An underwater energy storage system and method of installing the same

Abstract

An underwater energy storage system, comprising a source of compressed fluid, a plurality of energy storage units, each comprising at least one fluid storage container configured for storing compressed fluid therein, and a piping system fluidically connecting the source of compressed fluid to the at least one fluid storage container of each of the plurality of energy storage units to allow ingress and egress of the compressed fluid in and out of the at least one fluid storage container of each of the plurality of energy storage units, respectively. The piping system comprises a pipeline connected at a longitudinal end thereof to the source of compressed fluid, the pipeline comprising a plurality of branch connection provisions provided one after the other along the length of the pipeline, each of the branch connection provisions fluidically connecting at least one energy storage unit of the plurality energy storage units to the pipeline.

Description

[0001] An underwater energy storage system and method of installing the same

[0002] The present invention relates to an underwater energy storage system. In particular, the present invention relates to an underwater compressed air energy storage system, more particularly a subsea compressed air energy storage system. Further, the present invention relates to a method of installing an underwater energy storage system. In particular, the present invention relates to a method of installing an underwater compressed air energy storage system, more particularly a subsea compressed air energy storage system.

[0003] Compressed air energy storage (CAES) is a known technology for storing energy by compressing air and storing it in an underground reservoir, such as a depleted natural gas field or an aquifer. When the energy is needed, the compressed air is released and used to generate electricity through a turbine. One advantage of CAES is that it can be used to store excess energy generated by renewable sources, such as wind and solar, when they are producing more energy than is needed. This excess energy can then be used later when the renewable source is not producing enough energy.

[0004] As an alternative to storing compressed air in an aquifer or the like, underwater compressed air energy storage (UCAES) systems have been proposed, wherein compressed air is stored in large, reinforced bags that are anchored to the seafloor. Compressed air is injected into the bags to fill the bags with air. When electricity is needed, the compressed air is released from the bags and flows through a turbine, generating electricity as it expands. Because the bags are anchored to the seafloor, they are able to maintain a constant pressure during both compression and expansion, which can lead to higher efficiency compared to traditional CAES systems.

[0005] UCAES systems are an attractive option for certain energy storage applications. UCAES systems can be easily scaled up or down by adding or removing bags, allowing them to be customized for a wide range of energy storage needs. In addition, UCAES systems can be deployed in a variety of offshore environments, including shallow and deep waters, making them a versatile option for offshore energy storage. Also, UCAES systems can be integrated with existing (offshore) infrastructure, such as (offshore) wind farms or solar fields, allowing them to be used in conjunction with renewable energy generation technologies. Further, the bags used in UCAES systems are designed to withstand the harsh and corrosive underwater environment, making them a durable and long-lasting energy storage solution. UCAES systems are thus attractive for energy storage applications that require flexibility, scalability, and compatibility with existing (offshore) infrastructure. Moreover, UCAES systems can be configured not to produce greenhouse gas emissions or other pollutants, making them a clean energy storage option that can help reduce the carbon footprint of energy generation. Despite the advantages of UCAES systems, a widespread adoption of such systems has not yet occurred due to several challenges that complicate large-scale commercial implementation. One challenge is the difficulty of designing and maintaining the system in the harsh and complex underwater environment. In addition, the costs of constructing and deploying underwater compressed air energy storage systems are high, which make them, until now, typically less economically viable than other energy storage options.

[0006] It is an object of the present invention to provide an underwater energy storage system and a method of installing the same which at least partially alleviate the abovementioned problem.

[0007] The present invention provides thereto, according to a first aspect thereof, an underwater energy storage system, comprising a source of compressed fluid, a plurality of energy storage units, each comprising at least one fluid storage container configured for storing compressed fluid therein, and a piping system fluidically connecting the source of compressed fluid to the at least one fluid storage container of each of the plurality of energy storage units to allow ingress and egress of the compressed fluid in and out of the at least one fluid storage container of each of the plurality of energy storage units, respectively. The piping system comprises a pipeline connected at a longitudinal end thereof to the source of compressed fluid, the pipeline comprising a plurality of branch connection provisions provided one after the other along the length of the pipeline, each of the branch connection provisions fluidically connecting at least one energy storage unit of the plurality energy storage units to the pipeline.

