Hydrogen
285
at an operating pressure of typically less than 200 psi … .” This is 50 times
higher as compared to compressed air storage of the same capacity.
The metal hydrates are usually insulated with noncombustible materials
so that in the event of tank crack, the major portion of hydrogen still remains
stored in the hydrogen block, thus avoiding explosion. The hydrate need not
be replaced frequently and the process of charging and discharging can be
repeated an infinite number of times provided the metal hydrate does not
get contaminated. In case of contamination of metal hydrates, the hydrogen
can be reactivated by heating.
9.13.4 Onboard-Hydrogen Storage
An important alternative to hydrogen storage is onboard hydrogen production: using devices called reformers hydrogen can be extracted from a fossil
hydrocarbon, typically natural gas. This obviously implies a larger onboard
complexity and some gaseous polluting emissions by reformers, but has
the advantage of not requiring an infrastructure for distributing hydrogen.
Currently, two leading hydrogen supply systems onboard are envisaged: POX
and methanol steam reformer (MSR; Ogden, Steinbugler, and Kreutz 1999).
A detailed description of reforming systems could be found in Dicks (1996)
and Ahmed and Krumpelt (2001).
However, it is worth remarking in which ways fuel processors affect fuel
cell behavior and system performance. PEM FCs prefer to be fed with pure
hydrogen, since fuel reformer by-products, namely CO, CO 2 , and S, can poison the platinum catalyst obstructing reaction sites. Particularly, platinum
attracts carbon monoxide that may be removed adding 2% oxygen to a fuel
stream with 100 ppm of carbon monoxide. Unfortunately, this technique
experiences different drawbacks since hydrogen stream from reformers
contains 1000 ppm, while oxygen not reacting with CO will consume fuel.
Obviously, reformers increase fuel cell system cost, size and weight, and
introduces other issues related to their control and subsystems. Hydrogen
streams leaving the fuel reformer have to be prefiltered before approaching
fuel cells whereas hydrogen molar fraction effectively reacting at anode sites
achieves only 0.3% against 75% assured fueling of pure hydrogen (Ogden,
Steinbugler, and Kreutz 1999). Moreover, reformer efficiency lies in a range
between 62 (POX) and 69% (MSR), thus further reducing PEMFC system performance. In real applications, an accurate control is required since fuel processors experience slow start-up and inadequate transient response.
To be used in a vehicle, hydrogen must be stored onboard or produced
onboard. Main characteristics of the major Fuel cell typologies is given in
Table 9.8. Hydrogen can be stored in four different ways:
• In compressed form, CGH2: in high-pressure tanks or gas cylinders
made of ultra-light composite materials. They are used in prototype
fuel cell automobile and buses.
285
at an operating pressure of typically less than 200 psi … .” This is 50 times
higher as compared to compressed air storage of the same capacity.
The metal hydrates are usually insulated with noncombustible materials
so that in the event of tank crack, the major portion of hydrogen still remains
stored in the hydrogen block, thus avoiding explosion. The hydrate need not
be replaced frequently and the process of charging and discharging can be
repeated an infinite number of times provided the metal hydrate does not
get contaminated. In case of contamination of metal hydrates, the hydrogen
can be reactivated by heating.
9.13.4 Onboard-Hydrogen Storage
An important alternative to hydrogen storage is onboard hydrogen production: using devices called reformers hydrogen can be extracted from a fossil
hydrocarbon, typically natural gas. This obviously implies a larger onboard
complexity and some gaseous polluting emissions by reformers, but has
the advantage of not requiring an infrastructure for distributing hydrogen.
Currently, two leading hydrogen supply systems onboard are envisaged: POX
and methanol steam reformer (MSR; Ogden, Steinbugler, and Kreutz 1999).
A detailed description of reforming systems could be found in Dicks (1996)
and Ahmed and Krumpelt (2001).
However, it is worth remarking in which ways fuel processors affect fuel
cell behavior and system performance. PEM FCs prefer to be fed with pure
hydrogen, since fuel reformer by-products, namely CO, CO 2 , and S, can poison the platinum catalyst obstructing reaction sites. Particularly, platinum
attracts carbon monoxide that may be removed adding 2% oxygen to a fuel
stream with 100 ppm of carbon monoxide. Unfortunately, this technique
experiences different drawbacks since hydrogen stream from reformers
contains 1000 ppm, while oxygen not reacting with CO will consume fuel.
Obviously, reformers increase fuel cell system cost, size and weight, and
introduces other issues related to their control and subsystems. Hydrogen
streams leaving the fuel reformer have to be prefiltered before approaching
fuel cells whereas hydrogen molar fraction effectively reacting at anode sites
achieves only 0.3% against 75% assured fueling of pure hydrogen (Ogden,
Steinbugler, and Kreutz 1999). Moreover, reformer efficiency lies in a range
between 62 (POX) and 69% (MSR), thus further reducing PEMFC system performance. In real applications, an accurate control is required since fuel processors experience slow start-up and inadequate transient response.
To be used in a vehicle, hydrogen must be stored onboard or produced
onboard. Main characteristics of the major Fuel cell typologies is given in
Table 9.8. Hydrogen can be stored in four different ways:
• In compressed form, CGH2: in high-pressure tanks or gas cylinders
made of ultra-light composite materials. They are used in prototype
fuel cell automobile and buses.