ENERGY STORAGE AND RECOVERY

Abstract: Underground storage of energy, wherein a low-density fluid is pumped from an upper cavern to a lower cavern filled with a high-density fluid by means of an overground pump/turbine apparatus, as result of which high-density fluid is displaced out of the lower cavern to the upper cavern, and wherein subsequently, in order to retrieve the stored energy, the high-density fluid is allowed to return to the lower cavern displacing the low-density fluid to the upper cavern, which displaced low-density fluid drives the overground pump/turbine apparatus.

Description: ENERGY STORAGE AND RECOVERY The invention relates to a system for the storage and recovery of energy comprising an upper storage space, a lower storage space situated below the upper storage space, a connecting line between the lower part of the upper storage space and the lower part of the lower storage space, and a pump/turbine unit. Such a system is widely known. In the known system the upper storage space consists of a lake at a high location and the lower storage space consists of a lake at a lower location or of an underground cavern. Moreover, in the known system the pump/turbine unit is installed in the connecting line and located at the same height as the lower storage space in order, during normal operation, to be able to pump a liquid from the lower storage space to the upper storage space, or to be driven by the liquid flowing back from the upper storage space to the lower storage space. The pump of such a system is usually driven by an electric motor connected by an electric line to an electric network, and the turbine then drives an electric generator which is also connected by an electric line to an electric network. Such a system enables an excess of electrical energy to be used to pump the liquid to the upper storage space, while a shortage of electrical energy can be supplemented by electrical energy generated by the electric generator driven by the turbine, which is itself driven by liquid flowing back to the lower storage space. Since the pump/turbine unit is installed near the lower storage space, it is necessary in the case of the lower storage space being an underground cavern, in addition to this storage space to make a separate space for the pump/turbine unit and to make a separate shaft through which to run an electric line of sufficient size to carry the electrical energy to the electric motor and from the electric generator. This makes such a system extremely costly. The object of the invention is to overcome this drawback. Therefore, the system for the storage and recovery of energy according to the invention is characterized by an upper storage space at least partly filled with fluid having a relatively low density (the lighter fluid), a lower storage space located below the upper storage space and at least partly filled with fluid having a relatively high density (the heavier fluid), which fluids are hardly or not at all miscible with each other, a connecting line filled with heavier fluid and located between the lower part of the upper storage space and the lower part of the lower storage space, and a pump/turbine unit at approximately the same height as, or above, the upper storage space, the pump/turbine unit being connected to the upper part of the upper storage space and to the upper part of the lower storage space in order, during normal operation, to be able to pulp lighter fluid from the upper storage space to the lower storage space or to be driven by lighter fluid flowing fros the lower storage space to the upper storage space. In an advantageous embodiment of the invention, the lower storage space is an underground space. This can be formed in any hard rock. A particularly suitable rock is rock salt. This has the advantage that the underground space can be formed by dissolving the salt. In the case of very large volumes, it may be more advantageous if the upper storage space is also an underground space. In that case, it is favourable to form both underground storage spaces in rock salt. The invention also relates to a process for the storage and recovery of energy with the aid of the system according to the invention, characterized by the steps of a) pumping lighter fluid from the upper storage space to the lower storage space, which is at least partly filled with heavier fluid, with the aid of the pump/turbine unit, resulting in heavier fluid being displaced out of the lower storage space and flowing via the connecting line to the upper storage space; b) interrupting the pumping of lighter fluid; c) allowing lighter fluid, displaced from out of the lower storage space by heavier fluid flowing back from the upper storage space via the connecting line, to flow back to the upper storage space; d) letting the returning lighter fluid drive the pump/turbine unit; and e) interrupting the return flow of the lighter fluid. It will be appreciated that the words "pump" and "turbine" used in the specification and the claims can also refer to a "compressor" and an "expander" respectively. By way of example, the invention will now be discussed in more detail with reference to the figure in which a partial cross-section of the system is shown schematically. The system for storing and recovering energy comprises an upper underground storage space 1 formed in a salt dome 2 situated beneath an earth layer 3, a lower underground storage space 5 situated in the salt dome 2 below the upper storage space 1, and a connecting line 6 in the form of a partly curved leached-out bore-hole between the lower part of.the upper storage space 1 and the lower part of the lower storage space 5. The system further comprises a pump/turbine unit 8 located at ground level 7 above the upper storage space 1, the pump/turbine unit consisting of a pump 9 connected by a shaft 10 to an electric motor 13, and a turbine 14 connected by a shaft 15 to an electric