[0008] An important advantage of the pipeline comprising multiple branch connection provisions along the length of the pipeline is that it simplifies underwater installation as compared to systems which utilize one or more manifold stations to be coupled individually to one or more pipelines to each of which multiple energy storage units are to be connected. As these latter systems use multiple manifold stations and pipelines, each of these components need to be lowered, installed and connected individually on the seabed before the energy storage units can be connected, causing the installation of such systems to be labor intensive, time-consuming and expensive. In contrast, since the piping system of the underwater energy storage system according to the present invention integrates the pipeline and multiple branch connection provisions, which each serve as a manifold section, the system can be installed in an easy manner by laying the single pipeline in only one single pipelaying operation, typically using one single pipelaying vessel, and lowering the energy storage units next to the pipeline. Subsequently, the units can be each connected to the pipeline in an easy manner by for instance jumpers, using for instance a remotely operated vehicle. In principle, this provides a linear configuration of units along the pipeline, although the system is not limited to such configuration, even when using only one single pipeline. The linear configuration is typically advantageous as it allows installation of the system on an isobath extending from the coast in a substantially offshore direction and having a substantially constant elevation. In other words, the pipeline constitutes a substantially linearly extending “manifold”, which allows for an improved installation efficiency compared to using a network of multiple conventional manifold stations. The linearity of the system, due to the use of single pipeline, further allows pigging the piping system without the need for a complex pig system. The pig system only needs to be configured to be able to move a pig up and down through the straight pipeline.

[0009] The branch connection provisions may be provided at regular intervals along the pipeline, wherein the pitch between neighboring branch connection provisions preferably corresponds substantially to the optimal pitch between the energy storage units.

[0010] Preferably, each of the branch connection provisions fluidically connects the at least one energy storage unit to the pipeline through a respective jumper. Jumpers provide a reliable connection which can be made easily using, for instance, a remotely operated vehicle. In a preferred embodiment, the at least one energy storage unit is provided with the jumper pre-connected thereto. This eliminates the need to connect the jumper to the energy storage unit underwater, thereby further simplifying the overall installation of the system. Alternatively, the branch connection provision may be provided with the jumper pre-connected thereto.

[0011] In a preferred embodiment, a longitudinal end of the jumper remote from the least one energy storage unit is provided with a connector configured to connect the jumper to its respective branch connection provision and near the connector with means for picking up the jumper near the connector using a remotely operated vehicle (“ROV”). This enables using an ROV for the connection of each jumper to respective branch connection provision.

[0012] The energy storage units may each comprise a ballast and an anchoring means connected to the ballast for anchoring the at least one fluid storage container to the ballast. Thet anchoring means comprise a tether line, such as a chain or cable. The ballast may comprise a box caisson, preferably made from reinforced concrete, filled with granular material, such sand or rocks, and / or concrete. This way, the ballasts have a manageable installation weight and can be brought up to the required weight with the granular material, like sand, gravel or rocks, using for example a fall-pipe vessel, when installed on the seabed and before filling the at least one fluid storage container with the compressed fluid.

[0013] In a preferred embodiment, the jumper connects to the ballast and the anchoring means are configured to fluidically connect the at least one fluid storage container to the jumper. The anchoring means may be configured to adapt the length thereof so as to change the height at which the at least one fluid storage container is anchored above the ballast. This enables installing each of the fluid storage containers at substantially the same depth, so that the hydrostatic pressure on each of the fluid storage containers is substantially the same. The at least one fluid storage container may comprise two or more, preferably more than three, and more preferably more than five fluid storage containers. In a further preferred embodiment, the ballast has a hexagonal or a round shape. Preferably, the ballast has a hexagonal shape and the at least one fluid storage container comprises seven fluid storage containers anchored to the ballast. An advantage of the hexagonal shape of the ballast is that it allows to place multiple energy storage units close together with efficient use of the seabed surface. The resulting modularity of the energy storage units allows for configuration freedom on the seabed. Specifically, the ballasts may be configured to function as an artificial reef. Preferably, six of the seven fluid storage containers are anchored to the hexagonal ballast near the six vertices of the hexagonal ballast. The seventh fluid storage container may be anchored to the ballast near the center of the hexagon. This allows for an efficient distribution of fluid storage containers on the seabed near the pipeline, which may be in the form of bags, which are preferably balloon- or pear-shaped.

