(This is Part 12 of a series. Go back to Part 11.)
If solar power is one wing of the ultimate renewable energy solution, then hydrogen is probably the other. As discussed, thin-film solar cells of all sizes and shapes will power everything from houses to power plants to all sorts of gadgets. But what it probably will not power is the transportation system. For that, hydrogen will eventually be ideal.
As mentioned elsewhere, hydrogen is not an energy source, but rather, an energy carrier. Energy must be expended to obtain free hydrogen, but once obtained it has many advantages.
For example, when hydrogen is used in a fuel cell, energy is released with only water and heat as the by-products—no carbon dioxide or other pollutants are emitted. And if the hydrogen itself is produced by a renewable energy source such as solar, wind or geothermal power, then the energy (except for the original manufacturing processes) becomes virtually carbon-free and pollution-free.
But hydrogen still has significant technological hurdles to overcome. The foremost of these hurdles is volume. Though hydrogen by weight has an energy density 3 times that of gasoline, by volume gasoline is normally 3000 times more dense than hydrogen.
To overcome this limitation, hydrogen must be condensed somehow into a much smaller volume. Otherwise the fuel tank, in order to obtain an acceptable driving range of 300 miles, would have to be many times larger than the car itself. So engineers are looking for ways to condense the hydrogen.
The most direct way to obtain this condensation is to compress the hydrogen gas to several hundred times atmospheric pressure (5000 to 10,000 psi). But this extremely high pressure creates problems of its own, particularly in handling and refueling. Moreover, even at such prodigious pressures hydrogen still has only 15 percent of the energy density, by volume, of gasoline.
So researchers are looking beyond simple compression. One possibility is to cool the hydrogen gas down to -253 degrees, when it turns into a liquid. Liquid hydrogen has a high energy density and volume density, but because it must be kept at such a frigid temperature huge problems of refrigeration and handling become paramount. In addition, it takes a lot of energy to make and keep hydrogen that cold, so the energy return on investment goes way down.
A more promising alternative is to use metal hydrides, which absorb hydrogen into their molecular structure much like a sponge absorbs water. Metal hydrides can achieve volume densities even higher than liquid hydrogen and are stable in storage. Unlike liquid hydrogen (which must constantly use energy to stay frigid), metal hydrides can store their hydrogen without additional inputs of energy.
The big downside of metal hydrides is that they require a very high temperature (around 700 degrees F.) to release their hydrogen. This shortcoming can be overcome by using so-called complex hydrides, which release their hydrogen at a much lower temperature. However, they were thought until recently to require refueling in a factory.
Then, just in the past few years, reversible complex metal hydrides were discovered, which would allow autos to be refueled at the local service station. This is a very promising area, and though it's not quite ready yet, auto companies—including Toyota, Honda and GM—are pursuing it with vigor.
Other promising areas of development are storing hydrogen in liquid
chemical hydrides and in exotic carbon nanotube structures. Time will tell which of the technologies mentioned above will become the most efficacious, but it seems only a matter of time until the challenges of hydrogen storage are solved.
Then there's the challenge of infrastructure. Factories, refineries and delivery systems must be built, and tens of thousands of service stations must be converted to hydrogen delivery. All of that will take time, and so will the engineering challenges, so that humanity in the meantime is very likely to suffer through an energy deficit for some years or decades as reliable fossil fuels become more scarce and more costly.
However, we can assume that, over time, the technological challenges of solar cells, fuel cells and hydrogen storage will be solved, and when they are we can envisage a future when human beings can power their vehicles, appliances, factories and buildings in ways that remain truly sustainable, renewable and benevolent towards the biosphere.
(This is the end of Part 12. Go to Part 13.)
—jim sloman, 3.28.07
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