Hydrogen Efficiencies Introduction
Articles / Discussions on hydrogen efficiencies
Date: Apr 11, 2005 - 01:47 PM
Hydrogen System Efficiency |
Dr. Barry Pruden, Prodevel Inc.
It has become commonplace to discuss fuel cells and fuel cell systems in
connection with environmental protection and methods to eliminate the consumption
of fossil fuels. Many scientists bought into the concept of a hydrogen economy
several years ago when the focus was on hydrogen production from renewable
sources, with hydrogen promoted as the energy carrier of the future.
I have been surprised to see this concept reoriented to the production
of hydrogen from fossil fuels, while retaining the advantages set forth by
pioneers for hydrogen from renewable sources.
If hydrogen is to be produced from fossil fuel and used in hydrogen systems,
then it must stand on it's own merit in efficiency and cost. As well as compete
with fossil fuels on overall environmental and economic impact.
For example in large plants, the production of hydrogen from natural
gas carries a penalty of about 27%. In other words, only 73% of the energy
contained in the fossil fuel is now available for use. This production efficiency
and the environmental impact of the facility must be included in the evaluation
of the overall hydrogen system. Thus if the fuel cells are 40% efficient,
and power conditioning is 95% efficient, electricity from fuel cells will
(0.73 x 0.40 x 0.95)*100 = 27.7 % efficient and not 40%.
Finally, to maintain a level playing field, all efficiencies should
be based on the high heating value (HHV) of the fuel and products. This can
have a significant impact on the reported efficiency. The ratio of HHV to
lower heating value (LHV) for hydrogen is about 1.18, so that efficiencies
for fuel cells alone based on LHV will be higher by 18%. When H2O is a product
of combustion it can be designated as liquid water, which yields HHV, or
water vapor giving LHV. The difference for the specific stoichiometry is
the heat of vaporization of water. Hydrogen rich fuels (H2, natural gas)
have a high ratio of HHV/LHV.
Why focus on efficiency if hydrogen is so clean burning and non-polluting?
The answer is that the overall system has to be considered. If the hydrogen
system is less efficient than the fossil fuel system then there must be other
overriding factors to promote it. For all systems the natural gas fuel
is ultimately converted to carbon dioxide and water. It follows that
the system with the highest efficiency will contribute the least carbon dioxide
per unit of electricity. If cleaner cities are desired, then this could be
an overriding factor, provided rural areas are willing to accept the environmental
cost. Other pollutants, such as unburned hydrocarbons and oxides of
nitrogen will have to be considered in the final analysis.
The debate regarding hydrogen as a transportation fuel with onboard reforming
It is difficult to compare the benefits of this system with current technology
without some perceived bias. There is no doubt that current technology
is wasteful and non-optimal, but it could be improved given the right incentive.
It would, however, be unfair to claim general improvements (smaller, lighter
cars with lower payload, no A/C, no power steering or power brakes, and energy
saving tires and design) as part of the benefit for hydrogen systems. In
addition, the environmental benefit for hydrogen must include the environmental
costs of production, as hydrogen is a carrier and not a primary fuel, and
the cost for compression or liquefaction, as these are not negligible.
In parallel with the development of hydrogen as a transportation fuel there
have been systems developed for the production of electricity by fuel cells.
In this case the input is natural gas and the output is electrical power.
It is the purpose of this paper to compare conventional fossil-fuel
systems with fuel cells using integrated reforming for hydrogen generation.
This article comes from Hydrogen Prodevel
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