WO2005116299A2

WO2005116299A2 - Solar-operated electrolytic apparatus for producing hydrogen, and method for operating such an apparatus

Info

Publication number: WO2005116299A2
Application number: PCT/EP2005/005742
Authority: WO
Inventors: Gregor Lengeling
Original assignee: Gregor Lengeling
Priority date: 2004-05-28
Filing date: 2005-05-27
Publication date: 2005-12-08
Also published as: DE102004026281A1; WO2005116299A3

Abstract

The invention relates to an apparatus and a method for producing hydrogen by means of at least one solar cell (1) which is spatially assigned to at least one electrolytic cell (2) encompassing an electrolyte and is operated with the aid of highly focused sunlight (3). According to the invention, the solar cell (1) is configured so as to be cooled by the electrolyte penetrating the electrolytic cell (2) such that the solar cell can be operated more economically.

Description

Solar powered electrolysis device for generating hydrogen and method for operating such

The invention relates to a device for generating hydrogen by means of at least one solar cell, the solar cell being spatially assigned to at least one electrolysis cell containing an electrolyte.

The invention further relates to a method for generating hydrogen by means of at least one solar cell, the solar cell being spatially assigned to at least one electrolysis cell containing an electrolyte.

A generic device is described by JP 2000192275 A. The device described there for the electrolysis of water has a solar cell, the back of which rests flat on an electrolysis cell. The electrolysis cell has two chambers separated from each other by a permeable wall. An electrode connected to the anode or cathode of the solar cell projects into each of the two chambers. The solar energy converted from the solar cell into electrical energy is supposed to dissociate the water in the electrolysis cell into hydrogen and oxygen. Hydrogen and oxygen can be stored in a tank.

DE 20308393 UI describes a solar power plant in which the solar power plant is connected to an electrolysis device which makes it possible to produce oxygen and hydrogen in order to store the generated energy.

DE 19528681 C2 describes a device and a method for storing and utilizing, in particular solar energy. By means of solar energy there Hydroxides are heated and decomposed electrolytically to produce hydrogen and oxygen. The hydrogen formed can be burned again in a heat engine.

DE 2851225 C2 deals with a method for storing solar energy in the form of an electrochemically generated and combustible substance, for which the solar radiation is first converted into electrical current and then fed to an electrode or cathode, which are immersed in an electrolyte, for formic acid manufacture.

DE 4332789 is concerned with a method for storing energy, a mixture of hydrogen and carbon dioxide being converted in a reactor into methane and / or methanol.

From DE 4302089 AI the suggestion can be found to overheat water vapor to temperatures around 3000 Kelvin with the help of solar radiation so that it partially dissociates.

DE 19523939 C2 and DE 10027549 A1 describe solar-described systems for the production of hydrogen in which solar cells and electrolysis cells are used.

WO 2004/050961 AI presents a photoelectrochemical cell in which a photovoltaic electrode in a container with a transparent front is exposed to and operated by the radiation. US 4841731 A describes a method and a device for the electrolysis of water with the aid of solar energy. The heat of the separately operated solar cells should possibly be supplied to the electrolysis and possibly support it. If necessary, hydrogen and oxygen are converted in a heat engine belonging to the system in order to provide electrical energy.

EP 0670915 B1 describes a process for the preparation of water vapor by means of concentrated solar radiation, in which the water vapor produced is subsequently broken down into its constituents hydrogen and oxygen by means of electrolysis in the gas phase. The necessary electrical energy is to be provided by means of separately operated solar cells.

Starting from EP 0670915 B1 as the generic state of the art, the object of the invention is to further develop a generic device or a generic method with the aim of better economic use.

The object is achieved by the subject-matter claimed in claim 1. This is characterized in that a solar cell can be cooled by an electrolyte flowing through an electrolytic cell.

Advantageously, the electrolyte not only serves to remove the gases produced by electrolysis, but also to a particular extent for cooling, that is, the removal of the heat absorbed in the solar cell, as a result of which the claimed device can be operated more economically. It is proposed that the solar cell can initially and essentially be operated with strongly focused sunlight, that is to say it is arranged in particular in the focal point of a light-focusing device. This light bundling device can be a reflector or a lens system. A parabolic mirror is preferably used as the light bundling device. The parabolic mirror can have a light trapping cross section, in particular an area that is at least 200 times preferably at least 1000 times as large as the exposed area of the solar cell. In order to minimize the electrical losses, it is advantageous if the electrolysis cell is as close to the solar cell as possible. The solar cell is preferably carried by an electrolysis cell.

