Fuel Cells
339
so on. This approach yields a gravimetric storage density of 100% H 2 without
the weight of the container. However, if the mass of the tank is included, it is
reduced to about 10% of gravimetric storage density of H 2 . This has two major
disadvantages, namely an inevitable boil-off rate that is currently ~1%/day
and there is high-energy loss due to the refrigeration process that amounts
to 30% of the energy available by burning hydrogen. Until very recently, liquid hydrogen has also been considered to be a technologically viable option
for automotive application. But its drawbacks concerning efficient thermal
insulation needs to be addressed satisfactorily. Also, due to the low-operating
temperatures between 20 and 30 K, heat flow looks inevitable. This heat input
comprises three fractions, namely thermal conduction, convection, and thermal radiation. Among these, the thermal conduction through pipes and cables
to the inner storage vessel and the heat radiation from the environment to the
cryogenic liquid predominate. To achieve the low overall values of aforesaid
heat transfer, it is important to work with cylindrical tank structures as this
geometry is closer to the optimal surface-to-volume ratio. Additionally, it is
required to implement an efficient multilayer vacuum super insulation with
about 40 layers of metal foil. Wrapping the metal foil around the storage vessel in general and around the dome area in particular is time consuming and
highly demanding. The remaining heat input leads to enhanced evaporation
of the liquid hydrogen stored inside that eventually causes a pressure rise.
Typically, when a system pressure of about 1 MPa is reached, a valve needs to
be opened for venting hydrogen. The complexity of the liquid hydrogen storage system together with the challenge to reduce evaporation adds to liquid
hydrogen storage system costs that are not favorable over compressed gaseous
hydrogen storage systems. Also, the energy required to liquefy hydrogen is
about 30% of the chemical energy stored compared to just 15% for 70 MPa
compressed hydrogen with reference to the net calorific value of 120 MJ kg –1
hydrogen.
The second approach is to store hydrogen at high pressures. Since the volumetric storage density of the compressed hydrogen tank is rather low, the
packaging of such a fuel system into the existing mass production vehicle
architecture remains a challenge. Because of the comparatively high-operating
pressure of these vessels, a cylindrical design is mandatory. Despite limitations of the compressed gaseous-hydrogen approach, this option yields the
best overall technical performance and has the highest maturity for automotive applications.
A conventional steel cylinder stores about 1% gravimetric hydrogen.
Recent developments of fiber-reinforced resin have reached pressures of 700
bar, corresponding to about half the density of liquid hydrogen. Hydrogenfueled buses under Clean Urban Transport for Europe (CUTE) use hydrogen
stored in conventional steel cylinders at 150 bar.
Solid-state absorbers of hydrogen offer an impressive volumetric hydrogen
density. A short refueling-time requirement causes a significant engineering burden on the system. Considering a 6 kg H 2 tank system comprising a
339
so on. This approach yields a gravimetric storage density of 100% H 2 without
the weight of the container. However, if the mass of the tank is included, it is
reduced to about 10% of gravimetric storage density of H 2 . This has two major
disadvantages, namely an inevitable boil-off rate that is currently ~1%/day
and there is high-energy loss due to the refrigeration process that amounts
to 30% of the energy available by burning hydrogen. Until very recently, liquid hydrogen has also been considered to be a technologically viable option
for automotive application. But its drawbacks concerning efficient thermal
insulation needs to be addressed satisfactorily. Also, due to the low-operating
temperatures between 20 and 30 K, heat flow looks inevitable. This heat input
comprises three fractions, namely thermal conduction, convection, and thermal radiation. Among these, the thermal conduction through pipes and cables
to the inner storage vessel and the heat radiation from the environment to the
cryogenic liquid predominate. To achieve the low overall values of aforesaid
heat transfer, it is important to work with cylindrical tank structures as this
geometry is closer to the optimal surface-to-volume ratio. Additionally, it is
required to implement an efficient multilayer vacuum super insulation with
about 40 layers of metal foil. Wrapping the metal foil around the storage vessel in general and around the dome area in particular is time consuming and
highly demanding. The remaining heat input leads to enhanced evaporation
of the liquid hydrogen stored inside that eventually causes a pressure rise.
Typically, when a system pressure of about 1 MPa is reached, a valve needs to
be opened for venting hydrogen. The complexity of the liquid hydrogen storage system together with the challenge to reduce evaporation adds to liquid
hydrogen storage system costs that are not favorable over compressed gaseous
hydrogen storage systems. Also, the energy required to liquefy hydrogen is
about 30% of the chemical energy stored compared to just 15% for 70 MPa
compressed hydrogen with reference to the net calorific value of 120 MJ kg –1
hydrogen.
The second approach is to store hydrogen at high pressures. Since the volumetric storage density of the compressed hydrogen tank is rather low, the
packaging of such a fuel system into the existing mass production vehicle
architecture remains a challenge. Because of the comparatively high-operating
pressure of these vessels, a cylindrical design is mandatory. Despite limitations of the compressed gaseous-hydrogen approach, this option yields the
best overall technical performance and has the highest maturity for automotive applications.
A conventional steel cylinder stores about 1% gravimetric hydrogen.
Recent developments of fiber-reinforced resin have reached pressures of 700
bar, corresponding to about half the density of liquid hydrogen. Hydrogenfueled buses under Clean Urban Transport for Europe (CUTE) use hydrogen
stored in conventional steel cylinders at 150 bar.
Solid-state absorbers of hydrogen offer an impressive volumetric hydrogen
density. A short refueling-time requirement causes a significant engineering burden on the system. Considering a 6 kg H 2 tank system comprising a