Field of the Invention

The present invention relates to a storage apparatus and more specifically, but not exclusively, to an underground storage apparatus having a concrete liner for storing gaseous hydrogen.

Background to the Invention

It has previously been proposed to provide hydrogen storage in the form of an underground vessel. For example, US Patent No. 10,995,906 discloses a method of storing hydrogen involving forming an excavation in earth and constructing a storage tank comprised of integrated primary and secondary containment structures. However, the applicant has identified there may be disadvantages with such a storage facility, including practicality issues such as expensive construction, as well as safety issues including vulnerability to failure/leakage.

Examples of the present invention seek to provide a storage apparatus which ameliorates or at least alleviates one or more disadvantages of existing hydrogen storage apparatuses, or at least provides a useful alternative.

Summary of the Invention

In accordance with an aspect of the present invention, there is provided a gaseous hydrogen containment system comprising one or more sources of gaseous hydrogen located above, below, or partially below a ground surface that are connected via one or more access shafts or one or more incline tunnels to an underground tunnel excavated from any type of rock at a depth from the ground surface of no more than 200m and the tunnel has an internal diameter range of 1.8 to 16m and a total length of between 1 and 40km, and the tunnel is fitted with at least one bulkhead, wherein the interior surface of the excavated tunnel is backfilled and lined to provide a surface that is lined with at least one layer of reinforced concrete having a compressive strength of between 40-80MPa and having a thickness of between 100 and 700mm and the concrete layer facing the interior of the tunnel is coated with at least one elastomeric polymer layer, whereby the concrete layer and the rock surrounding the tunnel resists a pressure of between approximately 1-90 bar(a) from the gaseous hydrogen being stored in the tunnel, and in the event that a crack forms in the concrete layer, the crack is no more than 6 mm in width.

Preferably, the one or more sources of gaseous hydrogen are powered by one or more intermittent sources of energy.

In a preferred form, the underground tunnel is connected to a pressurized hydrogen supply system located above ground.

Preferably, the underground tunnel is divided into two or more sections to provide a pre-defined percentage of system redundancy. More preferably, the two or more sections comprise one or more spiral chambers, one or more parallel chambers or a combination thereof. Even more preferably, the chambers are round in cross-section.

In a preferred form, 150 - 5000 tonnes of gaseous hydrogen are stored in the entire length of the tunnel.

Preferably, the tunnel is excavated at a depth from the ground surface of no more than 100 m.

It is preferred that the tunnel is excavated at a depth from the ground surface of no more than 50m.

Optionally, the concrete layer is pre-stressed with steel strands. The strands may be coated with a protective polymer coating or galvanised with a non-corrosive metal.

Preferably, the concrete layer is fitted with a water sealing gasket and an open cell drainage system that drains water away from the elastomeric polymer layer. More preferably, a drainage mat layer is attached on the concrete layer facing the interior of the tunnel. Even more preferably, a mortar layer is coated on the drainage mat layer. In one particular form, the elastomeric polymer layer is coated on the mortar layer.

In a preferred form, the elastomeric polymer layer has a hydrogen permeability of equal or less than 20 g/m ² /day at a pressure range of approximately 30-60 bar(a). More preferably, the elastomeric polymer layer has a hydrogen permeability of equal or less than 10 g/m ² /day at a pressure range of approximately 30-60 bar (a).

Optionally, the tunnel is fitted with at least one flame retarder, arrestor, or a gate system to mitigate flame propagation.

In accordance with another aspect of the present invention, there is provided a gaseous hydrogen containment system comprising one or more sources of gaseous hydrogen located above, below, or partially below a ground surface that are connected via one or more access shafts or one or more incline tunnels to an underground tunnel excavated from any type of rock at a depth from the ground surface of no more than 200m and the tunnel has an internal diameter range of 1.8 to 16m and a total length of between 1 to 40km and the tunnel is fitted with at least one bulkhead, wherein the interior surface of the excavated tunnel is lined with at least one layer of reinforced concrete having a compressive strength of between 40-80 MPa and having a thickness of between 100mm and 700mm and the concrete layer facing the interior of the tunnel is coated with a steel layer, whereby the concrete layer and the rock surrounding the tunnel resists a pressure of between approximately 1-90 bar(a) from the gaseous hydrogen being stored in the tunnel, and in the event that a crack forms in the concrete layer, the crack is no more than 6mm in width.

Preferably, the one or more sources of gaseous hydrogen are powered by one or more intermittent sources of energy.

