GB2614756A

GB2614756A - Energy harvesting in subsea shuttle

Info

Publication number: GB2614756A
Application number: GB2200585.4A
Authority: GB
Inventors: Matias Dupuy Pablo; E Ellingsen Kjell; Roger Finserås Kjell; Francesco Marra; Skrøvseth Ola
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2022-01-18
Filing date: 2022-01-18
Publication date: 2023-07-19

Abstract

An autonomous underwater vehicle for transporting a cargo fluid, comprises an outer hull (10, Fig 1) defining an internal volume, a pressure communication channel (14, Fig 1) between the internal volume and the exterior of the outer hull, arranged to control the pressure in the internal volume based on the exterior pressure, one or more internal vessels 51 for containing the cargo fluid. The one or more internal vessels are arranged to further contain a buffer fluid 53. The vehicle further comprises an energy harvesting device 57, 59 and 62, coupled to an inlet 58 or an outlet 56 of the one or more internal vessels. The energy harvesting device being arranged to extract energy from an inflow of fluid into the one or more internal vessels when coupled to the inflow, or to extract energy from an outflow of fluid from the one or more internal vessels when coupled to the outflow. A method of extracting energy in an autonomous underwater vehicle is also disclosed.

Description

Energy harvesting in Subsea Shuttle

Field of the invention

The invention relates to underwater vehicles and the operation thereof, and more specifically to the charging of a power supply of autonomous underwater vehicles by energy harvesting.

Background

Research Disclosure 662093 (published 20 May 2019) describes a subsea shuttle system, using autonomous subsea shuttles for transportation and storage purposes.

Research Disclosure 677082 (published 21 August 2020) provides further detail regarding possible shuttle structure and support, applications, e.g., on/offloading of a payload, and the propulsion system of the subsea shuttle.

UK patent number 2585758 (published 22 December 2021) describes an underwater vehicle for transporting a fluid cargo, comprising an outer hull, one or more internal vessels for containing the fluid cargo; a pressure compensation system arranged to control the pressure of the cargo and a barrier for separating a first portion of the vessel from a second portion of the vessel, the barrier permitting pressure communication between the first and second portions.

UK patent number 2585488, published 4 August 2021, describes an assembly for loading or unloading an autonomous underwater vehicle.

The use of fossil fuels for transport continues to contribute to climate change. The operation of the subsea shuttle below the surface reduces energy consumption when compared to conventional surface vessels, because the operating conditions below the water surface are more stable than those at the surface, where weather, waves and other traffic disrupt a constant operation. The shuttle is envisaged be large, with a correspondingly large inertia, so any adjustments to the movement will require large amounts of energy. A more stable and constant operating process of the subsea shuttle process therefore provides a reduction in energy consumption. However, further options for saving energy consumption are desirable.

Statement of invention

According to a first aspect of the invention, there is provided an autonomous underwater vehicle for transporting a cargo fluid, comprising: an outer hull defining an internal volume; a pressure communication channel between the internal volume and the exterior of the outer hull, arranged to control the pressure in the internal volume based on the exterior pressure; one or more internal vessels for containing the cargo fluid; wherein the one or more internal vessels are arranged to further contain a buffer fluid; the vehicle further comprising: an energy harvesting device, coupled to an inlet or an outlet of the one or more internal vessels, the energy harvesting device being arranged to extract energy from an inflow of fluid into the one or more internal vessels when coupled to the inflow, or to extract energy from an outflow of fluid from the one or more internal vessels when coupled to the outflow.

The energy harvesting device may be coupled to the outlet, and may be arranged to receive the buffer fluid displaced by cargo fluid during a loading process.

The vehicle may further comprise a valve arranged at the outlet and between the energy harvesting device and the one or more internal vessels, for isolating the one or more internal vessels from the energy harvesting device.

The energy-harvesting device may be arranged to extract energy from the temperature of the fluid or the pressure of the fluid, or from a combination of temperature and pressure of the fluid.

The energy harvesting device may comprise a turbo expander. An inlet of the turbo expander can then be connected to an inlet of the shuttle, while an outlet of the turbo expander is connected to an inlet of the one or more vessels, and the turbo expander is rotatably connected to a generator.

