(Captain's log): I may as well run through some of the other alternate-energy pipe dreams and point out their flaws:
Geothermal: It works in Iceland, but Iceland is one big volcano. The number of actual sites in the US where geothermal is practical is small, and they're all isolated, and most of those are in national parks. (The best geothermal source we own is Yellowstone. The second best is Mauna Loa.) Geothermal releases huge amounts of sulphur gases, and if you use equipment to remove that it also consumes most of the energy. Even if fully developed, the energy generation would be miniscule by comparison to our total consumption.
Solar: I talked about that here. Solar can also be used for water heating and home heating in some areas, but the total energy involved is not that great in the grand scheme of things, and it requires an impressive amount of equipment to be purchased and installed per megawatt.
Wind: It isn't where we need it, and it isn't when we need it, and there ain't enough of it. The power grid has to adjust its energy generation to match consumption, and we can't turn the wind on when we need more energy. The source is diffuse and it requires a massive investment to make and install all the windmills. There are not all that many appropriate sites where the wind is regularly strong and a lot of the places where that's true (e.g. the Columbia River Gorge) are protected areas. Windmill farms are an eyesore, and they kill a lot of birds. (A lot of birds.) The equipment is large, complicated and will require a lot of repair to keep working; the resulting energy will be inadequate and unreasonably expensive per unit energy yield. And I'm still not convinced that it won't take years before any given windmill finally yields as much total energy as it took to make it, transport it and install it. Ireland is making a massive investment in wind power, but when they're finished and have fully deployed all sites it's only going to generate 520 megawatts, when the wind is blowing. That's one eighth of the power generated by The Dalles Dam.
Fusion: wake me when it actually works.
Solar satellites: In about a hundred years this will be fine. But until we have solved the minor problem of cheap bulk cargo delivery to orbit, it isn't feasible. (The only other way is to make them in orbit, using material from the Moon, and that is an even more massive problem. It would require establishment of a Moon colony, for one thing. And even that requires bulk cargo delivery from Earth to orbit.) As it currently stands, there are physical reasons why the cost in energy of putting such a satellite into orbit is huge, and it would take a long time before such a satellite paid for itself in energy terms, let alone financially.
Tides: Harnessing the power of tides isn't all that easy. The original proposal was to create pools which filled at high tide, and then drained out through fairly standard turbine electrical generators as the tide fell again. The energy generation is intermittent and not timed to usage. And there's a much larger problem having to do with how turbines work. As a general rule, efficiency of a turbine increases as a function of the pressure being fed through it, and in most parts of the world the tide only goes up and down a few feet. That doesn't provide enough head to actually make turbines work well even if you wait until absolute low tide before beginning to drain your pool. (The turbines in The Dalles Dam are designed to utilize 81 feet of head, which is to say that the surface of the reservoir should be 81 feet above the turbines. No place in the world has tides remotely like that except the Bay of Fundy.) Which means that to generate any kind of significant power this way you need a really big pool and a heck of a lot of turbines, because the conversion process will be extremely inefficient. I don't believe this can be scaled up to the point of creating enough energy total to actually make any difference, especially considering the ecological damage it would do to coastal waters where it was deployed. Another big problem with tides is that it's saltwater, which is highly corrosive to both metal and concrete.
Fission: Completely practical from an engineering standpoint. Completely useless from a political and economic standpoint, at least in the US. The cost of a new fission plant is stratospheric now because of the amount of liability insurance it would have to carry, even if for no other reason. And the regulatory burden is beyond belief. It's been something like twenty years since the last design start for a fission plant in the US, and since the WPPS fiasco, no investor will be willing to take the chance again until something drastically changes.
Coal: Coal works really well. That's why it's our primary source of energy.
UPDATE 20020925: Different people have written in to point out two major omissions from the above.
Hydrogen: (non-fusion, hydrogen as chemical energy) The problem here is that hydrogen is a fuel but not an energy source. Gasoline is both. But there's no substantial natural source of hydrogen wh