ENERGY STORAGE SYSTEM

Abstract

An energy storage system includes a vessel, an energy conversion device, and a connection line connecting the vessel with the energy conversion device. The energy conversion device includes a housing, a pump turbine, and a motor generator. The pump turbine includes a first shaft, and an impeller. The motor generator includes a second shaft, and a rotor. The second shaft is coupled to the first shaft to transmit torque between the first shaft and the second shaft. The connection line connects a low pressure opening with an opening disposed at the vessel to receive water from the vessel or discharge water into the vessel. The connection line includes a switching unit with a shut-off device and a non-return device connected in parallel, the non-return device enabling flow of the water only in a first direction from the vessel to the low pressure opening.

Claims

1. An energy storage system configured for installation at an underwater location, comprising: a vessel configured to store water at a low pressure; an energy conversion device configured to selectively convert between potential energy and electric energy; and a connection line connecting the vessel with the energy conversion device, the energy conversion device comprising a housing, a pump turbine unit arranged in the housing, and a motor generator unit, the housing comprising a low pressure opening configured to receive water at the low pressure, and a high pressure opening configured to discharge water at a high pressure, the pump turbine unit comprising a first shaft configured to rotate about an axial direction, and at least one impeller mounted on the first shaft configured to interact with the water, the motor generator unit comprising a second shaft configured to rotate about the axial direction, and a rotor disposed at the second shaft and configured to rotate relative to a stator, the second shaft coupled to the first shaft to transmit a torque between the first shaft and the second shaft, and the connection line configured to connect the low pressure opening with an opening disposed at the vessel to receive water from the vessel or discharge water into the vessel, the connection line comprising a switching unit with a shut-off device and a non-return device connected in parallel, and the non-return device configured to enable a flow of the water only in a first direction from the vessel to the low pressure opening.

2. The energy storage system in accordance with claim 1, wherein the low pressure opening is located at a greater depth than the opening.

3. The energy storage system in accordance with claim 1, wherein the shut-off device is a control valve.

4. The energy storage system in accordance claim 1, wherein the energy conversion device is a multistage pump comprising the housing, the pump turbine unit and the motor generator unit are arranged within the housing, the first shaft extends from a drive end to a non-drive end, and the drive end is coupled to the second shaft.

5. The energy storage system in accordance with claim 4, further comprising a mechanical seal configured to seal the pump turbine unit at the first shaft adjacent to the drive end, the mechanical seal having a process side facing the pump turbine unit, the process side being in fluid communication with the high pressure opening, so that a pressure prevailing at the process side is at least approximately the same as the pressure at the high pressure opening.

6. The energy storage system in accordance with claim 4, further comprising a balance drum fixedly connected to the first shaft adjacent to the non-drive end, the balance drum defining a front side facing the pump turbine unit and a back side, a relief passage disposed between the balance drum and a stationary part configured to be stationary with respect to the housing, the relief passage extending from the front side to the back side, and a balance line configured to recirculate pressurized water to the back side.

7. The energy storage system in accordance with claim 6, wherein the front side is in fluid communication with the low pressure opening, so that a pressure prevailing at the front side is at least approximately the same as the pressure at the low pressure opening.

8. The energy storage system in accordance with claim 4, wherein the first shaft is radially supported in a non-contacting manner during operation, the pump turbine unit comprises exactly one hydrodynamic radial bearing to support the first shaft, and the radial bearing is arranged at the drive end of the first shaft.

9. The energy storage system in accordance with claim 5, wherein the mechanical seal arranged adjacent to the drive end is the sole mechanical seal to seal the pump turbine unit at the first shaft.

10. The energy storage system in accordance with claim 1, wherein the pump turbine unit comprises a first stage impeller and a last stage impeller, and the first stage impeller is a double suction impeller.

11. The energy storage system in accordance with claim 4, wherein the multistage pump is a vertical pump with the first shaft extending in a direction of gravity, and the motor generator unit is arranged on top of the pump turbine unit.

12. The energy storage system in accordance with claim 1, wherein the motor generator unit is a liquid filled motor generator unit, and a barrier fluid can be supplied to the motor generator unit at a pressure that is at least as high as a pressure prevailing at the process side of the mechanical seal.

13. A method of operating an energy storage system configured for installation at an underwater location, the method comprising: providing an energy storage system comprising a vessel to store water at a low pressure, an energy conversion device to selectively convert between potential energy and electric energy, and a connection line connecting the vessel with the energy conversion device, the energy conversion device comprising a housing, a pump turbine unit arranged in the housing, and a motor generator unit, the housing comprising a low pressure opening to receive water at the low pressure, and a high pressure opening to discharge water at a high pressure, the pump turbine unit comprising a first shaft to rotate about an axial direction, the motor generator unit comprising a second shaft to rotate about the axial direction, and the second shaft coupled to the first shaft to transmit a torque between the first shaft and the second shaft, and selectively operating the energy storage system in a pump mode or in a turbine mode, in the pump mode the pump turbine unit operated to discharge the water from the vessel through the high pressure opening, and in the turbine mode the water enters the housing through the high pressure opening, drives the rotation of the first shaft and is discharged through the low pressure opening to the vessel, the pump mode started by operating the pump turbine unit with a zero flow until the rotational speed of the first shaft is sufficient to generate a positive flow from the low pressure opening to the high pressure opening.

14. The method in accordance with claim 13, further comprising using a check valve to end the operating of the pump turbine unit with the zero flow.

15. The method in accordance with claim 13, wherein the turbine mode is started by opening a control valve provided in the connection line.

16. The energy storage system in accordance with claim 1, wherein the pump turbine unit comprises a first stage impeller, a last stage impeller, and at least one intermediate stage impeller, and the first stage impeller is a double suction impeller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Aspects of the invention will be explained in more detail hereinafter with reference to embodiments and with reference to the drawings.

[0061] FIG. 1 is a schematic representation of an embodiment of an energy storage system according to the disclosure,

[0062] FIG. 2 is a schematic cross-sectional view of an embodiment of the energy conversion device of an energy storage system according to the disclosure,

[0063] FIG. 3 is a diagram showing the rotational speed versus the flow during the starting of the pump mode, and

[0064] FIG. 4 is a diagram showing the rotational speed versus the flow during the starting of the turbine mode.

