WO1993006367A1

WO1993006367A1 - A system for subterranean storage of energy

Info

Publication number: WO1993006367A1
Application number: PCT/EP1992/002193
Authority: WO
Inventors: Arnold W. J. Grupping
Original assignee: Grupping Arnold
Priority date: 1991-09-25
Filing date: 1992-09-23
Publication date: 1993-04-01
Also published as: NL9101618A

Abstract

A system for subterranean storage of energy comprises two caverns (4, 5) formed in a subterranean salt formation (3), both caverns (4, 5) being partially filled with salt solution, a gas supply for pressurizing a head space in each cavern (4, 5), a first channel (6, 7, 8, 9) interconnecting the liquid filled portions of the caverns (4, 5), a pumping/generating plant (10) with cooler (11) at the ground surface (1) connected between two branches (6, 7) and (8, 9) of the first channel, a second channel (12, 13) and a third channel (14, 15) connecting the first cavern (4) and the second cavern (5) with compressing/generating plants (16 and 17) respectively, these in turn being connected with gas storage systems (18 and 19). The pumping/generating plant (10) is operable for pumping salt solution through the first channel (6, 7, 8, 9) from the first cavern (4) to the second cavern (5) when supplied with energy, or for generating energy when driven by salt solution flowing through the first channel (6, 7, 8, 9) from the second cavern (5) to the first cavern (4). The compressing/generating plant (16) is operable for compressing gas through the second channel (12, 13) from the gas storage system (18) into the first cavern (4) when supplied with energy, or for generating energy when driven by the gas flowing back and expanding through the second channel (12, 13) to the gas storage system (18). The compressing/generating plant (17) is operable for compressing gas from the second cavern (5) through the third channel (14, 15) into the gas storage system (19) when supplied with energy, or for generating energy when driven by the gas flowing back and expanding through the third channel (14, 15) from the gas storage system (19) into the second cavern (5). The gas in the first cavern (4) maintains a pressure sufficient for driving the salt solution to the ground surface (1). The gas in the second cavern (5) maintains a pressure sufficient for establishing a pressure difference across the pumping/generating plant (10) for generating energy by salt solution flowing back through it.

Description

A SYSTEM FOR SUBTERRANEAN STORAGE OF ENERGY

There has been a long-standing need for a system for sto¬ ring energy, enabling the turbo-generators and the heat source of an electric power station to woik at a given constant load, in which, during the time that the energy demand is lower than the energy produced, the surplus energy is supplied to the system, and during the time that the demand is larger, the stored energy can be con¬ verted into electric energy again so as to supplement the shortage. Such a system will allow an optimum utilization of the heat source and the turbo-generators, which will lead to substantial savings, whilst also an efficient use of fuel, which will become scarce, can be made. It has already been proposed to use, for this purpose, caverns in salt formations that remain after the salt has been removed therefrom by introducing water, and the salt extraction therefrom has been discontinued because given maximally allowable dimensions have been reached. EP-B 0247690 describes an energy storage system that makes use of two of such caverns in a salt formation.

The invention provides an improvement to this known sys¬ tem, allowing to substantiall increase the amount of energy that can be stored in salt caverns. To this end, two caverns are washed out in a salt formation, the top parts of which are filled with high pressure gas, the bottom parts being filled with salt water and inter¬ connected by means of a first channel, which is also fil¬ led with salt water and in which an energy conversion assembly is installed at the ground surface, the top part of each cavern being connected, by means of additional energy conversion assemblies, installed at the ground surface, to two gas storage systems. By making use, according to the invention, of the gas expansion/compression that takes place when salt water flows between the two caverns, an additional energy storage/withdrawal is obtained. This additional energy storage/withdrawal is not possible in systems in which the gas-filled parts of the caverns are interconnected and consequently of the same pressure.

