Hydrogen Information
The following comments on hydrogen as a fuel were tyaken from Solar Hydrogen Beyond Fossil Fuels, World Resources Institute, 1989. A must read if you are interested in renewable energy, hydrogen and photovoltaics.
Hydrogen is produced from electrolysis - splitting water into hydrogen and oxygen by passing an electrical current through it. The above reference uses an efficiency of 84% for the electrolytic conversion process, but I recall higher potential efficiencies of up to 95% (no reference).
Hydrogen could be made by using excess capacity from existing electric power plants in the short term. In the longer term, if we replaced fossil fuels with hydrogen, an area about half the size of New Mexico would be required to replace 1986 U.S. fossil fuel consumption (15% efficiency photovoltaics, 84% efficient electrolyzers).
Hydrogen safety is based on a number of factors
1. Limits of flammability
The range of mixtures of fuel in air that will sustain a fire. Hydrogen will detonate over a much wider range than either methane or gasoline. The lower limit of flammability is the most important factor. Hydrogen's lower limit of 4% by volume is not much less than methane (5.3) and is higher that gasoline(1.1%). Leak detection is critical for all 3 fuels.
2. Minimum energy for ignition
Hydrogen has a significantly low ignition energy than methane or gasoline. However, the ignition energy for all 3 is extremely low.
3. Buoyant velocity/diffusion velocity in air
Buoyant refers to the rate at which fuels rise in air. Diffusion velocity to the rate at which they diffuse through the air. Since hydrogen is the lightest element it diffuses and disperses very quickly. Outdoor hydrogen or methane leaks disperse very quickly do not pose a problem. Gasoline does pose a problem
4. Leak rate
Rate of leakage through a crack. Hydrogen leaks about 3 times faster than methane. Gasoline vapours have a higher leak rate. An odourant should be added as with natural gas.
5. Material embrittlement
High temperatures found in hydrogen handling equipment in oil refineries can cause potentially dangerous embrittlement of metal. However at the lower temperatures and pressures found in a hydrogen energy system this is not a problem.
6. Toxicity
Hydrogen and methane are none toxic while gasoline is. All three gases are asphyxiants at high concentrations.
Since the 19th century hydrogen rich town gas manufactured from coal has been used as a home heating fuel. States switched form town gas to natural gas as recently as the 1940 and 50s. Safety levels were acceptable.
Conclusions - all three fuels pose safety risks and must be treated with respect.
See J. Hord "Is Hydrogen a Safe Fuel, International Journal of Hydrogen Energy v 3 p157 -176
From Beals, 1993, Environmental Impacts of Alternative Electricity Generation Technologies, Beals and Associates for Ontario Hydro
Natural gas powered fuel cells for electricity generation produce significant carbon dioxide, sulphur oxide and nitrogen oxide emissions. These emissions are similar to combustion turbine units. Solar hydrogen powered fuel cells emissions are significantly lower. Hydrogen produced from other electrical sources (nuclear, hydroelectric, coal) would have the emissions associated with these technologies. |