DETAILED DESCRIPTION

[0065] FIG. 1 shows a schematic representation of an embodiment of an energy storage system according to the disclosure. The energy storage system is designated in its entity with reference numeral 1 and is configured for an installation at an underwater location 200 below a water surface 100. The underwater location 200 can be for example a subsea location on the sea ground or a location at the ground of a deep lake. In the following description the term “depth” designates the vertical distance from the water surface 100. The underwater location 200 is preferably at a depth of at least 400 m. The underwater location 200 can be at a considerably greater depth, for example at a depth of at least 1000 m or at least 2000 m.

[0066] In the following description the terms “high”, “higher”, “low”, “lower” and the like refer to the respective depth. With respect to a location, for example the location of a component, the term “lower” means “at a greater depth” and the term “higher” means “at a smaller depth”. Thus, the relative designations “higher” and “lower” refer to the vertical distance from the water surface 100. A higher location is closer to the water surface 100 or higher as measured from the bottom of the lake or the sea or the water body than a lower location.

[0067] The energy storage system 1 comprises a vessel 2 arranged at the underwater location 200. The vessel 2 for storing water W is secured for example to the sea ground. The vessel 2 is configured to withstand the pressure prevailing at the underwater location 200. The vessel 2 is for example made of concrete and can be configured as a hollow sphere or as a hollow cylinder for receiving water W. If the vessel 2 is configured as a hollow sphere, its inner diameter is preferably at least 30 m, but can also be considerably larger, for example 100m or even more. The vessel 2 can also be configured as a cylindrical pipe or in another cylindrical shape. Particularly preferred, the vessel 2 has a volume of at least 100,000 m.sup.3. The energy storage system 1 can also comprise a plurality of vessels 2 arranged at the underwater location 200. It goes without saying that each vessel 2 is configured to withstand the hydrostatic pressure of the water prevailing at the underwater location 200. Since it is sufficient for the understanding of the disclosure, in the following description reference is made to only one vessel 2. However, as already the, the energy storage system 1 can also comprise a plurality of vessels 2.

[0068] The energy storage system 1 further comprises an energy conversion device 3 which is also arranged at the underwater location 200 in the proximity of the vessel 2 or at the vessel 2. A connection line 4 connects the vessel 2 with the energy conversion device 3, The energy conversion device 3 is configured for selectively converting potential energy in electric energy or electric energy in potential energy. The energy conversion device 3 comprises a pump turbine unit 5, a motor generator unit 6 and a housing 31, wherein the pump turbine unit 5 is arranged in the housing 31. It is possible to provide a separate housing for the motor generator unit, however it is preferred, as shown in FIG. 1, that the housing 31 is configured as a common housing 31, in which both the pump turbine unit 5 and the motor turbine unit 6 are arranged. The housing 31 is configured to withstand the pressure prevailing at the underwater location 200 as well as the pressure generated by the pump turbine unit 5.

[0069] The pump turbine unit 5 is operable in a pump mode for pumping water W out of the vessel 2 and in a turbine mode for being driven by the water W discharged into the vessel 2 from the environment at the underwater location 200. The motor generator unit 6 is operable in a motor mode for driving the pump turbine unit 5, when the pump turbine unit 5 is operated in the pump mode. Furthermore, the motor generator unit 6 is operable in a generator mode for generating electric energy, when the pump turbine unit 5 operates in the turbine mode and drives the motor generator unit 6.

[0070] The housing 31 has a low pressure opening 52 and a high pressure opening 53 for the water W, a first shaft 54 for rotating about an axial direction A, and at least one impeller 55, preferably a plurality of impellers 55, mounted on the first shaft 54 for interacting with the water. The axial direction A is defined by the longitudinal axis of the first shaft 54. The high pressure opening 53 is located at a depth D. Thus, the hydrostatic pressure of the water at the depth D is essentially the discharge pressure prevailing at the high pressure opening 53, against which the pump turbine unit 5 has to pump the water W out of the vessel 2 in the pump mode.

[0071] The pressure prevailing at the low pressure opening is referred to as “low pressure” and the pressure prevailing at the high pressure opening 53 is referred to as “high pressure”. The low pressure constitutes the suction pressure for the pump turbine unit 5, when operating in the pump mode. The low pressure is given by the water level of the water W in the vessel 2. In addition, the vertical distance between the lower end of the vessel 2 and the low pressure opening 52 contributes to the suction pressure. When, for example, the vessel 2 is a sphere with an inner diameter of 30 m, the maximum value of the low pressure is approximately three bar (when the vessel 2 is completely filled) plus the hydrostatic pressure of the water in the connection line 4 above the low pressure opening 52.

[0072] Optionally, the high pressure opening 53 can be provided with a first shut-off valve 531 for opening and closing the flow passage through the high pressure opening 53. When the first shut-off valve 531 is in the open position, the high pressure opening 53 is open and the water can pass through the high pressure opening 53. When the first shut-off valve 531 is In the closed position, the high pressure opening 53 is closed and the water cannot pass through the high pressure opening 53, i.e. the water can neither flow into the housing 31 nor can the water leave the housing 31 through the high pressure opening 53. Optionally, a further shut-off valve (not shown) can be provided at the low pressure opening 52 for opening and closing the flow passage through the low pressure opening 52. The further shut-off valve at the low pressure opening 52 can be provided in addition to or instead of the first shut-off valve 531.

[0073] In the pump mode the first shaft 54 is driven by the motor generator unit 6 and the impeller(s) 55 covey(s) the water W from the vessel 2 through the low pressure opening 52 to the high pressure opening 53, where the water W is discharged to the environment. In the turbine mode the water enters the housing 31 from the environment through the high pressure opening 53, drives the impeller(s) 55 and is discharged through the low pressure opening 52 into the vessel 2.

[0074] The motor generator unit 6 arranged in the housing 31 comprises a second shaft 62 for rotating about the axial direction A and a rotor 63 fixed to the second shaft 62 for rotating relative to a stator 64, which is arranged stationary with respect to the housing 31. The second shaft 62 is coupled to the first shaft 54 by a coupling 65 for transmitting a torque between the first shaft 54 and the second shaft 62. An electric power line 66 is provided, which connects the motor generator unit 6 with an energy unit 67 located at location 300 at or above the water surface 100 for example on a platform. The energy unit 67 can be connected to a grid. In the motor mode the motor generator unit 6 receives electric energy from the energy unit 67 through the electric power line 66. The electric energy is used to rotate the rotor 63 and the second shaft 62 relative to the stator 64. The second shaft 62 drives the rotation of the first shaft 54, so that the pump turbine unit 5 is operated in the pump mode. In the generator mode the second shaft 62 is driven by the first shaft 54 and the rotation of the rotor 63 relative to the stator 64 generates electric energy which is delivered to the energy unit 67 by the electric power line 66.

