EP2956666A1

EP2956666A1 - Geothermal energy extraction

Info

Publication number: EP2956666A1
Application number: EP14706322.6A
Authority: EP
Inventors: Robert Simpson
Original assignee: Thermogas Dynamics Ltd
Current assignee: Thermogas Dynamics Ltd
Priority date: 2013-02-18
Filing date: 2014-02-14
Publication date: 2015-12-23
Also published as: GB201302768D0; WO2014125288A1

Abstract

A geothermal energy extraction system (100;1000) comprising a conduit (200; 1020, 1030) within an excavated bore(10; 1010). The conduit(200; 1020, 1030) comprises a substantially vertical descending feed section(20; 1020) and an ascending extraction section(30; 1030). At least part of both vertical sections (20;1020, 30; 1030) are located in a geothermal zone (300; 1080) below surface. A plurality of branches(600; 1070)extending from the bore in the geothermal zone (300;1080). The branches (600;1070) are each filled with thermally conductive material.

Description

GEOTHERMAL ENERGY EXTRACTION

FIELD OF THE INVENTION

The present invention relates to a system and use for geothermal energy extraction. In particular the present invention relates to a system for geothermal extraction without emissions.

BACKGROUND TO THE INVENTION

Geothermal energy is thermal energy generated and stored in the Earth. Power obtained from naturally created geothermal energy can be cost effective, reliable, sustainable and environmentally friendly. However, the means of capturing geothermal energy can involve risk due to gaseous emissions and due to the risk of earthquake.

Common practice in geothermal energy extraction is to bore a deep hole through the Earth's structure to hot aquifers and to extract steam and hot water for use in heating and electricity production. Over decades local depletion has been experienced, for example at Larderello in Italy which began in 1913, Wairakei in New Zealand 1958 and Geyser in California USA 1960. Depletion is due to heat and water being extracted faster than replenishment. Water injection wells were provided to restore output from the production well, in whole or in part. However, the water injection wells were achieved by fracturing the geological sub structure. In areas with accessible hot rock structures, that lack an adequate aquifer, water injection is practiced to intentionally fracture the structure by hydraulic pressure and thermal shocking. Unfortunately, this procedure can cause earthquakes as was found in Basil, Switzerland which had more than ten thousand seismic events of up to 3.4 on the Richter scale over six days of water injection and was then shut down. Evidence of subsidence has been found in the Wairakei field in New Zealand and in Staufen in Germany. Existing geothermal electricity generating plants emit an average of 122 kilograms of CO₂ per megawatt hour, which although small compared to emissions from a coal plant, which can emit as high as 950 Kg/MWhr, is still significant for a large scale undertaking because one million tonnes of CO₂ emissions would result from 1GW^e every year. Typically, other pollutants also present include sulphur and hydrogen sulphide, methane, ammonia and particulate matter. It will be appreciated where there are emissions emission-control systems, which can be expensive, are required. Quantities of toxic elements including mercury, arsenic, boron and antimony can also be present and are commonly re-injected as cooled geothermal fluids. It is desirable to provide an improved method of extracting and using geothermal energy.

It is further desirable to provide a method of extracting and using geothermal energy with minimum, preferably zero gaseous emissions.

SUMMARY OF THE INVENTION

The present invention provides a geothermal energy extraction system comprising: a conduit within an excavated bore; wherein the conduit comprises a substantially vertical descending feed section; and a vertical ascending extraction section, wherein at least part of the vertical ascending and at least part of the descending section is located in a geothermal zone below surface and comprises a plurality of branches extending from the bore into the geothermal zone, wherein the branches are each filled with thermally conductive material.

Advantageously, the branches accentuate thermal effectiveness of the system, whereby they increase heat transfer from the area of geothermal activity to the fluid flowing inside the conduit. The branches may comprise graphite. Alternatively, the branches may comprise graphene comprising molecular orientation in the direction of greatest heat conductance. Alternatively the branches may comprise brass.

In a first embodiment of the present invention the vertical descending section may be within the vertical ascending section. The vertical descending section may open into the vertical ascending section.

In a second embodiment of the present invention a substantially horizontal section may be located between and in fluid communication with the vertical descending section and the vertical ascending section.