The energy-harvesting device may comprise a rotor and a generator.

The energy harvesting device may comprise an electrical generator, wherein the electrical generator is arranged to generate electrical energy when driven by a fluid overpressure, and wherein the electrical generator is arranged to pump fluid and generate an overpressure when run in reverse.

The energy-harvesting device may comprise a heat-exchanger or a thermoelectric generator.

The vehicle may further comprise a battery electrically coupled to the energy harvesting device for storing energy generated by the energy harvesting device.

According to a second aspect, there is provided a method of energy-harvesting in an autonomous underwater vehicle, the underwater vehicle having an outer hull and one or more internal vessels arranged within the outer hull, the outer hull defining an internal volume, the method comprising: loading or unloading the one or more vessels with a cargo fluid, wherein loading the one or more vessels with the fluid cargo displaces a buffer fluid initially provided within the one or more vessels; passing the buffer fluid, or the cargo fluid through or past an energy harvesting device; extracting energy from the buffer fluid or the cargo fluid with the energy-harvesting device, wherein the energy is extracted from the pressure and/or the temperature of the buffer fluid or the cargo fluid.

The energy harvesting device may be coupled to the one or more vessels to receive buffer fluid, wherein the buffer fluid is displaced by the cargo fluid during loading, and wherein the displacement gives rise to a pressure drop over the energy harvesting device.

The method may further comprise charging a battery with the energy extracted with the energy-harvesting device.

The method may further comprise isolating the one or more internal vessels from the energy harvesting device after loading of the one or more internal vessels for maintaining an overpressure within the one or more internal vessels.

The method may further comprise using the energy harvesting device for pumping fluid into the one or more vessels before or after the step of extracting energy from the buffer fluid.

The method may further comprise extracting energy during transport of the cargo fluid.

Figures Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an underwater vehicle containing a pressure vessel; Figure 2 is a schematic drawing of a turbo-expander and storage container, adapted for use in a subsea shuttle; Figure 3 is a schematic drawing of a rotor and storage container; Figure 4 is a schematic drawing of a heat exchanger and a storage container; Figure 5 is a schematic drawing of a storage container with two rotors; and Figure 6 is a flow diagram of a method.

Detailed description

The subsea shuttle enables improved energy management. The subsea transportation of a fluid cargo (i.e. liquid or gas) is implemented by providing a pressure compensation system to maintain or control the internal pressure of the fluid based on the external hydrostatic pressure. In one example of the shuttle, a double hull is used.

Further, pressure within the hull of the shuttle but outside the pressure vessel holding the fluid may also be maintained and controlled at or near external hydrostatic pressure. In this way, it is ensured that the internal pressure of the fluids remains the same as or similar to the external hydrostatic pressure, meaning that the vessel (containing the fluid) is not at risk of a collapse (or burst) failure.

Although the word 'sea' and 'seawater' are used throughout, these may equally be understood as 'lake' and 'freshwater', respectively, and the invention is envisaged to be used in any large body of water. Similarly, when the words 'seabed' or 'sea surface' are used, this is not intended to be limited to a sea in a strict sense but should also be understood to cover 'ocean bed' or 'ocean surface', or similar terms for any large body of water.

Figure 1 illustrates a schematic underwater vehicle according to an embodiment of the invention. The underwater vehicle (or 'shuttle') may be an autonomous underwater vehicle (AUV), or a remotely operated underwater vehicle (WV). The vehicle comprises an outer hull 10, having a hydrodynamic shape to reduce drag. An elliptical outer hull 10 is shown in Figure 1, but other hydrodynamic shapes known in the art are suitable. Within the outer hull 10, an internal pressure vessel is provided 12. The pressure vessel 12 may be fixed within the outer hull 10 using a frame or other rigid supports (not shown). In this way, a space between the outer hull 10 and pressure vessel 12 is formed. The outer hull 10 has a channel 14, which is in fluid communication, or pressure communication, with the surrounding water when the vehicle is submerged. When the vehicle is submerged, the space between the outer hull 10 and inner pressure 12 vessel at least partially fills with seawater, depending on the net buoyancy requirements. In some embodiments, a part of the outer hull 10 volume is occupied by one or more compartments for containing gas (e.g. ballast tanks), as discussed in more detail below. In this way, the gas remains separate to the seawater, meaning that sloshing is avoided. In some embodiments, the channel 14 is selectively closed (e.g. via operation of a valve) to allow or block fluid communication through the channel.