[0075] The electric power line 66 can be integrated into an umbilical line 60 connecting the underwater location 200 with the location 300 at or above the water surface 100 for example on a platform. Beside the exchange of electric energy through the electric power line 66, the umbilical line 60 can be used to supply operating materials, e.g. a barrier fluid for the motor generator unit 6, from the location 300 to the underwater location 200, or to discharge material from the underwater location 200 to the location 300 at or above the water surface 100. As an example, FIG. 1 shows a barrier fluid reservoir 69 at the location 300, from where a barrier fluid for the motor generator unit 6 is supplied through the umbilical line 60 to the underwater location 200.

[0076] Preferably, the pump turbine unit 5 is configured as a vertical pump turbine unit 5, meaning that during operation the first shaft 54 is extending in the vertical direction, which is the direction of gravity. Thus, the axial direction A coincides with the vertical direction.

[0077] Furthermore, the energy storage system 1 comprises the connection line 4 that is configured to connect the low pressure opening 52 of the pump turbine unit 5 with an opening 21 provided at the vessel 2. The opening 21 is preferably arranged at the bottom of the vessel 2 or at a location of the vessel 2 being arranged at the greatest depth of the vessel 2. In the pump mode the connection line 4 receives water W from the vessel 2 by the pumping action of the pump turbine unit 5. In the turbine mode the water W leaves the housing 31 through the low pressure opening 52 and is discharged through the connection line 4 and the opening 21 into the vessel 2.

[0078] The connection line 4 includes a switching unit 8 for opening and closing the flow passage between the vessel 2 and the low pressure opening 52 through the connection line 4. The switching unit 8 is arranged in the connection line 4 and has a first fluid opening 81 as well as a second fluid opening 82 for receiving and discharging the water. The first fluid opening 81 is in fluid communication with the opening 21 of the vessel 2, and the second fluid opening 82 is in fluid communication with the low pressure opening 52 of the pump turbine unit 5. The first fluid opening 81 and the second fluid opening 82 are connected to each other by two branches, namely a first branch 83 and a second branch 84. The two branches 83, 84 are arranged in parallel.

[0079] The first branch 83 comprises a non-return device 85, for example a check valve or a non-return valve. The non-return device 85 is configured to allow a flow of the water only in a first direction, namely in the direction from the first opening 81 to the second opening 82. Thus, the water can flow through the first branch 83 only from the vessel 2 to the low pressure opening 52. The non-return device 85 blocks a flow of water through the first branch 83 in a second direction, which is opposite to the first direction, namely from the low pressure opening 52 towards the vessel 2. The non-return device 85 has an opening pressure which is very small. Preferably, the opening pressure of the non-return device 85 is as small as possible, so that the non-return device 85 opens as soon as the pressure prevailing at the first fluid opening 81 becomes larger than the pressure at the second fluid opening 82. As it is common in the art, the opening pressure of the non-return device 85 denotes the minimum pressure difference across the non-return device 85, which is required to open the non-return device 85 to allow a flow of the water in the first direction. This opening pressure is preferably as small as possible.

[0080] The second branch 84 comprises a shut-off device 86 for opening and closing the fluid passage through the second branch 84. The shut-off device 86 has a closed position, in which the shut-off device 86 closes the flow passage through the second branch 84, and an open position, in which the shut-off device 86 allows a flow of water passing through the second branch 84.

[0081] Preferably, the shut-off device 86 is configured as a control valve 86, allowing to regulate the flow between the closed position (no flow) and the open position (maximum flow). Preferably, the control valve 86 is configured for continuously adjusting the flow between the closed position and the open position.

[0082] In embodiments of the energy storage system 1 comprising more than one vessel 2 it is possible to provide a separate connection line 4 with a separate switching unit 8 for each vessel 2. Thus, the energy conversion device 3 can be selectively connected with each of the vessels 2. It is also possible to provide a common connection line 4 with a single switching unit 8 and to connect each of the vessels 2 to the common connection line 4. In this embodiment for each vessel 2 an additional shut-off valve is provided to selectively open or closed the flow connection between the respective vessel 2 and the common connection line 4.

[0083] Optionally, the vessel 2 comprises a vent 22 extending from the vessel 2 to a location at or above the water surface 100. By the vent 22 the pressure prevailing in the interior of the vessel 2 above the water W is essentially the same as the atmospheric pressure at the water surface 100, meaning that the water W in the vessel 2 is exposed to the ambient pressure prevailing at the water surface 100.

[0084] Furthermore, the vessel 2 can comprise a controller (not shown in detail) for ensuring that the water level in the vessel 2 will not exceed a maximum level M. The controller can comprise a sensor (not shown) for checking the fill level of the vessel. During turbine mode the vessel 2 is filled with water W. As soon as it is detected that the vessel 2 is filled to the maximum level M, the controller will prevent a further flow of water W into the vessel 2, e.g. by closing the flow passage through the switching unit 8.

[0085] In addition, the controller or an additional controller preferably ensures that the water level in the vessel 2 will not fall below a minimum level L. During pump mode the vessel 2 is emptied until the level of the water W in the vessel 2 reaches the minimum level L. As soon as it is detected that the water level has fallen to the minimum level L, the controller will prevent a further flow of water W out of the vessel 2, e.g. by switching-off the pump-turbine unit 5 or by closing the first shut-off valve 531.

[0086] Preferably, the low pressure opening 52 is located at a greater depth I than the opening 21. Thus, when the vessel 2 is emptied to the minimum level L during the pump mode, there is always a sufficiently large suction pressure prevailing at the low pressure opening 52. The minimum suction pressure at the low pressure opening 52 is given by the difference X between the minimum level L and the depth I, at which the low pressure opening 52 is located. Thus, the difference between the depth at which the minimum level is located and the depth I at which the low pressure opening 52 is located, determines the minimum suction pressure during the pump mode.

