Hydrogen Storage Systems

Hydrogen storage is one of the most critical components of a hydrogen fueled PEMFC vehicle.  Some believe that the hydrogen storage system will be the major obstacle delaying deployment of a sucessful fuel cell vehicle. Conventional fuel tanks for the storage of hydrogen (H2) in fuel cell vehicles have several associated issues. 

• It must be a pressure vessel to store any significant amount of hydrogen.
• When stored at 5,000 or 10,000 psi a reasonable sized tank can't store enough hydrogen for the length of trip, about 300 miles, that is typical in current gasoline or diesel powered vehicles.
• There are concerns about the saftey of a pressurized tank in case of an accident.
• A complicated filling nozzle/tank nozzle arrangement is necessary for safety and to prevent spills.
• It is expensive, estimated to cost $2,000 to $3,000.

Several alternate means of storage have been proposed and are in various stages of being developed.  The large number of options being considered is because no technology has, as of yet, been proven to be a satisfactory solution.

• Improved Composite Tanks for Compressed Hydrogen
• Liquid Hydrogen Tanks
• Metal Hydrides
• Chemical Hydride Slurries
• Carbon Based Materials

COMPOSITE TANKS FOR COMPRESSED HYDROGEN

Despite their disadvantages, tanks are still the main means of storing hydrogen in test vehicles. The energy density of gaseous hydrogen can be improved by storing hydrogen at higher pressures. Advances in compression technologies are also required to improve efficiencies and reduce the cost of producing high-pressure hydrogen.

In a typical state of the art tank 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 main advantages with such composite tanks are their low weight, and that they are commercially available, well-engineered and safety tested, and have codes that are accepted in several countries for pressures in the range 350-700 bars (4,350 - 10,150 psi) . Composite tanks require no internal heat exchange and may be usable for cryogas. The main disadvantages are the large physical volume required, the ideal cylindrical shape makes it difficult to conform storage to available space, the high cost (500-600 USD/kg H2. A kg, 2.2 lbs, of H2 has about the same energy content of a gallon of gasoline), and energy penalties associated with compressing the gas to very high pressures. There are also some safety issues that still have not been resolved, such as the problem of rapid loss of H2 in an accident. The long-term effect of hydrogen on the materials under cyclic or cold conditions is not fully understood either.

Compressed hydrogen tanks (5000 psi and 10,000 psi) have been certified worldwide according to ISO 11439 (Europe) and NGV-2 (U.S.) and approved by TUV (Germany) and The High-Pressure Gas Safety Institute of Japan (KHK). 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

LIQUID HYDROGEN TANKS

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.  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 would be required before this would be a cost effective system. 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.

Although used in a few test vehicles, considering all these limitations, liquid hydrogen tanks are not a viable option at this time.

METAL HYDRIDES

Metal hydrides have the potential for reversible on-board hydrogen storage and release at low temperatures and pressures. Hydrogen, under pressure, would be used to regenerate the tank containing spent metal hydrides, combining with the metal to form more hydride. Ideally the waste heat from the PEM fuel cell, at less than 80 C (176 F), would be used to release the hydrogen from the media as needed.

Negative attributes of a metal hydride system are its weight and the fact that the reaction that occurs during regeneration is exothermic and the heat generated must be removed by a cooling system.

EERE has an active program to find and develop hydrides that have a high enough energy density and are capable of releasing the H2 at a temperature sufficiently low to use the waste heat from a PEM fuel cell.

ECD Ovonics has developed a proprietary nickel metal hydride system. The system is filled at 1,500 psi and provides nearly four times the hydrogen storage capacity of a similarly sized 5000 psig compressed hydrogen tank (more than twice that of a 10,000 psig tank). The pressure in the tank drops down to about 500 psi shortly after filling. They claim that they can install tanks of their design, using their hydride, that will take about the same space as a normal gas tank and will provide 300 miles of driving.

CHEMICAL HYDRIDE SLURRIES

A hydride slurry provides a promising means for storing transporting and producing hydrogen.  As a pumpable medium that is very safe to transport, it can be easily moved from tank to tank, can be easily metered and can be transported with the existing liquid fuel infrastructure.  The slurry stores at normal temperature and pressure. The chemical hydride slurry has a high energy density on materials basis (twice the volumetric energy density of liquid hydrogen and 11.7% hydrogen by mass) and provides significant safety features. 

