COMBINED SYSTEM FOR UNDERWATER DRONE Technical field

The present invention relates to a combined system for an underwater drone. The present invention also relates to an underwater drone and to a method of operating an underwater drone.

Background

NO20140331 A1 discloses a probe (underwater drone) for monitoring fish, cages, and water quality in cages for aquaculture/fish farming. The probe in NO20140331 A1 comprises a waterproof container, which in turn comprises a first part and a second part, which are telescopically displaceable relative to each other to change the volume of a buoyancy chamber located within the two parts. The probe in NO20140331 A1 becomes lighter than water when the parts of the container are pushed apart and will thus float to the surface. Similarly, probe becomes heavier than water when the parts are pulled together and will then sink to the bottom of the cage.

Furthermore, the probe in NO20140331 A1 may comprise various types of sensors, such as a temperature sensor for measuring the temperature of the water, an oxygen sensor for measuring the amount of dissolved oxygen in the water, and a salinity sensor to measure the amount of salt in the water.

US2011039169A1 discloses a power generating system for operating below a surface of a body of water includes a fuel cell stack configured to react hydrogen and oxygen to produce electricity. An oxygen source is configured to provide oxygen to the fuel cell stack. A hydrogen source is configured to provide hydrogen to the fuel cell stack. The hydrogen source is at least partially submerged in water and incorporates a non-hydride metal alloy that reacts with water to produce hydrogen from the water. In some embodiments in US2011039169A1 , a portion of the hydrogen produced or separate air forced from the reactor volume by immersion in the water is stored in a flotation volume to thereby increase a buoyancy in the water. 2

CN1 10487875A discloses a biosensor for measuring dissolved oxygen at different depths in a water body. The biosensor includes an anode connected by a wire; and the anode is arranged in a base filledwith sediments. The base seals the sediments by a cover plate arranged at the upper end surface. Telescopic rods are arranged at the two sides of the top surface of the base respectively; connecting rods are arranged at the upper ends of the telescopic rods; and a cathode is fixed between the connecting rods and one end of the cathode is connected with resistors arranged in the telescopic rods by wires. The other ends of the resistors are connected with the anode arranged in the base by wires; and the other end of the cathode is connected with a traction wire. The other end of the traction wire passes through a support rod arranged at the upper ends of the telescopic rods and is led out from the top of the support rod. The invention in CN110487875A aims at providing a biosensor based on a microbial fuel cell; and the dissolved oxygen contents at different depths of a water body are measured conveniently by adjusting the distance between the anode and the cathode.

W02020079769A1 discloses an electrochemical oxygen sensor characterized by having a positive electrode, a negative electrode, and a liquid electrolyte, the liquid electrolyte containing a chelating agent and ammonia, and the concentration of the ammonia in the liquid electrolyte being 0.01 mol/L or greater.

In US8689714B1, an electrochemical engine for buoyancy is provided with the engine having a water-tight and gas-tight chamber containing a volume of seawater. The electrochemical buoyancy engine contributes electrons for reduction of hydrogen protons in the seawater using a sufficient voltage applied to an anode and a cathode disposed in the seawater. The generated hydrogen gas is held in the chamber to provide the desired buoyancy and can be vented to adjust the buoyancy.

US2018002200A1 discloses a system in which submerged electrolytic cells provide electricity from the seawater, that directly energizes electro chemical cells that produce oxygen and hydrogen. The entire system is configured so that micro bubbles of oxygen are quickly adsorbed as they rise 3 toward the surface, increasing dissolved oxidation (DO) by adsorption into the water.

In US2010064958A1 , the concept of a buoyancy engine that exploits the enormous volume and pressure changes accompanying the reversible electrochemical interconversion of water to hydrogen and oxygen gases is applied to stealth buoys and underwater gliders.

Summary of the invention

A conventional galvanic oxygen sensor (that could be used in the aforementioned probe of NO20140331 A1) may use gold or platinum as cathode, and a special electrolyte liquid. The cathode and electrolyte liquid are situated behind a membrane that is configured to let some of oxygen from the (sea)water in and activate the cell such that a small current is generated, from which current the oxygen level may be determined. Although the gold and the special electrolyte liquid allow the conventional oxygen sensor to work for a relatively long time, the current that is generated is very low. Also, gold or platinum is expensive, and changing of the electrolyte liquid is very unpractical.