[0014] In a preferred embodiment, the at least one fluid storage container is made from a flexible material, so that the fluid storage container is collapsible. Preferably, the flexible material is inelastic.

[0015] In a preferred embodiment, the branch connection provisions are each configured to fluidically connect two energy storage units on each lateral side of the pipeline to the pipeline. Preferably, the energy storage units are arranged in two rows on either side of the pipeline in a mutually staggered configuration. This way, the spacing between the branch connection provisions can be reduced, thereby reducing the overall length of the pipeline. The system may further comprise fluid flow control means, such as an inlet and outlet valve, between the pipeline and the at least one fluid storage container of each energy storage unit to control the ingress and egress of the compressed fluid in and out of the at least one fluid storage container, respectively. The system may further comprise an umbilical for powering and / or communicating with the fluid flow control means. The umbilical may be arranged along, preferably on top of the pipeline. Preferably, the pipeline is provided with the umbilical pre-connected thereto. Each energy storage unit may comprise a connector box configured for connecting the umbilical thereto to connect the umbilical to the fluid flow control means. Preferably, the connector box is configured as a watertight junction box for air, power and / or communication. The remaining installation effort to be done using an ROV is then limited to the connecting of the (preferably pre-connected) jumper, consisting of an air, power and / or communication line to the respective branch connection provision. The connector box may further be configured to connect one or more measurement sensors for monitoring a process of storing and releasing the compressed fluid in and from the at least one fluid storage container to a transmitter for transmiting data from the measurement sensors to an onshore processing unit, wherein the transmiter preferably comprises a multiplexer.

[0016] In a preferred embodiment, the system further comprises a pig station connected to the longitudinal end of the pipeline opposite the longitudinal end of the pipeline that is connected to the source of compressed fluid. The pig station allows pigging the pipeline using pigs or scrapers to perform maintenance on the pipeline without blocking the flow of fluid in the pipeline. Specifically, the pig station may be configured to inspect and clean the pipeline and to remove water from the pipeline that may have entered via condensation or via a(n) (accidental) leakage. Preferably, the pipeline is provided with the pig station pre-connected thereto. This further simplifies the installation of the system on the seabed.

[0017] By providing a single pipeline which is provided with the branch connection provisions and, optionally, to which jumpers and / or the umbilical and / or the pig station are pre-connected, all these components can be installed in one single operation of laying the pipeline on the seabed. As an important advantage, the installation of the system is thereby significantly simplified, and the required amount of deep-sea installation work greatly reduced.

[0018] Preferably, the source of compressed fluid is located on land, and the plurality of energy storage units and the piping system are located below sea level. The source of compressed fluid may be a flow of compressed air generated by an onshore compressed air energy storage plant.

[0019] In a preferred embodiment, the plurality of energy storage units and the piping system are located at a depth of between 500 m and 1500 m below sea level, preferably between 700 m and 1200 m, more preferably at approximately 800 m below sea level.

[0020] Preferably, the compressed fluid comprises a compressed gas, preferably compressed air or compressed hydrogen gas.

