Heat exchanger

Abstract

A heat exchanger 5 is disclosed for the lower end of a pipe (3, fig 1) disposed in a geothermal well bore (2, fig 1). The heat exchanger includes one or more vanes 19 arranged to cause a working fluid

Description

Field The present specification concerns a heat exchanger for use in geothermal thermosiphon systems. Background The thermodynamic properties of carbon dioxide in supercritical form are well understood and utilised in various applications. Recently the interest in utilising supercritical carbon dioxide as the secondary working fluid for surface-based power generation systems has been established and relies on a fluid heat transfer process for electricity generation. This is used in heat pumps and geothermal power generation. Geothermal systems that generate power output in the hundreds of kilowatt hour to megawatt hour range do so as a direct function of the temperature of a working fluid such as water or brine delivered to surface. Well depths for these systems are dependent on the local geothermal gradient (temperature increase per kilometre depth) and are driven by the need to return a sufficient temperature to surface to drive a turbine via direct or binary cycle system(s). Where the geothermal gradient is sufficiently high, a direct cycle system can be used in which steam extracted from the reservoir drives reaction turbines. In areas of low geothermal gradient, these systems become less attractive due to the large depths needed to generate the required temperature at surface and surface mass flow to drive turbines, aside from the geological challenges associated with such large well depths. In such cases, unconventional enhanced geothermal systems may be considered. These involve stimulation of the high-pressure, high-temperature reservoir rocks expected at such depths (typically 4 to 6 km) and extraction of heat from the rocks by injecting / circulating working fluid through the reservoir in an open loop injector / producer well / sub-surface system. These systems do, however, inherently carry more risk and cost with increasing depth for both geological and engineering reasons. Advancements have been made for non-conventional geothermal resource exploitation with closed loop systems. These systems still rely on heat carried to surface for power generation via conventional turbines and as such still require significant depth for meaningful power output with large surface footprints. US 2021 / 140413 Al describes a surface turbine system having a Catherine wheel design which rotates by reaction to working fluid jetting from nozzles, similar to a rotating garden sprinkler system. US 2022 / 0282640 Al describes delivering conditioned working fluid to drive a steam reaction turbine. US 2021 / 062682 Al describes a downhole heat exchanger design which is retrofitted into existing geothermal wells to heat the working fluid to be used for electricity generation. US 2017 / 130703 Al describes optimising multiple well locations in a tiered subsurface formation with varying heat conductance properties to increase the transfer of heat energy to surface. Alvaro Amaya, Joseph Scherer, John Muir, Mehul Patel, and Brian Higgins: "GreenFire Energy Closed-Loop Geothermal Demonstration using Supercritical Carbon Dioxide as Working Fluid”, Proceedings of the 45th Workshop on Geothermal Reservoir Engineering, Stanford, CA, USA (10-12 February 2020) (hereinafter referred to as "Amaya2020") describes a thermosiphon using supercritical carbon dioxide as the working fluid. Summary According to a first aspect of the invention, there is provided a heat exchanger for the lower end of a pipe disposed in a geothermal well bore. The heat exchanger includes one or more vanes arranged to cause a working fluid exiting the lower end of the pipe to return up the geothermal well bore via a helical path between an outer radius of the pipe and an interior radius of the geothermal well bore. The helical path having a pitch length and a total length parallel to the geothermal well bore. In this way, a retention time of the working fluid (and hence thermal transfer) may be controlled by varying the pitch length of the one or more vanes, the number of vanes and / or the total length of the helical path parallel to the well bore. Property changes of the working fluid may be controlled to occur predominantly within the total length of the helical path, providing greater control over the operation of a thermosiphon implemented using the working fluid. The pipe may be positioned parallel and concentric to the bore. The heat exchanger may include two or more vanes. The heat exchanger may include three vanes. The two or more vanes may be interleaved / interdigitated with one another. At least one vane of the one or more vanes may span the total length of the helical path. Each (i.e. all) of the vanes may span the total length of the helical path. The one or more vanes may form an Archimedean screw structure between an outer radius of the pipe and an interior radius of the geothermal well bore. The Archimedean screw structure may define the helical path. The heat exchanger may be for the lower end of an insulated pipe. The insulated pipe may be a vacuum insulated pipe. The heat exchanger may also include one or more flow-splitting structures disposed to divide a flow of the working fluid received from the pipe into two or more flows. The heat exchanger may also include one or more flow re-directing structures configured to redirect flows of working fluid received from the pipe into the helical path. A