[0087] The operation of the energy storage system 1 will now be described. With exemplary character it is assumed that the vessel 2 is filled with water W up to the maximum level M. To “charge” the energy storage system, the motor generator unit 6 is operated in the motor mode and the pump turbine unit 5 is operated in the pump mode. The starting of the pump mode will be described in detail below. The motor generator unit 6 receives electric energy from the energy unit 67 through the electric power line 66 and drives the first shaft 54 with the impeller(s) 55. The hydrostatic pressure of the water W in the vessel 2 and the connecting line 4 generates the low pressure, i.e. the suction pressure prevailing at the low pressure opening 52. The hydrostatic pressure of the water at the underwater location 200 generates the high pressure, i.e. the discharge pressure prevailing at the high pressure opening 53. The pump turbine unit 5 conveys the water W from the low pressure opening 52 to the high pressure opening 53, where the water is discharged to the environment at the underwater location 200. As soon as the vessel 2 is emptied to the predefined minimum level L the pump mode is terminated for example by closing the fluid passage through the switching unit 8. The energy storage system 1 is “charged”.

[0088] For recovering electric energy from potential energy the energy storage system 1 is “discharged”. For this purpose the pump turbine unit 5 is operated in the turbine mode and the motor generator unit 6 is operated in the generator mode. For starting the turbine mode the fluid passage through the switching unit 8 is opened, for example by switching the shut-off device 86 to the open position. The hydrostatic pressure prevailing at the underwater location 200 at the depth D causes the water to flow through the high pressure opening 53 and to drive the impeller(s) 55 of the pump turbine unit 5. The water W is discharged through the low pressure opening 52 into the connecting line 4 and starts to fill the vessel 2. The first shaft 54 of the pump turbine unit 5 drives the second shaft 62 of the motor generator unit 6 and therewith causes the rotor 63 to rotate relative to the stator 64. By the rotation of the rotor 63 electric energy is generated, which is supplied through the electric power line 66 to the energy unit 67. The energy unit 67 can, for example, feed the electric energy to a grid or to a transmission line. As soon as the vessel 2 is filled, for example filled to the maximum level M, the turbine mode and therewith the generator mode is terminated for example by closing the first shut-off valve 531 and/or the fluid passage through the switching unit 8. The energy storage system 1 is “discharged”.

[0089] Reference is also made to the European patent application No. 21216018.8 of the same applicant, were various embodiments and configurations for an energy storage system are disclosed.

[0090] Referring now to FIG. 2, an embodiment of the energy conversion device 3 of the energy storage device 1 according to the disclosure will be described in more detail. FIG. 2 shows a schematic cross-sectional view of the embodiment of the energy conversion device 3.

[0091] It goes without saying that the embodiment shown in FIG. 2 is an example, only. The disclosure is not restricted to this configuration of the pump turbine unit 5 and the motor generator unit 6, respectively.

[0092] Basically, each centrifugal pump that can also be operated in a reverse direction, i.e. in a turbine mode, for driving the second shaft 62 of the motor generator unit 6 during operation in the generator mode, is suited as pump turbine unit 5 for the energy conversion device 3. The pump turbine unit 5 has to be configured such that it can withstand the environmental conditions at the underwater location 200. Furthermore, when operating in the pump mode the pump turbine unit 5 has to be strong enough to empty the vessel 2 against the hydrostatic pressure of the water prevailing at the underwater location 200, more particular at the high pressure opening 53.

[0093] Preferably, the pump-turbine unit 5 is configured as a multistage pump having a plurality of impellers 55 which are all mounted on the first shaft 54 in a torque proof manner. The pump turbine unit 5 can be configured, for example, in an analogous manner as it is known from water injection pumps at subsea locations in the oil and gas processing industry.

[0094] FIG. 2 shows the embodiment of the energy conversion device 3 comprising the motor generator unit 6 and the pump turbine unit 5, both arranged in the housing 31. The pump turbine unit 5 can be configured as a process fluid lubricated pump turbine unit 5. The term “process fluid lubricated pump turbine unit” refers to pumps or pump turbine units, where the process fluid, that is conveyed by the pump 1, here namely water, is used for the lubrication and the cooling of components of the pump turbine unit 5, e.g. the bearings. The process fluid lubricated pump turbine unit 5 does not require a lubricant different from the process fluid for the lubrication of the pump turbine unit components. The process fluid is the sole lubricant used in the pump. Regarding the energy storage device 1 the process fluid is water, for example fresh water, when the underwater location 200 is in a deep lake, or seawater, when the underwater location 200 is a subsea location. The term seawater comprises raw seawater, purified seawater, pretreated seawater, filtered seawater and so on.

[0095] In the embodiment shown in FIG. 2 the pump turbine unit 5 is not configured as a process fluid lubricated pump turbine unit 5, but the bearings are lubricated by the barrier fluid supplied to the motor generator unit 6.

[0096] The housing 31 surrounds the pump turbine unit 5 and the motor generator unit 6. It is also possible that the housing 31 is configured as a barrel housing 31, in which the pump turbine unit 5 and the motor generator unit 6 are inserted. The housing 31 of the pump turbine unit 5 and the motor generator unit 6 comprises the low pressure opening 52, which is the inlet during pump mode, and the high pressure opening 53, which is the outlet during pump mode. The low pressure i.e. the pressure of the water at the low pressure opening 52 during pump mode is referred to as suction pressure. The high pressure of the water, i.e. the pressure at the high pressure opening 53 during pump mode is referred to as discharge pressure. The discharge pressure is given by the hydrostatic pressure of the water prevailing in the environment of the high pressure opening 53.

[0097] The pump turbine unit 5 comprises the first shaft 54 extending from a drive end 541 to a non-drive end 542 of the first shaft 54. The first shaft 54 is configured for rotating about the axial direction A, which is defined by the longitudinal axis of the first shaft 54. The drive end 541 of the first shaft 54 is connected to the coupling 65 that is arranged between the pump turbine unit 5 and the motor generator unit 6.

[0098] The motor generator unit 6 comprises the second shaft 62 that is configured for rotating about the axial direction A. The second shaft 62 is connected to the coupling 65. During pump mode the second shaft 62 drives the first shaft 54. During turbine mode the first shaft 54 drives the second shaft 62.