Although pressure vessel 12 in Fig. 1 is illustrated as a longitudinal vessel in a horizontal orientation, in some configurations, the vehicle may also carry one or more longitudinal vessels in a vertical orientation. More specifically, a plurality of vertical vessels may be carried by the vehicle, perhaps as many as a few hundred, while the overall shape of the outer hull still has a longitudinal hull in a horizontal orientation during normal use.

At the stern (back end) of the vehicle, a propeller 16 is provided. The propeller is coupled to a power source and control unit (not shown) to enable autonomous and/or remote operation of the vehicle. An electric power source is preferably used.

The vehicle structure shown in Figure 1 has some similarities to a double hull structure used in some conventional submarines. However, a difference in the present technology is that instead of having a pressure hull maintained at or near to atmospheric pressure to accommodate personnel, the inner structure is maintained at a pressure similar to the external hydrostatic pressure. Advantageously, this means that the vehicle can be designed with a much lower collapse pressure capacity than a standard pressure hull, which results in significant weight savings (e.g. by reducing the need for stiffeners). By "similar" to the external hydrostatic pressure, it is meant that the internal pressure is kept suitably close to the external pressure so that the pressure differential dP (i.e. overpressure or under-pressure) is not too large. If the overpressure is too large, the vehicle may undergo a burst failure. Conversely, if the under-pressure is too large, the vehicle is at risk of a collapse failure. The collapse pressure threshold is sensitive to geometric imperfections in the vehicle shape, meaning that a smooth, curved shape is preferable.

The pressure vessel 12 described above provides temporary storage for a payload.

Both the pressure and temperature of the payload may be similar to the pressure and temperature of the external subsea environment once equilibrium is reached. However, pressure vessel 12 may also receive a payload which has an overpressure relative to the surrounding internal volume. As described above, the pressure within the cavity defined by the outer hull is preferably similar to the external hydrostatic pressure of the seawater During the loading process, the liquid payload displaces a buffer fluid in the pressure vessel 12 by way of an overpressure. The buffer fluid and payload may be separated from each other, for example by a piston barrier, by a membrane, or by another barrier. Alternatively, there may not be a physical barrier between the buffer fluid and the payload. For example, the two materials may have a different density and separated from each other under the influence of gravity. The gravity separation works better when the pressure vessel has a vertical orientation than when it has a horizontal orientation, although there will also be a separation in a horizontal orientation. A difference in viscosity may also separate the fluids, optionally in combination with a difference in density.

Another example of a displacement process without a barrier is when there is not necessarily a difference in density or viscosity, and the buffer fluid and cargo fluid could mix, such as under the influence diffusion or turbulence. The buffer fluid initially occupies most of the pressure vessel, and the buffer fluid is displaced by the cargo fluid when the cargo fluid enters the pressure vessel. The mechanism by which the buffer fluid is driven out of the vessel and past the energy harvesting device is that of 'first-in-first-out' combined with the separation of the inlet and outlet. Figure 5, which will be described in more detail in relation to another embodiment, shows an elongate pressure vessel with inlet and outlet at opposite ends.

