Hydrogen Prodevel

Hydrogen Efficiencies Introduction

Articles / Discussions on hydrogen efficiencies
Date: Apr 11, 2005 - 01:47 PM
Hydrogen System Efficiency

Dr. Barry Pruden, Prodevel Inc.

Introduction:

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 be:

(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 is ongoing.


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.


 




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