To dissipate the heat absorbed by the solar cell, it is advantageous if the back of the solar cell is connected to a good thermally conductive housing section of the electrolysis cell, essentially over the entire area. This housing section of the electrolytic cell preferably consists of a good thermally conductive metal. The conductive connection between the back of the solar cell and the corresponding surface of this thermally highly conductive housing section can be produced via a silver solder or another and in particular also electrically conductive material.

In a preferred embodiment, the electrolysis cell has one or more inflow openings for the electrolyte and one or more outflow openings for the electrolyte and the hydrogen / oxygen dissolved in the electrolyte. This has the advantage that electrolyte and reaction products dissolved in the electrolyte and heat transferred to the electrolyte can escape from this or these discharge openings at the same time. The electrolysis cell preferably has separate inlet openings for separate chambers. The two chambers of the electrolytic cell are separated from each other by a permeable wall. The anode is located in one of the chambers. The cathode is in the other chamber. This ensures that hydrogen and oxygen are generated in different chambers. The consequence of this is that the hydrogen in solutions is transported out of the electrolysis cell through one drain opening and the oxygen in solutions in the other drain opening. The anode and the cathode are made of a suitable material. These can be pure metals or suitable composites with metals. The anode or the cathode can additionally be mechanically structured and / or coated with suitable catalysts. The two chambers of the electrolytic cell are separated from each other by a permeable wall. This wall can be a purely passive and more mechanically acting wall, which allows the exchange of ions but not the gases generated by electrolysis. However, it can also consist of a solid electrolyte as in a PEM electrolysis cell and thus enable current flow as an ion source. Pure water is then inside the electrolysis cell. The anode and cathode are then electrically conductively connected to the solid electrolyte.

The invention is not limited to these two known permeable wall types. In a further embodiment of the invention, it is provided that the thermally conductive housing section forms structures that increase the contact area with the electrolyte. In particular, it is provided that structures of this type which increase the contact area with the electrolyte protrude from the walls of both chambers of the electrolysis cell. These structures can be pins or pins, for example. As a result of the increased contact area, not only is the chemical mass conversion increased, but also the heat dissipation in the electrolytes. The electrolyte can be dilute sodium hydroxide solution or dilute potassium hydroxide solution or another suitable chemical substance. However, the presence of is essential Water, which is to be split into its components hydrogen and oxygen. If the electrolysis cell has several chambers, it is provided that each chamber is assigned its own electrolyte circuit. The device preferably has two electrolyte circuits, each electrolyte circuit having a pump, a gas separator and a heat exchanger. With the pump, the electrolyte is conveyed through the circuit, wherein pressures between 70 and 120 bar are preferably generated within the electrolysis cell. In the gas separator, the hydrogen or oxygen transported out of the electrolysis cell is removed from the electrolysis liquid.

For example, the additional use of the thermoelectric effect provides the solar cell with a voltage which is sufficient to split water into its components, hydrogen and oxygen. The voltage generated is between 1.3 and 3.3 volts. This eliminates the need for a series connection of several solar cells, which is more complex in terms of production technology.

The thermal voltage which arises as a result of a thermoelectric effect between the hot and cooled surface of the solar cell is also preferably used. The active surface of the solar cell exposed to the highly concentrated sunlight can heat up to temperatures of 400 to 450 degrees. The cooled back of the solar cell has a temperature that is significantly lower. This temperature can be between 100 and 200 degrees. In particular if the arrangement is operated at pressures above normal pressure, an elevated temperature can be used for the aqueous electrolyte without the electrolyte boiling. The tension to dissociate water at elevated temperature decreases. The yield of the process can increase. As a result of this temperature difference, the voltage between the anode and cathode can be increased by an additional thermoelectric effect (Seebeck effect). With suitable materials, this additional voltage can assume considerable values above 100 mV. The solar cell then becomes a combined one Photo and thermoelectric device. It is also provided that several solar cells are placed in series in order to increase the total voltage. These solar cells can be placed side by side in such a way that they are suitably impacted by the highly concentrated sunlight. This solution also provides for the electrolysis to take place in the immediate vicinity of the solar cell. Here, too, the heat absorbed by the solar cell is dissipated by the electrolyte. In this device, a plurality of electrolysis cells electrically connected in series are preferably provided. In order to achieve the voltage required for the dissociation of water with only one p - n junction, it is provided that the semiconductor layers of the solar cell consist of a III-V component material. The required voltage of this single pn junction solar cell is achieved by a corresponding band gap engineering, in which the composition of the semiconductor layers from III-V components (AI, Ga, In, P, As, N) in a corresponding manner Be chosen. The back of the solar cell can be connected in an electrically conductive manner to a metallic housing half. One or more contacts can be arranged on the surface, which are connected in an electrically conductive manner to a second housing half. The two housing halves are electrically insulated from one another and each form the anode or cathode.