It is an object of the present invention to provide a combined system for an underwater drone that overcomes or at least alleviates one or more of the drawbacks mentioned above.

According to a first aspect of the present invention, this and other objects are achieved by a combined oxygen sensor and seawater battery and buoyancy system for an underwater drone, comprising: at least one cell, each cell comprising an anode and a cathode both exposable to seawater when the underwater drone is placed in the sea, wherein the anode comprises magnesium, and wherein a reaction between the magnesium and the seawater acting as an electrolyte generates hydrogen; oxygen sensor electronics connected to the at least one cell, wherein the oxygen sensor electronics is adapted to measure at least one voltage of the at least one cell, and to determine an oxygen level based on the measured at least one voltage; a buoyancy chamber adapted to capture at least some of the 4 generated hydrogen to create buoyancy for the underwater drone; and battery terminals connected to the at least one cell for powering at least one electrical device of the underwater drone.

The present invention is at least partly based on the understanding that if seawater (salt water) is used as electrolyte, no oxygen sensor membrane is needed. This will result in more electrical power. However, a conventional gold cathode would be consumed rather quickly. To this end, by replacing gold with magnesium, which is cheap compared to gold, more material can be consumed to give the oxygen sensor a sufficient lifetime. The present oxygen sensor will be very large compared to conventional oxygen sensors, but that is acceptable in the underwater drone application. Furthermore, the magnesium may give even more electrical power, which beneficially also can be used to power other electrical devices of the underwater drone. Moreover, the reaction between magnesium and seawater will generate hydrogen (H2), which advantageously can be collected and used for buoyancy of the underwater drone. Hence a combined oxygen sensor and seawater battery and buoyancy system is realized, which cleverly and efficiently combines three important functions of an underwater drone. No (other) separate oxygen sensor or battery or buoyancy system is needed.

It should be noted that CN109895981 A discloses a device for adjusting buoyancy of a submersible by using gas generated by a metal/seawater battery. However, there is no combined oxygen sensor in CN109895981 A.

The (magnesium) anode of each cell of the present system can be at least partly arranged in the buoyancy chamber. This allows generated hydrogen to be capture while allowing the system to be compact. Also the cathode of each cell can be at least partly arranged in the buoyancy chamber. The anode and cathode of each cell could alternatively be positioned (just) below the buoyancy chamber.

The buoyancy chamber can have an open bottom to allow seawater into the buoyancy chamber. The buoyancy chamber could for example have the shape of an upside-down cup mounted over the anode (and optionally 5 also over the cathode).

Furthermore, as indicated above, the oxygen sensor is membrane free. Namely, there is no membrane covering the aforementioned (open) bottom.

The measured at least one voltage may be at least one open circuit voltage, wherein the oxygen sensor electronics is adapted to determine the oxygen level based on the measured at least one open circuit voltage. Experiments performed by the applicant indicated a positive correlation between oxygen level and open circuit voltage. The experiments also indicated that the salinity of the seawater acting as electrolyte did not impact the open circuit voltage, which will simplify implementation of the oxygen sensor functionality of the present system.

The combined system may further comprise a relay device configured to temporarily disconnect at least one cell of the combined system when the open circuit voltage of said at least one cell is measured. In other words, the relay device may disconnect the at least one cell from the aforementioned at least one electrical device to allow the open circuit voltage to be measured by the oxygen sensor electronics.

In case at least one cell of the combined system is a plurality of cells, the relay device may be configured to temporarily disconnect at least one cell of the combined system when the open circuit voltage of said at least one cell is measured while at least one other cell of the combined system remains connected. In this way, the oxygen level can be determined without any interruption in power supply for the at least one electrical device.

Specifically, the relay device may be configured to temporarily disconnect the cells of the combined system one cell after the other, wherein the oxygen sensor electronics is adapted to measure an open circuit voltage over each disconnected cell for determining the oxygen level.

If for example the combined system has 10 cells, wherein each cell is disconnected 1 second every 10 seconds, the remaining cells can supply 90% of the battery’s nominal energy while at the same time 1 measurement per second is performed.

Furthermore, the oxygen sensor electronics may be adapted to 6 determine the oxygen level based on measured (open circuit) voltages of several cells. This may improve the accuracy of the determined oxygen level, because deviating voltages can be scrapped and/or other variations can be averaged.

