Hydrogen storage and transportation are important links in the development of hydrogen energy. The high cost of hydrogen transportation has become a bottleneck for large-scale application of hydrogen energy.
The hydrogen energy industry chain includes three key links – hydrogen production, storage/transportation, and utilization, as shown in the diagram below:
The biggest challenge for hydrogen energy development worldwide is storage and transportation. Finding safe, economical, efficient and feasible storage and transportation methods is the key to full life cycle application of hydrogen energy. Hydrogen storage and transportation includes storage of hydrogen gas as well as transportation of hydrogen energy.
Hydrogen Storage
The requirements for hydrogen storage technologies are safety, large capacity, low cost and easy access. Currently, there are 4 main hydrogen storage methods – low temperature liquid hydrogen, high pressure gaseous hydrogen, solid state materials, and organic liquid hydrogen. A comparison of the 4 main hydrogen storage methods is shown in the table below:
Among the 4 hydrogen storage technologies, high pressure hydrogen is the most mature and widely applied, but has limitations in density and safety. Solid state materials have huge potential but are still in research stage. Low temperature liquid hydrogen has advantages in gravimetric and volumetric density, but high cost due to liquefaction energy and insulation requirements. Organic liquid hydrogen has not achieved large scale commercialization due to cost and technical challenges.
High Pressure Gaseous Hydrogen Storage
High pressure gaseous hydrogen storage is a relatively mature technology and currently the most widely used hydrogen storage method globally. It involves compressing hydrogen gas into a high-pressure resistant container. The hydrogen is stored in gaseous state, and the storage amount is proportional to the pressure inside the tank. Gas cylinders are commonly used as the storage containers.
The advantages of this method are low energy consumption for storage, low cost (at lower pressures), and easy control of hydrogen release through pressure relief valves. Thus, high pressure gaseous storage has become a relatively mature hydrogen storage solution.
Currently, there are two main types of high pressure hydrogen cylinders worldwide – 35 MPa carbon fiber composite cylinders, and 70 MPa hydrogen cylinders. The hydrogen density is around 23 kg/m3 in 35 MPa cylinders, and around 38 kg/m3 in 70 MPa cylinders. A new patent by Toyota in 2017 proposed an all-composite light-weight fiber wound tank design that can reach 70 MPa with a gravimetric density of 5.7%. However, pressurization of the cylinders is costly, and safety decreases significantly with increasing pressure, with risks of leakage and explosion. Thus, safety needs to be improved.
Future development of high pressure hydrogen storage will focus on lighter weight, higher pressure, lower cost, and stable quality. New cylinder materials will be explored to match higher pressure storage needs, improving both safety and economics.
Low Temperature/Organic Liquid Hydrogen Storage
- Low temperature liquid hydrogen storage first liquefies hydrogen gas, then stores it in low temperature, vacuum-insulated containers. The advantage is the high volumetric energy density of liquid hydrogen – with a density of 70.78 g/L, about 850 times that of gaseous hydrogen at ambient conditions. Even when compressed, the storage density of gaseous hydrogen per unit volume is far lower than liquid storage. However, the boiling point of liquid hydrogen is extremely low (−252.78°C), creating a large temperature difference from the environment and placing high insulation requirements on the storage containers. Currently, the largest liquid hydrogen storage tank is at the Kennedy Space Center in the U.S., with a capacity of 12,000 L.
- Organic liquid hydrogen storage chemically binds hydrogen gas to aromatic organic compounds like toluene (TOL) through hydrogenation, forming saturated ring compounds like methylcyclohexane (MCH) containing hydrogen within the molecular structure. This allows storage and transportation in liquid form at ambient temperature and pressure. At the point of use, a catalyst can extract the needed amount of hydrogen gas through dehydrogenation, as shown in the diagram below:
Organic liquid hydrogen storage allows hydrogen to be transported and stored in liquid form at ambient temperature and pressure, which is safe and efficient. However, there are still technical bottlenecks like complex dehydrogenation technology, high dehydrogenation energy consumption, and a need to break through dehydrogenation catalyst technology. If these problems can be solved, organic liquid hydrogen storage will become one of the most promising technologies for large-scale application in the field of hydrogen energy storage and transportation.
For large-scale, long-distance hydrogen energy storage and transportation, low temperature liquid hydrogen storage has greater advantages. When transporting over 500 km, the delivery cost per kg of liquid hydrogen only increases by about US$0.3, while the cost for high-pressure gaseous transport will increase more than 5 times. Currently, low temperature liquid hydrogen is mainly used as a cryogenic propellant for aerospace applications. Some researchers have started looking at using liquid hydrogen as an automotive fuel, but there has been no substantial progress so far. The only demonstrated applications of liquid hydrogen storage technology are Kawasaki Heavy Industries’ liquefied hydrogen storage and Chiyoda Corporation’s organic chemical hydride storage. In organic liquid hydrogen storage, American chemists have developed a B-N based liquid hydrogen storage material that can work safely at room temperature, providing a solution to the hydrogen energy storage and transportation challenge. In the near future, China should step up R&D efforts on low temperature and organic liquid hydrogen storage technologies, developing low-cost and low-power dehydrogenation catalysts and low melting point storage media. This is of great significance to the global hydrogen energy industry layout and an important direction for the large-scale development of future hydrogen energy storage and transportation.
Solid Material Hydrogen Storage
Based on the hydrogen storage mechanisms, solid-state hydrogen storage materials can be divided into two categories: physically adsorbed materials and metal hydride materials. Metal hydrides are currently the most promising and rapidly developing solid hydrogen storage approach. The classification of solid hydrogen storage materials is shown in the diagram below:
Metal hydride hydrogen storage utilizes metal hydride materials to store and release hydrogen gas. Under certain temperature and pressure, hydrogen reacts with transition metals or alloys, absorbing hydrogen in the form of metal hydrides. Hydrogen can then be released by heating the hydride. Examples are LaNi5H6, MgH2 and NaAlH4.
Metal hydride hydrogen storage tanks have the following characteristics: high hydrogen storage density, easy operation, convenient transportation, low cost, high safety, and good reversibility. However, the mass efficiency is low. If the mass efficiency can be effectively improved, this storage method would be very suitable for use in fuel cell vehicles.
A comparison of different hydrogen storage technologies, summarizing their various characteristics, is shown in the diagram below:
As shown in the diagram, low temperature liquid hydrogen storage technology has limited applications and high costs, making its commercial application prospects not as good as the other 3 storage technologies in the long run. High pressure gaseous hydrogen storage is the most mature technology worldwide. Low temperature liquid and organic liquid hydrogen storage have good overall performance, but related technologies need breakthroughs to reduce costs. Currently, hydrogen refueling stations use high pressure gaseous hydrogen storage. In the long run, high pressure gaseous hydrogen will still be the mainstream in China. However, due to safety risks and low volume-to-capacity ratio, this technology is not perfect for application in hydrogen fuel cell vehicles, so its use may decline in the future. Solid hydrogen storage materials have excellent hydrogen storage performance and are the most ideal of the 4 methods, as well as a frontier research area for hydrogen storage. However, it is still at the technology breakthrough stage. Therefore, this technology could be a breakthrough to remove barriers for hydrogen energy storage and accelerate hydrogen industry development.
Hydrogen Transportation
Hydrogen transportation methods vary depending on the hydrogen storage state and transportation quantity, with the three main methods being gaseous hydrogen delivery, liquid hydrogen delivery, and solid hydrogen delivery.
Gaseous Hydrogen Delivery
Gaseous transportation is divided into tube trailers and pipelines. Tube trailers have high costs for long-distance large-volume transportation; while pipelines are an important way to achieve large-scale, long-distance hydrogen delivery. Pipeline transportation has high hydrogen capacity and low energy consumption, but requires large upfront investment in pipeline construction. In the early stages of pipeline development, blending hydrogen into natural gas can be actively explored.
Liquid Hydrogen Delivery
Liquid delivery is suitable for long-distance, large-volume transportation, using liquid hydrogen tanker trucks or dedicated liquid hydrogen barges. Liquid hydrogen delivery can improve the supply capacity of individual refueling stations. Japan and the U.S. have adopted liquid hydrogen tankers as an important means of transporting hydrogen to refueling stations. In 2009, Japan’s Chiyoda Corporation successfully developed key technologies for the LOHC (Liquid Organic Hydrogen Carriers) system. The world’s first hydrogen supply chain demonstration project adopted Chiyoda’s SPERA technology to explore the commercial demonstration of liquid organic hydrogen carriers, achieving 210 t/year hydrogen transportation capacity in 2020.
Solid Hydrogen Delivery
Hydrogen stored via metal hydrides can utilize more diverse transportation means. Barges, large tankers and other transportation tools can all transport solid state hydrogen.
The comparison shows that for transportation distances over 300 km, the cost ranking is LOHC < LH2 (liquid hydrogen tanker) < hydrogen pipeline < tube trailer. For under 50 km, hydrogen pipeline transportation has lower costs, making it suitable for small-scale transportation, such as chemical plant hydrogen pipelines and hydrogen transportation within isolated microgrids. As transportation distance increases, organic liquid and low temperature liquid hydrogen delivery costs become much more advantageous. Therefore, liquid delivery is more suitable for long-distance, large-scale hydrogen transportation, such as between provinces, from hydrogen production to consumption centers.
Regarding safety issues in hydrogen production, storage, and transportation, some researchers have proposed a “liquid sunshine” concept – using CO2 and hydrogen to produce methanol, which effectively solves the hydrogen storage problem. Methanol is an excellent liquid hydrogen carrier for storage and transportation, with excellent safety and convenience. This will also become a new solution to address the intermittency of renewable resources, providing an outlet for renewable energy curtailment from wind, solar and hydropower in remote areas unable to connect to the grid. It will also become another large-scale energy transportation method besides ultra high voltage transmission.
The “liquid sunshine” concept also expands carbon capture and sequestration technologies by enabling capture and recycling instead of just storage, forming a complete ecological carbon cycle. This will aid the advancement of global carbon neutrality goals. Therefore, to facilitate green energy development and solve renewable energy curtailment problems.