Preferably, the underground tunnel is connected to a pressurized hydrogen supply system located above ground. A pressurized electrolyser and/or a hydrogen compressor can be parts of the pressurized hydrogen supply system.

In a preferred form, the underground tunnel is divided into two or more sections to provide a pre-defined percentage of system redundancy. More preferably, the two or more sections comprise one or more spiral chambers, one or more parallel chambers or a combination thereof. Even more preferably, the chambers are round in cross-section.

Preferably, 150 - 5000 tonnes of gaseous hydrogen are stored in the entire length of the tunnel. In a preferred form, the tunnel is excavated at a depth from the ground surface of no more than 100m. More preferably, the tunnel is excavated at a depth from the ground surface of no more than 50m.

Optionally, the concrete layer is pre-stressed with steel strands, that are optionally coated with a protective polymer coating or galvanised with a non-corrosive metal.

In a preferred form, the concrete layer is fitted with water sealing gaskets and an open cell drainage system that drains water away from the steel layer.

Optionally, the tunnel is fitted with at least one flame retarder, arrestor or a gate system to mitigate flame propagation.

In accordance with a further aspect of the present invention, there is provided a storage apparatus for storing gaseous hydrogen, the storage apparatus including a source of gaseous hydrogen, a storage cavity located below a ground surface, and a conduit between said source and said storage cavity for supply of hydrogen from said source to said storage cavity, wherein said storage cavity is provided with a liner formed of a settable material.

The source of gaseous hydrogen may be located above, below, or partially below a ground surface

In some embodiments, the settable material may be concrete. The settable material may be reinforced concrete. The reinforced concrete may have a compressive strength of between 40 and 80MPa. The reinforced concrete may have a thickness of between 100 and 700mm.

The concrete layer and rock surrounding the storage cavity may be adapted to resist a pressure of between approximately 1-90 bar(a) from the gaseous hydrogen being stored in the storage cavity. The concrete layer and said rock surrounding the storage cavity may be arranged such that, in the event that a crack forms in the concrete layer, a width of the crack does not exceed 6mm.

The liner may include a polymeric internal layer, located internally of the reinforced concrete. The internal layer may be formed of an elastomeric polymer. In some embodiments, the storage cavity may be in the form of a tunnel. The tunnel may have a total length between 1 km and 40 km. The tunnel may be excavated from rock such that surrounding rock provides confinement against expansion of the storage cavity. The tunnel may be excavated at a depth from said ground surface not exceeding 200m. The tunnel may have an internal diameter within the range of 1.8 m to 16m.

The tunnel may be in the form of a spiral. The tunnel may be in the form of multiple parallel tunnel sections interconnected by a cross manifold. Alternatively or additionally, the tunnel may be fitted with at least one bulkhead.

Brief Description of the Drawings

The invention is described, by way of non-limiting example only, with reference to the accompanying drawings in which:

Figure 1 shows examples of the present invention in which the storage cavity is in the form of a spiral type tunnel;

Figures 2A and 2B show an example of the present invention in which the storage cavity is in the form of parallel tunnels;

Figure 3 shows a cross sectional view of a concrete lining with sprayed membrane;

Figure 4 shows a tortuous effect of fillers in a polymer barrier;

Figure 5 shows storage of hydrogen within a hydrogen process chain;

Figure 6A shows a graph of the volume of excavation (m ³ ) required to store 600 tonnes of gaseous hydrogen stored at 20°C at different pressures (bar(a)).

Figure 6B shows a graph of the tunnel length (m) required to store 600 tonnes of gaseous hydrogen stored at 20°C at different pressures (bar(a)) and with different internal tunnel diameters (06m; 08m; 012m);

Figure 6C shows a graph of balance depth, i.e. the depth at which ground pressure theoretically equals storage pressure. Tunnels can be shallower than shown in this graph, for a given pressure; Figure 7 depicts cost optimisation in relation to storage pressure ranges;

Figure 8 shows a principle process flow diagram of a gas containment and purging system;

Figure 9 shows a prestressing system of a storage apparatus in accordance with an example of the present invention;

Figure 10A and 10B shows an integrated steel liner option;

Figures 11A to 11C show examples of open cell drainage systems including use of mesh in a tunnel lining, mesh material detail, and cell shaped geotextile;

Figures 12A to 12C show examples of open cell drainage systems including foamed grout, perforated pipes, and a combination of foamed grout and perforated pipes; and

Figure 13 shows an open cell drainage system is similar to Figure 11 A.

