JP7809265B2 - Hydrogen Generation System - Google Patents

Description

The present invention relates to a hydrogen generation system that generates hydrogen by decomposing water, and includes a decomposition tank to which water to be decomposed is supplied, a photocatalyst that comes into contact with the water inside the decomposition tank, and a light source that irradiates the photocatalyst with light.

Demand for hydrogen is increasing in Japan due to factors such as the increasing popularity of hydrogen stations and the fact that hydrogen can be reacted with carbon dioxide to produce methane, which can be used for city gas, etc.

One possible method of producing hydrogen is to extract it from natural gas. However, in recent years, in order to address environmental issues, the goal has been to create a low-carbon society, and ultimately, to create a carbon-free society. Considering this, extracting hydrogen from natural gas is undesirable because carbon dioxide is emitted when extracting hydrogen from natural gas. Therefore, there is a need for a method of producing hydrogen with as little carbon dioxide emissions as possible.

One method of generating hydrogen without emitting carbon dioxide is to generate hydrogen by decomposing water. Examples include methods of generating hydrogen by electrolyzing water and methods of decomposing water using a photocatalyst. Methods of generating hydrogen by electrolyzing water require electricity and an electrolysis device to electrolyze the water. On the other hand, methods using a photocatalyst do not require electricity or an electrolysis device because water can be decomposed using only water and a photocatalyst. Therefore, methods of decomposing water using a photocatalyst have attracted attention as a method of generating hydrogen more easily and at lower cost, and research and development is ongoing to maximize the efficiency of hydrogen generation. One known method of generating hydrogen using a photocatalyst is disclosed in Patent Document 1, for example.

On the other hand, Japan, a country with few energy resources, needs measures to ensure a stable supply of energy. In this context, there is growing interest in utilizing unused energy, such as waste heat from industrial equipment, in order to improve energy efficiency and achieve energy conservation.

JP 2010-470 A

However, there has been a problem in that the amount of hydrogen obtained by decomposing water using a photocatalyst is very small. For example, a photocatalyst is prepared by supporting rhodium-chromium composite oxide ( _RhCrOx ) as a promoter on strontium titanate ( _SrTiO3 ) doped with aluminum (Al) ( _RhCrOx / _SrTiO3 :Al), and the photocatalyst is dispersed in water. The water with the dispersed photocatalyst is then irradiated with light (wavelength λ>300 nm) from a xenon lamp at room temperature (20°C). Under these conditions, the amount of hydrogen obtained is very small, at 230 μmol per 100 ml of water in 60 minutes. Therefore, a system that can generate hydrogen more efficiently is desired.

Furthermore, from the perspective of energy conservation, currently much of the low- to medium-temperature waste heat (around 90 to 150 degrees Celsius) emitted from generators such as gas and diesel engines is simply discarded. There are high hopes for utilizing low- to medium-temperature waste heat to achieve greater energy conservation.

The present invention aims to solve the above problems by providing a hydrogen generation system that can generate hydrogen efficiently and is energy-efficient.

In order to solve the above problems, the hydrogen generation system of the present invention has the following configuration.
(1) A hydrogen generation system that generates hydrogen by decomposing water, comprising a decomposition tank to which water to be decomposed is supplied, a photocatalyst (excluding a self-oxidizing semiconductor) that comes into contact with the water inside the decomposition tank, and a light source that irradiates the photocatalyst with light, further comprising a heating device that uses exhaust heat to heat the water, and the heating device heats the water to a temperature above room temperature but below 100 degrees Celsius, thereby preventing a decrease in hydrogen generation efficiency due to evaporation of the water. The photocatalyst is characterized in that it is fixed on the surface of a glass substrate to form a sheet, which is then independently installed inside the decomposition tank.

The hydrogen generation system described in (1) can generate hydrogen efficiently and is an energy-saving hydrogen generation system.

The applicant has experimentally confirmed that when the water to be decomposed is heated and decomposed using a photocatalyst, the water decomposition activity is improved and the hydrogen production rate is increased compared to when the decomposition is performed at room temperature. Therefore, in a hydrogen generation system, hydrogen can be produced more efficiently by heating the water to be decomposed using a heating device.

The heating device also uses waste heat to heat the water to be decomposed. Waste heat is unused energy, such as the waste heat from a generator, and by utilizing unused energy, it is possible to create a hydrogen generation system that is energy-efficient.

Furthermore, according to the hydrogen generation system described in ( 1 ), hydrogen can be generated efficiently and the hydrogen generation system can be designed with energy conservation in mind.