The horizontal section may be located in a geothermal zone below surface. In the first embodiment the branches may extend from the vertical ascending section to the geothermal zone.

In the second embodiment the branches may extend from the horizontal section to the geothermal zone.

In both embodiments the branches may be configured to conduct heat from the geothermal zone via conductive material contained in the branches. The thermally conductive material may comprise conductively oriented graphite or graphene. Alternatively, the thermally conductive material may comprise brass.

At least the vertical ascending extraction section may comprise thermal insulation material with low thermal conductivity between the conduit and a wall of the excavated bore. The ascending extraction section may comprise thermal insulation material with low thermal conductivity between the conduit and a wall of the excavated bore. The thermal insulation may have to sustain very high temperatures, for example 205 degrees centigrade or higher. The thermal insulation is provided such that heat loss from the conduit is minimised and to assist in maintaining the temperature of heated fluid passing from the horizontal section to the ascending extraction section and to exit.

The system may further comprise a demineralising device in fluid communication with input to the vertical descending section, wherein the demineralising device may be operable to feed demineralised water to the system.

The system may further comprise pumping means operable to maintain pressure of the demineralised water fed to the system within a predetermined range such that superheated steam exits from the vertical ascending section. Superheated steam may be extractable from the system, via the vertical ascending section.

The temperature of the superheated steam may be in the region of 205 degrees centigrade. Alternatively, the temperature of the superheated steam is greater than 205 degrees centigrade.

Superheated steam in the region of 205 degrees centigrade is extractable from the system. Superheated steam at a temperature greater than 205 degrees centigrade may be extractable from the system. Superheated steam at temperatures greater than 205 degrees centigrade may be used in, for example electricity generation. Superheated steam in the region of 205 degrees centigrade may be used in for example fuel production.

The system may further comprise a capping bed at the surface. The capping material may be clay.

Advantageously, the present invention provides a geothermal extraction system with zero gaseous emissions and with a reduced risk of causing earthquakes.

With a feed system configured to supply demineralised water to the descending vertical section and an extraction plant at exit from the ascending plant the system according to the present invention provides a closed system that is fed with demineralised water and produces superheated steam without the emissions that are characteristic of and associated with known deep geothermal energy extraction systems and processes.

The system utilises low or constant thermal gradients within the geological substructure thus avoids fracturing of the substructure which may cause internal collapse and settlement that is often recognised as earthquake activity.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a geothermal system according to an embodiment of the present invention;

Figure 2 illustrates a schematic representation of a combined two-stage fuel and oxygen process using the system of figure 1 ; and

Figure 3 illustrates a geothermal system according to an embodiment of the present invention.

BRIEF DESCRIPTION

Figure 1 represents a closed system 100 formed by an excavated bore 10 that extends deep into the Earth's structure such that the Earth's internal energy resource can be tapped into. The excavated bore 10 comprises two substantially vertical sections 20, 30, each of which extend from the surface to a horizontal section deep below the Earth's surface, for example 1500 to 6000 metres. A relatively shallow excavation, which produces good quality steam output is preferred keeps the excavation costs to a minimum. However, a deeper excavation is more likely to guarantee the temperature, but the cost is much higher. A first descending vertical section 20 is arranged to feed water to the system and the second ascending vertical section 30 is arranged to carry and extract superheated steam from the system. Each of the vertical sections are open at the surface 40, but are closed by means of the processing plant equipment 50, 60 required for effective operation to extract heat as a natural resource from the Earth's structure.

Some geothermal fields have impermeable capping within their geological strata. However, in some locations and situations there is no impermeable cap, particularly in areas of recent geologically volcanic activity, for example in Iceland. For areas such as these a capping bed of impermeable clay 80 may be included at surface to enable sealing of the bore hole 10 to its casing 20, 30. An impermeable cap 80 is important to ensure zero emissions during the production phase of the system.

The main bore hole 10 is drilled to penetrate the hot geological substructure 300. During the drilling/excavation process horizontal and vertical branches 600 are also drilled which extend further into the Earth's structure to utilise the heat resource by conduction. The depth and cross sectional dimension of the branches is marginal compared with the bore 10.