generator 16. The pump/turbine unit 8 is connected to the upper part of the upper storage space 1 by a first line 17, and to the upper part of the lower storage space by a second line 18. The first line 17 comprises a bore-hole provided with a tube 19, and the second line 18 comprises a bore-hole provided with a tube 20. The electric motor 13 is connected by an electric line (not shown) to an electric network (not shown) in order to draw electrical energy from it, and the electric generator 16 is connected by an electric line (not shown) to the electric network in order to supply electrical energy to it. The upper storage space 1 is at least partly filled with a fluid having a relatively low density (the lighter fluid), viz. brine 22 having a density of 1200 kg/m3, and the lower storage space 5 with a fluid having a relatively high density (the heavier fluid), viz. drilling mud 23 having a density of 1850 kg/m3. The lighter fluid is also to be found in the first line 17, the pump/turbine unit 8, the second line 18 and the upper part of the lower storage space 5; and the heavier fluid is also to be found in the connecting line 6 and in the lower part of the upper storage space 1. The numeral 24 refers to the interface between the two fluids in the upper storage space 1 and the numeral 25 to the interface between the two fluids in the lower storage space 5. The inlet of the pump 9 is connected to the first line 17 by means of a supply pipe 26 provided with a shut-off valve 27, and the outlet of the pump 9 is connected to the second line 18 by a discharge pipe 28 provided with a shut-off valve 29. The inlet of the turbine 14 is connected to the second line 18 by means of a supply pipe 30 provided with a shut-off valve 31, and the outlet of the turbine 14 is connected to the first line 17 by a discharge pipe 32 provided with a shut-off valve 33. In the stationary situation shown in the figure, the shut-off valves 27, 29, 31 and 33 are closed. If there is an excess of electrical energy, this is fed to the electric motor 13 to drive the pump 9, and the shut-off valves 27 and 29 are opened so that the lighter fluid is pumped from the upper storage space 1 to the lower storage space 5 via the first line 17, the pump 9 and the second line 18. This results in the heavier fluid being displaced out of the lower storage space 5 and flowing via the connecting line 6 to the upper storage space 1. This will cause the interface 24 to rise. When the quantity of heavier fluid in the upper storage space 1 is sufficiently large, the pumping of the lighter fluid will be interrupted by stopping the pump 9 and closing the shut-off valves 27 and 29. When it is necessary to recover the stored energy, for example in the case of a large demand for electrical energy, just the shut-off valves 31 and 33 will be opened. Lighter fluid, driven by heavier fluid, will flow back from the lower storage space 5 to the upper storage space 1 via the second line 18, the turbine 14 and the first line 17, and heavier fluid will flow back from the upper storage space 1 via the connecting line 6 to the lower storage space 5. The returning lighter fluid causes the turbine 14 to move, which drives the electric generator 16, and this in turn generates electrical energy which can be fed into the electric network. When sufficient heavier fluid has flowed out of the upper storage space 1, the return flow of the lighter fluid is interrupted by closing the shut-off valves 31 and 33. The system will then again be in its stationary situation. When interrupting the return flow of the lighter fluid, the fluid pressure near the shut-off valve 31 and the height difference between the interfaces 24 and 25 must be chosen such that the fluid pressure near the shut-off valve 33 remains slightly above atmospheric pressure, so that when lighter fluid is pumped out of the upper storage space 1, the supply of lighter fluid to the pump remains assured. It is subsequently possible to repeat the process of storing energy and then recovering it according to the above-described process. To prevent lumps of salt from the roofs of the storage spaces 1 and 5 blocking the connecting line 6, the system is further provided with catch-pits 34 and 35. In the system described above, the pump and turbine are separate, but the pump/turbine unit can also consist of a hydraulic machine which can function both as pump (or compressor) and as turbine, which hydraulic machine is connected to an electric machine which can function both as motor and generator. To prevent the lighter fluid from getting too hot during the operation of the system, this can be provided with a heat exchanger (not shown) to cool the lighter fluid. The upper storage space can also be situated at ground level, for example in the form of an above-ground storage tank or a lake. In that case, the above-ground pump/turbine unit will be at about the same height as the upper storage space. The lighter fluid can also be a liquid other than brine, for example liquefied petroleum gas or oil. Water can also be used, especially if the underground part of the system is situated in a rock which is not affected by water. A very great difference between the density of the lighter fluid and that of the heavier fluid is achieved if the lighter fluid is a gas, such as nitrogen or carbon dioxide. The heavier fluid is preferably a liquid, such as brine, drilling mud or a dispersion of sand or fly-ash in brine. In an advantageous embodiment of the invention, the fluids are hardly or not at all soluble in each other. However, to prevent mixing, intermediate layers can also be introduced between the fluids at the interfaces 24 and 25. The density of the material of the intermediate layer is higher than the density of the lighter fluid and lower than the density of the lighter fluid, and the material should be such that the material and each of the fluids are hardly or not at all soluble in