SOLUTION MINED CAVITY SYSTEM

Abstract

A solution mined cavity system and method is disclosed. The system comprises an underground cavity created by solution mining in salt deposits, an upper water reservoir, tubing means adapted to lead water from the upper water reservoir into the 5underground cavity and out of the underground cavity, and pumping means adapted to contribute to pumping water from the upper water reservoir via the tubing means out of the cavity.0

Claims

1. An underground hydroelectric energy production and storage system, where the system comprises: an underground cavity (8) created by solution mining in salt deposits; an upper water reservoir (12, 18, 27) with an upper water reservoir surface (30); tubing means (11, 19, 26, 28, 29, 31) adapted to lead water from the upper water reservoir (12, 18, 27) into the underground cavity (8) and out of the underground cavity (8); pumping means adapted to contribute to pumping water from the upper water reservoir (12, 18, 27) via the tubing means out of the cavity (8); and energy generating means adapted to generate energy based on water flowing through the tubing means.

2. The underground hydroelectric energy production and storage system according to claim 1, where the underground cavity (8) is consolidated to avoid further dissolution of salt, comprising one or more of the following steps: removing (all) salt in at least a part of the cavity (8); impregnating at least a part of the cavity (8); and arranging an impermeable material, e.g., membrane (10) in the cavity (8).

3. The underground hydroelectric energy production and storage system according to claim 1, where the tubing means is an open loop comprising an inlet tube (11) arranged for leading water into an open volume (14) at the bottom of the cavity (8), and an outlet tube arranged for leading water from the open volume (14) and out of the cavity (8).

4. The underground hydroelectric energy production and storage system according to claim 3, where the energy generating means comprises a turbine/generator combination (13) arranged above the open volume (14) and to be driven by water flow in the inlet tube (11), and the pumping means comprises a pump arranged in the open water volume (14) adapted to pump water out of the cavity (8) via the outlet tube.

5. The underground hydroelectric energy production and storage system according to claim 3, where the energy generating means comprises a turbine/generator/pump combination (16) arranged in the open volume (14) and to be driven by water flow in the inlet tube (11), and adapted to pump water out of the cavity (8) via the outlet tube.

6. The underground hydroelectric energy production and storage system according to claim 3, where the underground cavity not is consolidated, and where the pumping means is arranged in an upper part of the open water volume (14) less rich in dissolved salt than a lower part.

7. The underground hydroelectric energy production and storage system according to claim 1, where the tubing means comprises a closed hydraulic loop, the closed hydraulic loop comprising a communicating system adapted to preserve hydraulic head provided by the upper reservoir.

8. The underground hydroelectric energy production and storage system according to claim 7, where the closed hydraulic loop comprises a U tube with a first branch (19) corresponding to the inlet tube, and a second branch (21) corresponding to the outlet tube, and a bottom part connecting the two branches.

9. The underground hydroelectric system according to claim 7, where the cavity (8) is established with at least a lower part at a depth allowing heat transfer from the environment/surroundings of the cavity (8) to a thermal carrier fluid in the cavity (8).

10. The underground hydroelectric energy production and storage system according to claim 7, where the closed hydraulic loop is arranged to allow for thermally driven density differential pumping, and the pumping means comprises a heat exchanger, and where the closed hydraulic loop is arranged for receiving water from the upper water reservoir and leading the water via an inlet tube (11) through the heat exchanger (20) submerged in the thermal carrier fluid in the cavity (8), where the thermal carrier fluid in the cavity (8) is representing a thermal reservoir contributing to convection transport of heat from the surroundings, and contributing to pumping the water out via the outlet tube (21).

11. The underground hydroelectric energy production and storage system according to claim 10, where the energy generating means comprises a turbine arranged (above the surface (9)) and adapted to be driven by water flow in the outlet tube.

12. The underground hydroelectric energy production and storage system according to claim 7, where the closed hydraulic loop is arranged to allow for dissolved gas density differential pumping, the system comprising gas provision means arranged to saturate water in the inlet tube (11) with a gas, and the energy generating means comprising a turbine (39) arranged at the bottom part of the closed hydraulic loop.

13. The underground hydroelectric energy production and storage system according to claim 12, further comprising gas nucleation means comprising a structure (44) arranged in the outlet tube (36) to assist in nucleating bubbles of the gas out of the water.

14. A method for generating and producing energy by operating an underground hydroelectric energy production and storage system according to claim 1.

