Hydrogen Storage

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Hydrogen Storage
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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

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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

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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

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Criteria for storage

• Safety
• Ease of use

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Storage Methods

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

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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

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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

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Cont..

• Hydride tanks are already used in several
  prototypes. (DaimlerChrysler, ETA-ING,
  Fraunhofer-ISE, Linde, Motor Zeitler-Speinshart,
  GfE)

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Chemical Reactions

• Reference US department of energy

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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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Cont..
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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 -2530
  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

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Cont..

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

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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

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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

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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

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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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Cont..
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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

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Cont..

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

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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