As in conventional deep bore excavations the main bore hole 10 includes a hollow conduit 200 that extends through the hole and is secured in place with a casing structure 90, for example concrete. At the extraction side of the conduit 200 at least the vertical extraction section 70 of the bore 10, 70 includes insulation material of low thermal conductivity such that the heat within the bore 10, 70 on the extraction side is maintained and losses are minimal. Suitable insulation materials may be selected from the known groups of insulation material that exhibit low thermal conductivity and that are configured to withstand temperatures in excess of 205 degrees centigrade.

The branches 600 described above extend further into the Earths structure from the horizontal section 700 of the main bore hole, but are closed when the conduit 200 is in place. The branches 600 are used to generate more heat by including a conductive material therein which can transfer heat through the walls of the conduit 200 to the fluid flowing inside. The branches 600 are filled with conductive material which effectively plugs the branch 600. Graphite and graphene are examples of suitable material within each branch 600 having suitable thermally conductive properties. A further example of a material comprising suitable thermal conductivity properties is brass. It should be appreciated that the diameter of the branches 600 relative to the diameter of the main bore hole 10 is much smaller and as such is less invasive in the earth's structure. The branches 600 accentuate thermal effectiveness of the system. The process of extracting geothermal energy according to an embodiment of the present invention is done by feeding demineralised water via a suitable processing plant 50 into the conduit 200 and as the water is pumped through the conduit 200 the water is heated, by conduction, as it passes through the horizontal section 700 of the conduit 200 in the region of greatest geothermal activity. Heat is added to the flowing water directly via the conduit's presence in the hot substructure and also by conduction of heat via the plugged branches 600. The water is pumped through the system at approximately 18 bar (250psi) and is heated to generate superheated steam which is extractable at the surface 40 by suitable processing plant 60. The production of superheated steam from the system reduces, to zero, gaseous emissions normally associated with geothermal extraction. The annulus area 90 between the conduit 200 and the wall of the bore 10 is sealed with suitable material, for example concrete.

The system described above is a closed pressurised system where input fluid, demineralised water 5, is heated by conduction to produce superheated steam at the output. The superheated steam is output at a temperature of 205 degrees centigrade. However temperature as low as 100 degrees centigrade or as high as 900 degrees centigrade may be obtainable; this is dependent on the source of heating and the end use of the system, for example electricity generation or fuel production. Heat is conducted from the natural heat source in the location of the horizontal section 700 of the conduit 200 and also from the heat conducted via the branches 600.

On initial start-up and subsequent start-up demineralised water is introduced gradually to the input/descending side 20 of the conduit 200 such that the thermal gradient affecting the geological substructure is minimised. In full production, flow conditions shall be maintained by the processing plant 50, 60 located at the surface 40 such that a substantially constant thermal gradient is maintained. As such, potentially damaging thermal cycling is minimised.

A suitable application of the apparatus and process according to embodiments of the present invention is feeding superheated steam at 205 degrees centigrade to a reaction vessel described and illustrated in co-pending PCT patent application WO2013/124632 which describes the production of fuel and oxygen in a highly efficient way.

By way of example figure 2 corresponds with figure 6 of WO2013/124632 and shows a potential layout of an upstream fuel/oxygen production plant. CO₂ transported from a downstream CCGTEG plant enters the plant through pipe 61 and is heated to 205 °C by start-up heater 648 or by pre-heater 639 before joining with a stream of electrolysis- produced hydrogen 645 from pipe 643 and non-return valve 644. The combined stream 65, controlled at 205°C, then enters the Sabatier reactor tubes 66 (of which there are several, being fed by a manifold) where the CO₂ and H₂ react over a metal catalyst 67 to form CH₄ and H₂O. The metal catalyst would be the leading industrial standard for this reaction, currently Ru-doped AI2O3.