15. The underground hydroelectric energy production and storage system according to claim 2, where the tubing means is an open loop comprising an inlet tube arranged for leading water into an open volume at the bottom of the cavity, and an outlet tube arranged for leading water from the open volume and out of the cavity.

16. The underground hydroelectric energy production and storage system according to claim 15, where the underground cavity not is consolidated, and where the pumping means is arranged in an upper part of the open water volume less rich in dissolved salt than a lower part.

17. The underground hydroelectric energy production and storage system according to claim 4, where the underground cavity not is consolidated, and where the pumping means is arranged in an upper part of the open water volume less rich in dissolved salt than a lower part.

18. The underground hydroelectric energy production and storage system according to claim 5, where the underground cavity not is consolidated, and where the pumping means is arranged in an upper part of the open water volume less rich in dissolved salt than a lower part.

19. The underground hydroelectric energy production and storage system according to claim 2, where the tubing means comprises a closed hydraulic loop, the closed hydraulic loop comprising a communicating system adapted to preserve hydraulic head provided by the upper reservoir.

20. The underground hydroelectric system according to claim 8, where the cavity is established with at least a lower part at a depth allowing heat transfer from the environment/surroundings of the cavity to a thermal carrier fluid in the cavity.

Description

DESCRIPTION OF THE DIAGRAMS

[0025] The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of an [exemplary] embodiment of the invention given with reference to the accompanying drawings.

[0026] Embodiments of the present invention will now be described, by way of example only, with reference to the following diagrams wherein:

[0027] FIG. 1a shows a subsurface geological salt dome structure

[0028] FIG. 1b shows a subsurface geological salt dome structure with an inlet tube providing for a dissolution process and an evacuation tube

[0029] FIG. 1c shows a subsurface geological salt dome structure where salt has been removed leaving a cavern

[0030] FIG. 1d shows a closed hydraulic system

[0031] FIG. 2a-c shows three configurations comprising turbine/generator/pump equipment

[0032] FIG. 3 shows a thermally driven density differential pumping embodiment

[0033] FIG. 4a-c illustrate thermally driven extraction of mechanical energy

[0034] FIG. 5 illustrates the basic principle underlying dissolved gas density differential pumping

[0035] FIG. 6 illustrates a system based on the principle illustrated in FIG. 5

LIST OF REFERENCE NUMBERS IN THE FIGURES

[0036] The following reference numbers and signs refer to the drawings:

TABLE-US-00001 Number Designation 1 Salt dome 2 Layer of cap rock 3 Surrounding rock strata 4 Inlet tube 5 Evacuation tube 6 Brine 7 Incipient cavity 8 Cavern 9 Surface 10 Membrane 11 Penstock 12 Reservoir 13 Turbine/generator combination 14 Volume 15 Pumping unit 16 Turbine/generator/pump 17 Top part of undissolved salt 18 Bottom part of water volume 19 Vent 20 Heat exchanger 21 Closed loop tubing 22 Pool of water or brine 23 Turbine 24 25 Heated water 26 U tube 27 Water reservoir 28 Branch 29 Branch 30 Reservoir surface 31 Tube opening 32 Turbine 33 Point 34 Point 35 Branch of a U tube 36 Branch of a U tube 37 Connection tube of a U tube 38 Reservoir 39 Turbine 40 Recipient 41 Gas provision unit 42 Shaft 43 Ambient air 44 Structure

Description of Preferred Embodiments of the Invention

[0037] Various aspects of the disclosure are described more fully hereinafter with reference to the accompanying drawings. This disclosure may, however, be embodied in many different forms and should not be construed as limited to any specific structure or function presented throughout this disclosure. Rather, these aspects are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Based on the teachings herein one skilled in the art should appreciate that the scope of the disclosure is intended to cover any aspect of the disclosure disclosed herein, whether implemented independently of or combined with any other aspect of the disclosure. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, the scope of the disclosure is intended to cover such an apparatus or method which is practiced using other structure, functionality, or structure and functionality in addition to or other than the various aspects of the disclosure set forth herein. It should be understood that any aspect of the disclosure disclosed herein may be embodied by one or more elements of a claim.

[0038] The following description of a preferred embodiment of the invention is exemplary without limiting the invention or the application.

[0039] Below are delineated a set of steps according to the present invention for exploiting the great potential of UPHS using in-situ salt leached caverns, and optionally for combining this with geothermal energy extraction:

[0040] Step 1): Creation of underground cavern by solution mining of underground salt deposits (salt domes, salt pillows).