As a particular example of a setup in which the buffer fluid and cargo fluid are in direct contact with each other, the buffer fluid may be water and the cargo fluid may be an oilin-water mixture. At the outlet towards the energy-harvesting device, an oil-in-water meter is provided. Initially, only the water from the buffer fluid flows through the outlet due to displacement by the cargo fluid. Subsequently, some mixing of the buffer fluid and cargo fluid will take place, and gradually the concentration of the oil in the water leaving the outlet will increase. A threshold can be set and when the measured ratio of oil and water exceeds the threshold, the loading process is stopped to protect the energy-harvesting device. It is envisaged that this process would still enable 80% of water as the buffer fluid to be displaced while substantially avoiding the oil in water phase to leave the pressure vessel. Alternatively, if some oil in water content is acceptable in the outlet and/or the energy harvesting device, substantially all of the buffer fluid can be displaced, as well as some of the oil in water received through the inlet. The oil in water phase is preferably not released outside the vehicle to avoid any pollution of the environment, but stored in a part of the vehicle which is fluidly isolated from the environment, although there may be pressure communication as discussed previously.

In the absence of other processes influencing the temperature, the payload temperature is initially close to the temperature of the payload fluid source, such as a reservoir, and reaches equilibrium with the external temperature over time because the overall structure is relatively lightweight and does not include specific temperature insulation in some embodiments. The payload mass is typically large and the temperature exchange will take place over a much longer timescale when compared to the pressure balancing, in the absence of other processes influencing the temperature.

The temperature change will require a corresponding re-balancing of the fluid barrier, in particular when gasses are included which will contract under the influence of a dropping temperature.

The inventors have appreciated that when the original fluid pressure or temperature is higher than the final pressure or the final temperature, energy harvesting of the pressure or temperature drop can be used to charge batteries or drive a process within the shuttle, such as pumping or control operations. Although the source of the original fluid is not relevant for this particular process, a specific example of a source is a reservoir within the seabed formation. Both the source pressure and temperature may be higher than the final pressure and temperature. The pressure within a well is much higher than the hydrostatic pressure near the seabed in many practical situations, and such pressure difference may be harvested to reduce the overall energy consumption.

In a particular example, the payload is received at a higher pressure than the initial pressure within the pressure vessel before the payload is received. Initially, the pressure vessel may almost entirely be filled with a buffer liquid. The buffer liquid may be in equilibrium with the hydrostatic pressure outside the pressure vessel, which may in turn be in equilibrium with the pressure of the seawater outside the vehicle. When receiving the payload, the buffer liquid is displaced by the higher pressure of the payload. The buffer liquid is expelled from the pressure vessel, and when it is expelled from the pressure vessel, the buffer liquid drives an energy generator such as a turbine. The action of driving the turbine extracts the energy in the form of pressure from the buffer liquid, and a pressure drop occurs over the energy generator. The final pressure after the energy generator may be in equilibrium with the interior of the vehicle, and thereby with the surrounding seawater given that the interior and exterior of the outer hull are in equilibrium.

An example of a possible pressure of a hydrocarbon reservoir, or CO2 source, is: at 46 bara and a temperature of: 5 degC. An example of a hydrostatic pressure near the seabed at a depth of 200m from the water surface, is and a temperature is: 5 degC. An example of the total amount of liquid is 15000 m3. The stored energy in the sense of work that can be done is 10 MWh.

Examples of energy harvesting devices utilised in the subsea shuttle are now described, whereby any combination of these devices may be used to optimise the energy harvesting. The payload is described as comprising fluid, or fluids, which covers a mixture of liquid and gas phases. For example, a mixture of oil, water and gas may be transported, while the combined mixture is referred to as 'fluid'. The ratio of the phases may also vary during transport, with some of the gasses condensing into liquids under influence of a temperature drop.

A first example of an energy-harvesting device is a turbo-expander, illustrated in Fig. 2.

A turbo-expander 21 arranged between the incoming fluid flow 22 and the pressure vessel 12. Although reference is made to fluids, turbo-expanders are particularly suitable for use with a gas. The gas entering the shuttle has a temperature Ti, and a pressure P1, while the fluid leaving the turbo-expander has a lower temperature 12 and pressure P2. The turbo expander rapidly expands the volume of the gas, thereby dropping the temperature and the pressure of the gas. An expander wheel is arranged within the expander, and the expander wheel turns when the gas flows past it during expansion. The expander wheel is coupled to a drive shaft, which in turn can drive an electricity generator (not illustrated in the Figure). The output of the generator is used to charge batteries of the subsea shuttle. The turbo expander preferable works only for inflow of gas from a reservoir, and generally does not work in reverse, so a one-way valve is included in channel 23 and a separate outlet is provided for unloading the cargo from pressure vessel 12.