The above-mentioned object of the invention is further achieved by a method for generating hydrogen using a solar cell. The advantages of this method correspond to the advantages mentioned above with reference to the claimed device.

Embodiments of the invention are explained below with reference to the accompanying drawings. Show it: 1 shows a schematic representation of the structure of a device for generating hydrogen by means of a solar cell,

1 a in a schematic representation the structure of a device for generating hydrogen by means of a solar cell in a modified form,

2 shows a schematic representation of the structure of an electrolysis cell according to the invention which is directly connected to a solar cell,

3 shows a further exemplary embodiment of an electrolysis cell with an integrated solar cell in a top view,

Fig. 4 is a section along the line IV-IV in Fig. 3 and

5 shows a further exemplary embodiment of an apparatus having a plurality of individual electrolysis cells for generating hydrogen with an integrated solar cell arrangement.

1 shows a parabolic mirror 4. This reflector reflects the sunlight 3 and focuses it on a transducer arranged in the focal point of the reflector 4. This converter has a solar cell 1 which is intimately connected to an electrolytic cell 2. It is a combination of one or more solar cells with one or more electrolysis cells in such a way that both components can be operated with high synergy so that hydrogen or other electrolysis products can be provided directly using the energy of the sun.

The electrolyte flows through the electrolytic cell 1. 1 shows two electrolyte circuits 19, 20. Each electrolyte circuit 19, 20 contains a pump 15, a gas separator 16 and a heat exchanger 17. The pump 15 pumps the electrolyte into a feed line 19 ', 20'. The feed line 19 ', 20' conducts the electrolyte into chambers 12, 13 of the electrolysis cell 2 which are separated from a permeable wall 11. Electrolysis produces hydrogen within the chamber 12 and oxygen within the chamber 13. Oxygen and hydrogen are carried away by the electrolyte flowing through the discharge lines 19 ', 20' in order to be further compressed and stored if necessary. In the flow direction upstream or downstream of a heat exchanger 17 there can be a gas separator 16 in each of the electrolyte circuits 19, 20, by means of which the oxygen or hydrogen is separated from the electrolyte in order to be compressed and stored. The removal of the heat exchanger 17 can be used for other purposes.

As a result of this configuration, it is possible to operate the solar cell (1) with concentrated sunlight due to the very good cooling by the electrolyte with high concentration factors. The direct production of hydrogen as an energy source bridges the known daily and seasonal fluctuations in the "primary energy supply" of the sunshine, so that hydrogen is an easily stored energy source. Due to the high possible concentration factors of sunlight, there is enormous economic potential for the inexpensive production of hydrogen also for energy use. The application is not limited to the production of hydrogen by electrolysis. In principle, any desired electrolysis using the sun's energy is possible on its basis. In particular, combination power plants are possible, which make it possible to process sea or brackish water into drinking water on one side and to produce hydrogen as an energy source on the other side. The structure shown in FIG. 1 a essentially differs from the structure shown in FIG. 1 in that both chambers of the electrolytic cell 2 are fed by a pump 5. Accordingly, only one feed line 19 is required, which divides only immediately in front of the electrolytic cell 2. This has the advantage that the same in both chambers of the electrolytic cell 2 There is pressure and the permeable wall between the two chambers is minimally loaded.

