total length parallel to the geothermal wellbore; and The turbine is driven by the mass flow rate of the working fluid.

2. The thermosiphon system of claim 1, wherein the heat exchanger comprises two or more blades.

3. The thermosiphon system according to claim 1 or 2, wherein at least one of the one or more blades spans the total length of the helical path.

4. The thermosiphon system according to any one of claims 1 to 3, wherein one or more blades form an Archimedean spiral structure between the outer diameter of the pipe and the inner diameter of the geothermal well, wherein the Archimedean spiral structure defines the spiral path.

5. The thermosiphon system according to any one of claims 1 to 4, wherein the pipe is an insulated pipe.

6. The thermosiphon system according to any one of claims 1 to 5, wherein the heat exchanger further comprises one or more flow splitting structures configured to divide the working fluid flow received from the conduit into two or more streams.

7. The thermosiphon system according to any one of claims 1 to 6, wherein the heat exchanger further comprises one or more flow redirection structures configured to redirect working fluid flow received from the conduit into the helical path.

8. The thermosiphon system according to any one of claims 1 to 7, wherein the heat exchanger is attached to the lower end of the pipe.

9. The thermosiphon system according to any one of claims 1 to 7, wherein the heat exchanger is integrally formed with the lower end of the pipe.

10. The thermosiphon system according to any one of claims 1 to 9, wherein the pitch length of the spiral path is between 20 cm and 40 cm.

11. The thermosiphon system according to any one of claims 1 to 10, wherein the total length of the heat exchanger parallel to the geothermal well is between 5 m and 120 m.

12. The thermosiphon system according to any one of claims 1 to 11, wherein the heat exchanger comprises a cylindrical body, the inner diameter of which is equal to the inner diameter of the lower end of the pipe; The one or more blades extend from the outer diameter of the cylindrical body.

13. The use of a thermosiphon system for power generation, said thermosiphon system being the thermosiphon system according to any one of claims 1 to 12.

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