[0021] According to a second aspect, the provides a method of installing an underwater energy storage system according to any of the above embodiment, comprising steps of providing the source of compressed fluid on land, installing the pipeline on the seabed in one single pipelaying operation and connecting the pipeline to the source, installing near each of the plurality of branch connection provisions of the pipeline at least one of the plurality of energy storage units on the seabed, and connecting each energy storage unit installed on the seabed to its respective nearest branch connection provision of the pipeline installed on the seabed. An important advantage of the pipeline comprising multiple branch connection provisions along the length of the pipeline is that it simplifies underwater installation as compared to systems which utilize one or more manifold stations to be coupled individually to one or more pipelines to each of which multiple energy storage units are to be connected. Since the piping system integrates the pipeline and multiple branch connection provisions, the system can be installed in an easy manner by laying the single pipeline in one single pipelaying operation, typically using one single pipelaying vessel and lowering the energy storage units next to the pipeline. Subsequently, the units can be each connected to the pipeline in an easy manner by for instance jumpers, using for instance a remotely operated vehicle. In principle, this provides a linear configuration of units along the pipeline, although the system is not limited to such configuration, even when using only one single pipeline. The linear configuration is typically advantageous as it allows installation of the system on an isobath extending from the coast in a substantially offshore direction and having a substantially constant elevation. In other words, the pipeline constitutes a substantially linearly extending “manifold”, which allows for an improved installation efficiency compared to using a network of multiple conventional manifold stations.

[0022] In a preferred embodiment of the method, the step of connecting each energy storage unit installed on the seabed to its respective nearest branch connection provision of the pipeline comprises connecting a jumper between each energy storage unit and its respective nearest branch connection provision. Jumpers provide a reliable connection which can be made easily using, for instance, a remotely operated vehicle.

[0023] The method may further comprise pre-connecting the jumper to each of the plurality of branch connection provisions of the pipeline or to each of the plurality of energy storage units before installing the pipeline on the seabed or before installing the plurality of energy storage units on the seabed, respectively. This step of pre-connecting the jumper eliminates the need to connect each jumper to both the energy storage unit and its respective branch connection provision underwater, further simplifying the overall installation of the system. Importantly, by pre-connecting each jumper to a branch connection provision on the pipeline or to an energy storage unit, each jumper is “automatically” located near its respective energy storage unit or branch connection provision, respectively, when the pipeline and the energy storage unit are installed on the seabed. Preferably, the step of connecting the jumper between each energy storage unit and its respective nearest branch connection provision comprises connecting the jumper pre-connected to each of the plurality of branch connection provisions to its respective nearest energy storage unit or connecting the jumper pre-connected to each of the plurality of energy storage units to its respective nearest branch connection provisions of the pipeline. The step of connecting the jumper between each energy storage unit and its respective nearest branch connection provision may be carried out using a remotely operated vehicle (“ROV”).

[0024] In a preferred embodiment, the step of installing the plurality of energy storage units on the seabed comprises adjusting a tether line with which the at least one fluid storage container is anchored to a ballast of its respective energy storage unit so as to install each fluid storage container at approximately the same depth below sea level.

[0025] In a preferred embodiment, the method further comprises providing an umbilical for powering and / or communicating with fluid flow control means, such as an inlet and outlet valve, between the pipeline and the at least one fluid storage container of each energy storage unit to control the ingress and egress of the compressed fluid in and out of the at least one fluid storage container. The step of providing the umbilical may comprise pre-installing the umbilical on top of the pipeline before installing the pipeline on the seabed. Further, the method may comprise connecting the umbilical to a connector box on each energy storage unit, wherein each connector box connects the umbilical to the fluid flow control means. Preferably, the connector box is configured as a watertight junction box for air, power and / or communication. The remaining installation effort to be done using an ROV is then limited to the connecting of the (preferably pre-connected) jumper, consisting of an air, power and / or communication line to the respective branch connection provision. The connector box may further be configured to connect one or more measurement sensors for monitoring a process of storing and releasing the compressed fluid in and from the at least one fluid storage container to a transmitter for transmitting data from the measurement sensors to an onshore processing unit, wherein the transmitter preferably comprises a multiplexer.

[0026] The present invention is further illustrated by the following figures, which show a preferred embodiment of the underwater energy storage system according to the invention, and are not intended to limit the scope of the invention in any way, wherein: figure 1 shows a perspective view of an underwater energy storage system according to a preferred embodiment of the present invention as part of a compressed air energy storage plant; figure 2 shows a perspective close-up view of the plant shown figure 1; figure 3 shows a perspective close-up view of a pipeline of the underwater energy storage system shown in figure 1 close to the shore; figure 4 shows a perspective close-up view of the pipeline of the underwater energy storage system shown in figure 1 close to an energy storage unit of the system; figure 5 shows a perspective close-up view of a pig station at the distal end of the pipeline of the system opposite the end that is connected to the plant; figure 6 shows a perspective close-up view of an energy storage unit of the system connected to the pipeline of the system as shown in figure 1.

[0027] Figure 1 shows a compressed air energy storage plant 1 connected to a power grid 2 which in turn is connected to a wind power plant 4 and a solar power plant 5 comprising onshore and offshore windmills 6 and solar panels 7, respectively. The compressed air energy storage plant 1 includes compressors for compressing air, a system 10 for storing the compressed air under water, turbines, and generators for generating electricity from air released from the storage system 10.

[0028] During periods of low demand for electricity, excess energy from the power grid 2 is used to power the compressors, which compress air to high pressures of around 50 to 70 bar. The compressed air is subsequently transported through a pipeline 11 and stored at high pressure in bags 12 which are anchored to ballasts 13 arranged next to the pipeline 11 on either side thereof. The hydrostatic pressure of the water surrounding the bags 12 plays an important role in the underwater compressed air energy storage system 10. The hydrostatic pressure of the water helps to store compressed air at high pressure, reducing the size of the storage system 10. This is because the pressure of the water surrounding the compressed air increases with depth, which helps to maintain the pressure of the compressed air.

[0029] For example, if the underwater or subsea storage system 10 is located at a depth of 500 meters, the hydrostatic pressure of the water will be around 50 bar. This means that the compressed air stored in system 10 can be maintained at a pressure of around 50 bar without the need for additional compression. The hydrostatic pressure of the water also helps to reduce the energy losses associated with storing compressed air. When compressed air is stored in bags 12, there is a risk of energy losses due to heat transfer and other factors. However, the hydrostatic pressure of the water helps to maintain the pressure of the compressed air, reducing the risk of energy losses and improving the efficiency of the system 10.

[0030] The optimal depth for underwater compressed air energy storage depends on several factors, including the availability of suitable underwater locations, the size of the storage system 10, and the characteristics of the surrounding environment. Typically, the underwater compressed air energy storage system 10 is located at a depth of between 500 and 1500 meters. This depth range provides several advantages, one of which is the high hydrostatic pressure, as explained above. At depths of 500-1500 meters, the water pressure is high enough to store compressed air at high pressures, reducing the required size of the storage system 10. In addition, the water at these depths is typically cooler than surface water, which helps to reduce energy losses associated with storing compressed air. Moreover, as system 10 is located underwater, it is not or barely visible and therefore has no or only little impact on the surrounding environment compared to above-ground facilities.

[0031] However, the optimal depth for underwater compressed air energy storage varies depending on the specific location and project requirements. Factors such as the availability of suitable underwater locations, the distance to the power grid 2, and the cost of installation and maintenance play an important role. In this regard, it must be noted that the subsea compressed air energy storage system 10 is comprised of a long pipeline 11 which is provided with branch connections or manifold sections 14 one after the other along the length of the pipeline 11. As discussed above, the pipeline 11 not only simplifies underwater installation as compared to systems which utilize one or more individual manifolds and several pipelines, but also allows for a linear storage system configuration which is typically advantageous as it allows installation of the system 10 on an isobath extending from the coast in a substantially offshore direction and having a substantially constant elevation. In other words, the pipeline constitutes a substantially linearly extending “manifold”, which allows for an improved installation efficiency compared to using a network of multiple conventional manifold stations.

[0032] On either side of the pipeline 11, energy storage units 15 comprising hexagonal box caissons 16 made of reinforced concrete and filled with granular ballast material are arranged on the seabed 17. Anchored to each of the caissons 16 are seven bags 12 in which the compressed air from the plant 1 is stored. The bags 12 are made from a collapsible but inelastic PVC coated fabric, which comprises a woven base fabric, e.g. polyester, a bonding or adhesive agent and exterior PVC coatings. PVC coated fabric is strong and durable.

[0033] Referring to figure 6, ballast 13 includes a connector 26 for connecting a jumper 18 thereto, the opposite end of which is connected the branch connection 14 on the pipeline 11. The connector 26 on the ballast connects the interior of the bags 12 through tube members 19 to the jumper 18 which is connected to the pipeline 11. This way, the bags 12 are in fluid communication with pipeline 11 which is connected to the onshore compressed air energy storage plant 1.

[0034] Referring to figure 4, the longitudinal end 25 of the jumper 18 to be connected to the branch connection or manifold section 14 is configured to be picked up by an ROV 20 and designed for easy connection to the branch connection 14. Each branch connection 14 comprises two connection terminals 21 for connecting two jumpers 18 and accordingly two respective energy storage units 15 to the pipeline. Referring to figure 5, at the distal end 22 of the pipeline 11 opposite the end 23 that is connected to the plant 1, a pig station 24 is provided for inspecting and / or cleaning the pipeline 11 and to remove water from the pipeline that may have entered via condensation or via a(n) (accidental) leakage.

[0035] It should be noted that the embodiment shown illustrates rather than limits the present invention and that those skilled in the art will be able to design alternative embodiments without departing from the scope of the appended claims.

Claims

Claims1. An underwater energy storage system, comprising: a source of compressed fluid; a plurality of energy storage units, each comprising at least one fluid storage container configured for storing compressed fluid therein; and a piping system fluidically connecting the source of compressed fluid to the at least one fluid storage container of each of the plurality of energy storage units to allow ingress and egress of the compressed fluid in and out of the at least one fluid storage container of each of the plurality of energy storage units, respectively, wherein the piping system comprises a pipeline connected at a longitudinal end thereof to the source of compressed fluid, the pipeline comprising a plurality of branch connection provisions provided one after the other along the length of the pipeline, each of the branch connection provisions fluidically connecting at least one energy storage unit of the plurality energy storage units to the pipeline.

2. The system according to claim 1, wherein the branch connection provisions are provided at regular intervals along the pipeline.

3. The system according to claim 1 or 2, wherein each of the branch connection provisions fluidically connects the at least one energy storage unit to the pipeline through a respective jumper.

4. The system according to claim 3, wherein the at least one energy storage unit is provided with the jumper pre-connected thereto.

5. The system according to claim 3, wherein the branch connection provision is provided with the jumper pre-connected thereto.

6. The system according to claim 3 or 4, wherein a longitudinal end of the jumper remote from the least one energy storage unit is provided with a connector configured to connect the jumper to its respective branch connection provision and near the connector with means for picking up the jumper near the connector using a remotely operated vehicle (“ROV”).

7. The system according to any one of claims 1 to 6, wherein the energy storage units each comprise a ballast and an anchoring means connected to the ballast for anchoring the at least one fluid storage container to the ballast, wherein the anchoring means preferably comprise a tether line.

8. The system according to claim 7, wherein the ballast comprises a box caisson, preferably made from reinforced concrete, fdled with granular material, such as sand or rocks, and / or concrete.

9. The system according to claim 7 or 8, when dependent on any one of claims 3 to 6, wherein the jumper connects to the ballast and wherein the anchoring means are configured to fluidically connect the at least one fluid storage container to the jumper.

10. The system according to claim 7, 8 or 9, wherein the anchoring means are configured to adapt the length thereof so as to change the height at which the at least one fluid storage container is anchored above the ballast.

11. The system according to any one of claims 1 to 10, wherein the at least one fluid storage container comprises two or more, preferably more than three, more preferably more than five fluid storage containers.

12. The system according to claim 11, when dependent on any one of claims 7 to 10, wherein the ballast has a hexagonal or a round shape.

13. The system according to claim 12, wherein the ballast has a hexagonal shape and the at least one fluid storage container comprises seven fluid storage containers anchored to the ballast.

14. The system according to any one of claims 1 to 13, wherein the at least one fluid storage container is made from a flexible material, so that the fluid storage container is collapsible, preferably wherein the flexible material is inelastic.