Fig. 3 illustrates an energy-harvesting device, whereby primarily a pressure drop is used to generate electrical power. The incoming fluid flow passes a rotor 31 comprising impellers 32. The fluid pressure drives the rotation of rotor 31 after impact on the impellers 32. The rotor 31 is coupled to a generator 33, comprising a rotatable permanent magnet 34 surrounded by coils 35. The inflow 36 has a pressure P1 and a temperature Th, and the outflow of the device has a pressure P3 and a temperature T3.

Examples of input and output pressures are: 46 and 26 bara respectively, and a corresponding example of mechanical power output is above 0.1 MW. The energy-harvesting device of Fig. 3 can be implemented as a large rotor on the main incoming flow line, but in other embodiments the device can be small, in particular smaller than the turbo expander of the first embodiment. A small device can be the size of a few inches in diameter and can be provided in a plurality of flowlines distributed across the subsea shuttle. Although a distributed network of energy harvesting devices may give rise to larger overall losses, it provides an opportunity to harvest energy in more locations, in particular locations difficult to access with larger devices.

As described above, a barrier is provided to achieve pressure communication between two internal parts of the vessel, without fluid communication. When both parts contain primarily liquids, the amount of inflow in the first part will be similar to the amount of outflow from the second part because the liquids are relatively incompressible when compared to gas. Although Fig. 3 illustrates the rotor 31 arranged at the inlet of the first part, the rotor may also be arranged at the outlet of the second part of the vessel. An advantage of arranging the rotor at the outlet of the second part is that, in some embodiments, the fluids in the second part is seawater, so the rotor can be optimised for use with seawater, while a range of possible different payloads may be stored in the first part, each with different properties. It is more challenging to design a rotor which can operate with a range of fluids, including heavy oil, water, gas, etc. The second configuration will be described in more detail below. When the rotor is at the outlet, the pressure drop also occurs after the pressure vessel, and the vessel itself may be maintained at a higher pressure.

Figure 4 illustrates an embodiment in which excess thermal energy is harvested. A heat exchanger 41 is used to extract heat from the inflow channel 42, before the fluid is passed through channel 43 into pressure vessel 12. The initial inflow temperature is Ti, and the initial pressure is Pi, and after the heat-exchanger the fluid has a lower temperature Ta and a pressure Pa. The pressure P4 may be similar to or slightly smaller than the initial pressure Pi because the energy is primarily extracted from the temperature of the fluids. An example of a simple heat-exchanger is a set of fins arranged within the fluid flow to form a heat sink. The extracted heat can be used to expand a gas, which in turn can drive an electricity generator. Alternatively, the extracted heat can be used to melt hydrates or wax formed within the shuttle, or for any other suitable process relying on thermal energy. An example of a thermoelectric generator is a Seeback generator.

The energy harvesting process is preferably implemented in the outflow channel of the subsea shuttle, where a large pressure drop occurs in the buffer liquid in some loading scenarios. However, the process may also be implemented in other parts of the subsea shuttle where a pressure and/or temperature drop occurs.

In a preferred embodiment, the pressure vessel comprises a piston, or other barrier element, which separates two parts within the internal volume of the pressure vessel from each other. Although a barrier element is illustrated, the fluids may also be separated from each other by differences in density, viscosity, weight, or a combination thereof, without a physical barrier element, as explained above. The barrier element is therefore optional. Figure 5 illustrates pressure vessel 51, with a first part 52 and second part 53, separated by a moveable barrier element 54. The first of the two parts 52 contains the cargo fluid and is closed after loading, such that the cargo does not escape the pressure vessel. A two-way conduit 55 with a valve provides access to the first part 52. The second of the two parts 53 contains sea water, or another buffer liquid, and is in fluid communication with the seawater, or other liquid, within the shuttle surrounding the pressure vessel. The piston or barrier element 54 can move, such that the first volume can increase or decrease in size to maintain pressure equilibrium when conditions change. When the first volume increases, the second volume decreases given that the outside wall of the pressure vessel is rigid. When the second volume decreases, seawater, or other buffer fluid, is pushed out of the second volume into the surrounding space, or dedicated reservoir. When the second volume increases, buffer fluid is pulled into the second volume. An energy-harvesting device such as illustrated in Fig. 3 can be utilised to harvest the energy from the in-or outflow. If energy-harvesting devices are used which are optimised for flow in one direction, a pair of fluid conduits is used in combination with one-way valves, one for inflow 58 and the other one for outflow 56, each including an energy-harvesting device such as the one illustrated in Fig. 3. Inflow channel 58 is coupled to a rotor 59, and outflow channel 56 is coupled to a rotor 57.