flow-re-directing structure may have a "bull-nose" shape. A flow re-directing structure may take the form of a curved section configured to re-direct a flow of working fluid through a 180° change of direction within a radius of curvature. The heat exchanger is configured for attachment to the lower end of the pipe. The heat exchanger may be configured for releasable attachment to the lower end of the pipe. For example, with a flanged connection secured with bolts. The heat exchanger may be configured for permanent attachment to the lower end of the pipe. For example, by welding. The heat exchanger may be configured for attachment to the lower end of the pipe using a single pipe joint. Alternatively, the heat exchanger may be integrally formed with the lower end of the pipe. The heat exchanger may be for a working fluid in the form of supercritical carbon dioxide. The supercritical carbon dioxide may be "dry", which is to say not including water (from a practical perspective). The supercritical carbon dioxide may be "wet", which is to say including a non-negliglble quantity of water. The heat exchanger may be formed from corrosion resistant alloys. For example, chromium containing stainless steels. And grades of steel already known and used for carbon capture-and-storage will be suitable for forming the heat exchanger. Alternatively, the working fluid may be water or brine. When the working fluid Is aqueous, the temperature at the heat exchanger may be higher. Suitable materials may include, for example, steels known for use as steam conduits in conventional, non-geothermal power plants. Alternatively, the working fluid may be a substituted or unsubstituted hydrocarbon, for example propane or butane. Each of the one or more vanes may be welded to an annular section of the heat exchanger. The helical path may have a pitch length of between 10 and 40 cm. Unless explicitly stated to the contrary, all ranges defined herein are inclusive of the defined endpoints. The pitch length may preferably be between 20 cm and 35 cm. The pitch length may most preferably be 30 cm to a precision of two significant figures. Each vane may extend into the space between the outer radius of the pipe and the interior radius of the geothermal well bore by a radial distance of between 5 cm and 50 cm. In general, the relative size of the pipe should be selected such that a ratio of cross-section areas between the pipe and the space between the outer radius of the pipe and the interior radius of the geothermal well bore may be approximately equal to a ratio of density between incident colder working fluid descending from the lower end of the pipe and heated working fluid exiting the length of the heat exchanger. In other words, to ensure that the heat exchanger may maximise throughout of the working fluid without forming a constriction to flow. The total length of the heat exchanger parallel to the geothermal well bore may be between 5 m and 120 m. The total length may preferably be 100 m, to a precision of two significant figures. The total length may be formed from two or more modular sections. For example, each modular section may have a length of 20 m, to a precision of two significant figures. The heat exchanger may include, or take the form of, a cylindrical body having an interior diameter equal to an interior diameter of the lower end of the pipe. The one or more vanes may extend from an outer diameter of the cylindrical body. At a point of connection to the cylindrical body, each vane may make an angle of 90° to the cylindrical body. A thermosiphon system may include a geothermal well bore, a pipe extending down the bore, and the heat exchanger. The heat exchanger may be coupled to a lower end of the pipe. A turbine may be driven by the working fluid. The turbine may be above ground, or may be below ground closer to the surface of the geothermal well bore The working fluid of the thermosiphon system may be supercritical carbon dioxide. According to a second aspect of the invention, there is provided a method including coupling the heat exchanger of the first aspect to a pipe to form an assembly. The method also includes lowering the assembly into a geothermal well bore with the heat exchanger at the lowest end of the pipe. The pipe may be inserted into the geothermal well bore so that a temperature difference between the surface and the heat exchanger is at least 40 K, at least 50 K, at least 60 K, at least 70 K, or at least 80 K. The method of the second aspect may include features corresponding to any features of the heat exchanger and / or thermosiphon system of the first aspect. Definitions applicable to the heat exchanger and / or thermosiphon system of the first aspect (or features thereof), may be equally applicable to the method of the second aspect (or 5 features thereof). According to a third aspect of the invention, there is provided use of the heat exchanger of the first aspect, a thermosiphon system of claims including the heat exchanger of the first aspect, for generation of electricity. 