Favourable embodiments of the invention are the subject of the dependent claims. The invention will be elucidated below in more detail by reference to a drawing, showing in: fig. 1 a diagrammatic representation of the system accor¬ ding to the invention. fig. 2 a modified version of the system of fig. 1. figs. 3-6 modified embodiments of the system of fig. 2 or a portion .thereof.

Referring to fig. 1, the black arrows show the direction of flow of salt water and gas during periods of energy storage, whilst the white arrows show the direction of flow fluring periods of energy withdrawal. The ground surface is indicated at 1, the overlying formations at 2, and a subterranean salt formation at 3, the depths of the various formations not being shown on scale. From the ground surface 1, a first cavern 4 and a second cavern 5 haΛ'e been washed out at about the same depth in the salt formation 3 in the known manner. These caverns are partly filled with salt water 4',5' and for the remainder with gas. The lower parts of the caverns 4,5 are connected, by means of curved channels 6 and 8 in the salt formation 3, to vertical cased boreholes 7 and 9 of Large diameter, the latter communicating, through the overlying formations 2, with the ground surface 1. The curved channels 6 and 8 have been washed out in the salt formation 3 by means of deviated boreholes of small diameter (not shown). The boreholes 7 and 9 are connected, above the ground sur- face 1, to a hydro-electric assembly 10, consisting of a pump/turbine and a motor/generator, said assembly being adapted to operate either as a motor driven liquid pump or a turbine driven generator. One or more coolers 11 are included between the boreholes 7 and 9 above the ground surface 1, for cooling part of the salt water flow. The upper sides of the caverns 4 and 5 are connected to washed-ot vertical channels 12 and 14 in the salt for¬ mation 3 which, by means of cased boreholes of large diameter 13 and 15, communicate through the overlying formations 2 with the ground surface 1. The boreholes 13 and 15 may have served for forming the caverns 4 and 5 and the washed-out channels 12 and 14, or other bore¬ holes of small diameter (not shown) may have been used for this purpose.

Above the ground surface 1 the boreholes 13 and 15 have been_, connected to electro-mechanical assemblies 16 and 17 respectively, consisting of a compressor/expander and a motor/generator, said assemblies being adapted to ope- rate either as a motor driven gas compressor or as a gas expander driven generator.

The compressor/expander assemblies 16 and 17 are adapted for compressing gas from a gas storage system 18 (shown schematically) into the top of the first cavern 4 and from the top of the second cavern 5 into a gas storage system 19

(shown schematically) respectively, when electric energy is supplied and, on the other hand , producing electric energy by gas flowing back and expanding from the top of the first ca_¬ vern 4 into the gas storage system 18 and from the gas storage system 19 into the top of the second cavern 5. In this manner, additional energy can be stored in the system. The gas storage systems 18 and 19 may consist of constant pressure (liquid displacement) underground gas storage systems or expansion type underground gas storage systems. In addition, the gas storage system 18 may be the earth's atmosphere or a natural gas transport pipeline. The gas pressure in the first cavern 4 is chosen such that it is sufficient for overcoming the column pressure of the salt water in the channel 6 and the borehole 7 up to the ground surface 1, so that, at the pump suction of the as¬ sembly 10, the pressure will be positive. During the operation of the pump, the column pressure of the salt water in the channel 8 and the borehole 9 acts in the flow sense, so that the pump discharge needs to produce a pressure which is lower than the gas pressure in the second cavern 5. The gas pressure in the second cavern 5 should not be so high that the ground pressure would be overcome and cratering of gas to the ground surface 1 would occur. The boreholes 7, 9, 13 and 15 are, as is customary, pro- vided with a suitable casing that extends to at least into the top of the salt formation 3, so as to prevent collapsing of the overlying formations 2 and penetration of groundwater and the like, the casings 9, 13 and 15 extending into the salt formation 3 to such a depth as is ne essary to avoid cratering of gas or salt water to the ground surface 1. The channels 6, 8, 12 and 14 and the boreholes 7, 9, 13 and 15 are so wide that the frict¬ ion of the flowing salt water and gas will be small. The operation of this system is as follows: During periods of excess electric energy, the pump of the assembly 10, which is capable of generating a pressure at least equal to the gas pressure in the second cavern 5 minus the column pressure of the salt vater in the channel 8 and. the borehole 9, the compressor of the assembly 16, which is capable of generating a pressure at least equal to the gas pressure in the first cavern 4, and the compressor of the assembly 17, which is capable of generating a pres¬ sure at least equal to the gas pressure in the gas sto¬ rage system 19, are started more or less simultaneously. Salt water is sucked from the borehole 7 aftd pumped by the pump of the assembly 10 into the second cavern 5, the gas pressure in the first cavern 4 pushing salt water from this first cavern 4 via the channel 6 towards the borehole 7, while, simultaneously, gas is compressed and pumped from the gas storage system 18 (which may be the earth's atmos- -phere), by the compressor of the assembly 16, into the first cavern 4, and from the second cavern 5, by the compressor of the assembly 17, into the gas storage system 19. The running speed of the compressors of the assemblies 16 and 17, and of the pump of the assembly 10, must be ad - justed such that the desired pressures are maintained in the system.