Detailed Description

With reference to Figures 1 to 13 of the drawings, there is shown a storage apparatus 10 for storing gaseous hydrogen 12. The storage apparatus 10 includes a source 14 of gaseous hydrogen located above a ground surface 16, a storage cavity 18 located below the ground surface 16, and a conduit 20 between said source 14 and said storage cavity 18 for supply of hydrogen from the source 14 to the storage cavity 18. The storage cavity 18 is provided with a liner 22 formed of a settable material 24. Advantageously, the storage apparatus 10 uses surrounding earth and rock as well as the liner 22 to provide containment of the gaseous hydrogen 12.

The settable material 24 may be in the form of concrete 26. In particular, the settable material 24 may be in the form of reinforced concrete 26.

The storage cavity 18 may be located entirely below the ground surface 16, as shown in Figure 8.

As shown in Figure 3, the liner 22 may include a polymeric internal layer 28, located internally of the reinforced concrete 26. The internal layer 28 may be formed of an elastomeric polymer 30. With specific reference to Figure 3, there is shown ground material 32, backfill annulus grout 34, precast concrete segments 36, water sealing gaskets 38, a drainage mat layer 40, a sprayed mortar regulating layer 42 and a spray applied gas proof membrane 44.

The storage cavity 18 may be in the form of a tunnel 46. The tunnel 46 may have a total length of between 1 km and 40km. The tunnel 46 may be excavated from rock such that ground material 32 in the form of surrounding rock provides confinement against expansion of the storage cavity 18. The volume of ground material that is required to be excavated to store 600 tonnes of gaseous hydrogen at 20 °C and at different pressures is shown in Figure 6a. From approximately 20 bar(a), the volume of excavation required decreases at a lower rate and at 30 bar(a), the volume of excavation begins to flatten.

The tunnel 46 may be excavated at a depth from the ground surface 16 not exceeding 200m. The tunnel 46 may have an internal diameter within the range of 1 ,8m to 16m. Figure 6b shows that the internal diameter of the tunnel affects the required length of the tunnel. To store 600 tonnes gaseous hydrogen at 30 bar(a) pressure, for example, a tunnel having an internal diameter of 12 m does not need to be as long as a tunnel having an internal diameter of 6 m.

The depth of the tunnel is also a factor as the deeper the tunnel, the higher the permitted storage pressure, so a tunnel of given diameter can be shorter.

An economic benefit of the apparatus is that it may be optimised for the best cost benefit. For example, in Figure 7, the capital expenditure curve (CAPEX) and the operational cost curve (OPEX) is shown in one graph. The variable is the pressure range or amount of cushion gas on the horizontal axis. The sum of both curves (CAPEX + OPEX) shows an optimum at a pressure range of 30-15 bar or 600T cushion gas. The optimum point will be different from project to project, depending on external equipment constraints and geological conditions. As shown in Figure 8, the tunnel 46 may be fitted with at least one bulkhead 48. The reinforced concrete 26 may have a compressive strength of between 40 MPa and 80MPa. The reinforced concrete 26 may have a thickness of between 100mm and 700mm.

The concrete layer 26 and rock ground material 32 surrounding the storage cavity 18 may be adapted to resist a pressure of between approximately 1-90 bar(a) from the gaseous hydrogen 12 being stored in the storage cavity 18. The concrete layer 26 and the rock ground material 32 surrounding the storage cavity 18 may be arranged such that, in the event that a crack forms in the concrete layer 26, a width of the crack does not exceed 6mm.

The tunnel 46 may be in the form of a spiral 50, as shown in Figure 1. Alternatively, the tunnel 46 may be in the form of multiple parallel tunnel sections 52 interconnected by a cross manifold 54.

Accordingly, the storage apparatus 10 may provide a gaseous hydrogen containment system comprising one or more sources 14 of gaseous hydrogen 12 located above, below, or partially below a ground surface that are connected via one or more access shafts or one or more incline tunnels to an underground tunnel 46. The tunnel 46 may be excavated from any type of rock at a depth from the ground surface 16 of no more than 200m and the tunnel 46 may have an internal diameter range of 1.8m to 16m and a total length of between 1km to 40km. The tunnel 46 may be fitted with at least one bulkhead 48. An interior surface of the excavated tunnel 46 may be backfilled and lined to provide a surface that is lined with at least one layer of reinforced concrete 26 having a compressive strength of between 40MPa to 80MPa and having a thickness of between 100mm and 700mm. In one form, the concrete layer 26 facing the interior of the tunnel 46 is coated with at least one elastomeric polymer layer 28, whereby the concrete layer 26 and the rock ground material 32 surrounding the tunnel 46 resists a pressure of between approximately 1-90 bar(a) from the gaseous hydrogen 12 being stored in the tunnel 46. In the event that a crack forms in the concrete layer 26, the storage apparatus 10 is adapted such that the crack is no more than 6mm in width.