Having a plurality of cells, i.e. a plurality of magnesium anodes, may also increase the overall voltage of the seawater battery of the present system, which in turn can make it easier to collect the energy.

The combined system may further comprise a capacitor configured to store energy for powering of the at least one electrical device. The capacitor beneficially allows energy generated while the at least one electrical device of the underwater drone is off to be stored until the at least one electrical device is on and may need more electrical current than the cell(s) can momentarily deliver. The capacitor may for example be a supercapacitor.

The combined system may further comprise a buoyancy chamber valve in fluid communication with the buoyancy chamber, wherein the buoyancy chamber valve is configured to release generated hydrogen from the buoyancy chamber to regulate the buoyancy. For example, generated hydrogen may be released such that the underwater drone moves down in the seawater. In other words, the present system can conveniently allow the underwater drone to move both up and down in the seawater.

The combined system may further comprise a ballast chamber and a ballast chamber valve, wherein the ballast chamber valve is configured to allow seawater into the ballast chamber to counter magnesium weight loss. At least the aforementioned reaction between the magnesium and seawater generating hydrogen consumes magnesium, but this weight loss may beneficially be compensated for by the ballast chamber and valve, to keep the weight of the combined system/underwater drone constant, and thereby improve control of the buoyancy.

The anode(s) can (initially) comprise 0.2-1 kg, for example about 0.5 kg, of magnesium. 0.5 kg of magnesium is estimated to give enough electrical current to power electrical devices of the underwater drone for about six months and to generate enough hydrogen to rise about 5000 times, e.g. from 7 the bottom of a fish farming cage to the surface.

The cathode of each cell may comprise graphite or other inert conductive material.

The oxygen sensor electronics may be adapted to determine the oxygen level based on the measured at least one voltage by consulting a predetermined chart mapping oxygen level and cell voltage or by consulting a look-up table mapping voltages to corresponding oxygen levels based on empirical observations. The predetermined chart and/or look-up table may be programmed into the oxygen sensor electronics.

According to a second aspect of the present invention, there is provided an underwater drone, comprising: a combined oxygen sensor and seawater battery and buoyancy system according to the first aspect; and (the) at least one electrical device. The at least one electrical device may be (directly or indirectly) connected to the battery terminals of the combined oxygen sensor and seawater battery and buoyancy system. This aspect may exhibit the same features and/or technical effects as the first aspect, and vice versa.

The at least one electrical device (connected to said battery terminals) may comprise one or more of: a temperature sensor adapted to measure the temperature of the seawater, a salinity sensor adapted to measure the salinity of the seawater, a carbon dioxide sensor adapted to measure dissolved carbon dioxide in the seawater, a pH sensor adapted to measure the acidity of the seawater, a light sensor adapted to measure lighting conditions from sunlight or artificial lighting, a turbidity sensor adapted to measure visibility in the seawater, a pressure sensor adapted to measure the depth of the underwater drone, an echosounder, a camera, an acoustic transmitter and receiver (separate or transceiver), and wireless communication means.

According to a third aspect of the present invention, there is provided a method for operating an underwater drone according the second aspect placed in seawater, wherein the method comprises: generating hydrogen by a reaction between the magnesium of the anode of each cell and the seawater acting as an electrolyte; capturing at least some of the generated hydrogen in 8 the buoyancy chamber such that the underwater drone moves up from a submerged position towards the surface of the seawater; measuring, by the oxygen sensor electronics, at least one voltage of the at least one cell; determining, by the oxygen sensor electronics, an oxygen level based on the measured at least one voltage; and powering the at least one electrical device connected to said battery terminals. This aspect may exhibit the same features and/or technical effects as any of the previous aspects, and vice versa.

The oxygen level may be determined by the oxygen sensor electronics based on the measured at least one voltage by consulting a predetermined chart mapping oxygen level and cell voltage or by consulting a look-up table mapping voltages to corresponding oxygen levels based on empirical observations. The predetermined chart and/or look-up table may be programmed into the oxygen sensor electronics.

The underwater drone is preferably placed in seawater in a cage for aquaculture (fish farming cage).

Brief description of the drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention.

Fig. 1 is a schematic side view of a combined oxygen sensor and seawater battery and buoyancy system for an underwater drone according to an embodiment of the present invention.