When the energy demand of the network increases beyond the average load value, the assemblies 10, 16 and 17 are switched over to energy production, the salt water then flowing through the turbine of the assembly 10 (which may be the pump), driving its generator, while, simultaneous¬ ly, ,gas expands through the expander of the assembly 16 from the first cavern 4 into the gas storage system 18 (which may be the earth's atmosphere) , and through the expander of the assembly 17 from the gas storage system 19 to the se¬ cond cavern 5. Again, the running speed of the expanders of the assemblies 16 and 17. and of the turbine of the as¬ sembly 10, must be adjusted such that the (.eslre_! pres¬ sures are maintained in tne system. During pumping and/ or flowing back of the salt water, part of it may be cooled by the cooler(s) 11 to reduce its temperature, which increases due to friction . If necessary, the as¬ semblies 16 and 17 are equipped to periodically supply and/ or withdraw heat from the gas. The compressors of the as- semblies 16 and/or 17 may be used to fill the upper parts of the caverns 4,5 for the first time with gas, and re¬ plenish them from time to time thereafter, if necessary assisted by compressors of lower pressure rating (not shown). The system of fig. 2 is similar to that of fig. 1, with the exception of the position of the first cavern 4, the depth of which having been lowered to below that of the second cavern 5. This raises the gas pressure, required to drive the salt water in the channel 6 and the bore- hole 7 to the ground surface 1, so that the pressure difference across the compressor/expander of the assem¬ bly 16 increases, thereby increasing the energy storage capacity of the system.

If the depth of the first cavern 4 is lowered sufficient- ly, the gas pressure in it, required to drive the saltwa¬ ter in the channel 6 and the borehole 7 to the ground surfa¬ ce 1, rises to a value that equals the gas pressure in the second cavern 5. In that case, it is possible to wash out a connection in the salt formation 3 between the channels 12 and 14, as a result of which gas flows directly from the first cavern 4to the second cavern 5 and vice versa. Such a system is shown in EP-B 0247690 (US-A 4808029). In this system, however, no use can be made of the energy stora¬ ge capacity of the assemblies 16 and 17, with their gas storage systems 18 and 19.

If the depth of the first cavern 4 is lowered even fur¬ ther, the gas pressure required to drive the salt water in the channel 6 and the borehole 7 to the ground surface 1, rises to a value that is higher than the gas pressure in the second cavern 5, which makes it possible to connect the high pressure side of the assembly 17 to the channel 12, 13, whereby the first cavern 4 and the gas storage system 19 unite into one gas storage system 4. This is indica¬ ted in fig. 3, which shows part of the system of fig. 2. Fig. 4 shows the same details of the system of fig. 2 as shown in fig. 3, but now the assembly 16, together with its gas storage system 18, have been moved to the low pressure side of the assembly 17.