The applicant has experimentally confirmed that when the temperature of the water to be decomposed exceeds room temperature, the water decomposition activity improves, with the hydrogen production rate at 60 degrees Celsius being approximately twice that at room temperature, and the hydrogen production rate at 90 degrees Celsius being approximately three times that at room temperature. Furthermore, the applicant has experimentally confirmed that when the temperature of the water to be decomposed exceeds 100 degrees Celsius, evaporation proceeds significantly, which may actually decrease the efficiency of hydrogen production. Therefore, it is desirable to heat the water to be decomposed to a temperature above room temperature but below 100 degrees Celsius using a heating device.

Furthermore, the waste heat used to heat the water to be decomposed above room temperature and below 100 degrees Celsius is medium- to low-temperature waste heat of around 90 to 150 degrees Celsius. Because it can utilize medium- to low-temperature waste heat, much of which is discarded, it can make effective use of unused energy, making it a hydrogen generation system that takes energy conservation into consideration.

(3) In the hydrogen generation system described in (1) or (2), the heating device is a cogeneration system that recovers exhaust heat, a heat exchanger is connected to the decomposition tank, and the cogeneration system heats water via the heat exchanger using the recovered exhaust heat.

The hydrogen generation system described in (3) can be designed to be energy-efficient. For example, in a cogeneration system that uses a gas engine as the internal combustion engine, the exhaust heat from the gas engine can be recovered as steam or hot water. If the exhaust heat recovered by the cogeneration system is used to heat the water to be decomposed via a heat exchanger, unused energy can be effectively utilized, resulting in a hydrogen generation system that is energy-efficient.

(4) In the hydrogen generation system described in (1) or (2), the heating device is a cogeneration system that recovers waste heat as hot water, and the water is hot water.

The hydrogen generation system described in (4) can be designed to be energy-efficient. For example, in a cogeneration system that uses a gas engine as the internal combustion engine, hot water heated by the exhaust heat of the gas engine can be recovered. If this hot water is used as the water to be decomposed, unused energy can be effectively utilized, making it possible to create a hydrogen generation system that is energy-efficient.

The hydrogen generation system of the present invention can generate hydrogen efficiently and is also designed to save energy.

1 is a diagram illustrating a configuration of a hydrogen generation system 1 according to a first embodiment. FIG. 1 is a diagram illustrating a configuration of a hydrogen generation system 1 according to a second embodiment. FIG. 1 is a diagram showing the configuration of an experimental device for conducting a photocatalytic water decomposition experiment. 1 is a graph comparing the amount of hydrogen generated when the suspension is heated and when the suspension is at room temperature. 1 is a graph comparing the rate of hydrogen production when the suspension is heated and when the suspension is at room temperature.

(First embodiment)
A first embodiment of a hydrogen generation system according to the present invention will be described in detail with reference to the drawings.

1 is a diagram showing the configuration of a hydrogen generation system 1 according to a first embodiment. The hydrogen generation system 1 includes a decomposition tank 11 having a photocatalyst sheet (an example of a photocatalyst) 12 therein, a light source 13, a cogeneration system (an example of a heating device) 14, and a heat exchanger 15, and is a system for generating hydrogen (H ₂ O) by decomposing water (H ₂ O) using the photocatalyst sheet 12.

The decomposition tank 11 is sealed, and the internal space 112 is isolated from the outside air. The internal space 112 is configured to be able to store water to be decomposed (hereinafter referred to as water to be decomposed W). At least the end surface 111 of the decomposition tank 11, on the side where the light source 13 (described below) is located, is made of transparent resin or glass. This allows light 131 emitted from the light source 13 to pass through the end surface 111 and reach the internal space 112.

The decomposition tank 11 is also connected to a supply line 16 for supplying water W to be decomposed from a water source (not shown), allowing water to be poured into the decomposition tank 11. As the amount of water W to be decomposed stored in the decomposition tank 11 decreases as decomposition progresses, water W to be decomposed is continuously supplied from the water source. The amount of water W to be decomposed continuously supplied is adjusted appropriately depending on the rate at which the water W to be decomposed is decomposed. The water W to be decomposed can be water obtained from various water sources, such as tap water or recycled water, or treated water from sewage where sludge has been removed.