The rotor 57, and rotor 59 if present, are coupled to a generator 62, which in turn is coupled to a battery 63 for storing the generated electrical energy.

An example of pressure in the pressure chamber and before the energy harvesting device is 46 bara, which drops to 26 bara after the energy harvesting device. This provides power output of at least 0.1 MW for an envisaged loading scenario of the pressure vessel.

In a preferred embodiment, the initial pressure of the cargo fluid when entering the shuttle system is maintained within part 52, and also part 53 by way of the pressure communication, after the loading process has been completed. A valve 60 is closed before, or shortly after, the loading process has been completed to interrupt the energy harvesting process, in order to prevent the pressure from dropping. If a second rotor 59 is provided, a second valve 61 is provided to also close that channel. The conduit 58 and rotor 59 are optional. The two-way conduit 55 is also closed to prevent cargo fluid leaving part 52, and to prevent the pressure within part 52 from dropping. When the source of the cargo fluid is a well, the pressure of the cargo fluid is at, or slightly below the well pressure, and by closing valve 60 (and, if present, also valve 61), the overpressure is maintained during transport. One example of an advantage of maintaining an overpressure is the maintaining of well fluids primarily in the liquid phase, thereby avoiding expansion due to transition to a gas phase.

One example where a change of conditions occurs is when the temperature of the fluids within the pressure vessel is different from the surrounding temperature directly after loading, and gradually adjusts to reach the surrounding temperature during transport. In particular, when the fluids comprise a large gas portion, the corresponding adjustment in volume will be significant and the opportunity for energy harvesting will accordingly be significant. If the valves 60 and 61 are kept open, and there is no overpressure, the change in conditions can then give rise to further energy harvesting. However, if valves 60 and 61 are kept closed during transport, then there may be an increase or decrease in pressure during a change of the temperature, but such change in pressure can be accommodated by the pressure vessel without risk of failing.

One further example in the scenario where the valves 60 and 61 are kept open is the change of pressure when the subsea shuttle changes depth. The hydrostatic pressure is primarily determined by the fluid column above the shuttle. When the shuttle rises to a shallower depth, the pressure decreases, and the first volume expands, in particular when containing gas phase fluids, thereby creating an outflow of seawater of buffer fluid from the second volume to maintain the equilibrium, which in turn drives the rotor 57 of a generator 62. When the shuttle moves deeper, the reverse process takes place, and a generator 59 harvests the energy of the inflow into the second volume.

The operation of rising or lowering the shuttle will consume energy, but such rising or lowering operation may be required for operation, depending on the layout of the production facilities or for other reasons. Some of the energy is recovered by the energy harvesting, even if there is an overall loss of energy.

A further specific example of an energy harvesting device is a pump with a dual function, such that it can be used for harvesting energy when a fluid overpressure is applied, but it can be used to pump fluid when used as a pump. Examples are a recovery turbine or a reverse pump. The same device can be used to pump buffer fluid into the pressure vessel during offloading, thereby consuming electrical energy, and also for generating electrical energy during loading when buffer fluid is displaced by the incoming cargo fluid. In the example illustrated schematically in Fig. 5, the lower set of conduit 58, rotor 59 and valve 61 would not be required, if the top set these devices can work in two directions.