Fig. 5 shows the same details of the system as shown in fig. 4, but now the low pressure side of the assembly 16 is connected to the channel 14,15, whereby the se- cond cavern 5 and the gas storage system 18 unite into one gas storage system 5. The assemblies 16 and 17 can then be united into one assembly 20. In this embodiment of the system a closed gas/salt-water circuit of constant volume is created in which the pressures will fluctuate. Fig. 6 shows the system of fig. 5 but now the pressure fluctuations are eliminated by producing part of the salt water into an artificial salt water reservoir 21 at the ground surface 1 during expansion phases of the gas, pre- ferably at the low-pressure side of the pump/turbine 10. During the subsequent compression phases of the gas, this amount of salt water is produced back into the system. For this additional salt water transport, a pump or pump/tur¬ bine 22 is used. In case of a pump/turbine, a otor/gene- rator may be added to allow storage of extra energy in the system.

Additional advantages of the system shown in fig. 6 are that the cooler(s) 11 may be partly or wholly replaced by natural cooling in the salt water reservoir 21. Should natural cooling be excessive, an isolating layer of solid and/or liquid material may be placed on the salt water in this reservoir 21. In addition, the salt water in this reservoir 21 is partly de-gassed during each cycle. Because this reservoir 21 assumes part of the storage task of the first cavern 4, the second cavern 5 must be washed out to a larger volume than that of the first ca¬ vern 4. Alternatively, two shallow second caverns must be created .

In the embodiments shown in figs. 3-6, the running speed of the compressors/expanders and pump/turbines of the various assemblies must also be adjusted such that the desired pressures are maintained in the system. If more than one of the described cavern-pairs (4,5) for energy storage are located together, their respective as- semblies 10, 16, 17, 20 and/or 22, their gas storage sys¬ tems 18 and/or 19, and their salt water reservoirs 21 may be combined into larger units, which reduces costs. Should the turbine of the assembly 10 run too long, gas from the second cavern^" 5 might enter into the first chan¬ nel 6,7,8,9 and the assembly 10. To avoid this, the salt water volume must be larger than the combined volumes of the first cavern 4, the channels 6 and 8 and the boreholes 7 and 9. In that case, salt water will rise in the channel 12 and the borehole 13, so that the tur¬ bine of the assembly 10 will automatically come to a standstill.

If the salt formation 3 is of sufficient thickness, the embodiment shown in fig. 6 is probably the preferred one. In this embodiment, the assemblies 16 and 17, with their gas storage systems 18 and 19, have been replaced by a surface salt water reservoir 21 of limited size w^rith a pump/turbine or pump 22. Moreover, artificial cooling of _the salt water is reduced or eliminated altogether and the salt water is de-gassed automatically. The operation of the embodiments that are sketched in figs, 4-6 is elucidated in the following calculation. The maximum allowable temperature in underground salt caverns is about 343 (70 C) . At higher temperatures they shrink at unacceptable rates. On the other hand, gas temperatures must not decrease too much, since otherwise water vapour in it may freeze and plug and/or damage the installations. This limits the tem¬ perature drop due to gas-expansion to about 60 °C. Adiabatic expansion of e.g. air will then result in a

If the depth of the second cavern 5 is D, m, the allow_¬ able pressure gradient to avoid cratering to the ground surface 1 is 0.018 MPa/m and the gradient of saturated salt water is 0.012 MPa/m, then the gas pressure in the second cavern 5 equals 0.018 D, MPa and the pressure in the salt water at the high-pressure ide of the pump/ turbine 10 equals (0.018 D_j - 0.012 D_j) = 0.006 D. MPa. Neglecting the suction pressure of the pump 10, this is also the pressure drop across the pump/turbine 10. If the depth of the first cavern 4 is D₂ m, the gas pressure in it equals 0,012 D₂ MPa. The pressure drop across the compression/expansion-machine 16 or 20 then equals (0,012 D-, - 0,018 _χ) MPa. These pressure drops will be lower if the pressure grad- ient of the gas is taken into account.