Furthermore, the decomposition tank 11 is connected to a heating circuit 17 for circulating the stored water W to be decomposed. In other words, the water W to be decomposed discharged from the output port 113 of the decomposition tank 11 to the heating circuit 17 passes through a heat exchanger 15 (described below) and is returned to the internal space 112 of the decomposition tank 11 from the input port 114 of the decomposition tank 11. The decomposition tank 11 is also provided with a discharge hole 19 for recovering the hydrogen produced by decomposing the water W to be decomposed. The hydrogen recovered from the discharge hole 19 is temporarily stored, for example, in a storage tank (not shown), and then transported via a pipeline or transport vehicle to a hydrogen station or the like, where it is used as energy.

When the target water W is decomposed, oxygen (O ₂ ) is produced along with hydrogen, and the hydrogen and oxygen are discharged from the discharge hole 19. Therefore, it is conceivable to connect a separation device (not shown) equipped with, for example, a separation membrane to the discharge hole 19, separate the hydrogen and oxygen using the separation device, and then temporarily store the separated hydrogen in a storage tank as described above and use it as energy.

The photocatalyst sheet 12 is, for example, a glass substrate having a powdered photocatalyst immobilized on its surface and formed into a sheet. The photocatalyst sheet 12 is fixed in the internal space 112 of the decomposition tank 11 with the upper end surface 121 on which the photocatalyst is immobilized facing the end surface 111 of the decomposition tank 11 through which light 131 passes. At least the upper end surface 121 of the photocatalyst sheet 12 on which the photocatalyst is immobilized comes into contact with the water W to be decomposed that is poured into the decomposition tank 11. The photocatalyst may be, for example, aluminum (Al)-doped strontium titanate (SrTiO ₃ ) supported by an impregnation method with rhodium chromium composite oxide (RhCrO _x ) as a promoter (RhCrO _x /SrTiO ₃ :Al).

The photocatalyst is not limited to the above. Examples of photocatalysts include TiO ₂ , CaTiO ₃ , SrTiO ₃ , Sr ₃ Ti ₂ O ₇ , Sr ₄ Ti ₃ O ₁₀ , K ₂ La ₂ Ti ₃ O ₁₀ , Rb ₂ La ₂ Ti ₃ O ₁₀ , Cs ₂ La ₂ Ti ₃ O ₁₀ , CsLa ₂ Ti ₂ NbO ₁₀ , La ₂ TiO ₅ , La ₂ Ti ₃ O ₉ , La ₂ Ti ₂ O ₇ , La ₂ Ti ₂ O ₇ :Ba, KaLaZr _0.3 Ti _0.7 O ₄ , La _{4 CaTi5O17 ​ Titanium - containing oxides} such as _{KTiNbO5 , Na2Ti6O13} , _{BaTi4O9 , Gd2Ti2O7} , _{and Y2Ti2O7} , _zirconia ( _{ZrO2 )} , _{niobium -} containing oxides _{such as K4Nb6O17 , Rb4Nb6O17 , Ca2Nb2O7 , Sr2Nb2O7 , Ba5Nb4O15 , NaCa2Nb3O10 , ZnNb2O6 , Cs2Nb4O11 , and La3NbO7 , and Ta2O5 , K2PrTa5O 15 , K3Ta3Si2O13 , K3Ta3B2O12} , _{LiTaO3 , NaTaO3} , _KTaO3 , _AgTaO3 , _{KTaO3 : Zr} , _NaTaO3 :La _{, NaTaO3 : Sr , Na2Ta2O6 , K2Ta2O6 , CaTa2O6 , SrTa2O6 , BaTa2O6 , NiTa2O6 , Rb4Ta6O17 , Ca2Ta2O7 , Sr2 ​ Ta2O7 , K2SrTa2O7 ​ ​} , RbNdTa ₂ O ₇ , H ₂ La _2/3 Ta ₂ O ₇ , K ₂ Sr _1.5 Ta ₃ O ₁₀ , LiCa ₂ Ta ₃ O ₁₀ , KBa ₂ Ta ₃ O ₁₀ , Sr ₅ Ta ₄ O ₁₅ , Ba ₅ Ta ₄ O ₁₅ , H _1.8 Sr _0.81 Bi _0.19 Ta ₂ O ₇
Possible materials for use include tantalum-containing oxides such as LaTaO ₄ and La ₃ TaO ₇ , cerium-containing oxides such as CeO ₂ :Sr and BaCeO ₃ , tungsten-containing oxides such as PbWO ₄ , RbWNbO ₆ and RbWTaO ₆ , and oxynitrides such as (Ga1-xZnx)(N1-xOx), (Zn1+xGe)(N2Ox), LaMg1/3Ta2/3O2N and TaON.

