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Lecture 16. Hydrogen Storage. Problem Statement:. Most H 2 produced on an “as needed” basis; It is more convenient to store the fuel as H 2 ; This is especially true for portable applications; Needs to store energy for solar and wind power;
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Lecture 16. Hydrogen Storage • Problem Statement: • Most H2 produced on an “as needed” basis; • It is more convenient to store the fuel as H2; • This is especially true for portable applications; • Needs to store energy for solar and wind power; • H2 storage is extremely complex and difficult; • 6. H2 has a very low density and low boiling point.
Hydrogen Storage Methods • Storage as hydrogen (Physical methods) • Compression in gas cylinder • Storage as a cryogenic liquid • Storage in a metal adsorber – as a reversible metal hydride • Storage in carbon nanofibers and other adsorbents • 2. Storage as hydrogen generating chemicals
H2 Storage as Compressed Gases • Equation of State • PV = nRT • Where P is the pressure of H2 in a vessel, V is the volume of the vessel, n is • mole of H2 stored, R is gas constant, T is temperature. • For a given cylinder, the amount of H2 stored is proportional to the gas pressure. • Special gas cylinder for H2 storage • Storing H2 gas in pressurized cylinders is the most technically straightforward method and most widely used for small amounts of the gas.
H2 Storage as Compressed Gases 3. Special gas cylinder for H2 storage Composite gas cylinders are used to achieve high storage volumetric and gravimetric efficiency. Storage pressure as high as 700-1000 bar was reported.
H2 Storage as Compressed Gases 4. Comparison of regular and composite cylinders for H2 storage
H2 Storage as Compressed Gases 4. Comparison of regular and composite cylinders for H2 storage
H2 Storage as Compressed Gases • 5. H2 storage cylinder requirements: • Pressure as high as possible to achieve high efficiency • Weight of the cylinder as low as possible • The cylinder materials don’t react with H2 (Hydrogen embrittlement) • The cylinder materials should be impermeable to H2
H2 Storage as Compressed Gases • 6. Advantages and disadvantages of H2 storage in cylinders: • Simplicity • Indefinite storage time • No purity limit on the H2 • Can supply for low and variable H2 demand • Very low storage efficiency as compared with other fuel storages • Very expensive to supply fuels for fuel cell ($125/kWh, still cheaper than electricity from primary batteries) • Safety concerns
Hydrogen Storage as a Liquid Storage of H2 as a liquid (commonly called LH2) at about 22K is currently the only widely used method of storing large quantities of H2. The following table lists the details of cryogenic H2 container suitable for cars (120 l for BMW).
Hydrogen Storage as a Liquid The following table is a comparison of performance on liquid hydrogen and gasoline for automobile applications. Four times of the gasoline tank size is needed for liquid hydrogen to have the same driving distance before refueling.
Hydrogen Storage as a Liquid • Comparison of gaseous and liquid fuels
Hydrogen Storage as a Liquid Comparison of liquid fuels for cars:
Hydrogen Storage as a Liquid • System Requirements for LH2 Storage • Container must be strongly reinforced vacuum (or Dewar) flask; • To maintain a constant pressure, a relief /vent valve is needed; • A safety rapture disc is installed in case pressure relief fails; • Purge the tank with N2 before filling LH2; • Double wall design to minimize heat transfer to the tank; • All pipes containing LH2 must be insulated to avoid frostbite; • Avoid condensation of oxygen or air in the surrounds of LH2.
Hydrogen Storage as a Liquid • Liquid H2 Fueling Station (Popular in Germany)
Hydrogen Storage as a Liquid • Liquid H2 Fueling Station (Popular in Germany)
Hydrogen Storage in Carbons • Initial report on H2 adsorption in carbon nanofibers claims 67 wt.% capacity and could run fuel cell car for 5000 miles without refueling; • This result couldn’t be reproduced; • DOE set a benchmark of 6.5 wt.%, and 62 kg H2/m3 for H2 storage for fuel cell cars. This would provide enough H2 in a car to run ~300 miles. • The mechanism of H2 storage in carbonaceous materials is not fully understood. It may or may not hold the promise for H2 storage.
Hydrogen Storage in Carbons • Three different types of carbon nanofibers: graphitic nanofibers, single-walled carbon nanotubes and multi-walled carbon nanotubes • Carbon nanaofilers are prepared from hydrocarbons or CO over catalysts • Graphitic nanofibers (GNF) have a diameter of 5-100 nm • Single-walled carbon nanotubes (SWNT) has a diameter of 1-2 nm • Multi-wall carbon nanotubes (MWNT) has a diameter of 5-50 nm • Different doping techniques were used to improve H2 capacity
Hydrogen Storage in Carbons • Comparison of H2 Storage in Carbon Nanofibers
High Pressure Adsorption Unit 77 K – 700 K 0 bara – 200 bara Volumetric Equilibrium Kinetics
High Pressure Adsorption Unit 77 K – 773 K 0 bara – 500/2000 bara Automatic, gravimetric Equilibrium Kinetics