[0099] The coupling 65 is configured for transferring a torque between the first shaft 54 and the second shaft 62. Preferably the coupling 65 is configured as a flexible coupling 65, which connects the second shaft 62 to the first shaft 54 in a torque proof manner, but allows for a relative movement between the second shaft 62 and the first shaft 54, e.g. lateral movements. Thus, the coupling 65 transfers the torque but no or nearly no lateral vibrations. The flexible coupling 65 can be configured as a mechanical coupling, a magnetic coupling, a hydrodynamic coupling or any other coupling that is suited to transfer a torque between the second shaft 62 to the first shaft 54.

[0100] The pump turbine unit 5 comprises a plurality of impellers 55. The plurality of impellers comprises at least a first stage impeller 55a fixedly mounted on the first shaft 54 as well as a last stage impeller 55b fixedly mounted on the first shaft 54. The first stage impeller 55a is the impeller 55a next to the low pressure opening 52 and the last stage impeller 55b is the impeller pressurizing the water to the discharge pressure during pump mode. Optionally, the pump turbine unit 5 further comprises one or more intermediate stage impeller(s) 55. Each intermediate stage impeller 55 is arranged between the first stage impeller 55a and the last stage impeller 55b when viewed in the direction of increasing pressure during pump mode, i.e. the direction of the main fluid flow through the pump turbine unit 5 during pump mode. In the embodiment shown in FIG. 2 three intermediate stage impellers 55 are provided, i.e. the pump turbine unit 5 is configured as an five stage pump. It goes without saying that the number of five stages is only exemplary. The pump turbine unit 5 can be designed also as a multistage pump having more or less than five stages.

[0101] The first stage impeller 55a is configured as a double suction impeller. All intermediate stage impellers 55 and the last stage impeller 55b are configured as single suction impellers 55, Configuring the first stage impeller 55a as a double suction impeller has the advantage that the required NPSH (net positive suction head) for the first stage is considerably lower as compared to a single suction design of the first stage impeller. Therewith, the risk of cavitation is strongly reduced. As it is known in the art, a double suction impeller is an impeller having two suction sides. Referring to the representation in FIG. 2, the fluid flows against the first stage impeller 55a both from the axially upper side and from the axially lower side of the first stage impeller 55a.

[0102] The pump turbine unit 5 is designed with an inline arrangement of all impellers 55a, 55b. In an inline arrangement all impellers are arranged one after another on the first shaft 54 in such a manner that the axial thrust generated by the action of the rotating impellers 55a, 55b has the same direction for each particular impeller 55a, 55, 55b. In addition, the main flow of the fluid from the low pressure opening 52 towards the high pressure opening 53 is always directed in the same direction, namely in upward direction according to the representation in FIG. 2.

[0103] In other embodiments (not shown) the impellers 55a, 55, 55b are arranged in a back-to-back arrangement. The pump turbine unit 5 comprises then a first set of impellers 55a, 55 and a second set of impellers 55, 55b wherein the first set of impellers 55a, 55 and the second set of impellers 55, 55b are arranged on the first shaft 54 such, that the axial thrust generated by the first set of impellers 55a, 55 is directed opposite to the axial thrust generated by the second set of impellers 55, 55b.

[0104] The back-to-back arrangement has the advantage that the axial thrust acting on the first shaft 54, which is generated by the first set of impellers 55a, 55 counteracts the axial thrust, which is generated by the second set of impellers 55, 55b. Thus, the two axial thrusts compensate each other at least partially.

[0105] As it is shown in FIG. 2 the pump turbine unit 5 is configured as a vertical pump, meaning that during operation the first shaft 54 is extending in the vertical direction, which is the direction of gravity. Thus, the axial direction A coincides with the vertical direction. The motor generator unit 6 is arranged above the pump turbine unit 5. During pump mode the motor generator unit 6, exerts a torque on the drive end 541 of the first shaft 5 for driving the rotation of the first shaft 54 and the impellers 55, 55a, 55b about the axial direction A.

[0106] A direction perpendicular to the axial direction is referred to as radial direction. The term ‘axial’ or ‘axially’ is used with the common meaning ‘in axial direction’ or ‘with respect to the axial direction’. In an analogous manner the term ‘radial’ or ‘radially’ is used with the common meaning ‘in radial direction’ or ‘with respect to the radial direction’. Hereinafter relative terms regarding the location like “above” or “below” or “upper” or “lower” or “top” or “bottom” refer to the usual operating position of the energy conversion device 3. FIG. 2 shows the embodiment of the energy conversion device 3 in its usual operating position.

[0107] In other embodiments (not shown) the pump turbine unit 5 can be configured as a horizontal pump, meaning that during operation the first shaft 54 is extending perpendicular to the vertical direction, which is the direction of gravity. Thus, the axial direction A is perpendicular to the vertical direction.

[0108] In the embodiment of the energy conversion device 3 shown in FIG. 2 the first shaft 54 of the pump turbine unit 5 is supported by shaft bearings 153, 154. With respect to the axial direction A the first shaft 54 is supported by an axial bearing 153. Preferably the axial bearing 153 is configured as a hydrodynamic bearing, and even more preferred as a tilting pad bearing 153. The axial bearing 153 is arranged near the drive end 541 of the first shaft 54. Furthermore, the pump turbine unit 5 comprises a radial bearing 154 for supporting the first shaft 54 with respect to the radial direction. The radial bearing 154 is arranged near to the drive end 541 of the first shaft 54, more precisely between the axial bearing 153 and the drive end 541 of the first shaft 54. Preferably, the radial bearing 154 is configured as a hydrodynamic bearing, and even more preferred as a radial tilting pad bearing.

[0109] A radial bearing is also referred to as a “journal bearing” and an axial bearing, is also referred to as an “thrust bearing”.

[0110] In the pump turbine unit 5 shown in FIG. 2 the lubrication and the cooling of both the axial bearing 153 and the radial bearing 154 is realized by the barrier fluid that is supplied to the motor generator unit 6. The barrier fluid is supplied from the barrier fluid reservoir 69 (FIG. 1) through the umbilical line 60 to the underwater location 200. The motor generator unit 6 is configured as a liquid filled motor generator unit 6, wherein the motor generator unit 6 is filled with the barrier fluid.