10 The method of the third aspect may include features corresponding to any features of the heat exchanger and / or thermosiphon system of the first aspect. Definitions applicable to the heat exchanger and / or thermosiphon system of the first aspect (or features thereof), may be equally applicable to the method of the third aspect (or 15 features thereof). Brief Description of the Drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 schematically illustrates a thermosiphon system; Figure 2A is a schematic cross-section of a heat exchanger; Figure 2B is a schematic cross-section of a heat exchanger in the plane denoted by the dashed line A-A' in Figure 2A; Figure 3 schematically illustrates flows of working fluid past vanes of a heat exchanger; Figure 4A is a schematic cross-section of a heat exchanger in the plane denoted by the dashed line B-B' in Figure 4B; Figure 4B is a schematic side view of a heat exchanger with the outer pipe cut away; and Figure 5 presents a calculated variation of vane pitch as a function of thermal conductance against the primary vertical axis, and a calculated variation of the ratio between a total travel distance and a vertical travel distance against the secondary vertical axis. Detailed Description of Certain Embodiments In the following description, like parts are denoted by like reference numerals. A thermosiphon system using supercritical carbon dioxide as the working fluid, as demonstrated by Amaya2020, has potential to be used in a wider range of locations, such as locations having a lower geothermal gradient, and / or using well bores drilled to relatively shallower depth than conventional geothermal electricity generation systems. It may also be possible to install such thermosiphon systems in existing well bores originally drilled for other purposes (for example oll / gas extraction, geological exploration and so forth), if a temperature differential of more than about 40-60 K Is realised between the bottom of the well and the surface. The mass-flow of the carbon dioxide may be used to drive a turbine at (or closer to) the surface to generate electricity. To fully realise the potential of such thermosiphon systems, it will be necessary to control and optimise the retention time of the working fluid at the bottom of the well, in order to control the change in properties (pressure, density etc.) between the cool, dense working fluid descending the well and the heated, reduced density working fluid ascending the well. The present specification concerns heat exchangers which may be installed at the bottom of a well in order to allow controlling and optimising retention time and heat transfer to a working fluid of a thermosiphon. Heat exchangers according to the present specification may provide such benefits to thermosiphons using supercritical carbon dioxide as a working fluid (for example the thermosiphon described in Amaya2020), and will be described in relation to an example system using supercritical carbon dioxide as the working fluid. However, it shall be apparent from the following discussions that the design and operating principles of the heat exchangers according to the present specification are applicable to any thermosiphon system, regardless of the substance used as the working fluid. Supercritical carbon dioxide does represent a preferred example, due to the large changes in properties such as density which result from relatively small (for example, tens of K) changes in temperature. Unconventional geothermal applications as described in Amaya2020 may be particularly suitable for remote locations where there is limited / unreliable power grid supply and the local geothermal gradient / sub-surface conditions are not viable for traditional geothermal systems to be economic, for example, electrical vehicle (EV) power stations in remote locations to replace conventional petrol stations or geographies hard to reach with existing power systems infrastructure. However, since local geothermal gradients and other geological factors can vary widely, it is important to be able to control retention time and thermal transfer rates at the well base, for example to allow systems across a wide range of locations to have a reduced range of surface pressure, temperature and mass flow, in order to enable using standardised surface turbines (or at least one of a number of standardised arrangements), instead of requiring each installation to have a turbine customised to the working fluid properties which might be expected using a simple termination as described in Amaya2020. Such considerations will provide reductions in costs of installation and maintenance, which will be important if unconventional geothermal systems are to realise their potential in practice. Referring to Figure 1, a schematic illustration of a thermosiphon system 1 is shown. In the thermosiphon system 1, a working fluid is circulated around a closed loop flow path. Relatively cool and high-density working fluid descends down a well bore 2 in a pipe 3 under gravity to the well base 4. At the well base 4, a heat exchanger 5 is used to facilitate heat transfer from the surrounding rocks to the working fluid, thereby causing a decrease in the density of the working fluid and generating a natural convection current that delivers a surface mass flow stream of the working fluid back up the well bore 2 in a well bore annulus 6 surrounding the pipe 3. The heat exchanger 5 serves to extend the portion of the return path in the region having the highest temperature, that is, around the lower end of the pipe 3 at the well base 4, which increases the time available for heat transfer to the working fluid (referred to hereinafter as "residence time"). Additionally, the heat exchanger 5 can help to generate turbulent flow in that region, increasing the uniformity of the temperature of the working fluid. The precise design of the heat exchanger 5 may vary, and one example is described further in relation to Figures 2A, 2B, 3, 4A, and 4B. In all cases, however, the heat exchanger 5 is designed to allow sufficient residence time and heat conduction