There are a lot of things that go into making sustainable energy.  When debating the merits of different types of energy, you often times hear about the long term sustainability of oil... especially peak oil theory.  Though "peak oil" is not a proven fact, it is certainly compelling.  One of the problems with "peak oil" theory seems to be the same problem that plagued Malthus Theory... timing.  It is highly susceptible to new technologies and discoveries, which make predicting exactly when peak oil occurs very hard.

But what of other energy sources?  Most people look to technologies such as solar power, and even to hydrogen fuel cells as a "sustainable" energy source.  After all, unlike oil, the Sun will keep pumping out energy for billions more years, and hydrogen is the most abundant element in the universe.  The problem with both of these however, is that you need more than merely the sun and pure hydrogen in order to convert them to useful forms of energy.  The New Scientist has an excellent article regarding the problems that will plague these new energy sources.

The first problem is with solar energy.  Current low efficiency solar cells use silicon, but are only 25% efficient at converting solar radiation to electricity.  That's not efficient enough to be useful as a long term energy source.  There are solar cells that are about 40% efficient, but they use indium, which is a very rare earth metal.  It is estimated that there is only a 10 year supply of Indium.  One of the problems is that Indium is also used in the production of LCD screens, which are booming right now.

Hydrogen fuel cells also utilize rare earth metals as a catalyst in the chemical reaction that creates electricity.  In this case, it uses platinum, which is even more rare than Indium.  The supply is estimated to last only 15 years, only because it is not in as heavy demand as Indium.

Now then, all of this could change, just like peak oil timing changes.  The problem that's important here is that "peak indium" and "peak platinum" are coming along much faster than "peak oil".  They also discuss the sustainability of biofuels.  Read the whole thing.