The slurry is slow to ignite and is protected from unwanted reactions with ambient moisture by an oil coating on the metal hydride particles. Oil is added to the slurry to prevent ignition of the hydride due to inadvertent exposure to water or water vapor. On sufficiently humid days the hydride would absorb water from the air producing hydrogen and heat which will ignite the hydrogen.  When mixed with mineral oil the hydride cannot absorb moisture fast enough to ignite and become a hazard. The slurry with oil added is much safer than gasoline when exposed to an open flame.

A mobile hydrogen generator system would be installed in the vehicle to produce hydrogen as needed from the hydride slurry.  When hydrogen is needed the slurry is metered into a chemical reaction vessel containing water.  The reaction between water and the chemical hydride produces hydrogen which is fed directly to the fuel cell. The water that is produced in the fuel cell is recycled and used for the reaction with the slurry to produce the hydrogen.

When the slurry is depleted of hydrogen, instead of just filling up the tank you would first pump out the depleted slurry and then "fill-up" with fresh slurry.  The depleted slurry would be recycled and recharged with hydrogen.

Tests were first conducted with lithium hydride which indicated that a slurry with the required properties could be produced and a prototype generator system was developed and tested. Cost estimates indicated that such a system would be competitive with gasoline. Analysis indicated that a magnesium hydride might be more suitable. 

Safe Hydrogen, LLC has developed a system using magnesium hydride which provides a 300-plus mile driving range to a car in a fuel tank about 20% larger than the average gasoline tank.  They claim that their system is energy-cost competitive with gasoline.  According to Safe Hydrogen "their slurry - a liquid mix not unlike thick paint - both stores and generates 99.999 percent pure hydrogen on demand by the addition of water. This is achieved by a very simple and low cost mixing system using any available water. Additionally, the Safe Hydrogen slurry provides the handling and safety benefits of a non-explosive and non-flammable storage format." 

CARBON BASED MATERIALS

This category includes a range of carbon-based materials such as carbon nanotubes, aerogels, nanofibers (including metal doped hybrids), as well as metal-organic frameworks, conducting polymers and clathrates.  The materials absorb carbon on their surface. The initial focus was on single-wall nanotubes (SWNT) and a limit of about 3 wt. % was found by EERE. They believe greater storage capacity may be achieved by forming porous metal-organic frameworks with the carbon, which seems to be the favored material at the present time. Metal-organic frameworks are exceptionally porous at the molecular scale, with surface areas of more than 3,000 square meters per gram, according to Omar Yaghi, a chemistry professor at the University of Michigan. The goal of EERE is to demonstrate 6 wt. % storage capacity by 2006 on their way to meeting a 9 wt. % target by 2010.

Honda had this phrase in their announcement of their newest model of their fuel cell car, the 2006 FCX: Honda has now developed a new approach to expanding storage capacity, a newly developed hydrogen absorption material in the tank doubles capacity to 5 kg of hydrogen at 5000 PSI, extending cruising range to 350 miles, equivalent to that of a gasoline-engine car. The tank is located behind the rear seat. No other details were given, but I presume that by increasing the tank pressure to 5,000 PSI they were able to increase the capacity of absorbent material to an acceptable value, the same value as most cars using a simple pressure vessel.  This is the same pressure as is used in most cars using a simple pressure vessel and has the accompanying disadvantages of the high pressure, but requires much less volume than a simple tank.

A company called AMMINEX A/S, a start-up company founded by researchers at the Technical University of Denmark, claims to have an experimentally proven concept at the 100g - 1kg scale that has a energy density of 9.1 wt. % of H2.

Revision: March 4, added comment about the Honda storage tank.

Resources:

Hydrogen Storage, US DOE EERE, Hydrogen, Fuel Cells & Infrastructure Program, Technologies
ECD Ovonics Motive Solutions, Rochester Hills, MI, USA, website
Safe Hydrogen, LLC, Lexington, MA USA, website
Hydrogen Transmission/Storage with Metal Hydride-Organic Slurry and Advanced Chemical Hydride/Hydrogen for PEMFC Vehicles, A. McClaine et al, Thermo Technologies, Waltham, MA, 2000
AMMINEX A/S, Longbow, Denmark, website

Technocrat tags: hydrogen, hydrogen storage, fuel cells, energy, technology

Comments

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