Fig. 2 is a schematic side view of an underwater drone comprising a combined oxygen sensor and seawater battery and buoyancy system according one or more embodiments of the present invention.

Fig. 3 is an exemplary and partly schematic circuit diagram of components of the drone and combined system of fig. 2.

Fig. 4 is a flow chart of a method for operating an underwater drone according to an aspect of the present invention. 9

Fig. 5a-b illustrates the underwater drone moving up and down in a cage for aquaculture.

Fig. 6 shows test results of salinity and cell voltage.

Fig. 7 shows test results of oxygen level and cell voltage.

Fig. 8 is a schematic side view of a combined seawater battery and buoyancy system in an underwater drone for aquaculture.

Detailed description of the invention

Fig. 1 is a schematic side view of a combined oxygen sensor and seawater battery and buoyancy system 10 according to an embodiment of the present invention.

The combined system 10 may be incorporated in an underwater drone. The combined system 10 may for example be incorporated in an underwater drone of the type disclosed in the aforementioned document NO20140331 A1 , but where the combined system 10 replaces the dedicated oxygen sensor, the battery, and the waterproof container with two telescopically displaceable parts. The content of NO20140331 is herein incorporated by reference.

The combined system 10 comprises at least one (battery) cell, in fig. 1 a single cell. The cell comprises an anode 12 and a cathode 14. The anode 12 and cathode 14 may be shaped like rods. The anode 12 and cathode 14 may be arranged parallel to each other at a distance d. The anode 12 and cathode 14 are exposed to seawater 16 when the underwater drone (and hence the combined system 10) is placed in the sea. The seawater 16 contains dissolved salt. On average, seawater 16 has a salinity of about 3.5% (35 g/l). The seawater 16 acts an electrolyte of the cell.

The anode 12 comprises or consists of magnesium, Mg. The anode 12 can initially comprise 0.2-1 kg, for example about 0.5 kg, of magnesium. The cathode 14 may comprise graphite or other inert conductive material.

The combined system 10 further comprises oxygen sensor electronics 18 connected to the cell. The oxygen sensor electronics 18 is adapted to measure a voltage of the cell. This voltage could be referred to as a cell 10 voltage. The oxygen sensor electronics 18 may for example include a voltmeter electrically connected to the anode 12 and cathode 14. The oxygen sensor electronics 18 is further adapted to determine an oxygen level, namely the amount of dissolved oxygen O2 in the seawater 16, based on the measured (cell) voltage. This will be described in more detail hereinbelow.

The combined system 10 further comprises a buoyancy chamber 20. The buoyancy chamber 20 could have a volume in the range of 150-200 ml (for a 3 kg underwater drone). The anode 12 and cathode 14 are in fig. 1 partly arranged in the buoyancy chamber 20. Namely, lower portions of the anode 12 and cathode 14 protrude through an open bottom 22 of the buoyancy chamber 20. The open bottom 22 allows seawater 16 (as well as hydrogen 28) into the buoyancy chamber 20. The buoyancy chamber 20 could for example have the shape of an upside-down cup mounted over the anode 12 and cathode 14. Notably, contrary to conventional oxygen sensors, no membrane covers the open bottom 22.

The combined system 10 further comprises battery terminals 24a-b connected to the cell for powering at least one electrical device 26 of the underwater drone. Specifically, a positive battery terminal 24a is electrically connected to the cathode 14, and a negative battery terminal 24b is electrically connected to the anode 12. The at least one electrical device 26 may be electrically connected to the battery terminals 24a-b, as illustrated in fig. 1.

In operation (discharge) of the combined system 10, a first reaction between the magnesium of the anode 12 and the seawater 16 generates hydrogen, H ₂ :

Mg + 2H ₂ 0 Mg(OH) ₂ + H ₂ (1 )

The generated hydrogen is designated by reference sign 28. At least some of the generated hydrogen 28 is captured by the buoyancy chamber 20, to create buoyancy for the underwater drone. Accordingly, with sufficiently generated hydrogen 28 captured by the buoyancy chamber 20, the 11 underwater drone can rise in the seawater 16, e.g. from the bottom of a fish farming cage to the surface.