each other. A suitable material is freon: the trade name for a group of substances consisting of ethane or methane in which one or more hydrogen atoms are replaced by fluorine or chlorine. Another suitable material is a mixture of diesel oil and freon, or a solution of polyvinyl chloride in a suitable solvent, for example tetrahydrofurane or methyl chloride. The material of the intermediate layer can also be a slurry, for example particles of polyvinyl chloride in diesel oil, or haematite in diesel oil with oleic acid as surfactant to cause the haematite particles to be wetted. The intermediate layer can also consist of a layer of polyvinyl chloride particles. A suitable system for storing and recovering 100 MW of electrical power would have underground storage spaces of 5 about 5 x 105 m9 to 6 x 105 m3, the height difference between the storage spaces being about 1000 m and the density of the lighter fluid being 1200 kg/m3 and that of the heavier fluid being 1850 kg/m3. In order to reduce hydraulic losses, the areas of the passageways of the connecting line and of the first and second line should be as large as possible, at least 3 m2. Instead of an electric motor 13 to drive the pump 9, the system can also comprise a turbine (not shown) to drive the pump 9. The turbine is itself driven be a compressed gas, for example natural gas. As already discussed in the foregoing, in an advantageous embodiment of the invention a gas such as air and a liquid such as brine may be used as the lighter fluid and heavier fluid respectively. When using an expander, it appears that the volume of the air flowing from the lower storage space to the upper storage space has considerably increased at the low pressure outlet of the expander. Subsequently, part of this expanded air can be used in a conventional compressed air energy storage unit located at the earth's surface. Another advantageous possibility is to store (part of) the expanded air in a second set of underground storage spaces. In this manner, a compensation of gas expansion is provided.

Claims: CLAIMS 1. A system for the storage and recovery of energy characterized by an upper storage space at least partly filled with fluid having a relatively low density (the lighter fluid), a lower storage space located below the upper storage space and at least partly filled with fluid having a relatively high density (the heavier fluid), which fluids are hardly or not at all miscible with each other, a connecting line filled with heavier fluid and located between the lower part of the upper storage space and the lower part of the lower dtorage space, and a pump/turbine unit at approximately the same height as, or above, the upper storage space, the pump/turbine unit being connected to the upper part of the upper storage space and to the upper part of the lower storage space in order, during normal operation, to be able to pump lighter fluid from the upper storage space to the lower storage space or to be driven by lighter fluid flowing from the lower storage space to the upper storage space. 2. The system as claimed in claim 1, characterized in that the lower storage space is an underground space. 3. The system as claimed in claim 2, characterized in that the upper storage space is an underground space. 4. The system as claimed in claim 2 or 3, characterized in that the pump/turbine unit is situated above the upper storage space. 5. The system as claimed in any one of claims 1-3, characterized in that the pump/turbine unit is located at or near ground level. 6. The system as claimed in any one of claims 1-5, characterized in that the lighter fluid is a gas, for example carbon dioxide or nitrogen. 7. The system as claimed in any one of claims 1-5, characterized in that the lighter fluid is a liquid, for example liquefied petroleum gas, oil, water or brine. 8. The system as claimed in any one of claims 1-7, characterized in that the heavier fluid is a liquid, for example brine, drilling mud, or a dispersion of sand or fly-ash in brine. 9. The system as claimed in any one of claims 1-8, characterized in that the fluids are separated from each other by an intermediate layer of a material with a density lying between the densities of the fluids in which material the fluids can hardly or not at all dissolve. 10. The system as claimed in claim 9, characterized in that the material of the intermediate layer is a liquid, for example freon, a mixture of diesel oil and freon, or a solution of polyvinyl chloride in a suitable solvent, for example tetrahydrofurane or methyl chloride. 11. The system as claimed in claim 9, -characterized in that the material of the intermediate layer is a slurry, for example particles of polyvinyl chloride in diesel oil, or haematite in diesel oil with oleic acid. 12. The system as claimed in claim 9, characterized in that the intermediate layer consists of particles of polyvinyl chloride. 13. A method for the storage and recovery of energy with the aid of the system as claimed in any one of claims 1-12, characterized by the steps of a) pumping lighter fluid from the upper storage space to the lower storage space, which is at least partly filled with heavier fluid, with the aid of the pump/turbine unit, resulting in heavier fluid being displaced out of the lower storage space and flowing via the connecting line to the upper storage space; b) interrupting the pumping of lighter fluid; c) allowing lighter fluid, displaced from out of the lower storage space by heavier fluid flowing back from the upper storage space via the connecting line, to flow back to the upper storage space; d) letting the returning lighter fluid drive the pump/turbine unit; and e) interrupting the return flow of the lighter fluid.

Inventors: HOLSCHER, HUGO, QUIST, BERNARDUS BASTIAAN, VERSCHUUR, EKE
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1986
Main classification: F03B17
IPC codes: F01K27, F02C6, F03B17, F03G7, F03B13