for the working fluid in the vicinity of the well base for to experience a controlled property change to a lower density state in order to deliver the mass flow stream back up the well bore 2 to the surface. Although a thermosiphon may be obtained using nearly any fluid, in order to generate a large enough mass flow to allow extraction of useful work, it is preferable to use a working fluid which exhibits a large change in density for a temperature difference on the order of tens of degrees K. One particularly preferred example to provide the working fluid is supercritical carbon dioxide. Other supercritical fluids may also be used to provide the working fluid for example, water / brine or hydrocarbons such as butane and propane (though noncombustible working fluids are preferred). The mass flow stream of returned working fluid at the surface is used to drive a turbine 9 before being recirculated through the system 1, that is, directed back down the well bore 2 in the pipe 15. For example, when the working fluid is supercritical carbon dioxide, the mass flow stream at the surface may be between 10 and 12 kg.s 1 at a pressure between 1000 to 1500 psi (6.9 MPa to 10 MPa). Upstream of the turbine 9, an upstream turbine selector valve 10 is provided in the flow path. Likewise, downstream of the turbine 9, a downstream turbine selector valve 11 is provided in the flow path. The upstream selector valve 10 and downstream turbine selector valve 11 are connected by a turbine bypass 12 and can be used to route all or part of the mass flow stream through the turbine bypass 12. A flow measuring station 13 is provided downstream of the turbine bypass 12 in the flow path to monitor the properties of the working fluid prior to re-entering the pipe 3 to descend the well bore 2. The measuring station 13 may include any suitable measurement instruments including, but not limited to, a Pitot tube installation (not shown) connected to a pressure transducer (not shown) for total stream pressure measurement, a stream total pressure thermocouple (not shown) for total stream temperature measurement, a single-phase velocity Coriolis flow meter (not shown) for stream flow rate measurement and so forth. The pressure transducer and the thermocouple may be used to determine working fluid density from thermodynamic charts. The Coriolis meter may be used to measure both mass flow and working fluid density. To start the system 1, the turbine 9 is bypassed using the turbine bypass 12 until a suitably strong thermosiphon has been created. This is achieved using the upstream selector valve 10 and the downstream selector valve 11. In particular, once a thermosiphon has been established and has a sufficient mass flow (measured by the measuring station 13), for example around 10 kg.s1 for working fluid in the form of supercritical carbon dioxide, the downstream selector valve 11 can be opened fully. The upstream turbine selector valve 10 can then be adjusted to deliver increasing amounts of carbon dioxide flow to an inlet 14 of the turbine 9 until the upstream turbine selector valve 10 is fully opened. The amount of working fluid (for example carbon dioxide) in the thermosiphon system 1 should be increased until a desired mass flow rate is achieved as measured by the measuring station 13. Addition of too much working fluid may cause a reduction of mass flow rate (for example due to increasing friction), though a desired flow rate may be obtained by optimising the total quantity of working fluid. The turbine 9 is connected to an electrical generator 15 via a driveshaft 16 and coupling 17. Optionally, the generator 15 may be disconnected from the turbine 9 at times when no electricity generation is required. In cases in which the system 1 can deliver more than the required mass flow to drive a single turbine 9, it is possible to have turbines 9 and generators 15 installed in a parallel configuration to increase the power output. In such cases, doubling the mass flow can double the electrical power generated. The turbine 9 need not be located at the surface. In some examples, the turbine 9 and generator 15 may be located underground, for instance just below the surface where temperatures tend to be more stable than above ground. The density of the working fluid will be increased following passage through the turbine 9. In some examples, additional expansion and cooling of the working may be provided by further introducing an expander (not shown) and / or a condenser (not shown) to the flow path. This can help to improve the performance of the turbine 9. For example, a condenser can serve to a cause the working fluid (such as supercritical carbon dioxide) to have a higher density in the case that the turbine outlet does not achieve the desired conditions, for example if the working fluid has not achieved a sufficient fluid density to sustain the thermosiphon delivery of the surface mass flow stream. To reduce (or even prevent) gradual heating of the relatively high-density working fluid (for example supercritical carbon dioxide) before reaching the well base 4, the pipe 3 may be insulated. When insulation is used, the thermal conductance of the pipe 3 is preferably less than 0.1 W.mK T Such a thermal conductance can be provided in a variety of ways and with a variety of materials, for example with vacuum insulated tubing (VIT) already used in the oil and gas industry. The depth of the well bore 2 is based on the local geothermal gradient, and should be deep enough that a temperature differential sufficient to generate the surface mass flow