Combined Hydrogen Efficiency
The net efficiency of a hydrogen fuel cell system depends on the entire fuel
pathway – the source of the hydrogen (reformation of some form of organic matter or
electrolysis) and the means by which the hydrogen was derived (i.e. regular vs. high
temperature electrolysis), any hydrogen transportation (pipeline being the only reasonable
one, as trucking is far too energy intensive for the low energy delivered), storage method
(compression, liquefaction, or solid), and the efficiency of the final fuel cell system. Once
the efficiencies of each individual stage have been determined though (as has been done
in this section), the combined efficiency of the various pathways can be fairly readily
calculated.
The efficiencies of the most likely pathways are calculated based on the
individual efficiencies determined in this section, and tabulated below in Table Error! No
text of specified style in document.-1. The net efficiencies include the efficiency of
hydrogen generation, hydrogen storage (compression, liquefaction, or sodium
borohydride), and fuel cell conversion back to electricity (optimistically considered to be
50%, just below the DOE’s goal for average efficiency at 25% of peak power), combined
with electric motor efficiency (90%). Ac-DC conversion is assumed to be 80% efficient,
the same as was assumed for Li-ion battery efficiencies. For low temperature electrolysis,
the highest efficiency of current low temperature electrolysis systems was used (73%)
rather than the maximum calculated efficiency (78.4%), which assumes negligible current
density (thus negligible hydrogen production rate). Overall, the potential net gain from
achieving the theoretical limit to electrolysis efficiency is minor (the net electrical energy
efficiency would increase from 30% to 32%).
Table Error! No text of specified style in document.-1 - Combined Hydrogen fuel cell
efficiencies for various pathways. Net energy efficiency accounts for the percentage
of primary energy available for propulsion, while electrical energy efficiency ignores
the efficiency with which electricity is produced from primary energy (and is thus
useful for comparing to electric vehicle options and compressed air storage).
Hydrogen Production
Electrolysis
High-Temp Electrolysis
High-Temp Electrolysis
High-Temp Electrolysis
Natural Gas Reformation
Natural Gas Reformation
Storage
Near-Isothermal
Compression
Near-Adiabatic
Compression
Liquefaction
Sodium Borohydride
(regenerated borate)
Near-Adiabatic
Compression
Sodium Borohydride
(regenerated borate)
Net Energy Efficiency
(Electrical Energy efficiency)
10.6% (30%)
14% (32%)
10% (22.5%)
9% (20%)
23.5%
15%
For the electrolysis generation options, the efficiency listed factors in the
efficiency of electricity generation, with the percentage of electrical energy generated at
the powerplant ultimately being available for propulsion being given in parentheses (this
therefore would not factor in the efficiency of the powerplant, and would be the
efficiency to be compared to electrochemical battery efficiencies, and mechanical energy
storage efficiencies). The primary efficiency though is useful for comparing electrolysisbased hydrogen to reformation based hydrogen. For low temperature electrolysis,
powerplant efficiency is assumed to be 35%, while for high-temperature electrolysis the
combined efficiency for hydrogen generation from primary energy is assumed to be 45%
(roughly also the efficiency at which electricity could be generated at the temperatures
required for efficient high-temperature electrolysis systems).
Electrolysis can be either the high or low temperature version, where the use of
high-temp electrolysis restricts the storage options somewhat. High-temperature
electrolysis would need to be performed at centralized power plants, and the hydrogen
therefore shipped via pipeline (with losses optimistically assumed to be equivalent to
electric transmission losses, and therefore they can be neglected). As building hydrogen
pipelines to every home would be unrealistic, isothermal compression (which generally
means very slow compression) of hydrogen produced by high-temperature electrolysis is
not feasible.
As is shown in the table, reformation of natural gas at a large centralized plant,
transfer of hydrogen via new hydrogen pipelines, and near-adiabatic compression at
filling stations gives the highest overall efficiency among the various hydrogen pathways.
This therefore indicates that a hydrogen economy would at least initially likely rely on
hydrogen produced from natural gas. Dwindling natural gas supplies (and therefore rising
costs), and a growing concern about associated environmental problems would likely
gradually push the focus to other sources.
Biomass gasification could produce hydrogen with efficiencies somewhat lower
than steam reformation of natural gas (accounting only for the efficiency of the biomass
gasification itself, not energy input for growing and harvesting crops), due to additional
energy requirements based on the water content of biomass and relatively lower hydrogen
content. But, gasification of biomass generally requires more energy than coal
gasification due to the water content of crops. Further, as stated previously, as
gasification produces hydrogen and carbon monoxide (and CO2) which can together be
used to produce synthesis fuels through Fischer Tropsch synthesis (yielding hydrocarbon
fuels – gasoline, diesel, etc.), that would likely be a preferable path than producing
hydrogen from biomass, due to the substantially greater energy density and suitability for
current infrastructure.
Therefore, overall, the most appealing “clean” hydrogen pathways would revolve
around either high temperature electrolysis and near adiabatic compression (which would
require an expensive hydrogen infrastructure), or low temperature local electrolysis with
near isothermal compression.