The combined system 10 could further comprise a buoyancy chamber valve 30 in fluid communication with the buoyancy chamber 20. The buoyancy chamber valve 30 may be configured to release generated hydrogen 28 from the buoyancy chamber 20, to regulate the buoyancy. For example, generated hydrogen 28 may be released through the valve 30, such that the underwater drone moves down in the seawater 16. The buoyancy chamber valve 30 may be positioned at or near the top of the buoyancy chamber 20.

Furthermore in operation of the combined system 10, a second reaction consumes oxygen, O2, from the seawater 16:

2Mg + 0 ₂ + 2H ₂ 0 2Mg(OH) ₂ (2)

This second reaction contributes to an electrical potential over the aforementioned cell. Therefore, the oxygen sensor electronics 18 may determine the amount of dissolved oxygen O2 in the seawater 16 based on the measured (cell) voltage.

Specifically, the oxygen sensor electronics 18 may measure an open circuit voltage of the cell, and determine the oxygen level based on the measured open circuit voltage. Experiments performed by the applicant indicated a positive correlation between oxygen level and open circuit voltage, see EXPERIMENTS hereinbelow.

Based on a measured voltage (e.g. open circuit voltage = 0.7 V), the oxygen sensor electronics 18 may determine the oxygen level (e.g. 4 mg/I) for example by consulting a predetermined chart mapping oxygen level and cell voltage (like fig. 6) or by consulting a look-up table mapping voltages to corresponding oxygen levels based on empirical observations. The predetermined chart and/or look-up table may for example be programmed into the oxygen sensor electronics 18. The current temperature of the seawater 16 could have some effect, so the oxygen sensor electronics 18 12 could have at least one of: different predetermined charts for different temperatures, look-up tables for different temperatures, and a function wherein oxygen level depends on open circuit voltage and temperature. Accordingly, the oxygen sensor electronics 18 could be adapted to determine the oxygen level based on the measured at least one voltage and on the (current) temperature. The temperature of the seawater 16 could be measured by a temperature sensor 26a of the underwater drone.

To measure the open circuit voltage, the combined system 10 may further comprise a relay device 32. The relay device 32 is configured to temporarily disconnect the cell from the at least one electrical device 26 when the open circuit voltage of the cell is measured by the oxygen sensor electronics 18. To time this, the relay device 32 may be controlled by the oxygen sensor electronics 18. The relay device 32 could for example disconnect the cell for 1 second every 10 seconds, allowing one measurement of the cell’s open circuit voltage every ten seconds (=measurement frequency of 0.1 Hz). The measurement frequency of the present combined system 10 is preferably in the range of 0.1-2 Hz, more preferably 1 -2 Hz.

Turning to fig. 2, fig. 2 shows an underwater drone 100 comprising a combined oxygen sensor and seawater battery and buoyancy system 10 according one or more embodiments of the present invention.

The underwater drone 100 has a generally elongated shape. The underwater drone 100 is typically uncrewed. The underwater drone 100 is capable of moving vertically up and down (indicated by arrows 102a and 102b, respectively) in seawater 16 due to the combined system 10. The combined system 10 may also determine the oxygen level of the seawater 16, and power the least one electrical device 26 of the underwater drone 100. Accordingly, the underwater drone 100 may be devoid of a dedicated (conventional) oxygen sensor and a further (conventional) battery.

The least one electrical device 26 of the underwater drone 100 may for example include various sensors, such as a temperature sensor 26a, a salinity sensor 26b, a light sensor 26c, a camera 26d, an acoustic transmitter 13 and receiver 26e, etc., as well as wireless communication means 26f. The electrical devices 26 of the underwater drone 100 may be electrically connected to the battery terminals 24a-b of the combined system 10.

The combined system 10 in fig. 2 is similar to that of fig. 1. However, the combined system 10 in fig. 2 comprises a plurality of cells, wherein each cell comprises a magnesium anode 12 and a cathode 14. The combined system 10 in fig. 2 could comprise N cells, wherein N is in the range 2-20, for example 10 cells. The total weight of the magnesium of the N anodes 12 can initially be in the range of 0.2-1 kg, for example about 0.5 kg.