15. The system according to any one of claims 1 to 14, wherein the branch connection provisions are each configured to fluidically connect two energy storage units on each lateral side of the pipeline to the pipeline.

16. The system according to any one of the preceding claims, further comprising fluid flow control means, such as an inlet and outlet valve, between the pipeline and the at least one fluid storage container of each energy storage unit to control the ingress and egress of the compressed fluid in and out of the at least one fluid storage container, respectively.

17. The system according to claim 16, further comprising an umbilical for powering and / or communicating with the fluid flow control means.

18. The system according to claim 17, wherein the umbilical is arranged along, preferably on top of the pipeline.

19. The system according to claim 17 or 18, wherein each energy storage unit comprises a connector box configured for connecting the umbilical thereto to connect the umbilical to the fluid flow control means.

20. The system according to claim 19, wherein the connector box is configured to connect one or more measurement sensors for monitoring a process of storing and releasing the compressed fluid in and from the at least one fluid storage container to a transmitter for transmitting data from the measurement sensors to an onshore processing unit, wherein the transmitter preferably comprises a multiplexer.

21. The system according to any one of claims 1 to 20, further comprising a pig station connected to the longitudinal end of the pipeline opposite the longitudinal end of the pipeline that is connected to the source of compressed fluid.

22. The system according to any one of the preceding claims, wherein the source of compressed fluid is located on land, and wherein the plurality of energy storage units and the piping system are located below sea level.

23. The system according to any one of the preceding claims, wherein the plurality of energy storage units and the piping system are located at a depth of between 500 m and 1500 m below sea level, preferably between 700 m and 1200 m, more preferably at approximately 800 m below sea level.

24. The system according to any one of the preceding claims, wherein the compressed fluid comprises a compressed gas, preferably compressed air or compressed hydrogen gas.

25. A method of installing an underwater energy storage system according to any of claims 1 to 24 , comprising steps of: providing the source of compressed fluid on land; installing the pipeline on the seabed in one single pipelaying operation and connecting the pipeline to the source;installing near each of the plurality of branch connection provisions of the pipeline at least one of the plurality of energy storage units on the seabed; and connecting each energy storage unit installed on the seabed to its respective nearest branch connection provision of the pipeline installed on the seabed.

26. The method according to claim 25, wherein the step of connecting each energy storage unit installed on the seabed to its respective nearest branch connection provision of the pipeline comprises connecting a jumper between each energy storage unit and its respective nearest branch connection provision.

27. The method according to claim 26, further comprising pre-connecting the jumper to each of the plurality of branch connection provisions of the pipeline or to each of the plurality of energy storage units before installing the pipeline on the seabed or before installing the plurality of energy storage units on the seabed, respectively.

28. The method according to claim 27, wherein the step of connecting the jumper between each energy storage unit and its respective nearest branch connection provision comprises connecting the jumper pre-connected to each of the plurality of branch connection provisions to its respective nearest energy storage unit or connecting the jumper pre-connected to each of the plurality of energy storage units to its respective nearest branch connection provisions of the pipeline.

29. The method according to any one of claims 26-28, wherein the step of connecting the jumper between each energy storage unit and its respective nearest branch connection provision is carried out using a remotely operated vehicle (“ROV”).

30. The method according to any one of claims 25 to 29, wherein the step of installing the plurality of energy storage units on the seabed comprises adjusting a tether line with which the at least one fluid storage container is anchored to a ballast of its respective energy storage unit so as to install each fluid storage container at approximately the same depth below sea level.

31. The method according to any one of claims 25 to 30, further comprising providing an umbilical for powering and / or communicating with fluid flow control means, such as an inlet and outlet valve, between the pipeline and the at least one fluid storage container of each energy storage unit to control the ingress and egress of the compressed fluid in and out of the at least one fluid storage container.

32. The method according to claim 31, wherein the step of providing the umbilical comprises pre-installing the umbilical on top of the pipeline before installing the pipeline on the seabed.

33. The method according to claim 31 or 32, further comprising connecting the umbilical to a connector box on each energy storage unit, wherein each connector box connects the umbilical to the fluid flow control means.

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