Utilization of Hydrogen Using Hydrogen Storage Alloys

The use of renewable energy is being promoted around the world to prevent global warming. Since the amount of power generated by renewable energy fluctuates, surplus electricity can be effectively utilized by converting it into hydrogen. There are many control problems associated with the supply and use of hydrogen in, for example, heavy goods vehicles. It is hoped the following “perspective” will encourage papers on the modeling and control engineering problems, where advance controls may play an important future role. Renewable energy, particularly solar and wind power, is increasing in Japan, but the cost is high, and their adoption has not progressed as much as in other countries. Therefore, a model is being considered that involves importing large quantities of clean hydrogen from overseas. To reduce the cost of transporting hydrogen, the introduction of high-density materials such as liquefied hydrogen, chemical hydride, ammonia, and synthetic methane is being considered. On the other hand, reducing CO2 emissions in domestic industry is also an urgent issue. Switching to fuels that do not emit CO2 could be a fundamental solution, but infrastructure development and fuel costs are issues, and it will not be realized for a while. There are growing expectations for local production for local consumption, such as energy self-sufficiency and effective use of CO2. We are therefore developing applications using hydrogen storage alloys. The first application is the development of a chemical compressor that uses hydrogen storage alloys. Hydrogen storage alloys have the property of absorbing hydrogen at room temperature and releasing hydrogen at high temperatures. By installing multiple containers filled with hydrogen storage alloys and repeating cooling and heating in sequence, hydrogen can be continuously compressed to high pressure. If waste heat can be effectively utilized, hydrogen can be compressed without using electricity. Compared to conventional mechanical hydrogen compressors, there is no noise, and because it has no sliding parts, it is also expected to reduce maintenance costs [1, 2]. The second application is the demonstration of hydrogen transportation and utilization using hydrogen storage alloys. Generally, the place where renewable energy is generated and the place where the power is used are different, so it is necessary to convert into hydrogen using water electrolysis equipment and then transported in cases where the power infrastructure is not in place. Electricity is required to compress or liquefy hydrogen, resulting in large energy losses. Additionally, the High-Pressure Gas Safety Act applies to their operation and transportation in Japan. The developed storage alloy tank is about the same weight as a general hydrogen cylinder and can store the same amount of hydrogen even at pressures of less than 1 MPaG, which is not apply to the High-Pressure Gas Safety Act. Hydrogen is released using the exhaust heat of the fuel cell, these transportation and utilization systems are safe and convenient. These systems use nano-sized iron-titanium hydrogen storage alloys manufactured by NASU DENKI-TEKKO. It is manufactured using a special alloy manufacturing method called mechanical alloying and has a nano-level fine crystal structure [3]. The initial hydrogen reactivity (initial activity), which was a drawback of conventional iron-titanium alloys [4], has been significantly improved. While taking advantage of the inexpensive characteristics of iron-titanium alloys, this is a highly practical alloy that is not classified as a hazardous material under Japanese Fire Service Act [5]. The absorption and desorption characteristics of hydrogen storage alloys will be explained [6]. The reaction in which a hydrogen storage alloy absorbs hydrogen and produces a metal hydride is an exothermic reaction, while the reaction in which a metal hydride decomposes, and releases hydrogen is an endothermic reaction. The PCT characteristic diagram is commonly used as a method for showing the characteristics of hydrogen storage alloys. P is the hydrogen pressure, C is the amount of hydrogen contained in the alloy, and T is the temperature. Figure 1 is an isothermal diagram of hydrogen pressure and hydrogen content at constant temperature. When the temperature is kept constant and the hydrogen pressure is increased, hydrogen dissolves in the alloy and a solid solution phase (α phase) is formed. The amount of hydrogen in solution increases in proportion to the logarithm of the pressure (a → b). This relationship is called Sieverts' law. At point b, part of the α phase reacts with hydrogen to form a metal hydride phase (β phase). During the phase transformation from the α phase to the β phase, there is a region where the hydrogen pressure is constant (b → c). This pressure is called the plateau pressure and is an indicator of the properties of the hydrogen storage alloy. Once all the α phase has transformed into β phase, the hydrogen pressure and hydrogen content increase proportionally again (c → d). If the temperature is held constant, the plateau pressure is uniquely determined. This indicates that the plateau pressure can be controlled by changing the temperature. The plateau pressure may be slightly different during the hydrogen absorption and desorption processes, in which case hysteresis occurs. Hysteresis is thought to be caused by large distortions in the crystal structure that occur during phase transformation between the solid solution phase (α phase) and the hydride phase (β phase), as well as the occurrence of internal lattice defects such as dislocations and stacking faults [7]. To utilize hydrogen storage alloys, this hysteresis must be taken into consideration. Most hydrogen storage alloys are intermetallic compounds consisting of a combination of element A, which forms a stable hydride, and element B, which forms an unstable hydride, and are classified into several groups based on their composition and structure. For example, AB5 type alloys such as LaNi5, which is a typical hydrogen storage alloy, AB2 type alloys (Laves phase alloys) that use Ti or Zr for A, inexpensive AB type alloys such as TiFe, and V-based solid solution alloys with a body-centered cubic lattice (BCC) structure are some of them. Even in the same temperature range, the plateau pressure differs depending on the alloy composition, so a chemical compressor can be designed by selecting an appropriate hydrogen storage alloy. The hydrogen release pressure from the hydrogen storage alloy increases exponentially as the temperature rises. It is possible to increase the pressure of hydrogen using heat by this property. High-pressure hydrogen can be released by heating the alloy to a high temperature after low-pressure hydrogen is absorbed in a hydrogen storage alloy as a heat-driven compressor. Figure 2 is a schematic diagram of the hydrogen compression cycle. After absorbing low-pressure (PL < 1 MPa) hydrogen at room temperature (point 1 → 2), the alloy is heated (point 2 → 3), and hydrogen is released at an exponentially increased pressure in response to the heating temperature. As heating continues, the amount of hydrogen in the alloy decreases and hydrogen is released outside until it reaches the left end of the plateau region (point 3 → 4). When the temperature is lowered to room temperature (point 4 → 1), the pressure also decreases and returns to the initial state. This is the compression cycle for one reactor system, and the amount of hydrogen discharged in one cycle is ΔC = C2 − C4. By setting up multiple systems and operating them in cycles, it can be used as a compressor with a high compression ratio. This chemical compressor can achieve a compression ratio of more than 10 times with heat of about 200°C. Assuming that the volume is constant based on the equation of state for an ideal gas, and considering that the pressure increases by just under 1.6 times when the temperature is raised from 25°C to 200°C, the high performance of the chemical compressor can be seen. We have focused on the properties of this hydrogen storage alloy and are developing a high-pressure hydrogen compressor using this alloy with the team shown in Table 1. The specifications of a prototype were a low hydrogen pressure of less than 1 MPa before compression, and 19.6 MPa after compression, which is the pressure found in commonly used cylinders and curdles. The hydrogen storage alloy used in the compressor is a nano-sized iron-titanium alloy manufactured by NASU DENKI-TEKKO. The practical use of this inexpensive alloy has been increased by significantly improving the initial hydrogen reactivity. Figure 3 shows a schematic diagram of the chemical compressor. The prototype was tested at Hydrogen Energy Test and Research Center (HyTReC) in Fukuoka. Figures 4 and 5 are photos of the prototype. In the test operation, hydrogen was absorbed in a reactor containing hydrogen storage alloys, and the pressure increase was measured as the reactor temperature was increased. Figure 6 shows the pressure increase as the reactor temperature was increased. The target pressure of 19.6 MPa was achieved at approximately 230°C. Furthermore, the compressor was cycled 50 times to verify durability, and the results are shown in Figure 7. During the operation period, no changes were observed in the absorption and discharge capacity, and it was confirmed that the discharge pressure was 19.6 MPa and the hydrogen flow rate was 1 Nm3/h or more. We have manufactured the second prototype shown in Figure 8. To popularize this compressor, it is desirable to use the waste heat at a lower temperature, and