The aforementioned relay device 32 may here be configured to temporarily disconnect one cell of the combined system 10 from the electrical devices 26 when the open circuit voltage of that cell is measured while the other cells of the combined system 10 remains connected. Specifically, the relay device 32 may be configured to temporarily disconnect the cells of the combined system 10 one cell after the other, wherein the oxygen sensor electronics 18 is adapted to measure an open circuit voltage over each disconnected cell for determining the oxygen level. Accordingly, relay device 32 can comprise a plurality of individually controllable sub-relays 32I-N, one sub-relay per cell, see fig. 3. The sub-relays 32I-N may be controlled by the oxygen sensor electronics 18. Analogously, the oxygen sensor electronics 18 may include one voltmeter V _I-N per cell.

If for example the combined system has ten cellsi-N, wherein each cell is disconnected 1 second every 10 seconds, the remaining cells can supply 90% of the overall nominal energy, while at the same time 1 measurement per second is performed (=measurement frequency of 1 Hz). In fig. 3, the cellsi-N are serially connected, but cel is shown temporarily disconnected (sub-relay 32 ₂ is open rather than (normally) closed like the other sub-relays) when the open circuit voltage is measured by V2. The cellsi- _N being serially connected allows the overall voltage level to be raised.

Furthermore, the oxygen sensor electronics 18 may determine the oxygen level based on measured voltages of several cells of the combined system 10. This may improve the accuracy of the determined oxygen level. 14

The oxygen sensor electronics 18 may for example scrap any voltage deviating too much from other voltages measured during one cycle. One cycle could be N measurements from N different cells over N seconds, where N may be 10 as in the above example. The oxygen sensor electronics 18 could also average the measured voltages for one cycle, and determine an oxygen level based on the average voltage.

The combined system 10 in fig. 3 may further comprise a capacitor 34. The capacitor 34 may be configured to store energy generated by the cells during discharge for powering the at least one electrical device 26. The capacitor 34 for example allows energy generated while the at least one electrical device 26 is turned off to be stored until the at least one electrical device 26 is turned on and may need more electrical current than the cells can momentarily deliver. The capacitor 34 may be connected to a voltage pump / power harvest electronics 36, which in turn is connected to the cells-i-

N.

Moving on, in the combined system 10 of fig. 2, the anode 12 and cathode 14 of each cell are positioned just below the buoyancy chamber 20.

A protection 104 of the underwater drone 100 protects the anode 12 and cathode 14 of the cells but lets seawater 16 through. The protection 104 is preferably constructed so that seawater 16 can completely freely flow inn and out through the protection 104, but so that large foreign objects would not be able to pass, thus protecting the anodes 12 and cathodes 14 from damage. The protection 104 may for example be configured as a cage. The protection 104 should not be confused with, or construed as, a conventional oxygen sensor membrane.

The combined system 10 in fig. 2 may further comprise a ballast chamber 106 and a ballast chamber valve 108. The ballast chamber 106 may be formed in a housing 110 of the underwater drone 100. The ballast chamber 106 could have a volume in the range of 400-500 ml. The ballast chamber valve 108 is configured to allow seawater 16 from the outside into the ballast chamber 106, to counter magnesium weight loss that occurs due to the aforementioned first and second reactions. The ballast chamber valve 15

108 may be connected to, and controlled by, a control unit 114 of the underwater drone 100. The aforementioned buoyancy chamber valve 30 may also be connected to, and controlled by, the control unit 114.

The housing 110 of the underwater drone 100 may further accommodate the electrical devices 26, the oxygen sensor electronics 18, and the capacitor 34 in a (top) compartment 112 separate from the ballast chamber 106. In fig. 2, the ballast chamber 106 is positioned between compartment 112 and buoyancy chamber 20.

Turning to figures 4 and 5a-b, in operation of the underwater drone 100 comprising the combined system 10 of figs. 2-3 (or the combined system 10 of fig. 1), the underwater drone 100 may initially be placed in the sea at S1. Specifically, the underwater drone 100 may be placed in seawater 16 in a cage 200 for aquaculture. The cage 200 could also be referred to as a fish farming cage 200. Aquaculture refers to the cultivation of aquatic organisms (such as fish 202 or shellfish), especially for food.

As the underwater drone 100 is placed in seawater 16, the anode 12 and cathode 14 of the cell(s) are exposed to the seawater 16, whereby hydrogen 28 is generated by the aforementioned first reaction. At least some of the generated hydrogen 28 is captured in the buoyancy chamber 20.