[0041] Step 2): Consolidation of cavern.

[0042] Step 3): Installation of turbine, generator, etc. for UPHS operation.

[0043] Step 4): UPHS operations: Generation of energy by running water from the surface through the turbine and collecting the water at the bottom of the cavern. Storage of energy by pumping out water from the cavern to the surface.

[0044] A detailed description shall now be given with reference to FIG. 1a which shows an example involving a structure of relevance in the present context, namely a salt dome. This is a subsurface geological structure in the general form of a vertical pillar (1) of salt (typically dominated by NaCl), extending from a depth of several kilometers and to the surface (9), where it may be exposed or covered by a layer of cap rock (2) up to some hundreds of meters thick. The pillar diameter may range up to 1 km. or more. The salt dome is enveloped by surrounding rock strata (3), e.g. limestone, sandstone and dolomite.

[0045] Step 1) starts with preparatory work that includes a geological survey to ascertain suitability of a given site (e.g.: seismic tests and exploratory drilling). In cases where the salt dome is not exposed to the surface, there follows excavation, drilling and blasting from surface ground level to access the top of the salt deposit or salt dome. Tubing for transport of water and dissolved salts is then inserted into the body of salt to be removed. The simplified drawing in FIG. 1b shows one inlet tube (4) providing the water for the dissolution process, and an evacuation tube (5) for transporting the dissolved salt away from the pool of brine (6) collected at the bottom of the incipient cavity (7). The evacuated brine is either disposed of, stored or refined for sale. Water of zero or low salinity (seawater may be used) is sprayed and circulated in a controlled pattern inside the cavity to dissolve the salt and extend the cavity volume outwards and downwards to achieve the shape and size desired for the subsequent operations inside the cavity. For the present purposes, a preferred solution is as shown in FIG. 1c, where the salt has been removed from the upper part of the salt dome, leaving a cavern (8) with exposed rock walls which may be left as they are or may be sealed against water transmission. Removal of the brine may be achieved by direct pumping as implied in FIG. 1c (pump not shown), or alternatively by means of an unbroken hydraulic loop which reduces the need for pumping energy, cf. FIG. 1d: Here, the entire cavern (8) and the inlet and evacuation tubes (4), (5) form a closed hydraulic system with a water pressure head corresponding to a level above the surface (9). Water inserted into the tube (4) exerts pressure which is transmitted throughout the hydraulic system, forcing water and brine to evacuate through the tube (5). Since the brine is heavier than pure water, a pump shall be required to assist in the lifting process, but the pumping energy shall typically be small. This basic principle has been widely used in solution mining, and may be combined with elevated temperature thermal encapsulation of the tube (5) to avoid clogging due to aggregation or crystallization of the materials carried in the brine.

[0046] Step 2). Consolidation of cavern: The size and shape of the cavern shall in part be dictated by the geology and in part by how the mining of the salt is carried out. Structural considerations must be made to ensure against collapse, and for the present purposes it shall generally be advantageous to avoid further dissolution of salt during operations, cf. FIG. 1c. This may be achieved by removing all salt in at least one part of the salt deposit, leaving the surrounding rock to define the cavern walls. Alternatively, a cavern may be excavated within a limited part of a salt deposit, and an impermeable membrane may be applied on the walls against the salt. In the case of salt dome excavations, the available depth of the salt pillar shall typically exceed the optimal depth of the cavern, in which case the floor of the cavern may be covered by a membrane (10) to avoid leaching from the salt underneath.

[0047] Step 3): Installation of turbine, generator, etc., cf. FIGS. 2a,b,c: Three configurations are shown: In FIG. 2a a penstock (11) carries water from a reservoir (12) at a high level to a turbine/generator combination (13) at a low level, with water collecting in a volume (14) below. During the charge/discharge of the UPHS system, the water level in the water volume (14) shall rise and fall, and the turbine/generator is positioned as low as possible without being submerged during any part of the charge/discharge cycle, in order to maximize the amount of energy extracted. A pumping unit (15) evacuates water from the bottom of the water volume (14) during the charging cycle. In order to prevent unwanted pressure effects in the cavity (8), a vent (19) to the atmosphere is provided. In FIG. 2b the turbine/generator/pump (16) are submerged in the water volume (14). During the discharge (energy production) part of the UPHS cycle, the turbine works against a rising head in the water volume (14), which in this configuration can fill the whole cavity (8) to provide increased energy storage. With the turbine submerged throughout the charge/discharge cycles, the turbine may be run in reverse to act as a pump during the charging cycle. The configuration shown in FIG. 2c is similar to that shown in FIG. 2b, but in this case the bottom of the cavity (8) is not sealed by an impermeable membrane. Rather, it is defined by the top part (17) of the undissolved salt below. The bottom part (18) of the water volume (14) is now rich in dissolved salt, which may to a certain extent be admixed into the water passing through the turbine and pump (16). However, the concentrated layers of brine shall be heavy and may be kept near the bottom by proper design and operation of the system. The salt that is ultimately removed may have a beneficial effect by enlarging the cavity volume (8).