we are working on development to lower the temperature at which hydrogen is compressed. To realize a hydrogen society, it is essential to further promote the development of technologies for “producing,” “storing,” “transporting,” and “using” hydrogen. We are developing a hydrogen delivery and utilization system that uses hydrogen storage alloys. The concept is shown in Figure 9. This system is a series of processes in which hydrogen is stored in metal hydride tanks filled with hydrogen storage alloys, transported to the destination where electricity is required, the hydrogen is extracted, and electricity is generated by fuel cells. The duties of each company in the hydrogen delivery and utilization system are as shown in Table 2. Hydrogen produced from carbon-neutral city gas using a hydrogen generator installed at Mitsubishi Kakoki was filled into metal hydride tanks, transported, and connected to fuel cells to provide power for various events. As shown in Equation (1), hydrogen storage alloys generate reaction heat as they absorb and release hydrogen. When using hydrogen storage alloys for storage, this reaction heat must be removed [8]. Conventionally, water-cooled/water-heated systems [9, 10] have been used to remove this reaction heat, but we have developed an air-cooled/air-heated system [11]. There is no need to install heat transfer medium piping on the tank, so this system is designed with a simple structure. The weight of the metal hydride tank has been reduced by optimizing the design of the tank thickness and the amount of hydrogen storage alloys. The tank size is Φ139.8 mm × length 1200 mm, and it weighs 65 kg. The hydrogen storage capacity per cylinder is 5 Nm3. In addition, this storage alloy has excellent durability even when repeatedly storing and releasing hydrogen, and the tank can be transported in an upright position, so it can be handled in the same way as a hydrogen cylinder. Hydrogen is stored in metal hydride tanks at less than 1 MPaG, so the High-Pressure Gas Safety Act does not apply. The fuel cell system used was the Hydro X Power manufactured by Nippon Filcon, with an output of 3.5 kW. It can generate electricity for 8 h by four metal hydride tanks. Hydrogen can be released efficiently from the hydrogen storage alloy by utilizing the exhaust heat from the fuel cell to heat the metal hydride tank, thereby contributing to energy savings in the supply chain. Figure 10 shows a photograph of hydrogen storage and supply. Demonstration tests were performed at 10 locations from August 2023 to July 2024. Figure 11 shows an example of the demonstration test. Although hydrogen is difficult to store in the alloy at high temperature, it was confirmed that it is possible to fill the alloy with hydrogen by simply operating the fan, even in the summer. On the other hand, hydrogen is difficult to release at low temperatures, but it was confirmed that electricity can be generated even at 3°C in winter. The system was able to keep up with fluctuations in electricity demand and continue supplying power without interruption at each demonstration site. One possible application for the chemical compressor is to pressurize hydrogen produced from renewable energy using waste heat from factories. This compressor has the potential to efficiently produce high-pressure hydrogen. We will continue to develop alloys and scale them up, aiming to build a hydrogen infrastructure with excellent energy efficiency. A future issue for the chemical compressors will be to develop control modeling for managing the various exhaust heat properties in response to the required hydrogen pressure and flow rate. Hydrogen delivery systems that use hydrogen storage alloys are easy to store and transport and have a significant advantage in supplying electricity on a relatively small scale, such as during disasters or events, where the location and period of use are not fixed. This system will require optimization studies that add hydrogen transportation to the EMS related to renewable energy and hydrogen conversion. Japan has many remote islands, and energy self-sufficiency is an issue. The construction of offshore wind power plants is planned, and it is expected that some of the power generated will be supplied to remote islands. To effectively utilize renewable energy on remote islands where infrastructure is underdeveloped, it is effective to convert surplus electricity into hydrogen and transport the metal hydride tanks to the electricity usage destination. We are considering a utilization model for a remote island with Nagasaki University and Tokyo Sangyo, as shown in Figure 12, and will proceed with demonstration experiments. In the future, we will explore the possibility of commercializing hydrogen storage alloy technology, aiming to realize a sustainable society that utilizes hydrogen energy.