Energy contained in 1 kg of H2 is equivalent to 1 gallon of gasoline
Fuel cells are more efficient than internal combustion engines, and require 5-8 kg of hydrogen on board.
H2 is an energy carrier and not a fuel
H2 has a low volumetric energy density
Hydrogen reacts explosively with oxygen above the ignition temperature of 450 oC at atmospheric pressure.
How to store hydrogen
- Compressed hydrogen gas
- Liquid hydrogen (cryogenic, 20 K at atmospheric pressure)
- Hollow glass microspheres
- Metal hybrides
Know the 2015 DOE targets in red
DOE 2015 targets
- Gravimetric energy density should be larger than 9 wt% or 10.8 MJ/kg
- Volumetric energy density should be larger 81 g of H2/L or 9.72 MJ/L
- Refueling time of hydrogen tanks as short as refueling with current fuels such as diesel or gasoline. 2.5 minutes for 5 kgof h@
- Autonomy of 300 miles without refueling.
Compressed Hydrogen – hydrogen is in the gas phase at a pressure of 10,000 psi. The gravimetric density of hydrogen is 143 MJ/kg, and the Volumetric density is 5.6 MJ/L at 700 bar. Advantages: simple, few losses, up to 700 bar. The disadvantages are that there is a high compression cost (up to 20% of the stored energy).
Hydrogen embrittlement – metals become brittle and fracture when they are exposed to hydrogen. In high temperatures, hydrogen diffuses in the metal, and leads to crack development and propagation. High strength steels and nickel and titanium alloys are the most susceptible. As hydrogen diffuses through steel, it reacts with carbon and forms methane, which initiates cracks in the steel.
Aluminum compressed hydrogen tanks have a maximum pressure of 175 bar. Steel tank can have a maximum pressure of 200 bar. Carbon reinforced composites with a polymer liner can have up to 700 bar of maximum pressure. The polymer liner disallows hydrogen from diffusing.
Liquid Hydrogen – requires cryogenic storage at 20 K, with super thermal insulation. Gravimetric density is 5.22 MJ/kg, and volumetric is 22.9 MJ/L. Advantages: increase energy storage density. Cost of Dewar decreases exponentially with volume. Disadvantages: high compression and cooling cost (40%). Heat transfer during long-term storage.
Ammonia H2 storage – second most commonly produced chemical in the world. Ammonia is toxic at standard temperature and pressure. The infrastructure for making and transporting it is already in place.