is experienced at the well base 4 (relative to the well exit 7). In addition to the quantity of working fluid in the thermosiphon system 1, the mass flow rate can also be adjusted by varying design parameters of components of the system 1, for example, the cross-sectional areas of the pipe 3 and the annulus 6. The design parameters of the thermosiphon system 1, such as diameters for the pipe 3 and annulus 6, can be evaluated through thermo-fluid simulation using a digital model. Parameters such as the dimensions and length of the heat exchanger 5 (described further hereinafter) may also be determined using the digital model. The digital model should be based on the thermodynamic cycle properties of the working fluid (for example supercritical carbon dioxide), pressure, temperature and associated density, and an analytical model of the system flow process. The digital model may take any suitable form, including without limitation a finite element model. Whilst it Is not necessary for the working fluid to undergo a phase change (for example liquid to gas) at any point around the closed cycle of the thermosiphon system 1, in some examples the working fluid may undergo a phase change. Referring to Figure 2A, a schematic cross-section of an example of the heat exchanger 5 is shown. Working fluid (for example supercritical carbon dioxide) descends through the lower part of the pipe 3 before being re-directed back towards the surface by re-directing element 18 disposed below the exit of the pipe 3. The re-directing element 18 in the illustrated example is part of the heat exchanger 5, and takes the form of a hemispherical curved section 18 (sometimes termed a "bull-nose" shape) connected to an outer pipe 21 (or "shield") extending part-way back up the pipe 3 within the well annulus 6. The re-directing element 18 is not essential, but may help improve circulation of the working fluid compared to simply relying on the well base 4 to redirect downward momentum of the working fluid. Referring also to Figure 2B, a schematic cross-section of the heat exchanger 5 is shown in the plane denoted by the dashed line A-A' of Figure 2A. The heat exchanger 5 has vanes 19 arranged to cause the working fluid exiting the lower end of the pipe 3 to return up the geothermal well bore 2 via a helical path between an outer radius of the pipe 3 and an interior radius of the geothermal well bore 2 (or a casing of the well bore 2 when drilled into porous rocks). The vanes 19 of the illustrated example are connected between the outer pipe 21 (or "shield") and the outer cylindrical surface of the pipe 3. The working fluid follows the helical path defined by the vanes, and in so doing follows an extended path (compared to moving vertically up the well annulus 6). In this way, the time available for heat transfer from surrounding rocks to the working fluid in the well bore 2 Is increased. Additionally, the vanes 19 are formed of the same metal materials as other components of the heat exchanger 5 and pipe 3, which allows for conduction of heat to larger contact area with the working fluid. Although the example heat exchanger 5 shown in Figure 2B includes three vanes 19 are, the number of vanes 19 need not be three. For example, there may be one, two, or more than three vanes 19. The heat exchanger 5 also has internal flow splitters 20 disposed to divide a flow of the working fluid received from the pipe into two or more flows. This may help the redirecting element 18 to divert the working fluid into the annulus 6. Although three flow splitters 20 are shown in the example of Figure 2B, the number of flow splitters 20 need not be three. In the example shown in Figures 2A and 2B, the vanes 19 of the heat exchanger 5 are contained between the exterior of the pipe 3 and the outer pipe (shield) 21 to ensure that all of the working fluid passes through the helical path. Although the vanes 19 are shown in Figure 2B as extending between the pipe 3 and the outer pipe 19, this need not be the case. For example, the vanes 19 could instead span only a part of this distance, and protrude from either the pipe 3 and / or the outer pipe 22. Moreover, the vanes 19 may be segmented (consisting of separate portions connected to a support but not to each other), interleaved, and / or interdigitated. In other examples, the outer pipe 21 may be omitted and the vanes 19 may be sized to just fit within the diameter of the well bore 2 (or casing thereof, for example when drilled into porous rocks). This may allow some of the working fluid to bypass the helical path, though this may be acceptable provided a sufficient fraction does follow the helical path and that a desired change in properties of the working fluid (principally density) is obtained between the exit of the pipe 3 and the end of the heat exchanger 5. The heat exchanger 5 is formed from corrosion resistant alloys. For example, chromium containing stainless steels. Alternatively, grades of steel already known and used for carbon capture-and-storage will be suitable for forming the heat exchanger. Referring to Figure 3, a schematic illustration of the working fluid flowing past vanes 19 of the example heat exchanger 5 is shown. The configuration of vanes 19 of the heat exchanger 5 shown in Figure 3 is such that the working fluid follows a helical path 22 between an outer radius of the pipe 3 and the outer pipe (shield) 21, which in turn is within an interior radius of the well bore 2. By directing the working fluid (for example supercritical carbon dioxide) flow within the heat exchanger 5 along the helical path 22 instead of, for example, a path parallel to the well bore 2 axis, the time available for heat transfer to the working fluid is increased. For example, if a helical path 22 of 67 m length is formed by the vanes 19 within a straight length of 20 m parallel to the well bore 2 axis, the path length of within the heat exchanger 5 can be increased by a factor of 67 / 20 = 3.35, thereby increasing retention time. In addition to defining the helical path, the vanes 19 also serve to provide improved heat transfer by increasing the surface area of the working fluid which is in contact with surfaces heated by conduction from the surrounding rocks.. The flow rotation along the helical path 22 may also cause the heavier fluid to move towards the outside of the heat exchanger 5 as the working fluid moves along the helical path. Since this region of the well bore 2 annulus 6 is inherently hotter being nearer to heat source (the hot rocks), this may further improve the thermal transfer in the heat exchanger 5. Referring also to Figures 4A and 4B, an example of the heat exchanger 5 is shown which spans a larger number of rotations of the helical path 22. Figure 4B is a schematic side view of the example heat exchanger 5 with the outer pipe 21 cut away, and Figure 4A is a schematic cross-section in the plane denoted by the dashed line B-B' in Figure 4B. In the example heat exchanger 5 illustrated, the vanes 19 define an Archimedean screw structure 23 for returning the working fluid (for example supercritical carbon dioxide) up through the well bore 2 annulus 6 via the helical path 22. This can significantly increase the heat transferred to the working fluid. Figure 4A shows the Archimedean screw structure 23 having three vanes 25, but the number of vanes need not be three. For example, the number of vanes may be one, two, or more than three. The pitch P of the vanes 19 and the overall length H of the heat exchanger 5 parallel to the well bore 2 axis may be selected to provide a desired improvement in heat transfer to the working fluid. This will also increase retention time, due to the increased length along the helical path 22 (compared to the straight length H). The example heat exchanger 5 illustrated in Figures 4A and 4B includes three vanes 19 having a vane depth of around 10 cm arranged at a right angle to the pipe 5 (this is then the space between pipe 3 and outer pipe (shield) 21), a pitch P of about 30 cm, and a heat transfer coefficient between about 5 and 10 W.mK4. Referring also to Figure 5, a method of determining heat exchanger vane pitch is described. In an example in which 200 kW of electrical energy is desired at the surface, if an efficiency of 50% is assumed and the working fluid is supercritical carbon dioxide, 400 kW of heat transfer would be required in the well bore 2 to provide the required mass flow stream of carbon dioxide to the turbine 9. Heat transfer to the supercritical carbon dioxide is achieved by surface heat conductance along the surfaces of vanes 19 of the heat exchanger 5. Heat conductance may also occur from outer pipe 21, though preferably the pitch P, length H and number of vanes 19 should be such that heat conductance via vanes 19 dominates. Consequently, heat conductance from sources other than vanes 19 will be omitted from the following calculations. For a heat exchanger 5 of length H with a helical vane 19 wrapped around the lower part of a pipe 3 of diameter D, the length L of the helical vane 19 is: L = -pjCjiD')2 + P2 (i: re-arranging for the pitch of the vane P: The vane conductance area required to deliver the 400 KW heat transfer of this example is a function of the surface heat conductivity of the vane 19, temperature difference between the vane 19 surface and the supercritical carbon dioxide due to the geothermal gradient, and the surface area of the vane 19. Typically, a heat conductance for a flat plate near to a heat source varies from 2 to 20 W.m2AT, where AT is the temperature difference. For a heat conductivity of 10 W.m 2 AT and a temperature difference of AT=50 K, a vane 19 surface area of 400,000 / 10 / 50 = 800 m2 would be required. For a vane 19 having a width W (radial distance between pipe 3 and outer pipe 21) equal to 0.2 m, this equates to a vane length of L = 800 / 0.2 / 3 / 2 = 667m for a three-vane heat exchanger 5 with two heat transfer surfaces per vane 19. For a heat exchanger 5 of length H, the vane pitch 24 can be calculated from Equation (2). Figure 5 presents calculated vane 19 pitch P values for a three-vane heat exchanger 5 of 1-1 = 150 m length for a range of conductance values. Vane 19 pitch P can be increased for higher geothermal gradients and higher conductance values. The vane 19 pitch P may be around 0.2 m. The retention multiple is the ratio L / H. Modifications It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design and use of methods and apparatuses for geothermal energy generation and / or thermosiphons, and which may be used Instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. 5 Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or 10 not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. A heat exchanger for the lower end of a pipe disposed in a geothermal well bore, comprising one or more vanes arranged to cause a working fluid exiting the lower end of the pipe to return up the geothermal well bore via a helical path between an outer radius of the pipe and an interior radius of the geothermal well bore, the helical path having a pitch length and a total length parallel to the geothermal well bore.

