Breakthrough Enables Safe Transport of Hydrogen for Fuel-Cell Vehicles

Breakthrough Enables Safe Transport of Hydrogen for Fuel-Cell Vehicles

A proprietary membrane technology allows for bulk hydrogen to be transported in the form of ammonia and then reconverted back to hydrogen for a fuel-cell vehicle.

Scientists are now testing the fuel-cell-powered Toyota Mirai that uses ultra-high purity hydrogen, produced using CSIRO membrane technology. CSIRO, an acronym for the Commonwealth Scientific and Industrial Research Organization, is an independent Australian federal government agency responsible for scientific research.

Hydrogen is difficult to transport and store. Gaseous hydrogen can be transported by pipeline, but it has a tendency to damage steel, and needs considerable pipe-wall thickness to ensure it doesn’t escape. Hydrogen is very flammable and difficult to ship because of its low density. These logistical issues have always been a stumbling block.

In contrast, ammonia can be stored at room temperature and be easily converted back into hydrogen by passing it over a catalyst to release hydrogen and nitrogen gas. CSIRO has successfully road-tested its ammonia-to-hydrogen technology for a fuel-cell-powered Toyota vehicle.

The key to the CSIRO project rests in a different approach based on a proprietary membrane separator technology designed by Dr. Michael Dolan. Its vanadium-alloy membrane is tipped to transform the hydrogen separation process, as well as enable the use of ammonia as a means of carrying the ultralight hydrogen. The thin metal membrane allows hydrogen to pass while blocking all other gases, and using decomposed ammonia feedstock, it enables H2 conversion in a single step. It permits a small plant—with no moving parts—to work in continuous operation.

Ammonia (NH3) has a high capacity for storing hydrogen atoms—17.6% by weight, and at a volumetric density 45% greater than liquid H2. It has often been proposed as a carrier method, given that it’s stable and can be stored in pressure tanks in much the same way as propane or other fuels. Yet the large amount of energy needed to create and/or separate ammonia molecules and unfavorable economics has discounted any further practical use—until now.

Dr. Dolan says “Our design philosophy has been to use inexpensive materials and mass-production techniques (like metal tube extrusion and electroplating) as much as possible. The membrane substrate itself is a dense tube of a permeable, inexpensive [vanadium] alloy which is drawn down to a wall thickness of ~0.2 mm, and diameter of 10 mm. A catalytic layer is then deposited on the inner and outer surfaces.”

1. Decomposed ammonia passes through CSIRO’s metal membrane to produce pure hydrogen.

The ammonia is stored in the tank at ambient temperature. Therefore, most of the ammonia is a liquid. However, some vaporizes, which creates a pressure in the tank of 5 to 10 atmospheres, depending on the temperature. Vapor from the tank is then heated to 400°C and passes through a catalyst bed that then decomposes ammonia into nitrogen and hydrogen gas. That mixture is subsequently passed over the membrane. The hydrogen passes through the membrane, and the nitrogen doesn't (Fig. 1). This conversion process takes place at the refueling station.

The system requires heat to drive the endothermic decomposition process, and the loss of pressure means the hydrogen must then be fed into a compressor for use in fuel-cell applications (although it could be used at ambient pressures in stationary power generation).

Dolan added, “As with most gas separation processes, the recovery of the valuable product (H2) is never 100%. We will typically operate with around 85% recovery, but this depends on residence time and desired output.” That said, there are methods of improving its efficiency. “The unrecovered energy will not be wasted. The off-gas, which contains mostly nitrogen (N2) with unrecovered H2 and unreacted NH3, can be combusted to create the heat required for ammonia decomposition, or it can be fed to a second device, like a high-temperature fuel cell, internal combustion engine, or turbine for power generation,” confirmed Dolan.

CSIRO chief executive Dr. Larry Marshall said, “This is a watershed moment for energy, and we look forward to applying CSIRO innovation to enable this exciting renewably sourced fuel and energy-storage medium a smoother path to market.”

“The oil and gas industry, too, is now waking up to the potential of hydrogen as a commercial opportunity, and investment in technologies such as CSIRO’s now will play a key role in the development of a supporting economy. Dolan believes the change is already visible; “Renewable H2 will become more important in the longer term, and most of the major established oil and gas companies have already announced major investment and partnerships to facilitate this transition. Using ammonia as a hydrogen carrier is one of the greatest emerging opportunities.”

Toyota Mirai

The Toyota Mirai (Japanese for "future") is a hydrogen fuel-cell vehicle (FCV) with an output power density of 3.0 kW/L, delivering an output of more than 100 kW. It uses Toyota's proprietary, small, lightweight fuel-cell stack and two 70-MPa high-pressure hydrogen tanks placed beneath the specially designed body. (Toyota started development of FCV technology in 1992.)

The FCV concept also applies portions of Toyota's Hybrid Synergy Drive technology, including the electric motor, power control unit, and other parts and components from its hybrid vehicles, to improve reliability and minimize cost. Furthermore, the hybrid technology is used to work together with the fuel cell.

At low speeds, such as city driving, the FCV runs just like any all-electric car by using the energy stored in its battery, which is charged through regenerative braking. At higher speeds, the hydrogen fuel cell alone powers the electric motor. When more power is needed, for example during sudden acceleration, the battery supports the fuel-cell system as both work together to provide propulsion.

2. The Mirai cutaway shows the power control unit and the electric traction motor in the front, the fuel-cell stack and hydrogen storage tank in the middle, and the NiMH rechargeable battery above in the rear.

The Mirai uses the stack, FC boost converter, and high-pressure hydrogen tanks (Fig. 2). The system accelerates Mirai from 0 to 97 km/h (0 to 60 mph) in 9.0 seconds and delivers a passing time of 3 seconds from 40 to 64 km/h (25 to 40 mph). Refueling takes between 3 and 5 minutes, and Toyota expected a total range of 480 km (300 miles) on a full tank.

A button labeled H2O opens a gate at the rear of the Mirai, dumping the water vapor that forms from the hydrogen-oxygen reaction in the fuel cell. The exhausted  H2O, or water volume, is 240 mL per 4 km running. The Toyota Fuel Cell System features both fuel-cell technology and hybrid technology, and includes proprietary Toyota-developed components including the fuel cell (FC). (One kg of hydrogen is roughly equivalent to one U.S. gallon of gasoline.)

3. Mirai's fuel-cell stack consists of 370 (single-line stacking) cells, with a cell thickness of 1.34 mm and weight of 102 g.

The electric traction motor delivers 113 kW (152 hp) and 335 Nm of torque. The Mirai has a 245-V (1.6 kWh) sealed nickel-metal-hydride (NiMH) traction rechargeable battery pack. Electricity-generation efficiency is enhanced through the use of 3D fine mesh flow channels. These channels, arranged in a fine 3D lattice structure, enhance the dispersion of air (oxygen), thereby enabling uniform generation of electricity on cell surfaces. Each stack comprises 370 (single-line stacking) cells, with a cell thickness of 1.34 mm and weight of 102 g. The Mirai has a new compact (13-liter), high-efficiency, high-capacity converter developed to boost voltage generated in the Toyota FC Stack to 650 V (Fig. 3).

The Mirai has two hydrogen tanks with a three-layer structure made of carbon-fiber-reinforced plastic consisting of nylon 6 and other materials (Fig. 4). The tanks, which store hydrogen at 70 MPa (10,000 psi), have a combined weight of 87.5 kg (193 lb) and 5-kg capacity.

4. Mirai's high-pressure hydrogen tank has a rechargeable battery pack on top.

Safety

The Mirai was subjected to extensive crash testing to evaluate a design specifically intended to address frontal, side, and rear impacts, and to provide excellent protection of vehicle occupants. A high level of collision safety also was achieved to help protect the fuel-cell stack and high-pressure tanks against body deformation.

The high-pressure hydrogen tanks demonstrate excellent hydrogen permeation prevention performance, strength, and durability. Hydrogen sensors provide warnings and can shut off tank main stop valves. The hydrogen tanks and other hydrogen-related parts are located outside the cabin to ensure that if hydrogen leaks, it will dissipate easily.

The vehicle structure is enhanced with carbon-fiber-reinforced polymers and designed to disperse and absorb impact energy across multiple parts. This helps to ensure high-impact safety performance that protects the Toyota FC Stack and high-pressure hydrogen tanks during frontal, side, or rear impacts.

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