[0111] The energy conversion device 3 further comprises a mechanical seal 155 for sealing the pump turbine unit 5 at the first shaft 54. The mechanical seal 155 is a seal for the rotating first shaft 54. As it is known for mechanical seals as such, the mechanical seal 155 comprises a rotor part (not shown) fixed to the first shaft 54 and rotating with the first shaft 54 as well as a stationary stator part (not shown) fixed with respect to the housing 31. During operation the rotor part of the mechanical seal 155 and the stator part of the mechanical seal are sliding along each other—usually with a fluid film between the seal faces—for providing a sealing action to prevent the process fluid (water) from escaping from the pump turbine unit 5 along the first shaft 54. The mechanical seal 155 is arranged with respect to the axial direction A between the last stage impeller 55b and the axial bearing 153.

[0112] The mechanical seal 155 has a process side facing the pump turbine unit 5. The process side is in fluid communication with the high pressure opening 53, so that the pressure prevailing at the process side is at least approximately the same as the high pressure prevailing at the high pressure opening. The mechanical seal 155 seals between the part of the housing 31 which is filled with the process fluid (water) and the part of the housing 31 which is filled with the barrier fluid. According to the representation in FIG. 2 the part above the mechanical seal 155 is filled with the barrier fluid and the part below the mechanical seal 155 is filled with the process fluid (water). The barrier fluid pressure is adjusted to a value which is larger than the high pressure prevailing at the process side of the mechanical seal 155. Thus, any leakage through the mechanical seal 155 is always directed towards the process side of the mechanical seal 155. The barrier fluid can leak through the mechanical seal 155 into the pump turbine unit 5, but the water cannot pass through the mechanical seal 155 from the process side to the motor generator unit 6. Any leakage of the barrier fluid through the mechanical seal 155 will be compensated or replaced from the barrier fluid reservoir 69 through the umbilical line 60. The mechanical seal 155 separates the part of the energy conversion device 3, that is filled with the process fluid (water) from the part of the energy conversion device 3, that is filled with the barrier fluid.

[0113] The barrier fluid is supplied to the motor generator unit 6 at a pressure which is at least as high as, and preferable a few bars higher than, the high pressure, so that the water cannot pass through the mechanical seal 155 and therewith cannot enter the motor generator unit 6.

[0114] Preferably, the barrier fluid pressure is adjusted to a value which is only slightly larger than the high pressure, e.g. approximately 2-5 bar, so that the pressure difference across the mechanical seal 155 is quite small. A small pressure difference over the mechanical seal 155 results in a small leakage of barrier fluid through the mechanical seal 155.

[0115] At the process side the mechanical seal 155 is exposed to the high pressure (discharge pressure), which is given by the hydrostatic pressure of the water at the underwater location 200. At least at high water depths the pressure can be considered as constant. This means, that it is easy to keep a stable pressure difference across the mechanical seal 155, which is beneficial for longevity of the mechanical seal 155 and regarding the overall barrier fluid consumption.

[0116] The energy conversion device 3 further comprises a balance drum 17 for at least partially balancing the axial thrust that is generated by the impellers 55a, 55, 55b during operation of the pump turbine unit 5. The balance drum 17 is fixedly connected to the first shaft 54 and arranged adjacent to or at the non-drive end 542 of the first shaft 54. The balance drum 17 defines a front side 171 and a back side 172. The front side 171 is the side or the space facing the first stage impeller 55a of the pump turbine unit 5. The front side 171 is in fluid communication with the low pressure opening 52. Thus, at the front side 171 a pressure prevails that is at least approximately the same as the low pressure prevailing at the low pressure opening 52. The back side 172 is located at the other side of the balance drum 17, according to the representation in FIG. 2 below the balance drum 17. The balance drum 17 is surrounded by a stationary part 126, so that a relief passage 173 is formed between the radially outer surface of the balance drum 17 and the stationary part 126. The stationary part 126 is configured to be stationary with respect to the housing 31. The relief passage 173 forms an annular gap between the outer surface of the balance drum 17 (which is also referred to as a throttle bush in a back-to-back configuration) and the stationary part 126 and extends from the front side 171 to the back side 172.

[0117] A balance line 19 is provided and configured for recirculating pressurized water to the back side 172. The balance line 19 extends from the back side 172 to the process side in front of the mechanical seal 155, where a pressure prevails which is at least approximately the same as the high pressure. Thus, by the balance line 19 and neglecting smaller friction losses along the balance line 19 the back side 172 is exposed to a pressure which is essentially the discharge pressure, i.e. the high pressure. A pressure drop exists across the balance drum 19, because the front side 171 is exposed essentially to the low pressure prevailing at the low pressure opening 52, and the back side 172 is exposed to a pressure, that is approximately the same as the high pressure. The pressure drop over the balance drum 17 results in a force that is directed upwardly in the axial direction A (according to the representation in FIG. 2) and therewith counteracts the downwardly directed axial thrust generated by the impellers 55a, 55, 55b of the pump turbine unit 5.

[0118] Providing the balance drum 17 at the non-drive end 542 of the first shaft 54 has the advantage that the balance drum 17 can be additionally used as a hydrostatic support device for providing a radial support to the first shaft 54 at the non-drive end 542. A hydrostatic support device is preferably configured to provide the support by the Lomakin effect. Different from hydrodynamic radial bearings, which require a rotation of the first shaft 54 to generate the radial bearing forces, a hydrostatic support device does not require a rotation of the first shaft 54 for supporting the first shaft 54 with respect to the radial direction, but a pressure drop across the hydrostatic support device with respect to the axial direction A. As it is known in the art, for example, the Lomakin effect requires a pressure drop along an annular gap for the fluid arranged between the first shaft 54 and a stationary part surrounding the first shaft. The conventional hydrodynamic radial bearing does not require a mentionable pressure drop across the radial bearing, but needs the rotation of the first shaft 54.

[0119] Since the balance drum 17 provides for a pressure drop along the annular relief passage 173 arranged between the balance drum 17 and the stationary part 126, the balance drum 17 can be used as a hydrostatic support device for radially supporting and centering the first shaft 54 by the Lomakin effect.

[0120] Therefore, it is not necessary, to provide a separate radial bearing, such as a hydrodynamic bearing, at the non-drive end 542 of the first shaft.

[0121] Furthermore, there is no need for an additional mechanical seal at or near the non-drive end 542 of the first shaft.

