The article by Joan Ogden proposes that available hydrogen technologies can address future energy and environmental challenges. More energy, though, is needed to produce a quantity of hydrogen than can be obtained from it by combustion or by reactions in a fuel cell. Alternative fuels such as hydrogen and methanol are actually energy storage media or secondary energy carriers rather than fuels in the traditional sense. Ordinarily, we think of fuels as substances that, when burned, release more energy than is required to produce them. In addition to specifying the heat of combustion of an alternative fuel, giving its production energy value would also be helpful—and would require specifying the process of production.
In the US, 90–95% of hydrogen is produced by steam reforming, a chemical process that makes hydrogen from a mixture of water and a hydrocarbon feedstock. Theoretically, the energy that must be supplied to the process is the difference between the heat of combustion of the resulting hydrogen and the heat of combustion of the reformed feedstock. This difference sets the lower limit on the energy required to produce an alternative fuel. In practice, the overall efficiency of the process—that is, the energy content of the hydrogen produced divided by the total energy consumed by the reforming process—is approximately 65%. 1 The efficiency of the more costly electrolysis process is approximately the same (62.5%), although some commercial producers claim efficiencies as high as 80%. In other words, to produce an amount of hydrogen with the energy content of 1 MJ, about 1.6 MJ of energy must be expended. But only 0.167 MJ of energy must be expended to produce a quantity of gasoline with an energy content of 1 MJ; there is thus a substantial gain of available energy. 2
The US Census Bureau reports that 132 million cars were registered in the US in 1999, and those cars used 73.2 billion gallons, or 208 × 109 kg of gasoline. Using the value for the high heat of combustion for gasoline of 47.3 MJ/kg or 13.14 kWh/kg, this amounts to a total of 2.73 × 109 MWh. With an overall efficiency of 25% for automobiles with internal combustion engines, 682 × 106 MWh is actually used for propulsion. With a fuel-cell efficiency of 50% and electric motor efficiency of 90%, the energy supplied to fuel-cell-powered cars would have to be at least 1.52 × 109 MWh.
The amount of hydrogen with this energy content is 115.7 × 109 kg. Producing that much hydrogen requires 262 × 109 kg of octane, or about 92.3 × 109 gallons of gasoline, somewhat more than the quantity now used by US automobile traffic, as noted previously. Clearly, use of hydrogen produced by reformation does not free us from dependence on hydrocarbons.
To produce hydrogen with 1.52 × 109 MWh energy content by electrolysis would, according to the hydrogen production efficiency value of 62.5%, require 2.42 × 109 MWh. The total generation of electrical energy in the US in 1999 was 3.68 × 109 MWh, with the winter peak load of 849 GW, according to the Census Bureau. US electrical energy production would have to be increased by at least 65% to supply enough energy for those 132 million US autos to be fuel-cell–powered. If the power plants ran 24 hours a day to supply electrical energy for the electrolyzers, their capacity would have to be 276 GW above the existing generating capacity. If electrolyzers were to operate off peak, proportionally higher additional capacity would have to be installed to meet the demand. Most US power plants are fueled by coal, fuel oil, and natural gas, the only fuels available in sufficient quantities to meet the demand. Nuclear energy appears to be out of the question because of prejudice and possibly because personnel who would conscientiously operate nuclear power plants may not be available on the scale needed.
The prospects for solar energy, frequently offered as a solution, do not appear encouraging. The stated goal of the US Department of Energy was to achieve a capacity of 1400 MW from US-made photovoltaic systems worldwide by the year 2000. Compare this to the 849 GW of US winter peak load. Photovoltaic installations on the scale required for hydrogen production may also have problems with the toxicity of their metallic components. Therefore, an environmental impact assessment of the recycling and disposal of photovoltaic cells is desirable.