The structure of an electrolysis cell 2 having a solar cell 1 is described in principle in FIG. 2. There, the electrolysis cell 2 has two chambers which are separated from a permeable wall 11 through which an ion exchange can take place. The chamber 12 has an electrically conductive chamber wall 21 which forms a cathode. Chamber 13, which is essentially a mirror image of chamber 12, also forms an electrically conductive chamber wall 22 which forms the anode. The chamber 21 also has a high thermal conductivity and is connected over a large area to the rear side 1 ″ of a solar cell 1. On the active surface 1 ′ of the solar cell 1 there are contacts 18 which are connected to the anode 22 via an electrical conductive connection 23. A feed line 7 opens into the chamber 12, through which H ₂ O and possibly KOH are introduced into the chamber 2. The chamber 13 also has a feed line 8 through which H ₂ O and optionally KOH is introduced into the chamber 13. In the Hydrogen is produced by electrolysis in chamber 12. This hydrogen is discharged together with the H ₂ O or the KOH which may be present through a discharge line 9. Oxygen is produced in electrolytic chamber 13. This is discharged together with the water through discharge line 10.

The surface 1 'of the solar cell 1 is exposed to concentrated sunlight. As a result, electrical current is generated in the solar cell, which is formed from an n-layer 24 and a p-layer 25, and operates the electrolysis. In addition, 1 heat is absorbed in the solar cell. This heat is emitted via the cathode 21 in the electrolyte, which is located in the chamber 12. The flow through the chamber 12 causes the heat to be removed. The arrangement of the cathode and the anode in the exemplary embodiment is chosen purely arbitrarily. They can also be exchanged. The surface 1 'of the solar cell 1 can heat up to over 400 degrees. As a result of the cooling, the temperature on the rear 1 'is only 100 to 200 degrees Celsius. As a result of the Seebeck effect, an additional thermoelectric voltage occurs between the two surfaces 1, 1 'with suitable materials. This can be well over 100 mV. Depending on the choice of the material of the solar cell, the selected polarity of the substrate of the solar cell and, if the solar cell 1 is suitably mounted on the electrolytic cell 2, it is possible that this thermoelectric effect in temperature gradients between the two surfaces of the solar cell contributes significantly to the overall efficiency of the arrangement.

According to the invention, solar cells of any kind can be used. It is not absolutely necessary to use III-V solar cells. IV component solar cells made of silicon, for example, can also be used. Techniques can be used in which the semiconductor of one polarity, which together with a semiconductor of the other polarity, forms a solar cell in such a way that at least one side serves directly as an electrode of an electrolysis cell. This method is referred to in the literature as photolysis, photo splitting or photoelectrolysis.

What is essential is the direct assembly of the solar cell 1 and the direct electrical connection of the solar cell 1 to an associated electrolysis cell 2, so that the electrical energy from the solar cell 1 is used directly for the electrolysis and the thermal contact between the solar cell 1 and the electrolysis cell 2 is one effective cooling of the solar cell 1 leads. Instead of one solar cell 1, it is of course also possible to use a plurality of solar cells 1 next to one another.

In practice, voltages between 1.3 V and 3.3 V are required for the electrolysis of water to hydrogen and oxygen. A solar cell is used which generates this voltage in particular using the above-mentioned thermoelectric effect. However, it is also possible to connect several solar cells in series in order to achieve the required voltage. It is thus possible to mount a series connection of 4 to 8 classic silicon solar cells on the side of the electrolysis cell facing the sun and to connect them electrically in an appropriate form.

The active cooling of the solar cell not only ensures a high synergy effect in the operation of the two components solar cell and electrolysis cell. It also ensures that the thermoelectric effect is harnessed. The thermodynamic starting situation, in particular with regard to the second law of thermodynamics, achieves a high yield of electrolysis.

It is also planned to use highly efficient "multi-junction solar cells" or tandem solar cells. Depending on the design, these components deliver an output voltage of more than 3 V and are often produced on germanium substrates. The desired thermoelectric effect can also occur here under concentrated sunlight if the substrate is of suitable polarity.

3 and 4, an embodiment of an electrolysis cell 2 is shown, which consists of two half-shells 5, 6 manufactured as turned parts. These two housing halves 5, 6 consist of an electrically and thermally highly conductive material. In particular, the housing half 5 supporting the solar cell 1 can consist of platinum.