[0122] The motor generator unit 6 comprises an electric motor 141, the second shaft 62 extending in the axial direction A, and a plurality of second shaft bearings, namely an axial second shaft bearing 143 and two radial second shaft bearings 144. The electric motor 141 comprising the rotor 63 and the stator 64 (see e.g. FIG. 1) can be operated in the generator mode, wherein the second shaft 62 is driven by the first shaft 54 during turbine mode. In the generator mode the rotation of the rotor 63 inside the stator 64 produces electric energy, which is transmitted by the electric power line 66 to the energy unit 67. During the motor mode the electric motor 141 receives electric energy by the electric power line 66 and rotates the second shaft 62 about the axial direction A for driving the first shaft 54 of the pump turbine unit 5 operating in the pump mode.

[0123] Referring to the representation in FIG. 2 the axial second shaft bearing 143 and one of the radial second shaft bearings 144 are arranged above the electric motor 141, and the other of the radial second shaft bearings 144 is arranged below the electric motor 141, namely between the electric motor 141 and the coupling 65 with respect to the axial direction A. Preferably, each of the second shaft bearing 143, 144 is configured as a hydrodynamic bearing. The second shaft bearings 143, 144 are lubricated and cooled by the barrier fluid with which the motor generator unit is filled.

[0124] The electric motor 141 of the motor generator unit 6 comprises the inwardly disposed rotor 63 (see e.g. FIG. 1), which is connected to the second shaft 62 in a torque proof manner, as well as the outwardly disposed stator 64 surrounding the rotor 63 with an annular gap between the rotor 63 and the stator 64. The rotor 63 can constitute a part of the second shaft 62 or is a separate part, which is rotationally fixedly connected to the second shaft 62, so that the rotation of the rotor 63 drivers the second shaft 62 (motor mode) or vice versa (generator mode). The electric motor 141 can be configured as a cable wound motor. In a cable wound motor the individual wires of the stator 64, which form the coils for generating the electromagnetic field(s), are each insulated, so that the motor stator 64 can be flooded even with an electrically conducting fluid. The cable wound motor does not require a dielectric fluid as barrier fluid for cooling the stator 64. Alternatively, the electric motor 141 can be configured as a canned motor. When the electric drive 141 is configured as a canned motor, the annular gap between the rotor 63 and the stator 64 is radially outwardly delimited by a can that seals the stator 64 hermetically with respect to the rotor 63 and the gap. Thus, any barrier fluid flowing through the gap cannot enter the stator 64. When the electric motor 141 is designed as a canned motor the electric motor 141 is filled with the barrier fluid. Preferably the entire motor generator unit 6 is filled with the barrier fluid.

[0125] Preferably, the electric motor 141 is configured as a permanent magnet motor or as an induction motor.

[0126] The electric motor 141 can be designed to operate with a variable frequency drive (VFD), in which the speed of the drive, i.e. the frequency of the rotation is adjustable by varying the frequency and/or the voltage supplied to the electric motor 141. Preferably, the VFD is provided in the energy unit 67. However, it is also possible that the electric motor 141 is configured differently, for example as a single speed or single frequency drive.

[0127] Optionally, the high pressure opening 53 can be configured to extend from the housing 31 to a location which is a distance away from the housing 31 of the pump turbine unit 5, for example at a certain elevation from the sea ground, in order to avoid the intake of sand or other solid material, in particular during the turbine mode.

[0128] The switching unit 8 is particularly advantageous for starting the pump mode, which will be explained now referring to FIG. 3. Without loss of generality it is assumed that the energy storage system 1 is fully “discharged”, i.e. the vessel 2 is filled with water up to the maximum level M. The pump turbine unit 5 is at standstill and completely filled with water at the high pressure, i.e. the hydrostatic pressure of the water prevailing at the underwater location 200. The control valve 86 of the switching unit 8 is in the closed position. Since the pump turbine unit 5 is filled with water at the high pressure, the high pressure also prevails at the low pressure opening 52 as well as at the second opening 82 of the switching unit 8.

[0129] For starting the pump mode the pump turbine unit 5 is started by supplying electric energy to the motor generator unit 6, so that the second shaft 62 starts to drive the rotation of the first shaft 54 with the impellers 55a, 55, 55b. According to the disclosure, the pump mode is started with operating the pump turbine unit 5 with a zero flow until the rotational speed of the first shaft 54 is sufficient to generate a positive flow from the low pressure opening 52 to the high pressure opening 53. A positive flow in the pump mode designates a flow that is discharged through the high pressure opening 53 of the pump turbine unit 5. A negative flow or a reverse flow designates a flow which is discharged through the low pressure opening 52.

[0130] FIG. 3 is a diagram showing the rotational speed S of the first shaft 54 on the vertical axis versus the flow F discharged through the high pressure opening 53 on the horizontal axis. The intersection of the two axis corresponds to zero flow and zero rotational speed. As already said a positive flow, i.e. on the right side of the vertical axis S, is a flow discharged through the high pressure opening 53 and a negative flow, i.e. on the left side of the vertical axis S, is a flow discharged through the low pressure opening 52. A positive rotational speed S, i.e. above the horizontal axis F, designates a rotation in a first direction, in which the impellers 55a, 55, shall convey the water from the low pressure opening 52 to the high pressure opening 53 (pump mode), and a negative rotational speed S, i.e. below the horizontal axis F, designate a rotation in a second direction opposite to the first direction. In the turbine mode the impellers 55, 55b shall rotate in the second direction, i.e. with negative rotational speed.

[0131] In addition, FIG. 3 shows the dashed curve MP, which indicates a constant pressure difference line. In particular MP shows the constant pressure difference line for a pressure difference generated by the pump turbine unit, which corresponds to 100% of the pressure difference the pump turbine unit 5 shall generate at the duty point. Thus, MP shows the curve on which the pump-turbine unite 5 generates 100% of the duty pressure difference (duty head) In the pump mode the curve MP indicates the operation of the pump turbine unit 5 with the nominal operating rotational speed (duty point speed). Furthermore, FIG. 3 shows the zero torque curve ZT at which the pump turbine unit 5 delivers or generates a torque of zero. The zero pressure rise curve ZP is the curve at which the pressure rise, i.e. the difference between the pressure at the low pressure opening 52 and the high pressure opening 53, equals zero.