The light source 13, for example, is a xenon lamp, and emits short-wavelength ultraviolet light (for example, wavelength λ>300 nm) as light 131. The light 131 emitted from the light source 13 passes through the end surface 111 of the transparent decomposition tank 11 and is irradiated onto the upper end surface 121 of the photocatalyst sheet 12, on which the photocatalyst is immobilized.

The photocatalyst immobilized on the photocatalyst sheet 12 generates hydrogen and oxygen (O ₂ ) from the target water W by being irradiated with light 131 while in contact with the target water W. The generated hydrogen and oxygen are then collected through the exhaust holes 19. The photocatalyst can generate hydrogen semi-permanently as long as the target water W and light 131 are present. Therefore, unlike methods of generating hydrogen by electrolyzing water, no electricity is required. This allows for cost-effective and easy hydrogen generation. The light source 13 is not limited to a xenon lamp; a mercury lamp, halogen lamp, laser, sunlight, or other suitable light source may be selected depending on the photocatalyst used, or a combination of these light sources may be used. Furthermore, the light 131 irradiated on the photocatalyst sheet 12 is not limited to ultraviolet light; visible light or medium- or long-wavelength infrared light may be selected depending on the photocatalyst used.

The cogeneration system 14 recovers waste heat from an internal combustion engine 141, such as a gas turbine or gas engine that serves as a power generator, as hot water. The hot water is heated by a heat exchanger 142, using, for example, jacket water J, which cools the internal combustion engine 141 by circulating through a jacket water circulation path 143. The temperature of the hot water is heated to approximately 90 degrees Celsius. This hot water at approximately 90 degrees Celsius corresponds to so-called medium- to low-temperature waste heat. The cogeneration system 14 is also connected to a heat medium circuit 18, which uses the recovered hot water as a heat medium H.

The heat exchanger 15 is connected to the heating circuit 17 and the heat transfer medium circuit 18, and performs heat exchange between the water to be decomposed W flowing through the heating circuit 17 and the heat transfer medium (hot water) H flowing through the heat transfer medium circuit 18.

The water W to be decomposed is discharged from the output port 113 of the decomposition tank 11 to the heating circuit 17, where it is heated to, for example, 90 degrees Celsius by heat exchange with the heat transfer medium (hot water) H in the heat exchanger 15. The heated water W to be decomposed is then returned to the decomposition tank 11 from the input port 114. The heat transfer medium (hot water) H discharged from the cogeneration system 14 to the heat transfer medium circuit 18 is cooled by heat exchange with the heat transfer medium (hot water) H in the heat exchanger 15. The cooled heat transfer medium (hot water) H is then returned to the cogeneration system 14. This cooled hot water as the heat transfer medium H can be used again to cool the jacket water J of the internal combustion engine 141 of the cogeneration system 14.

The water W to be decomposed in the decomposition tank 11 is maintained at 90°C by the water W to be decomposed heated in the heat exchanger 15. Therefore, the decomposition of the water W to be decomposed by the photocatalyst sheet 12 is carried out when the water W to be decomposed is at 90°C. When the water W to be decomposed is decomposed at 90°C, the hydrogen production rate is approximately three times faster than at room temperature, allowing for efficient hydrogen production.

The temperature of the water W to be decomposed in the decomposition tank 11 is not limited to 90 degrees Celsius, but may be any temperature within the temperature range above room temperature and below 100 degrees Celsius. This is because the rate of hydrogen production is improved in this temperature range compared to when the water W to be decomposed is at room temperature. The applicant has confirmed through experiments that the rate of hydrogen production is improved when the temperature of the water W to be decomposed is within the temperature range above room temperature and below 100 degrees Celsius.

The following describes the experiments conducted by the applicant. Figure 3 shows the configuration of the experimental apparatus 7 used to conduct experiments on water decomposition using a photocatalyst. The experimental apparatus 7 is composed of a water tank 71, a hot stirrer 72, a reaction vessel 73, a light source 74, and a trap 75.

The reaction vessel 73 is a light-transmitting vessel and contains a suspension S. The suspension S is prepared by adding 20 mg of a powdered photocatalyst (RhCrO _x /SrTiO ₃ :Al) to 100 mL of water to be decomposed, and then dispersing the powdered photocatalyst by ultrasonic treatment for one minute. Because the photocatalyst is dispersed in the suspension S, when irradiated with light 741 (described later), the suspension S decomposes the water to be decomposed and generates hydrogen.