With an allowable pressure drop factor 2 of the gas, it follows that 0,012 D₂/0,018 _j ^ 2, so that D. __≥ 1/3 D Because the pressure drop of the gas must be positive, it also follows that (0.012 D₂ - 0.018 D^^O, so that D. 2/3 D₂.

If the amount of salt water, moved during an expansion

3 phase, equals V and the gas pressure at the high-pressure side must remain constant, the expansion-machine 17 ex¬ pands the same volume of gas. With a pressure drop factor 2 the expanded gasvolume increases to 2 * ( (343 - 60)/343) __

3 * V = 1,65 V m . In this calculation effects of supercom- pressibility have been neglected. Since in the second cavern

5 room is created for only V 3 gas, 0,65 V m3 low-pressure gas of 283 K will have to be further expanded (after heat- ing) by the expansion-machine 16. If salt water is with-

•a drawn, 1.65 V m of it will flow through the turbine 10, of

^■ 3 which 0.65 V m will flow through the turbine 22 to the sal water reservoir 21, the remainder flowing into the cavern 5.

In a closed system of constant volume, as sketched in fig.- 5, the gas pressure at both sides of the expansion- machine 20 will increase, but the pressure drop across it remains the same. This pressure drop is determined by the depth difference between the caverns, the gradient of the salt water and the allowable gradient: (0.012 D₂ - 0.018 D_j) MPa. The pressure at both sides of the pump/turbine 10 in¬ creases by the same amount.

Claims

C A I M S

1. A system for subterranean storage of energy, in which, in a subterranean salt formation (3), two caverns (4,5) are formed by washing by means of water, the lower sides thereof being interconnected by means of a first channel (6,7,8,9), filled with salt water, and the upper sides being connected to the ground surface (1) by means of a second channel (12,13) and a third channel (14,15), from which caverns (4,5), by introducing gas under pres¬ sure through the second channel (12,13) and the third channel (14,15), a part of the salt water has been re¬ moved, in which first channel (6,7,8,9) a pump/turbine (10) is included which is adapted for pumping salt water through the first channel (6,7,8,9) from the first ca¬ vern (4) towards the second cavern (5) when electric energy is supplied, and, on the other hand, producing electric energy by salt water flowing back through the first¹ channel (6,7,8,9) from the second cavern (5) to¬ wards the first cavern (4), the pump/turbine (10) being located at the ground surface (1) and connected, by means of two branches (6,7) and (8,9) of said first channel

(6,7,8,9) to the lower sides of the first cavern (4) and the second cavern (5) respectively, one or more coolers (11) being included in the first channel (6,7,8,9) at the ground surface (1) to cool part of the salt water, whereby the gas pressure in the second cavern (5), minus the hydrostatic pressure of the salt water in the branch (8,9) of the first channel (6,7,8,9) is higher than the gas pressure in the first cavern (4) , minus the hydros¬ tatic pressure of the salt water in the branch (6,7) of the first channel (6,7,8,9), the gas pressure in the first cavern (4) being chosen such that the salt water is driven upward from said first cavern (4) , via the branch (6,7) of the first channel (6,7,8,9), towards the ground Surface (1), the gas pressure in the second cavern (5) being chosen such that no cratering of gas towards the ground surface (1) from said second cavern (5) can occur, and that the top parts of the two branches (6,7) and (8,9) of said first channel (6,7,8,9) and the top parts of the second and third channel (12,13) and (14,15) consist of cased boreholes (7,9,13,15) through the overlying forma¬ tions (2), extending to such depths in the salt formation (3), that no cratering of salt water and/or gas can occur to the ground surface (1), the second channel (12,13) and the third channel (14,15) being connected to assemblies (16) and (17) respectively, each assembly consisting of one or more compressor/expander units and one or more motor/generator units, said assemblies (16) and (17) be¬ ing located at the ground surface (1), the assembly (16) being adapted for compressing and pumping gas through the second channel (12,13) from a gas storage system (18) in¬ to the top part of the first cavern (4) when electric ener- gy is supplied and, on the other hand, producing electric energy by gas flowing back and expanding through the second channel (12,13) from the first cavern (4) towards the gas storage system (18), the assembly (17) being adapted for compressing and pumping gas through the third channel (14, 15) from the top part of the second cavern (5) into a gas storage system (19) when electric energy is supplied and, on the other hand, producing electric energy by gas flo¬ wing back and expanding through the third channel (14,15) from the gas storage system (19) into the top part of the second cavern (5), all channels being chosen so wide that the friction resistance of flowing salt water and gas will be small, the running speed of the pump/turbine of the as¬ sembly (10) and of the compressors/expanders of the assem¬ blies (16) and (17) being adjusted such that the desired pressures are maintained in the system. 2. The system of claim 1, characterised in that the first cavern (4) is located at a greater depth than the second cavern (5) (Fig.