The housing half 5 has a plane, circular surface 29 surrounded by an edge, on which the solar cell 1 is applied. The back 1 ′ of the solar cell 1 is connected to the surface 29 over the entire surface by means of a suitable electrically conductive and thermally conductive medium. The housing half 5 has a circular opening in its axis of symmetry. The solar cell 1 has a shape-adjusted opening that it is aligned with this opening.

The housing half 6, which is firmly connected to the housing half 1, has a central contact pin 28 which projects through the above-mentioned opening. The opening is lined with a ceramic tube 27, so that the two chambers 12 and 13 are sealed off from the outside.

The contact pin 28 is connected to a contact 18 of the solar cell 1 with a conductor 23 dimensioned according to the currents to be encountered, so that the housing half 5 forms the cathode and the housing half 6 the anode (or vice versa).

To isolate the two housing halves 5, 6, which are made in particular of metal, a separation plane is provided between the housing halves 5, 6, an insulation layer which can be formed in particular by a permeable wall 11. This permeable wall 11 separates the two chambers 12, 13 from one another in such a way that the hydrogen gas generated in the chamber 12 cannot penetrate into the chamber 13, or only insignificantly. The same applies to the oxygen gas generated in the chamber 13 with respect to the chamber 12.

In order to increase the heat and current-conducting surface of the chamber walls 13, 14, the chamber wall forms pegs 14 which run parallel to one another and extend transversely to the direction of extension of the permeable wall 11. The ends of the pins 14 can be in contact with the permeable wall 11, so that the position of the permeable wall 11 is stabilized by the pins 14. In particular, the direct electrical contact between the membrane and Electrodes the use of an ion-supplying PEM membrane as electrolyte and the use of pure water as a water source.

In this exemplary embodiment too, each of the two chambers 12, 13 has an individual feed line 7, 8 or an individual drain line 9, 10.

As can be seen in particular from FIG. 3, two circular contacts 18 ′, 18 are arranged on the surface 1 ′ of the solar cell 1 and are connected to one another by conductor tracks 26.

The exemplary embodiment shown in FIG. 5 shows an apparatus which has a solar cell arrangement 1 consisting of a plurality of individual solar cells 30. The individual solar cells 30 are electrically connected in series, so that a total voltage is present at the contact 18. Here, the solar cell arrangement 1 lies in an electrically insulated but thermally conductive system on the cathode 21, which is conductively connected to a pole of the solar cell arrangement 1. The other pole 18 is connected to the anode 22 via a conductor 23. In this exemplary embodiment, a plurality of individual electrolysis cells, which are electrically connected in series, are arranged one above the other, which overall form an electrolysis cell arrangement 2. Each of the individual electrolysis cells has a chamber 12 and a chamber 13 with separate feed lines 7, 8, the feed lines 7, 8 opening into the respective chambers 12 and the respective chambers 13 being connected to one another. The same applies to the leads 9, 10 of the individual chambers 12, 13, which are separated from one another in the manner described above by a permeable wall 11. The bottoms 31 of the respective individual electrolysis cells form electrical conductive connections between the electrolysis cells arranged one above the other. They form the anode on one side and the cathode on the other side. The number of individual solar cells 30 or the individual electrolysis cells are coordinated with one another such that the total voltage of the solar cell arrangement 1 is optimal for the dissociation of water into hydrogen and oxygen.

In small systems, the solar cell according to the invention can be operated under normal pressure. In medium-sized systems, it is intended to work at higher liquid pressures, for example at pressures of approximately 20 bar. In large systems, it is advantageous to work at liquid pressures of 70-120 bar. At such high pressures, subsequent compression of the hydrogen or oxygen produced can be omitted. Due to the use of highly concentrated sunlight, currents in the range between 300 and 3000 amperes can be generated. The solar cell can have a p-n junction or an n-p junction. But it is also possible to use thermoelectric solar cells. In particular at high liquid pressures, the voltage that can be achieved there can be sufficient to dissociate water.

Claims

1. Device for generating hydrogen by means of at least one solar cell (1), the solar cell (1) being spatially assigned to at least one electrolysis cell (2) containing an electrolyte and being operable with highly concentrated sunlight (3), characterized in that the solar cell ( 1) can be cooled by the electrolyte flowing through the electrolysis cell (2).

