Hydrogen Storage

Introduction  Hydrogen is widely regarded as the most promising alternative to carbon-based fuels: it can be produced from a variety of renewable resources (e.g. wind and solar), and - when coupled with fuel cells - offers near-zero emissions of pollutants and greenhouse gases  Developing hydrogen as a major energy carrier, will require solutions to many scientific and technological challenges

Challenges  Conventional storage solutions include liquefaction or compression, however there are energy efficiency and major safety concerns associated with both these options  A more promising alternative is solid-state hydrogen materials: however the conventional alloys used, such a LaNi5, have very poor gravimetric hydrogen storage densities

Cont…  Another challenge, is delivering high purity hydrogen. Polymer Electrode Membrane (PEM) fuel cells are sensitive to gas impurities and, for prolonged exposure, require a very pure hydrogen feed  For automotive applications, a dense thin-metal membrane purifier has a number of advantages: it is compact, has a low capital cost, and offers a one-stage high-purity hydrogen output. However, the thin-metal membrane alloys currently used (Pd-Cu and Pd-Ag) are relatively thick (~25 microns) and need to be operated at high temperatures, making them unacceptably expensive in material and operating costs

Criteria for storage  Safety  Ease of use

Storage Methods Different storage Methods-  Metal Hydride tanks  Compressed Hydrogen  Liquid Hydrogen  Chemically Stored Hydrogen  Carbon Nanotubes  Glass Microsphere  Liquid Carrier Storage

Metal hydride Tanks  Metal hydrides are specific combinations of metallic alloys that act similar to a sponge soaking up water  Posses the unique ability to absorb hydrogen and release it later, either at room temperature or through heating of the tank  The total amount of hydrogen absorbed is generally 1% - 2% of the total weight of the tank  The percentage of gas absorbed to volume of the metal is still relatively low, but hydrides offer a valuable solution to hydrogen storage

Cont… Advantages  Metal hydrides offer the advantages of safely delivering hydrogen at a constant pressure  The alloys act as a sponge, which absorbs hydrogen, but it also absorbs any impurities introduced into the tank by the hydrogen. The result is the hydrogen released from the tank is extremely pure Disadvantages  Tank's lifetime and ability to store hydrogen is reduced as the impurities are left behind and fill the spaces in the metal that the hydrogen once occupied

Cont..  Hydride tanks are already used in several prototypes. (DaimlerChrysler, ETA-ING, Fraunhofer-ISE, Linde, Motor Zeitler- Speinshart, GfE) DaimlerChryslerETA-ING Fraunhofer-ISELindeMotor Zeitler- SpeinshartGfEDaimlerChryslerETA-ING Fraunhofer-ISELindeMotor Zeitler- SpeinshartGfE

Chemical Reactions Reference :US department of energy

Compressed Hydrogen  Hydrogen can be compressed into high-pressure tanks. This process requires energy to accomplish and the space that the compressed gas occupies is usually quite large resulting in a lower energy density when compared to a traditional gasoline tank  A hydrogen gas tank that contained a store of energy equivalent to a gasoline tank would be more than 3,000 times bigger than the gasoline tank  Hydrogen can be compressed into high-pressure tanks. High-pressure tanks achieve 6,000 psi, and therefore must be periodically tested and inspected to ensure their safety Disadvantages  Compressing or liquefying the gas is expensive

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Liquid Hydrogen  Hydrogen does exist in a liquid state, but only at extremely cold temperatures. Liquid hydrogen typically has to be stored at 20o Kelvin or C.  The temperature requirements for liquid hydrogen storage necessitate expending energy to compress and chill the hydrogen into its liquid state  The storage tanks are insulated, to preserve temperature, and reinforced to store the liquid hydrogen under pressure

Cont..  Linde Cryogenic Tank used in GM fuel cell car  Robotic arm filling liquid hydrogen to a BMW 5 Series hydrogen car

Limitations  The cooling and compressing process requires energy, resulting in a net loss of about 30% of the energy that the liquid hydrogen is storing  The margin of safety concerning liquid hydrogen storage is a function of maintaining tank integrity and preserving the Kelvin temperatures that liquid hydrogen requires. Combine the energy required for the process to get hydrogen into its liquid state and the tanks required to sustain the storage pressure and temperature and liquid hydrogen storage becomes very expensive comparative to other methods

Chemically Stored Hydrogen  Hydrogen is often found in numerous chemical compounds. Many of these compounds are utilized as a hydrogen storage method  The hydrogen is combined in a chemical reaction that creates a stable compound containing the hydrogen. A second reaction occurs that releases the hydrogen, which is collected and utilized by a fuel cell. The exact reaction employed varies from storage compound to storage compound

Carbon Nanotubes  Carbon nanotubes are microscopic tubes of carbon, two nanometers (billionths of a meter) across, that store hydrogen in microscopic pores on the tubes and within the tube structures  Similar to metal hydrides in their mechanism for storing and releasing hydrogen, the advantage of carbon nanotubes is the amount of hydrogen they are able to store  Carbon nanotubes are capable of storing anywhere from 4.2% - to 65% of their own weight in hydrogen

Cont..  The US Department of Energy has stated that carbon materials need to have a storage capacity of 6.5% of their own body weight to be practical for transportation uses  Carbon nanotubes and their hydrogen storage capacity are still in the research and development stage. Research on this promising technology has focused on the areas of improving manufacturing techniques and reducing costs as carbon nanotubes move towards commercialization

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Glass Microspheres  Tiny hollow glass spheres can be used to safely store hydrogen. The glass spheres are warmed, increasing the permeability of their walls, and filled by being immersed in high-pressure hydrogen gas  Spheres are then cooled, locking the hydrogen inside of the glass balls. A subsequent increase in temperature will release the hydrogen trapped in the spheres

Cont.. Advantages  Microspheres have the potential to be very safe, resist contamination, and contain hydrogen at a low pressure increasing the margin of safety

Liquid Carrier Storage  This is the technical term for the hydrogen being stored in the fossil fuels that are common in today's society. Whenever gasoline, natural gas methanol, etc.. is utilized as the source for hydrogen, the fossil fuel requires reforming  The reforming process removes the hydrogen from the original fossil fuel. The reformed hydrogen is then cleaned of excess carbon monoxide, which can poison certain types of fuel cells, and utilized by the fuel cell