Let me preface this: I know that this has been suggested before, and that it's not exactly a new concept. Certainly it would require dramatic changes in the way our society is organized, and a lot of people would be unhappy with the requisite changes, but I think that in the long run it would prove to be a source of energy which is clean, inexhaustible, and rather pleasing for those of us with the ability to feel sympathy for other human beings.
Now, it wouldn't power all of the U.S., and while simple, it is not easy to implement. Nevertheless:
Step 1: Unearth Ayn Rand's coffin.
Step 2: Link it to as large a turbine as is feasible given current standards of material manufacturing and engineering.
Step 3: Reform U.S. law and social norms to fit with the common model of social democracy, including but not limited to a universal guarantee of human rights, including the right of all humans to the basic necessities of life, such as clean water, healthful food, medical care, housing, &c., as well as strict limitations on predatory business practices.
Step 4:
Unlimited PowerI know it's not the most conventional of clean-energy proposals, but please give it a little bit of consideration.
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On a more serious note, Leo has presented one of the better points: We should focus first on removing the worst offenders rather than finding an ideal solution. Coal is absolutely terrible for the environment, both in terms of power production and extraction methods. The main issue with eliminating it is something I'm unfortunately well aware of, living in a coal state: there's massive support for the industry, not just in the lobbies, but on the popular level (due to the jobs it creates). It's also, to my recollection, the fossil fuel which will take the longest to run out; IIRC the current estimate puts it at around 250 years, as opposed to a handful of decades for oil and less than a century for natural gas.