However, with no or a very small amount of hydrogen 28 captured in the buoyancy chamber 20 at launch of the underwater drone 100, the underwater drone 100 is designed to sink (S2) to a position S3 at the bottom 204 of the cage 200.

While at the bottom 204 of the cage 200 at S3, the electrical devices 26 of the underwater drone 100 may be off. Energy generated by the cell(s) may then be stored in the capacitor 34. The underwater drone 100 may stay at the bottom 204 of the cage 200 for a duration of 10-60 minutes, for example.

When sufficient hydrogen 28 has been generated and captured in the buoyancy chamber 20, the underwater drone may move up from the submerged position S3 at the bottom 204 of the cage 200 towards the surface 206 of the sea/seawater 16, see S4. The distance from the bottom 204 of the 16 cage 200 to the surface 206 could be in the range of 20-60m, for example 50 meters.

During the ascent (S4), the oxygen sensor electronics 18 may measure at least one cell voltage, and determine the oxygen level of the seawater 16 based on the measured voltage(s). Also during the ascent, the various electrical devices 26 of the underwater drone 100 may be powered by the combined system 10, directly by the cell(s) and/or by the capacitor 34. For example, the temperature sensor 26a may measure the temperature of the seawater 16 at different depths, the camera 26d can be used to take images of fish 202 in the cage 10, etc.

Once the underwater drone 10 has reached the surface 206 at S5, data captured by the sensors 26a-e may be transmitted preferably via the wireless communication means 26f (also powered by the combined system 10) to a unit or facility remote of the underwater drone 10.

Next time the underwater drone 10 is to descend from the surface 206 to the bottom 202, generated hydrogen 28 may be released from the buoyancy chamber 20 through the buoyancy chamber valve 30, such that the underwater done 100 (again) moves down in the seawater 16 at S2. Thereafter, S3-S5 may be repeated.

As operation of the underwater drone 100 and its combined system 10 progresses, magnesium of the anode(s) 12 will be consumed, resulting in a weight loss. To counter this weight loss, the ballast chamber valve 108 allows seawater 16 into the ballast chamber 106, to keep the weight of the underwater drone 100 constant. Over the course of the lifetime of the magnesium anode(s) 12, more and more seawater 16 will be let into the ballast chamber 106 by the ballast chamber valve 108. The control unit 114 could use at least one of accumulated power generation (by the combined system 10) and rise speed of the underwater drone 100 to calculate the amount of water 16 needed to be added to the ballast tank 106, and to control the ballast chamber valve 108 accordingly. The amount of water 16 in the ballast tank 106 should counter the loss of magnesium such that 0.74g/ml of water 16 needs to be in the ballast tank 106 for every 1.74g / 1ml of 17 magnesium that is lost. The amount of magnesium lost can be indirectly estimated from the total energy produced or directly derived from the calculated buoyancy based on rise speed combined with salinity and temperature reading (the density of seawater 16).

EXPERIMENTS

In the following, reference signs placed in parentheses refer to similar or corresponding components in the drawings.

Setup: In a tank of about 40 litres, a magnesium rod (12) of 20mm diameter and length 200mm was mounted. The rod's initial weight was 113g. Next to it with a shortest distance (d) of about 8mm a graphite rod (14) of diameter 5mm and length 150mm was mounted. Each rod was electrically connected to a voltmeter (V), and a relay (32) was used to connect and disconnect a load (26) of 65 Ohm. Over the magnesium rod a transparent cup (20) was mounted to collect hydrogen (28). An optical oxygen sensor and an inductive salinity/temperature sensor were also mounted in the tank. A circulation pump was also placed in the tank. The tank was closed with a lid to reduce aerating of the water. A logger collected measurements from the sensors every 10-12 seconds and also recorded the voltages. The load was connected for 5 seconds before the load voltage is recorded, and disconnected for 5-7 seconds before the open circuit (no load) voltage was recorded.

Test 1 - Salinity: By starting the logger with fresh water in the tank and then adding 1 kg of salt one can see the effects of changing salinity as the salt is dissolved in water. The expected resulting salinity when all salt is dissolved in water was 25 ppt. As the test results shown fig. 6 show, the salinity did not seem to impact the voltages without load at all, and for the with-load-voltage above a certain threshold the salinity no longer impacts the voltage level as the salinity continues to rise. The salinity eventually rises to 24ppt, almost the expected level.