[0048] In parallel with the opportunities for storage of unprecedented amounts of electrical energy as described above, excavation of deep salt deposits opens up possibilities for tapping into vast geothermal resources, i.e. by transferring heat from the cavity environment to a thermal carrier fluid (e.g. water or brine) circulating in the cavity. This step may occur when significant depths have been reached during the latter stages of the solution mining process in Step 1) and/or after the cavern volume has been established and the UPHS system has been installed. An early description of geothermal heat extraction from deep salt deposits was given in: C. H. Jacoby and D. K. Paul “Salt domes as a source of geothermal energy”, Society of Mining Engineers, pp.34-39, May 1974.

[0049] The excavation procedure described above was with reference to an essentially isothermal environment, which is the case in the early stages of the mining operation and at shallow depths. However, deep salt deposits can provide access to great amounts of geothermal energy: Efficient solution mining methods exist that allow reaching great depths, salt domes can exhibit high vertical thermal gradients, and thermal diffusivities of salt in the deposits that may substantially exceed those of the surrounding rock. As shall now be described, an unbroken hydraulic loop, i.e. a communicating system of cavities and tubing that preserve the hydraulic head provided from a surface reservoir, can be combined with access to geothermal heat to extract mechanical energy, even in cases where the geothermal source is low grade in terms of available temperatures. This process shall in the following be referred to as thermally driven density differential pumping, and is described with reference to FIGS. 3, 4a-c:

[0050] FIG. 3 shows an example of a closed loop system where cold water is fed from a surface reservoir (18) through a tube (19) and heat exchanger (20) located at depth in the salt dome. The heat exchanger is in this case submerged in a pool of water or brine (22) at the bottom of the cavity (8) which represents a thermal reservoir and assists in convection transport of heat from the surrounding rock and underlying salt plug (24). As shall be described in more detail below, the water in the closed loop tubing (19), (20), (21) shall be lifted to the surface via the tube (21), and mechanical energy is extracted by a turbine (23). Heated water (25) exiting from the turbine can be made available for thermal uses (space heating, desalination, etc). The thermally driven extraction of mechanical energy is illustrated in FIGS. 4a-c: In FIG. 4a a U tube (26), is filled with water up to a level defined by the free surface (30) of a water reservoir (27) via the top of the left branch (28) of the U tube. In a quiescent state, water fills the right branch (29) of the U tube to the same level as shown. In FIG. 4b, the water in the lower part of the U tube has been exposed to heating, which lowers the local water density in the bottom part of the U tube by thermal expansion. Over time and in the absence of a directed flow in the U tube, the hot water would rise symmetrically in both branches of the U tube. In FIG. 4c, an imbalance has been introduced, whereby water in the U tube flows from left to right. As shown, the left branch (28) is replenished by cold water from the reservoir, and the hot water now fills the right branch (29), with excess water spilling out of the right branch tube opening (31) at a height Δh above the reservoir surface (30) and driving a turbine (32). This process would continue as long as the system receives cold water from the reservoir to fill the left branch and thermal energy to maintain an elevated temperature in the right branch. The maximum height h.sub.2 to which the right branch water level can be lifted in this manner can be estimated as follows, with reference to FIG. 4c: Assume that the average density of the cold water in the left branch (28) is ρ.sub.1. At the bottom of the left branch, i.e. at a point located at (33), the local hydrostatic pressure is generated by a water column of height h.sub.1 below the free surface (30) of the reservoir (27), and the hydrostatic pressure at (33) is:

p.sub.1=ρ.sub.1 g h.sub.1.   Eq.1

[0051] In the right branch (29) the water is warmer and thus has a lower density ρ.sub.2. If the water column in the right branch has a height h.sub.2 above a point located at (34) which is at the same depth as the point located at (33), the hydrostatic pressure at (34) is:

p.sub.2=ρ.sub.2 g h.sub.2   Eq.2

[0052] At near-equilibrium, there is very little flow in the U tube, and one has:

p.sub.1p.sub.2=ρ.sub.1 g h.sub.1=ρ.sub.2 g h.sub.2   Eq.3

h.sub.2=h.sub.1 ρ.sub.1/ρ.sub.2   Eq.4

Δh=h.sub.1(ρ.sub.1/ρ.sub.2−1)   Eq.5

[0053] If there is a volume flow F [m.sup.3 s.sup.−1] through the U tube, the maximum mechanical power that can be extracted by a turbine (32) is:

P=F g h.sub.1(ρ.sub.1−ρ.sub.2).   Eq.6

[0054] Here, the density differential (ρ.sub.1−ρ.sub.2) shall depend on the temperature difference between the left and the right branches in the U tube, and the volume flow shall depend on several factors, including the density differential and the cross sectional area of the U tube. An important feature of thermally driven density differential pumping as described above is that it may run in parallel with UPHS operations.

[0055] FIGS. 5,6 illustrate the basic principles underlying dissolved gas density differential pumping. This is another type of density differential pumping which can operate with or without thermal input and which employs the access to a deep underground cavity provided by solution mining as described in the present invention: As shown in FIG. 5, a U tube consisting of a left (35) and a right (36) vertical branch and a horizontal connection tube (37) is filled with water which is drawn from a reservoir (38). In operation, the water flows down the left branch (35), through a turbine (39) and rises in the right branch (36) before flowing from the top of the right branch and into a recipient (40). During its transit down the left branch, the water is saturated with dissolved gas which is provided from a unit (41) and dispensed from a shaft (42) which extends vertically inside the left branch. The gas may be ambient air (43) as indicated in FIG. 5, but other gases, e.g. CO.sub.2, may be used for special purposes. Depending on the pressure and temperature, a certain amount of gas can be dissolved in the water, as described by Henry's law, which indicates that dissolved concentrations increase with increasing pressure and lower temperatures:

X.sub.gas=P.sub.gas/K   Eq.7

[0056] Here, X.sub.gas is the equilibrium mole fraction of solute (gas) in the liquid phase:

X.sub.gas=(No. of moles in solute)/(no. of moles in solute+No. of moles in solvent),   Eq.8

[0057] P.sub.gas is the partial pressure of the gas, and K is Henry's constant. K is a characteristic parameter for each type of gas, and is strongly temperature dependent. Thus, as air is brought into contact with the water in the left branch, an increasing amount of air shall be dissolved as it experiences increased hydrostatic pressure at increasing depths.

[0058] The water experiences an abrupt loss of hydrostatic pressure as it passes through the turbine (39) and thereafter rises in the right branch (36). This implies that the dissolved gas shall be brought closer to supersaturation at a given point in the right branch than at a corresponding depth in the left branch, and gas bubbles start to nucleate out of the water. To assist in this process, a structure (44) has been inserted in contact with the rising water in the right branch. The bubbles rise in the right branch through buoyancy and convection in the water flow, and expand as they encounter lower hydrostatic pressure higher up in the water column. This provides a pumping action and at the same time a density dilution effect on the water in the right branch. The net effect is a lower hydrostatic pressure at the outlet of the turbine compared to that at the inlet, and the turbine delivers power. During the process described here, the expanding bubbles perform mechanical work and the gas within would cool down if they were not in intimate thermal contact with the water. In practice, the bubble gas temperature remains close to that of the water, which experiences a moderate cooling effect during its transit through the system. Thermal effects connected with the insertion of gas and dissolution of gas in the left branch, and with nucleation in the right branch shall generally have a minor impact on the energy produced by the turbine. However, by employing cold water in the left branch, more gas can be dissolved to increase energy production. Furthermore, heating of water delivered to the right branch shall facilitate nucleation and lowering of density in the right branch, cf. FIG. 6, where the U tube is partially heated by warm water or brine (22). Thus, geothermal energy may be harvested as described above in connection with thermally driven density differential pumping to enhance energy production in the present case of dissolved gas density differential pumping.

[0059] Thus, systems as described here may generate revenues by several different modalities separately or in combination, i.e.:

[0060] Sale of pumped hydro energy storage capacity

[0061] Sale of geothermal energy

[0062] Sale of leached materials