The powdered photocatalyst was produced as follows. First, _SrTiO3 :Al was synthesized as follows. 10 mmol (1.8349 g) of strontium titanate ( _SrTiO3 ) and 0.2 mmol (0.0204 _g ) of aluminum oxide ( _Al2O3 ) were weighed and mixed thoroughly in an agate mortar for 15 minutes. 100 mmol (15.85 g) of block-shaped strontium chloride ( _SrCl2 ) was then added and mixed for 15 minutes while grinding to produce a powder sample. The powder sample was then transferred to an alumina crucible, covered, and heated to 1150°C at a rate of 10°C/min in an electric furnace and fired for 10 hours. After firing, the sample was cooled to room temperature, the alumina crucible was removed from the electric furnace, and the powder sample was washed with water until the washings were nearly neutral. The resulting mixture was then dried overnight at 60°C to complete the synthesis of _SrTiO3 :Al. Next, the co-catalyst rhodium-chromium composite oxide ( _RhCrOx ) was supported on the synthesized _SrTiO3 :Al by impregnation as follows. First, sodium hexachlororhodate(III) n-hydrate ( _{Na3RhCl6.nH2O} ) was used as the Rh source, and chromium nitrate _nonahydrate (Cr( _NO3 ) _3.9H2O ) was used as the Cr source. Both Rh and Cr were weighed out to _0.1 wt% metal. Then, strontium titanate ( _SrTiO3 ) and a small amount of water were added to an evaporating dish, and the mixture was stirred with a glass rod over a hot water bath until all the water had evaporated. _The mixture was then fired in an electric furnace at 350°C for 1 hour. The sample thus obtained is the above-mentioned powdered photocatalyst (RhCrO _x /SrTiO ₃ :Al).

A stirring bar 761 is also placed inside the reaction vessel 73. The stirring bar 761 is rotated by the stirrer 76 to evenly disperse the photocatalyst powder in the suspension S.

Furthermore, the reaction vessel 73 is equipped with an input port 731 and an output port 732. Argon gas for bubbling the suspension S flows into the reaction vessel 73 through the input port 731. Gases such as hydrogen and oxygen generated by the decomposition of water are discharged from the output port 732. A trap 75 is connected to the output port 732, and the gas discharged from the trap is quantified using a gas chromatograph.

Reaction vessel 73 is placed in water tank 71. Water is placed in water tank 71, and the temperature of the water (hereinafter simply referred to as water temperature) is controlled by a hot stirrer 72 equipped with a temperature sensor 721. A stirrer 722 is also placed in water tank 71. The stirrer 722 is rotated by the hot stirrer 72 to maintain a uniform temperature of the water in water tank 71.

The light source 74 is, for example, a xenon lamp, and is installed approximately 3 cm from the reaction vessel 73. The light source 74 can irradiate the suspension S in the reaction vessel 73 with light 741 (wavelength λ > 300 nm).

Using the experimental device 7 configured as described above, an experiment was conducted as follows.

First, a reaction vessel 73 containing suspension S was placed in a water tank 71 filled with water. Argon gas was bubbled through suspension S for one hour to remove air from the reaction vessel 73. The water temperature was then controlled to 30°C, 50°C, 60°C, 70°C, 80°C, or 90°C using a hot stirrer 72, and suspension S was heated. While suspension S was heated, light 741 was irradiated onto suspension S using a light source 74. The amount of hydrogen generated at each temperature (30°C, 50°C, 60°C, 70°C, 80°C, and 90°C) was quantified using a gas chromatograph. The amount of hydrogen generated at each temperature was then compared with the amount at room temperature. Note that when the suspension S is at room temperature, this refers to a case where the reaction vessel 73 containing the suspension S is placed in a water tank 71 with a water temperature of 20 degrees Celsius.

The results of this experiment will be explained using Figures 4 and 5. Figure 4 is a graph comparing the amount of hydrogen generated when suspension S is heated and when suspension S is at room temperature. Figure 5 is a graph comparing the rate of hydrogen generation when suspension S is heated and when suspension S is at room temperature.

The horizontal axis of the graph shown in Figure 4 represents the time that suspension S is irradiated with light 741, and the vertical axis represents the amount of hydrogen generated. It can be seen that there is a proportional relationship between time and the amount of hydrogen generated at each temperature, and that the amount generated increases as the temperature rises. Here, the slope determined from the relationship between time and the amount of hydrogen generated at each temperature is taken as the hydrogen production rate (μmol/h).