[0132] The two axis define four quadrants Q1-Q4. The upper right quadrant Q1 is the quadrant, in which the pump mode should take place. More precisely, the region of Q1 in which the pump mode should take place is delimited by the vertical axis S and the zero pressure rise curve ZP The lower left quadrant Q3 is the quadrant where the turbine mode should take place. More precisely, the turbine mode should take place in the region of Q3 which is delimited by the horizontal axis F and the zero torque curve ZT. The upper left quadrant Q4 and most of the lower right quadrant Q2 are regions, where the pump turbine unit 5 operates in energy dissipating modes. For example, in quadrant Q4 the pump turbine unit 5 would operate with a negative flow F and a positive rotational speed S, meaning that the impellers 55a, 55, 55b rotate in the first direction, where the impellers 55a, 55, 55b should convey the fluid from the low pressure opening 52 to the high pressure opening 53, but the flow F is directed from the high pressure opening 53 to the low pressure opening 52. This occurs for example when the pump mode shall be started against a reverse (negative) flow F. If one were to start the pump mode, e.g. by simply opening the control valve 86 and starting the rotation of the first shaft 54, the rotation of the impellers 55a, 55, 55b were started against the negative flow through the pump turbine unit 5. This corresponds to an energy dissipating mode.

[0133] To avoid these energy dissipating modes the disclosure proposes to start the pump mode with the zero flow until the rotational speed S of the first shaft 54 is sufficient to generate a positive flow. This is illustrated by the curve PS in FIG. 3 indicating the starting of the pump mode. As it can be seen, when starting from the point of zero rotational speed S and zero flow F, the first shaft 54 is accelerated, i.e. the rotational speed S of the first shaft 54 increases and the flow F remains zero. When the rotational speed S of the first shaft 54 approaches the operating rotational speed, i.e. the duty point speed, the curve PS, still at zero flow, approaches the curve MP. Now, the pump turbine unit 5 is capable to create a pressure difference sufficiently high for causing a positive flow from the low pressure opening 52 to the high pressure opening 53. The end of the curve PS, which is located in the quadrant Q1 and on the dashed curve MP, indicates the duty point, at which the pump-turbine unit 5 operates with the duty point speed, and generates the duty point flow and the duty point pressure head (pressure difference).

[0134] Usually, the pump turbine unit 5 will heat-up during operation at zero flow conditions. However, this does not constitute a problem, because known pumps, for example, can be operated at zero flow conditions for at least 30-40 seconds and a typical time to accelerate the first shaft 54 from standstill to a typical rotational speed (duty point speed) of e.g. 1600 rpm is at most 10 seconds.

[0135] To keep the pump turbine unit 5 at zero flow until the first shaft 54 has at least approximately reached the duty pint speed, the switching unit 8 is used. As already said, prior to starting the pump mode, the control valve 86 of the switching unit 8 is in the closed position. Since the pump turbine unit 5 is filled with water at the high pressure, the high pressure also prevails at the low pressure opening 52 as well as at the second fluid opening 82 of the switching unit 8. The first fluid opening 81 of the switching unit 8 is exposed to the low pressure generated by the water W in the vessel 2 and in the connection line 4 upstream of the first fluid opening 81. It is obvious that the water cannot pass through neither the first branch 83 nor the second branch 84. The second branch 84 is blocked by the closed control valve 86. Furthermore, the low pressure cannot open the non-return device 85 against the high pressure at the second fluid opening 82. Thus, there is no flow through the switching device 8 and consequently zero flow through the low pressure opening 52.

[0136] When the first shaft 54 of the pump turbine unit 5 is accelerated the impellers 55a, 55, 55b generate an increasing pressure rise, whereby the pressure at the low pressure opening 52 and therewith the pressure at the second fluid opening 82 of the switching unit 8 decreases. When the first shaft 54 approaches its duty point speed the pressure rise generated by the pump turbine unit 5 is as large that the pressure at the low pressure opening 52 and therewith the pressure at the second fluid opening 82 drops below the pressure prevailing at the first fluid opening 81 of the switching unit. Thus, the pressure at the first fluid opening 81 of the switching unit 8 becomes larger than the pressure at the second fluid opening 82 of the switching unit 8. As soon as the pressure difference passes the opening pressure of the non-return device 85 (the opening pressure being very small), the non-return device 85 opens automatically and the water can flow through the first branch 83 to the low pressure opening 82. The positive flow through the pump turbine unit 5 starts and therewith the emptying of the vessel 2. Thus, the flow of water through the non-return device 85 starts nearly instantaneously, when the pressure drop across the non-return device 85 changes its direction or its sign. i.e. when the pressure prevailing at the second fluid opening 82 becomes smaller than the pressure at the first fluid opening 81.

[0137] Once the pump turbine unit 5 is at its operating rotational speed (duty point speed) and the non-return device 85 has opened, it is possible to additionally switch the control valve 86 in the open position to reduce the overall flow resistance, which is advantageous in view of the energy efficiency.

[0138] FIG. 4 is a diagram showing the rotational speed S of the first shaft 54 on the vertical axis versus the flow F on the horizontal axis in an analogous manner as FIG. 3. However, in FIG. 4 the starting of the turbine mode is illustrated, namely by the curve TS. Without loss of generality it is assumed that the energy storage system 1 is fully “discharged”, i.e. the vessel 2 is empty or at a minimum level. The pump turbine unit 5 is at standstill and completely filled with water at the high pressure, i.e. the hydrostatic pressure of the water prevailing at the underwater location 200. The control valve 86 of the switching unit 8 is in the closed position. The first branch 81 of the switching unit 8 is not used for the turbine mode, since the non-return device 85 blocks a flow in the second direction, i.e. towards the vessel 2.

[0139] To start the turbine mode, the control valve 86 in the second branch 82 is slowly opened from the closed position to the open position. This renders possible to very smoothly start the turbine mode, in which the water enters the pump turbine unit 5 through the high pressure opening 53, drives the rotation of the first shaft 54 for generating electric energy by the motor generator unit 6 operating in the generator mode, and is discharged through the low pressure opening 52 and the switching unit 8 into the vessel 2. The turbine mode is started slowly by slowly opening the control valve from the closed position to the open position. Thus, the full pressure difference between the high pressure and the low pressure is gradually transferred from across the control valve 86 to across the impellers 55a, 55, 55b of the pump turbine unit 5.