Figure 6 illustrates method steps for carrying out the energy harvesting in a subsea shuttle. The steps comprise: loading or unloading a cargo fluid (Si); passing the cargo fluid or buffer fluid through or past an energy harvesting device (53); and extracting the energy from the buffer fluid or the cargo fluid (53). The extracted energy can be stored in a battery, or other energy storage medium. If heat is extracted, for example by a heat sink described previously, the extracted heat can be stored in a medium with a large heat capacity. In a particular example of the method, the buffer fluid is displaced by the cargo fluid during loading, and the displacement gives rise to a pressure drop over the energy harvesting device.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS: 1. An autonomous underwater vehicle for transporting a cargo fluid, comprising: an outer hull defining an internal volume; a pressure communication channel between the internal volume and the exterior of the outer hull, arranged to control the pressure in the internal volume based on the exterior pressure; one or more internal vessels for containing the cargo fluid; wherein the one or more internal vessels are arranged to further contain a buffer fluid; the vehicle further comprising: an energy harvesting device, coupled to an inlet or an outlet of the one or more internal vessels, the energy harvesting device being arranged to extract energy from an inflow of fluid into the one or more internal vessels when coupled to the inflow, or to extract energy from an outflow of fluid from the one or more internal vessels when coupled to the outflow.
2. The vehicle of claim 1, wherein the energy harvesting device is coupled to the outlet, and is arranged to receive the buffer fluid displaced by cargo fluid during a loading process.
3. The vehicle of claim 1 or 2, further comprising a valve arranged at the outlet and between the energy harvesting device and the one or more internal vessels, for isolating the one or more internal vessels from the energy harvesting device.
4. The vehicle of any one of the preceding claims, wherein the energy-harvesting device is arranged to extract energy from the temperature of the fluid or the pressure of the fluid, or from a combination of temperature and pressure of the fluid.
5. The vehicle of claim 1, wherein the energy harvesting device comprises a turbo expander and wherein an inlet of the turbo expander is connected to an inlet of the shuttle, wherein an outlet of the turbo expander is connected to an inlet of the one or more vessels, and wherein the turbo expander is rotatably connected to a generator.
6. The vehicle of any one of the preceding claims, wherein the energy-harvesting device comprises a rotor and a generator.
7. The vehicle of any one of the preceding claims, wherein the energy harvesting device comprises an electrical generator, wherein the electrical generator is arranged to generate electrical energy when driven by a fluid overpressure, and wherein the electrical generator is arranged to pump fluid and generate an overpressure when run in reverse.
8. The vehicle of any one of the preceding claims, wherein the energy-harvesting device comprises a heat-exchanger or a thermoelectric generator.
9. The vehicle of any one of the preceding claims, further comprising a battery electrically coupled to the energy harvesting device for storing energy generated by the energy harvesting device.
10. A method of energy-harvesting in an autonomous underwater vehicle, the underwater vehicle having an outer hull and one or more internal vessels arranged within the outer hull, the outer hull defining an internal volume, the method comprising: loading or unloading the one or more vessels with a cargo fluid, wherein loading the one or more vessels with the fluid cargo displaces a buffer fluid initially provided within the one or more vessels; passing the buffer fluid, or the cargo fluid through or past an energy harvesting device; extracting energy from the buffer fluid or the cargo fluid with the energy-harvesting device, wherein the energy is extracted from the pressure and/or the temperature of the buffer fluid or the cargo fluid.
11. The method of claim 10, wherein the energy harvesting device is coupled to the one or more vessels to receive buffer fluid, wherein the buffer fluid is displaced by the cargo fluid during loading, and wherein the displacement gives rise to a pressure drop over the energy harvesting device.
12. The method of claim 10 or 11, further comprising charging a battery with the energy extracted with the energy-harvesting device.
13. The method of any one of claims 10 to 12, further comprising isolating the one or more internal vessels from the energy harvesting device after loading of the one or more internal vessels for maintaining an overpressure within the one or more internal vessels.
14. The method of any one of claims 10 to 13, further comprising using the energy harvesting device for pumping fluid into the one or more vessels before or after the step of extracting energy from the buffer fluid.
15. The method of any one of claims 10 to 14, further comprising extracting energy during transport of the cargo fluid.