The horizontal axis of the graph shown in Figure 5 represents the water temperature in the water tank 71, and the vertical axis represents the hydrogen production rate. Comparing the hydrogen production rate at each temperature, it is clear that the production rate increases as the temperature rises, with the production rate at 60 degrees Celsius being approximately twice the production rate at 20 degrees Celsius and the production rate at 90 degrees Celsius being approximately three times the production rate at 20 degrees Celsius. On the other hand, it was confirmed that when the water temperature exceeds 100 degrees Celsius, evaporation of the suspension S progresses significantly, which may actually reduce the efficiency of hydrogen production.

From the above experimental results, in order to efficiently generate hydrogen in the hydrogen generation system 1, it is preferable to set the temperature of the water W to be decomposed in the decomposition tank 11 above room temperature and below 100°C. Note that room temperature was set to 20°C in the experiments conducted by the applicant, but according to JIS Z 8703, it refers to a range of 5°C to 35°C. Therefore, in reality, the temperature of the water W to be decomposed is heated to above 35°C.

Furthermore, as mentioned above, the water W to be decomposed is heated to a temperature above room temperature but below 100 degrees Celsius by the heat medium H, which uses the exhaust heat recovered from the cogeneration system 14. Therefore, the hydrogen generation system 1 can be made energy-efficient in that it can make effective use of unused energy.

The steamed cogeneration system 14 heats the water W to be decomposed using hot water, which is waste heat. However, it is also possible to recover the waste heat from the internal combustion engine 141 as steam, and pass this steam through the heat medium circuit 18 as a heat medium H to heat the water W to be decomposed. The steam as waste heat is obtained from an exhaust gas steam boiler using the exhaust gas from the internal combustion engine 141, and its temperature is approximately 150 degrees Celsius. This steam at approximately 150 degrees Celsius corresponds to so-called medium- to low-temperature waste heat.

As described above, according to the hydrogen generation system 1 according to the first embodiment,
(1) The hydrogen generation system 1 comprises a decomposition tank 11 to which water to be decomposed (water to be decomposed W) is supplied, a photocatalyst (e.g., a photocatalyst sheet 12) that comes into contact with the water to be decomposed W inside the decomposition tank 11, and a light source 13 that irradiates light 131 onto the photocatalyst (photocatalyst sheet 12), and is characterized in that it comprises a heating device (e.g., a cogeneration system 14) that uses exhaust heat to heat the water to be decomposed W.

The hydrogen generation system 1 described in (1) can generate hydrogen efficiently and can be designed to be energy-efficient.

The applicant has experimentally confirmed that when photocatalytic decomposition is performed on water to be decomposed while it is heated, the water decomposition activity is improved and the hydrogen production rate increases compared to when the decomposition is performed at room temperature. Therefore, in the hydrogen generation system 1, hydrogen can be produced more efficiently by heating the water to be decomposed (water to be decomposed W) using a heating device (cogeneration system 14).

The heating device (e.g., cogeneration system 14) uses exhaust heat to heat the water to be decomposed (water to be decomposed W). Exhaust heat is, for example, unused energy such as the exhaust heat of a generator (e.g., internal combustion engine 141), and by utilizing this unused energy, it is possible to create a hydrogen generation system 1 that is energy-efficient.

(2) In the hydrogen generation system 1 described in (1), the heating device (cogeneration system 14) heats the water (water W to be decomposed) to a temperature above room temperature and below 100 degrees Celsius.

The hydrogen generation system 1 described in (2) can generate hydrogen efficiently and can be designed to be energy-efficient.

The applicant has experimentally confirmed that when the temperature of the water to be decomposed exceeds room temperature, the water decomposition activity improves, with the hydrogen production rate at 60 degrees Celsius being approximately twice that at room temperature, and the hydrogen production rate at 90 degrees Celsius being approximately three times that at room temperature. Furthermore, the applicant has experimentally confirmed that when the temperature of the water to be decomposed exceeds 100 degrees Celsius, evaporation progresses, and the efficiency of hydrogen production may actually decrease. Therefore, it is desirable to heat the water to be decomposed (water to be decomposed W) to a temperature above room temperature but below 100 degrees Celsius using a heating device (cogeneration system 14).

Furthermore, medium- to low-temperature waste heat of approximately 90 to 150 degrees Celsius is appropriate for the waste heat used to heat the water to be decomposed (water to be decomposed W) above room temperature and below 100 degrees Celsius. Because it is possible to utilize medium- to low-temperature waste heat, much of which is discarded, it is possible to make effective use of unused energy, making the hydrogen generation system 1 energy-saving.