The product stream of CH₄ and H₂O gives up part of its heat to the water in pipe 619. The water is fed to the demineralised water being fed to the system as illustrated in figure 1 and 3 (described further below). The ChU leaves the condenser 612 through pipe 613, through which it passes to a CO₂ scrubber 614 used to remove any unreacted CO₂ from the Sabatier reactor tubes 66. The purified ChU is then taken off through pipe 615 for transportation to the downstream CCGTEG plant. The Sabatier reaction needs a temperature of at least 200°C to proceed and gives its highest product yield at around 300°C, above which increasing temperatures begin to favour the back reaction, reducing product yield and finally, at greater than 500°C, stopping the forward reaction from occurring. The highly exothermic nature of the Sabatier reaction therefore means that, without temperature regulation, the reactor tubes would quickly heat to temperatures that would prevent a sustainable process. To prevent this, the cooled H₂0 619 from condenser 612 is returned to demineralisation device according to embodiments of the present invention.

In this embodiment the optimal steam conditions for injection into the heat transfer reactor 625, are obtained by extracting superheated steam from the system according to figure 1 and figure 3 (described below). For start-up electric heating 637 will be used. Steam 620, from the system according to embodiments of the present invention, is used to heat the Sabatier reactor tubes 66, to a temperature of 205°C. As the H2O rises through heat transfer reactor 625 across the Sabatier reactor tubes 66, it is heated to 300°C. The steam continues to rise gaining heat by passing successively over tubes 641 and 642 which contain the hot electrolysis products hydrogen and oxygen respectively. Further heating may also be applied by heater 653 for temperature control of the steam entering electrolysis cells 621.

The heated, high-pressure steam then passes into the electrolysis cells 621. The high- pressure superheated steam enters the cell at the cathode 621 a. This is envisaged to be a solid-state electrode providing the best efficiency that current technology is capable of, at present a NiZr cement. A DC electricity supply 621d generated from local renewable energy source(s) will drive the electrolysis of H₂0 into H₂ and O₂. The H₂ forms at the cathode 621a and is taken off through pipe connection 621e to pipe 622. The O^2" ions migrate through the solid-state electrolyte 621 b to the anode 621c, where they give up electrons and form O2 molecules. Both the electrolyte and the anode will, like the cathode, reflect the optimal efficiencies available using current technology. At present, the electrolyte is envisaged to be yttria-stabilised zirconia, while the anode would be made from strontium-doped lanthanum manganite. At the anode, the 02 produced is taken off through pipe connection 621f to pipe 626.

It is inevitable that sizable quantities of unreacted H₂O will also enter the take-off connections 621a and thus pipe 622 and 624. The H₂ will therefore be separated from this steam in a separation chamber using a hydrogen-porous membrane 623. The H₂ will pass through the membrane and then enter pipe 62 and into the steam heating tubes 641 within the vessel 625 exiting via tube 63 into a Rankine cycle boiler 64 before joining the stream of C0₂ as previously described. The steam from the separation chamber 623 is passed back into the vessel 625 through a non-return valve 647 contained in tube 624 thereby to maintain the inlet steam to the electrolysis cells 621 at high temperatures approaching 900°C.

The electrolysis product oxygen exiting from collection manifold 626 is passed into the abovementioned steam heating tubes 642 within the vessel 625 then passing out to the collection manifold 627 and into a Rankine cycle boiler 628 to be cooled to approximately 30°C as outgoing product at pipe 616.

The Rankine cycle boiler 64 heated by the hydrogen stream 63 has its boiler tubes fed from condensate water pipe 634. Steam is generated in the boiler 64, which passes to steam turbine 629 generating DC electricity in generator 633. In a similar manner, Rankine cycle boiler 628 heated by the oxygen stream 627 is fed by condensate 634 and raises steam for turbine 630 generating further electricity in the DC generator 633. Exhaust steam from the turbines 629 and 630 pass into condensers 631 and 632 respectively to form condensate in 634 to be pumped by feed pumps to boilers 64 and 628 to continue the Rankine cycle.

Yet further gains in efficiency can be achieved by the steam bridges 650 and 649 which connect to pipeline 651 feeding into the main heat transfer reactor 625. This would enable the reduction or elimination of Rankine condenser loss in condensers 631 and 632. The steam bridges 650 and 649 situated within steam turbines 629 and 630 contain valves to divert steam into pipeline 651 and to shut off steam flow to the low pressure side of the turbines 629 and 630. The high pressure side of turbines 629 and 630 would provide steam at 18 bar (250psi) into the steam bridges, this being the pressure of saturated steam at the temperature required of 205°C for inlet 625.

In order to pre-heat the CO₂ 61 entering the fuel/oxygen production process bled steam 638 from turbine 629 is passed to a heat exchanger 639. Condensate and low enthalpy steam is recovered from the heat exchanger 639 by pipe 640 and recovered into the condenser 631.

The DC generator 633 generates electricity to enable further electrolysis for further product production thereby to increase conversion of energy from the source energy (i.e. non-fossil source) to chemical energy in the methane and oxygen, potentially more than an efficiency of 70%.

Thus it can be seen that the present invention provides a high efficiency combined electrolysis and Sabatier reaction apparatus and system. Excess heat produced during the Sabatier reaction is used to heat water that is then electrolysed. In order to further elevate the temperature of the water to be electrolysed, the high temperature outputs from the electrolysis process are passed though the water, which makes use of the high heat energy content of the electrolysis products to further raise the temperature of the water input. This increases overall efficiency of the electrolysis process. Furthermore, an additional potential loss of energy in the form of heat is avoided by using the still relatively hot oxygen and hydrogen, even after heating the electrolysis input water, to drive electricity generation via an additional generation stage, e.g. through a Rankine cycle. These features provide a highly efficient system for combining electrolysis and generation of methane. Of course, many of the efficiency increasing features can be used independently from each other, where appropriate, but the optimum efficiency is achieved where all the features are combined.

A further embodiment of the present invention is illustrated in figure 3. The extraction system 1000 comprises a substantially vertical main bore 1010, into which is located an input conduit 1020 and an output conduit 1030, where the input conduit 1020 is contained within the output conduit 1030 and where the input conduit 1020 opens into the bottom of the output conduit 1030.

The input conduit 1020 is fed with demineralised water 1040 from the surface and exits at the bottom of the bore 1050 into a region of thermal activity where the water is heated to produce superheated steam 1060, which is extracted at the surface by processing plant (not illustrated). As described above with respect to figure 1 the system 1000, as illustrated in figure 3, uses thermal branches 1070, which extend further into the earth's structure, into the hot resource 1080 from the main bore 1010. These branches 1070 can be substantially vertical, substantially horizontal or extend angularly further into the earth's structure 1080 from the main bore 1010. The borehole 1010 and the conduits 1020, 1030 are suitably sealed 1090, for example by concrete.

To maximise conductance of heat to the output conduit 1030 such that superheated steam 1060 is produced the branches 1070 each contain material of high thermal conductivity, for example graphite, such that heat is transferred from the earth's natural heat source 1080 via the branches 1070 to the output conduit 1030. The system includes insulation material 1090 about the output conduit 1030 such that heat transferred from the hot resource 1080 is maximised and heat lost, for example to the input conduit 1020 is minimised.

As described above with reference to figure 1 , some geothermal fields have impermeable capping within their geological strata. However, in some locations and situations there is no impermeable cap, particularly in areas of recent geologically volcanic activity, for example in Iceland. For areas such as these a capping bed of impermeable clay 1100 may be included at surface to enable sealing of the bore hole 1100 to its casing 1020, 1030. An impermeable cap 1100 is important to ensure zero emissions during the production phase of the system 1000.

In both embodiments described, with reference to figures 1 and 3, a closed system is provided. The closed system inputs demineralised water and the production process allows the extraction/take off of superheated steam. Each system described uses branches containing material of high thermal conductivity and thermal insulation to maximise heat transfer to the steam and also to protect the geological structure.

Embodiments of the present invention, described above provide a geothermal energy extraction system having zero emissions and one which minimises the risk of causing earthquakes. The system is described producing superheated steam, which can then be utilised to produce fuels as described above with reference to figure 2. By suitable processing means, referenced in WO2013/124632, fuels such as petroleum, diesel oil and aviation fuel can be produced from the superheated steam output from the system. Chemical products, such as methanol, ethylene glycol, polythene, styrene and PVC by suitable processing means are also obtainable. It should be appreciated that electricity production and district heating are also practical applications of the superheated steam.

In the examples described, the closed system is fed with demineralised water and discharges superheated steam such that the emissions, characteristic of current geothermal energy extraction, are not produced.

Low or constant thermal gradients within the geological substructure are achieved to avoid the fracturing of the substructure that can cause internal collapse and settlement potentially large enough to be classed as earthquakes.

The system according to the present invention will be produced using controlled methods which should reduce the risk of fracturing the substructure apart. As such, during many decades of future steady state production there will be no need to cause fracturing in response to depletion that would otherwise potentially cause earthquakes.

Indeed the Carbon Credit Scheme arising from the Kyoto Protocol could be invoked and would aid the economics of potential developments. In this context the efficiency of geothermal electricity production becomes important. Currently efficiencies vary from 10 to 30% being dependent on the quality of the steam/water produced. It could be expected that the present invention used in conjunction with WO2013/124632 as described above could potentially more than double these efficiencies. With double efficiency 2 million Tonnes of CO₂ emissions per GW Year could be saved if replacing existing geothermal electricity generation. A further claim of Carbon Credits if replacing coal electricity generation is also possible. In such case arguably 10 million Tonnes per GW Year of Carbon Credits in total could be claimed with a value of $10 million per GW Year in the United States. At present the Protocol for Carbon Credits are under review.

In summary, the embodiments of the present invention provide emission free and earthquake free geothermal energy extraction. It is achieved by a single drilled hole passing down to and through a hot substructure before returning to the surface. Plugged conduction avenues, for example graphite plugs, accentuate thermal effectiveness. A closed system of demineralised water insertion and subsequent superheated steam production is obtained with none of the emissions characteristic of known geothermal energy extraction systems. Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.

Claims

CLAI MS

1. A geothermal energy extraction system comprising: a conduit within an excavated bore; wherein the conduit comprises a substantially vertical descending feed section; and a vertical ascending extraction section, wherein at least part of the vertical ascending and at least part of the descending section is located in a geothermal zone below surface and comprising a plurality of branches extending from the bore into the geothermal zone, wherein the branches are each filled with thermally conductive material.

2. A geothermal energy extraction system according to claim 1 , wherein the vertical descending section is within the vertical ascending section.

3. A geothermal energy extraction system according to claim 2, wherein the vertical descending section opens into the vertical ascending section.

4. A geothermal energy extraction system according to claim 1 , further comprising a substantially horizontal section located between and in fluid communication with the vertical descending section and the vertical ascending section.

5. A geothermal energy extraction system according to claim 4, wherein the horizontal section is located in a geothermal zone below surface.

6. A geothermal energy extraction system according to claim 4 or 5, wherein the branches extend from the horizontal section and the branches are configured to conduct heat from the geothermal zone via conductive material contained in the branches.

7. A geothermal energy extraction system according to any preceding claim, wherein the thermally conductive material comprises graphite or graphene comprising molecular orientation in the direction of greatest heat conductance.

8. A geothermal energy extraction system according to any of claims 1 to 7, wherein the thermally conductive material comprises brass.

8. A geothermal energy extraction system according to any preceding claim, wherein at least the vertical ascending extraction section comprises thermal insulation material with low thermal conductivity between the conduit and a wall of the excavated bore.

9. A geothermal energy extraction system according to any preceding claim, further comprising a demineralising device in fluid communication with input to the vertical descending section, wherein the demineralising device is operable to feed demineralised water to the system.

10. A geothermal energy extraction system according to claim 9, further comprising pumping means operable to maintain pressure of the demineralised water fed to the system within a predetermined range such that superheated steam exits from the vertical ascending section.

11 . A geothermal energy extraction system according to any preceding claim, wherein superheated steam is extractable from the system, via the vertical ascending section.

12. A geothermal energy extraction system according to claim 10 or 11 , wherein the temperature of the superheated steam is in the region of 205 degrees centigrade.

13. A geothermal energy extraction system according to claim 10 or 11 , wherein the temperature of the superheated steam is greater than 205 degrees centigrade.

14. A geothermal energy extraction system according to any preceding claim, further comprising a capping bed at the surface,

15. A geothermal energy extraction system according to claim 14, wherein the capping material is clay.