Test 2 - Oxygen saturation: The voltage level drops as the oxygen level in the tank falls as shown in fig. 7. The measured oxygen saturation 18 trails some behind the voltage drop, which could be due to the distance from the sensor to the rods. The graphs are scaled so to overlay the open circuit voltage over the measured oxygen level.

Hydrogen production and rod condition: Hydrogen was recorded produced at a rate of about 70-370 ug/h, but both circulation and whether or not the load was connected, seemed to impact this. With circulation and the intermittent load hydrogen production was in the high end of this range. A significant proportion of the hydrogen was lost in the test setup, so hydrogen production in an actual implementation is expected to be higher, e.g. up to 1000 ug/h. The production of hydrogen never stopped for any of the conditions during testing. There were some changes to the surface texture of the magnesium rod and for a time more visible in the area where it was close to the graphite rod, but no clear indication of build-up of any kind or this causing a slowdown in the reactions.

Calculations: For the circulated situation the power generation is (U ^A 2/R) 5.5 mW or a little less than half that, 2.5mW considering that the load is only connected less than half the time. This would for a use case of 5 min on per hour give about 25mW or 5mA at 5V per rod. Hydrogen production rate of 370 ug/h would correspond to about 4ml/h in volume at surface pressure, 1ml/h at 30m depth per rod. About 1000 ug/h would mean 3ml/h at 30m depth per rod. Calculations and assumptions indicated that the 130g magnesium rod would last about 100-200 days.

Conclusions: The experiments indicated a correlation between oxygen level and open circuit voltage. It was unexpected that the salinity level did not seem to impact the voltages above a certain threshold, but this could simplify implementation. Power production, rate of magnesium oxidation, hydrogen production, and voltage levels were all within ranges acceptable for the implementation considered.

The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended 19 claims. For example, the system of fig. 1 could also include at least one of: a capacitor 34, a voltage pump 36, and a ballast chamber 106 and ballast chamber valve 108.

Furthermore, there is envisaged a combined system 10’ and underwater drone 100’ placeable in seawater 16’ in a cage 200’ for aquaculture without the oxygen sensor functionality / oxygen sensor electronics 18 and any relay device 32, see fig. 8. This underwater drone 100’ could instead comprise a separate oxygen sensor 26g, which may be of conventional type.

Flence, according to a fourth aspect, there is provided a combined seawater battery and buoyancy system 10’ for an underwater drone 100’ placeable in seawater 16’ in a cage 200’ for aquaculture, comprising: at least one cell, each cell comprising an anode 12’ and a cathode 14’ both exposable to seawater 16’ when the underwater drone 100’ is placed in the sea, wherein the anode 12’ comprises magnesium, and wherein a reaction between the magnesium and the seawater 16’ acting as an electrolyte generates hydrogen 28’; a buoyancy chamber 20’ adapted to capture at least some of the generated hydrogen 28’ to create buoyancy for the underwater drone 100’; and battery terminals connected to the at least one cell for powering at least one electrical device 26’, 26f, 26g of the underwater drone 100’.

According to a fifth aspect, there is provided an underwater drone 100’, comprising: a combined seawater battery and buoyancy system 10’ according to the fourth aspect; and at least one electrical device 26’, 26f, 26g connected to the battery terminals of the combined seawater battery and buoyancy system 10’.

According to a sixth aspect, there is provided a method for operating an underwater drone 100’ according the fifth aspect placed in seawater 16’ in a cage 200’ for aquaculture, wherein the method comprises: generating hydrogen 28’ by a reaction between the magnesium of the anode 12’ of each cell and the seawater 16’ acting as an electrolyte; capturing at least some of the generated hydrogen 28’ in the buoyancy chamber 20’ such that the underwater drone 100’ moves up from a submerged position towards the 20 surface 206’ of the seawater 16’; and powering the at least one electrical device 26’, 26f, 26g connected to the battery terminals.

The fourth to sixth aspects may exhibit the same (further) features and/or technical effects as any of the first to third aspects of the invention. For example, the combined system 10’ could further comprise the ballast chamber 106’ and ballast chamber valve 108’, wherein the ballast chamber valve 108’ is configured to allow seawater 16’ into the ballast chamber 106’ to counter magnesium weight loss.