(3) In the hydrogen generation system 1 described in (1) or (2), the heating device is a cogeneration system 14 that recovers exhaust heat, a heat exchanger 15 is connected to the decomposition tank 11, and the cogeneration system 14 heats water (water to be decomposed W) via the heat exchanger 15 using the recovered exhaust heat (e.g., hot water or steam).

The hydrogen generation system 1 described in (3) can be configured as a hydrogen generation system 1 that takes energy conservation into consideration. For example, if the cogeneration system 14 uses a gas engine as the internal combustion engine 141, the exhaust heat of the gas engine can be recovered as steam or hot water. If the exhaust heat recovered by the cogeneration system 14 is used to heat the water to be decomposed (water to be decomposed W) via the heat exchanger 15, unused energy can be effectively utilized, making it possible to configure a hydrogen generation system 1 that takes energy conservation into consideration.

Second Embodiment
Next, a second embodiment of the hydrogen generation system of the present invention will be described with reference to the drawings, focusing on differences from the first embodiment.

2 is a diagram showing the configuration of a hydrogen generation system 5 according to a second embodiment. The hydrogen generation system 5 includes a decomposition tank 51 having a photocatalytic sheet 52 therein, a light source 53, a cogeneration system (an example of a heating device) 54, and a temperature regulator 55, and is a system for generating hydrogen by decomposing water (H ₂ O) using the photocatalytic sheet 52.

The decomposition tank 51 is sealed, and the internal space 512 is isolated from the outside air. The internal space 512 is configured to be able to store water W to be decomposed. At least the end surface 511 of the decomposition tank 51, on the side where the light source 53 (described below) is located, is made of transparent resin or glass. This allows light 531 emitted from the light source 53 to pass through the end surface 511 and reach the internal space 512.

The decomposition tank 51 also has a supply hole 513 connected to the supply channel 56, and the water to be decomposed W supplied from the supply channel 56 is poured into the internal space 512 through the supply hole 513. The decomposition tank 51 also has a drainage hole 515.

The decomposition tank 51 also has a discharge hole 514 for collecting the hydrogen produced by decomposing the target water W. The hydrogen collected from the discharge hole 514 is temporarily stored, for example, in a storage tank (not shown), and then transported via a pipeline or transport vehicle to a hydrogen station or the like, where it is used as energy.

The photocatalyst sheet 52 (an example of a photocatalyst) is identical to the photocatalyst sheet 12 according to the first embodiment. The photocatalyst sheet 52 is fixed to the internal space 512 of the decomposition tank 51 with the upper end surface 521, on which the photocatalyst is immobilized, facing the end surface 511 of the decomposition tank 51 through which light 531 passes. At least the upper end surface 521 of the photocatalyst sheet 52, on which the photocatalyst is immobilized, comes into contact with the water W to be decomposed that is poured into the decomposition tank 51. As with the first embodiment, instead of forming the photocatalyst into a sheet, photocatalyst powder may be dispersed in the water W to be decomposed in the decomposition tank 51 to form a suspension within the decomposition tank 51.

The light source 53 and the light 531 emitted from the light source 53 are identical to the light source 13 and light 131 in the first embodiment. The light 531 passes through the end surface 511 of the transparent decomposition tank 51 and is irradiated onto the upper end surface 521 of the photocatalyst sheet 52, on which the photocatalyst is immobilized. The photocatalyst immobilized on the photocatalyst sheet 52 produces hydrogen and oxygen from the water W by being irradiated with the light 531 while in contact with the water W to be decomposed. The produced hydrogen and oxygen are then discharged through the discharge hole 19.

The cogeneration system 54 recovers waste heat from an internal combustion engine 541, such as a gas turbine or gas engine that serves as a power generator, as hot water HW. The hot water HW is heated by a heat exchanger 542, using jacket water J, which cools the internal combustion engine 541 by circulating through a jacket water circulation path 543, as a heat source. The temperature of the hot water HW is heated to approximately 90 degrees Celsius. This hot water HW at approximately 90 degrees Celsius corresponds to so-called medium- to low-temperature waste heat. A hot water outlet path 57 is connected to the cogeneration system 54, and the hot water HW passes through the hot water outlet path 57 and is discharged from the cogeneration system 54.

