Gaseous and Liquid Hydrogen Storage
Today's state-of-the-art for hydrogen storage includes 5000- and 10,000-psi compressed gas tanks and cryogenic liquid hydrogen tanks for on-board hydrogen storage.
Compressed Hydrogen Gas Tanks
The energy density of gaseous hydrogen can be improved by storing hydrogen at higher pressures. This requires material and design improvements in order to ensure tank integrity. Advances in compression technologies are also required to improve efficiencies and reduce the cost of producing high-pressure hydrogen.
Carbon fiber-reinforced 5000-psi and 10,000-psi compressed hydrogen gas tanks are under development by Quantum Technologies and others. Such tanks are already in use in prototype hydrogen-powered vehicles. The inner liner of the tank is a high molecular weight polymer that serves as a hydrogen gas permeation barrier. A carbon fiber-epoxy resin composite shell is placed over the liner and constitutes the gas pressure load-bearing component of the tank. Finally, an outer shell is placed on the tank for impact and damage resistance. The pressure regulator for the 10,000-psi tank is located in the interior of the tank. There is also an in-tank gas temperature sensor to monitor the tank temperature during the gas filling process when heating of the tank occurs.
The driving range of fuel cell vehicles with compressed hydrogen tanks depends, of course, on vehicle type, design and the amount and pressure of stored hydrogen. By increasing the amount and pressure of hydrogen, a greater driving range can be achieved but at the expense of cost and valuable space within the vehicle. Volumetric capacity, high pressure and cost are thus key challenges for compressed hydrogen tanks. Refueling times, compression energy penalties and heat management requirements during compression also need to be considered as the mass and pressure of on-board hydrogen are increased.
Issues with compressed hydrogen gas tanks revolve around high pressure, weight, volume, conformability and cost. The cost of high-pressure compressed gas tanks is essentially dictated by the cost of the carbon fiber that must be used for light-weight structural reinforcement. Efforts are underway to identify lower-cost carbon fiber that can meet the required high pressure and safety specifications for hydrogen gas tanks. However, lower-cost carbon fibers must still be capable of meeting tank thickness constraints in order to help meet volumetric capacity targets. Thus lowering cost without compromising weight and volume is a key challenge.
Two approaches are being pursued to increase the gravimetric and volumetric storage capacities of compressed gas tanks from their current levels. The first approach involves cryo-compressed tanks. This is based on the fact that, at fixed pressure and volume, gas tank volumetric capacity increases as the tank temperature decreases. Thus, by cooling a tank from room temperature to liquid nitrogen temperature (77°K), its volumetric capacity will increase by a factor of four, although system volumetric capacity will be less than this due to the increased volume required for the cooling system.
The second approach involves the development of conformable tanks. Present liquid gasoline tanks in vehicles are highly conformable in order to take maximum advantage of available vehicle space. Concepts for conformable tank structures are based on the location of structural supporting walls. Internal cellular-type load bearing structures may also be a possibility for greater degrees of conformability.
Compressed hydrogen tanks [5000 psi (~35 MPa) and 10,000 psi (~70 MPa)] have been certified worldwide according to ISO 11439 (Europe), NGV-2 (U.S.), and Reijikijun Betten (Iceland) standards and approved by TUV (Germany) and The High-Pressure Gas Safety Institute of Japan (KHK). Tanks have been demonstrated in several prototype fuel cell vehicles and are commercially available. Composite, 10,000-psi tanks have demonstrated a 2.35 safety factor (23,500 psi burst pressure) as required by the European Integrated Hydrogen Project specifications. Learn more about high-pressure hydrogen tank testing.
Liquid Hydrogen Tanks
The energy density of hydrogen can be improved by storing hydrogen in a liquid state. However, the issues with LH2 tanks are hydrogen boil-off, the energy required for hydrogen liquefaction, volume, weight, and tank cost. The energy requirement for hydrogen liquefaction is high; typically 30% of the heating value of hydrogen is required for liquefaction. New approaches that can lower these energy requirements and thus the cost of liquefaction are needed. Hydrogen boil-off must be minimized or eliminated for cost, efficiency and vehicle range considerations, as well as for safety considerations when vehicles are parked in confined spaces. Insulation is required for LH2 tanks and this reduces system gravimetric and volumetric capacity.
Liquid hydrogen (LH2) tanks can store more hydrogen in a given volume than compressed gas tanks. The volumetric capacity of liquid hydrogen is 0.070 kg/L, compared to 0.030 kg/L for 10,000 psi gas tanks.
Liquid tanks are being demonstrated in hydrogen-powered vehicles and a hybrid tank concept combining both high-pressure gaseous and cryogenic storage is being studied. These hybrid (cryo-compressed tanks) insulated pressure vessels are lighter than hydrides and more compact than ambient-temperature, high pressure vessels. Because the temperatures required are not as low as for liquid hydrogen, there is less of an energy penalty for liquefaction and less evaporative losses than for liquid hydrogen tanks.
Learn about DOE's Compressed/Liquid Hydrogen Tanks R&D.