The one or more sources 14 of gaseous hydrogen 12 may be powered by one or more intermittent sources of energy. The underground tunnel 46 may be connected to a pressurized hydrogen supply system 14 located above ground. The underground tunnel 46 may be divided into two or more sections to provide a pre-defined percentage of system redundancy. The two or more sections may comprise one or more spiral chambers, one or more parallel chambers or a combination thereof. The chambers may be round in cross-section. In one form, 150 - 5000 tonnes of gaseous hydrogen 12 are stored in the entire length of the tunnel 46. The tunnel 46 may be excavated at a depth from the ground surface 16 of no more than 100m. More specifically, the tunnel 46 may be excavated at a depth from the ground surface 16 of no more than 50m.

The concrete layer 26 may be pre-stressed with steel strands, that are optionally coated with a protective polymer coating or galvanised with a non-corrosive metal. The concrete layer 26 may be fitted with a water sealing gasket 38 (see Figure 3) and an open cell drainage system that drains water away from the elastomeric polymer layer 28. The drainage mat layer 40 may be attached to the concrete layer 26 facing the interior of the tunnel 46. The mortar layer 42 may be coated on the drainage mat layer 40. The elastomeric polymer layer 28 may be coated on the mortar layer 42.

The elastomeric polymer layer 28 may have a hydrogen permeability of equal or less than 10 g/m ² /day at a pressure range of approximately 30-60 bar(a). The tunnel 46 may be fitted with at least one flame retarder, arrestor, or a gate system 58 to mitigate flame propagation.

As will be appreciated from the above, there is provided a gaseous hydrogen containment system comprising one or more sources 14 of gaseous hydrogen 12 located above, below, or partially below a ground surface that are connected via one or more access shafts or one or more incline tunnels to an underground tunnel 46 excavated from any type of rock at a depth from the ground surface 16.

It is to be understood that the term “containment” is used herein to mean a loss of hydrogen of approximately no more than 0.4% of the storage mass of hydrogen per day, and preferably a loss of hydrogen of approximately no more than 0.2% of the storage mass of hydrogen per day.

It is also to be understood that the term “partially below ground” is used herein to mean that the structure of the one or more sources of gaseous hydrogen that may include a pressurized hydrogen supply system is located both above and below the ground surface. Preferably, the part of the structure that is below the ground surface (i.e. underground) is at a depth from the ground surface that is not greater than the depth of the underground tunnel.

In one form, the depth from the ground surface 16 of the underground tunnel 46 is no more than 200m and the tunnel 46 has an internal diameter range of 1.8m to 16m and a total length of between 1km and 40km. The tunnel 46 may be fitted with at least one bulkhead 48. The interior surface of the excavated tunnel 46 is lined with at least one layer of reinforced concrete 26 having a compressive strength of between 40MPa and 80MPa and having a thickness of between 100mm and 700mm. The concrete layer facing the interior of the tunnel 46 may be coated with a steel layer 60, whereby the concrete layer 26 and the rock ground material 32 surrounding the tunnel 46 resists a pressure of between approximately 1 bar(a) and 90 bar(a) from the gaseous hydrogen 12 being stored in the tunnel 46. In the event that a crack forms in the concrete layer 26, the storage apparatus 10 may be adapted such that the crack is no more than 6mm in width.

The one or more sources 14 of gaseous hydrogen 12 may be powered by one or more intermittent sources of energy. The underground tunnel 46 may be connected to a pressurized hydrogen supply system 14 located above ground.

The underground tunnel 46 may be divided into two or more sections to provide a pre-defined percentage of system redundancy. The two or more sections may comprise one or more spiral chambers, one or more parallel chambers or a combination thereof. The chambers may be round in cross-section. In one form, 150 - 5000 tonnes of gaseous hydrogen 12 are stored in the entire length of the tunnel 46. The tunnel 46 may be excavated at a depth from the ground surface 16 of no more than 100m. More specifically, the tunnel 46 may be excavated at a depth from the ground surface 16 of no more than 50m.