2. Device according to claim 1, characterized in that the solar cell (1) is arranged in the focal point of a light bundling device, in particular a reflector (4), the light trapping cross-section of which is at least 200 times preferably at least 1000 times as large as the solar cell area (1 ') ,

3. Device according to one or more of the preceding claims, characterized in that the light-sensitive active surface (1 ') of the solar cell (1) opposite back (1 ") substantially thermally conductive over the entire surface with a well thermally conductive housing section (5) Electrolyte cell (2) is connected or forms it.

4. Device according to one or more of the preceding claims, characterized in that the electrolyte cell (2) one or more inflow openings (7, 8) for the electrolyte and one or more outflow openings (9, 10) for the electrolyte and the dissolved in the electrolyte Has hydrogen / oxygen.

5. The device according to one or more of the preceding claims, characterized in that the electrolyte cell (2) forms two chambers (12, 13) separated by a permeable wall (11), each chamber (12, 13) having its associated inflow opening (7 , 8) and drain opening (9, 10), the anode (23) in one and the cathode (21) in the other chamber (12, 13).

Device according to one or more of the preceding claims, characterized in that the permeable wall (11) is a PEM membrane or another membrane which provides ions and allows the current to flow through its ion provision and ion conductivity, so that pure water can be used.

7. The device according to one or more of the preceding claims, characterized in that the thermally conductive housing section (5) forms the contact surface to the electrolyte-increasing structures (14).

8. The device according to one or more of the preceding claims, characterized by one or more electrolyte circuits (19, 20), each with a pump (15), a gas separator (16) and optionally a heat exchanger (17).

9. The device according to one or more of the preceding claims, characterized by using the thermoelectric effect between the hot, the focused sunlight (3) exposed active surface (1 ') and the rear, cooled side (1 ") of the solar cell (1) ,

10. The device according to one or more of the preceding claims, characterized by a thermal use of the waste heat of the heat exchanger (17).

11. The device according to one or more of the preceding claims, characterized in that the at least one solar cell (1) is formed, preferably by additional utilization of the thermoelectric effect, to provide electrical voltages at a level required for the dissociation of water, especially at levels of 1.3 to 3.3 V.

12. The device according to one or more of the preceding claims, characterized in that the electrolytic cell (2) is designed for both chambers or for one chamber and the permeable partition wall as a pressure vessel, preferably with an electrolyte pressure of over 70 bar with a pressure between 70 and can be operated at 120 bar.

13. The device according to one or more of the preceding claims, characterized by a plurality of individual solar cells (30) electrically connected in series in spatially direct association with a plurality of electrolytic cells (2) electrically connected in series.

14. The device according to one or more of the preceding claims, characterized in that the solar cell has at least one III-V semiconductor layer.

15. The device according to one or more of the preceding claims, characterized in that the back (1 ") of the solar cell (1) is electrically conductively connected to a first electrically conductive housing half (5) and one or more on the surface exposed to sunlight (1 ' ) the solar cell (1) arranged contacts (18) is connected to a second electrically conductive housing half (6), the two housing halves (5, 6) being insulated from one another.

16. The device according to one or more of the preceding claims, characterized in that the solar cell (s) belongs to the class of so-called tandem or multijunction solar cells.

17. A method for generating hydrogen by means of at least one solar cell (1), the solar cell (1) being spatially assigned to at least one electrolytic cell (2) containing an electrolyte and the active surface (1 ^! ) Of the solar cell (1) with highly concentrated sunlight (3) is acted on, characterized in that the electrolyte flows through the electrolysis cell (2) for cooling.

18. The method according to claim 17, characterized by at least one electrolysis circuit (19, 20) comprising a pump (15) and a gas separator (16) and a heat exchanger (17), the waste heat of which is used.

19. The method according to one or more of claims 17 and 18, characterized in that the thermoelectric effect between the hot, sunlit surface (l ¹ ) and the cooled back (1 ") of the solar cell (1) is used to to supply the voltage required for the dissociation of water or to add a thermoelectric voltage to a photoelectric voltage.

20. The method according to one or more of claims 17 to 19, characterized in that an electrolyte pressure of over 70 bar, preferably a pressure between 70 and 120 bar, is built up within the electrolysis cell (2).

21. The method according to one or more of claims 17 to 20, characterized in that an electrolysis process other than the electrolytic decomposition of water takes place.