2).

3. The system of claim 2, characterised in that the gas pressure in the second channel (12,13) at the ground surface (1) is higher than that in the third channel (14,15), whereby the high pressure side of the assembly (17) is connected to the second channel (12,13), so that the gas storage system (19) becomes part of the gas- filled part of the first cavern (4), said assembly (17) being adapted for compressing and pumping gas from the second cavern (5) through the third channel (14,15) and the second channel (12,13) to the first cavern (4) when electric energy is supplied and, on the other hand, pro- ducing electric energy by gas flowing back and expanding from the first cavern (4) through the second channel (12,13) and the third channel (14,15) to the second cavern (5) (Fig. 3).

4. The system of claim 3, characterised in that the assembly (16), with its gas storage system (18), is moved to the low pressure side of the assembly (17) (Fig. 4).

5. The system of claim 3, characterised in that the low pressure side of the assembly (16) is connected to the third channel (14,15), so that the gas storage sys- tem (18) becomes part of the gas-filled part of the se¬ cond cavern (5), while the assemblies (16) and (17) are united into a combined assembly (20) (Fir. 5^Λ.

6. The system of any one of claims 1-4, characteri¬ sed in that the gas storage system (18) consists of the Earth's atmosphere or of a natural gas transport pipeline 7. The system of claim 5, characterised in that the first channel (6,7,8,9) is connected to an above-ground salt water reservoir (21), preferably at the low-pressure side of the assembly (10), in order to transport salt water from this first channel (6,

7,8,9) to this salt water reservoir (21) and back by an assembly (22), consisting of a pump/turbine . coupled to a motor/generator, during each expansion/compression cycle of the gas.

8. The system of claim 7, characterised in that the turbine/generator part of the assembly (22) is replaced by one or more adjustable valves or other means to regu¬ late the velocity of the salt water to the salt water reservoir (21) .

9. The system of the claims 7 and 8, characterised in that the cooler(s) (11) partly or wholly consist of the salt water reservoir (21).

10. The system of claim 9, characterised in that an isolation layer of solid and/or liquid material is present on the^" salt water reservoir (21).

11. The system of any one of claims 7-10, character¬ ised in that the second cavern (5) is larger than the first cavern (4).

12. The system of claim 11, characterised in that the second cavern (5) is split up into two smaller caverns in the salt formation (3).

13. The system of any one of claims 1-12, character¬ ised in that the volume of the salt water is larger than the combined volume of the first cavern (4) and the first channel (6,7,8,9).

14. The system of any one of claims 1-13, character¬ ised in that the compressors of the assemblies (16), (17) and/or (20) are used for filling the top part of the ca¬ verns (4,5) for the first time with high pressure gas, and replenishing them thereafter from time to time, if necessary assisted by one or more additional compressors of lower pressure rating.

15. The system of any one of claims 1-14, character¬ ised in that two or more pairs of caverns (4,5) are lo- cated in a subterranean salt formation (3), whereby their respective assemblies (10,16,17,20,22), their gas storage systems (18,19) and/or their salt water reservoirs (21) are combined into larger units.