The concrete layer 26 may be pre-stressed with steel strands, that are optionally coated with a protective polymer coating or galvanised with a non-corrosive metal. The concrete layer 26 may be fitted with a water sealing gasket 38 (see Figure 3) and an open cell drainage system that drains water away from the elastomeric polymer layer 28. Examples of open cell drainage systems are shown in Figures 11A-11C, Figures 12A-12C and Figure 13. The drainage system in Figure 13 shows the principle of water drainage through the permeable layer with collection of water in a pipe located in the bottom or invert of this layer. The system also shows the venting of hydrogen through the same permeable layer with collection of hydrogen in a pipe located at the top or crown of this layer.

The drainage mat layer 40 as shown in Figure 3 may be attached to the concrete layer 26 facing the interior of the tunnel 46. The mortar layer 42 may be coated on the drainage mat layer 40. The elastomeric polymer layer 44 may be coated on the mortar layer 42.

The elastomeric polymer layer designated as 28, 30 or 44 may have a hydrogen permeability of equal or less than 20 g/m ² /day at a pressure range of approximately 30-60 bar(a). Preferably, the elastomeric polymer layer designated as 28, 30 or 44 has a hydrogen permeability of equal or less than 10 g/m2/day at a pressure range of approximately 30-60 bar(a). The tunnel 46 may be fitted with at least one flame retarder, arrestor, or a gate system 58 to mitigate flame propagation.

In one example, there is provided a system using excavated underground space in the form of concrete lined tunnels for hydrogen storage.

The underground space comprises tunnels, shafts and caverns. Concrete lined tunnels are the main storage element. The excavation method for these underground spaces may comprise any one or a combination of: drilling and blasting; use of a tunnel boring machine (TBM); use of pipe jacking equipment; top down drilling or raise boring. The main tunnel solutions will be either a spiral type tunnel or multiple parallel tunnels, as shown in Figure 1 and Figure 2 A and 2B.

Hydrogen gas containment can be achieved by using a combination of containment components and lining layers. A main containment component can be a sprayed on polymer based membrane, which has low hydrogen permeability.

Containment components and lining layers as shown in Figure 3, may be a combination of the following: surrounding earth and rock 32; primary concrete layer 26 or 36;

• permeability layer for ground water drainage and leak detection 40;

• regulating layer 42; and

• polymeric membrane 28, 30 or 44.

Containment components may include:

1. primary containment by polymer based membrane, using the “tortuous effect”, Figure 4 (alternatively spiral welded steel layer);

2. diverting and collecting leakage by a permeable layer or channels;

3. secondary containment by the structural concrete lining and a bulkhead to seal off the tunnel; and/or

4. secondary containment by the rock mass.

Main intent of the design is to provide hydrogen gas storage to overcome the intermittency of supply in the process chain from delivered hydrogen to a carrier plant, see Figure 5. The cause of the hydrogen intermittency is the intermittency of the renewable power supply. The hydrogen storage is located between the hydrogen source (such as one or more electrolysers) and the carrier plant (such as a liquefication or ammonia plant). There is the potential however for this design to be extended to suit extended storage applications such as weekly or monthly supply chain or strategic storage and being separate to the hydrogen process or value chain. The system can be purged with nitrogen in several cycles. A schematic purging system is shown in Figure 8. The system will not use a vacuum for purging or in any other operation at any point of time.

Examples of the present invention may provide the following advantages.

1. A pressure vessel design is not required, because the surrounding rock is designed to take some of the structural containment load, with the installed liner membrane providing another containment layer. 2. Costly, above surface steel structures are avoided.

3. Underground installation is intrinsically safer, better protected against containment faults and fire disasters.

4. Underground installation is much better protected against sabotage or terrorist attacks.

5. Does not require extensive acquisition of land.

6. Reduced maintenance compared to overground tank storage; winter maintenance is minimized.

7. The product quality is better maintained for long time storage because of stable temperatures and dry environment inside the underground space.

8. Compared to salt caverns, this storage is drier and cleaner, and the hydrogen does not require extensive drying or purification processes after withdrawal.

9. Compared to salt caverns, this storage solution is structurally stable at atmospheric pressure and doesn’t require a minimum cushion gas and pressure to maintain rock stability. Cushion gas can however, be beneficial in reducing the cost of compression.

10. Compared to unlined mined caverns, this storage solution is drier and cleaner, and the hydrogen doesn’t require extensive drying or purification processes after withdrawal. It is also at a much lesser depth underground, thus more economical.

11. Compared to concrete lined mined caverns, this storage solution is exploiting the round tunnel shape as a means to optimize the design in terms of depth underground and in minimizing the structural lining requirements.

Advantageously, the use of the combining effect of rock mass and concrete lining for pressure containment at reasonable geological depth may reduce lining and containment cost. A numerical (FEA) model may be used for designing an example with optimised efficiency.

An embodiment of the present invention may have the following features.

1. The division of the storage tunnel in several sections, e.g. number of spirals or number of parallel tunnels will increase redundance of the storage facility and enable continuous operation over its lifetime.

2. The use of a tunnel boring machine (TBM) to excavate the storage volume will enable fast and cost-efficient construction.

3. The design of a concrete lining with pre-defined flexibility will be used to limit crack allowance < =3mm. If the rock mass cannot support the crack allowance a prestressed ring design may be used (see Figure 9). The crack opening is controlled by design parameters like concrete thickness, amount of reinforcing bar (rebar) or amount of internal pre-stressing. This crack limitation by design will only enable the use of a flexible poly-membrane for hydrogen containment.

4. Same as above, by controlling the crack opening by design parameters, the storage can be used for frequent cycling, e.g. daily cycling. The stresses of frequent pressure cycling will be absorbed by the different design elements, e.g. stiffness of the rock mass, stiffness of the concrete lining and tensile strength of the steel elements (pre-stress strands, Figure 9). By controlling the crack opening, the storage system can be made suitable for use with intermittent renewable energies.

5. The use of a flexible polymer-based containment membrane, which can bridge any lining faults <=6mm and will not fail on minor flexible movements, can be used for hydrogen containment. In one example, a polymeric membrane may be required to fulfil a list of performance criteria (functions), as follows: • hydrogen permeability of the membrane is <= =10g/m ² /day at approximately 30bar(a); or alternatively <=20g/m ² /day at approximately 30 bar(a)

• 0-<=6 mm gap cycling possible with the membrane remaining elastic during loading and unloading of hydrogen;

• thickness of the membrane is approximately 5mm;

• crack bridging by the membrane is possible up to and including 6mm; and/or

• the life span of the membrane can be up to and including 50y which can be reapplied if necessary.

6. The use of a combination of containment membrane and drainage layer (see Figure 3 and Figure 10A) will isolate the water drainage of the surrounding rock mass from the function of the sealing membrane. This will facilitate the performance of the sealing membrane and achieve an economically reasonable and safe containment outcome. Hydrogen loss will be minimized.

7. The system can be designed for a maximum storage pressure of 60bar(a), which limits the cost of compression and also enables the use of a polymeric membrane. Higher storage pressures such as 90 bar(a) can be facilitated with the use of an integrated steel liner option.

8. The use of an integrated steel liner option 22 (Figures 10A and 10B) will extend the application range for higher pressure ratings and weaker geological conditions, where joint or crack opening is not suitable for the application of a polymeric membrane.

9. The use of a polymeric membrane or a steel liner 60 will enable storage of gaseous hydrogen in a drier and cleaner environment when compared to a salt cavern, which eliminates the need to dry or purify the hydrogen when it is withdrawn form storage. 10. The use of an optimization algorithm for tunnel diameter, length, depth, pressure and divisions will achieve the most economical solution for a given storage volume, as shown in Figures 6a-c. The algorithm for this optimization is:

O unptt. with volume being a function of storage pressure, length being a function of tunnel diameter and depth being a function of storage pressure.

11. The use of an optimization algorithm of pressure and cushion gas volumes will achieve the most economical solution for an operational model with given operational pressure and storage volume requirement, as shown in Figure 7. The algorithm for this optimization is: upt. with cost of construction being a function of volume and cost of compression being a function of final pressure. In general, cost of construction represents CAPEX and cost of compression represents OPEX.

12. The storage apparatus or storage system can have safety features to prevent flame propagation. These features can include bulkheads and flame retarders incorporated into the tunnel as shown in Figure 8. These features can make the storage tunnel safer to operate than other storage solutions.

13. The storage apparatus or containment system can be accessible by men, under atmospheric conditions for inspection, maintenance or certification purposes. This is a distinctive feature separating it from geological storage solutions like salt caverns or depleted reservoirs.

14. All parts of the storage system can be installed underground, which can minimize the vulnerability to environmental or terrorist impact.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not by way of limitation. It will be apparent to a person skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the present invention should not be limited by any of the above described exemplary embodiments. The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

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