2. The heat exchanger of claim 1, comprising two or more vanes.

3. The heat exchanger of claims 1 or 2, wherein at least one vane of the one ormore vanes spans the total length of the helical path.

4. The heat exchanger of any one of claims 1 to 3, wherein the one or more vanes form an Archimedean screw structure between an outer radius of the pipe and an interior radius of the geothermal well bore, wherein the Archimedean screw structure defines the helical path.

5. The heat exchanger of any one of claims 1 to 4, for the lower end of an insulated pipe.

6. The heat exchanger of any one of claims 1 to 5, further comprising one or more flow-splitting structures disposed to divide a flow of the working fluid received from the pipe into two or more flows.

7. The heat exchanger of any one of claims 1 to 6, further comprising one or more flow re-directing structures configured to redirect flows of working fluid received from the pipe into the helical path.

8. The heat exchanger of any one of claims 1 to 7, wherein the heat exchanger is configured for attachment to the lower end of the pipe.

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

10. The heat exchanger of any one of claims 1 to 9, for a working fluid in the form of supercritical carbon dioxide.

11. The heat exchanger of any one of claims 1 to 10, wherein the helical path has a pitch length of between 20 and 40 cm.

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

13. The heat exchanger of any one of claims 1 to 12, comprising a cylindrical body having an interior diameter equal to an interior diameter of the lower end of the pipe;wherein the one or more vanes extend from an outer diameter of the cylindrical body.

14. A thermosiphon system comprising a geothermal well bore, a pipe extending down the bore, a heat exchanger according to any one of claims 1 to 13, the heat exchanger coupled to a lower end of the pipe, and a turbine driven by the working fluid.

15. The thermosiphon system of claim 14, wherein the working fluid is supercritical carbon dioxide.

16. A method, comprising:coupling the heat exchanger of any one of claims 1 to 13 to a pipe to form an assembly;lowering the assembly into a geothermal well bore with the heat exchanger at the lowest end of the pipe.

17. Use of the heat exchanger of any one of claims 1 to 13 or the thermosiphon system of claims 14 or 15 for generation of electricity.

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