The temperature regulator 55 is connected to the hot water outlet passage 57 and the supply passage 56. The hot water HW introduced into the temperature regulator 55 from the hot water outlet passage 57 is adjusted by the temperature regulator 55 to any temperature above room temperature and below 100°C. The adjusted hot water HW is discharged into the supply passage 56 as water to be decomposed W. The water to be decomposed W discharged into the supply passage 56 is then poured into the internal space 512 of the decomposition tank 51 through the supply hole 513. In this way, the hot water HW as waste heat is used as water to be decomposed W, so that water to be decomposed W is continuously supplied to the decomposition tank 51 while the internal combustion engine 141 is operating. Excess water to be decomposed W that is not decomposed in the decomposition tank 51 is discharged through the drain hole 515 of the decomposition tank 51.

The temperature of the water W to be decomposed is regulated by the temperature regulator 55, so that the water W to be decomposed in the decomposition tank 51 is kept at a desired temperature above room temperature and below 100 degrees Celsius. Therefore, the decomposition of the water W to be decomposed by the photocatalyst sheet 52 is carried out when the temperature of the water W to be decomposed is above room temperature and below 100 degrees Celsius.

As mentioned above, the applicant has experimentally confirmed that the generation rate increases when the temperature of the water W to be decomposed exceeds room temperature, with the generation rate at 60 degrees Celsius being approximately twice that at 30 degrees Celsius and the generation rate at 90 degrees Celsius being approximately three times that at 30 degrees Celsius. Furthermore, as mentioned above, the applicant has experimentally confirmed that when the water temperature exceeds 100 degrees Celsius, evaporation of the suspension S progresses, which may actually reduce the efficiency of hydrogen generation. Therefore, in the hydrogen generation system 5, the water W to be decomposed is decomposed when its temperature is above room temperature and below 100 degrees Celsius, thereby enabling efficient hydrogen generation.

As described above, according to the hydrogen generation system 5 according to the second embodiment,
(4) In the hydrogen generation system 5 described in (1) or (2), the heating device (e.g., the cogeneration system 54) is a cogeneration system that recovers exhaust heat as hot water HW, and the water (water W to be decomposed) is hot water HW.

The hydrogen generation system 5 described in (4) can be an energy-saving hydrogen generation system 5. For example, if the cogeneration system 54 uses a gas engine as the internal combustion engine 541, hot water HW heated by the exhaust heat of the gas engine can be recovered. If this hot water HW is used as the water to be decomposed (water to be decomposed W), unused energy can be effectively utilized, making it possible to provide a hydrogen generation system 5 that is energy-saving.

The above first and second embodiments are merely illustrative and do not limit the present invention in any way. Naturally, therefore, various improvements and modifications are possible within the spirit and scope of the present invention. For example, while photocatalyst sheets 12, 52 are used as an example of a photocatalyst, the photocatalyst does not necessarily have to be formed into a sheet. For example, photocatalyst powder may be dispersed in the water W to be decomposed in the decomposition tank 11, 51 to form a suspension in the decomposition tank 11, 51, thereby bringing the water W to contact with the photocatalyst. In this case, the water W to be decomposed is continuously supplied to the decomposition tank 11, 51 to maintain the concentration of the photocatalyst in the suspension. Furthermore, a stirring device is required to evenly disperse the photocatalyst powder in the suspension.

1 Hydrogen generation system 11 Decomposition tank 12 Photocatalytic sheet (an example of a photocatalyst)
13 Light source 14 Cogeneration system (an example of a heating device)
131 Light W Decomposition target water

Claims (3)

A hydrogen generation system that generates hydrogen by decomposing water, the hydrogen generation system comprising: a decomposition tank to which water to be decomposed is supplied; a photocatalyst (excluding a self-oxidizing semiconductor) that comes into contact with the water inside the decomposition tank; and a light source that irradiates the photocatalyst with light,
a heating device that uses exhaust heat to heat the water;
the heating device heats the water to a temperature higher than room temperature and lower than 100 degrees Celsius to prevent a decrease in hydrogen production efficiency due to evaporation of the water;
the photocatalyst is fixed on the surface of a glass substrate to form a sheet, and the sheet is independently installed inside the decomposition tank;
A hydrogen generation system characterized by: The hydrogen generation system according to claim 1 ,
the heating device is a cogeneration system that recovers the exhaust heat;
a heat exchanger is connected to the decomposition tank;
the cogeneration system heats the water via the heat exchanger using the recovered exhaust heat;
A hydrogen generation system characterized by: The hydrogen generation system according to claim 1 ,
the heating device is a cogeneration system that recovers the exhaust heat as hot water;
the water is hot water;
A hydrogen generation system characterized by: