which leads me to wonder if some of those involved are idealists or scam artists
That's a bit of a jump. Maybe they're getting ahead of themselves in promoting it, but the only way this stuff is ever going to reach a practical stage is if the public is interested in it, and it can be a viable commercial enterprise. It's kind of a catch-22 in that respect.
It's not like the technology doesn't work. It's just not ready for you and me. Geothermal heat works, so do solar panels and hybrid cars, once you get past the initial cost. With geothermal, that's kind of unavoidable, because you're drilling and/or exacavating; the unit itself isn't much more than an air conditioner. Hybrid cars are a little bit more complicated than their gas-only counterparts, but a good part of the cost is just for novelty. It's something new, so they'll first sell it to people who'll pay a lot to have one, then the price will gradually come down for the rest of us.
The other part of it is demand, and that is also a catch-22. Right now, the only thing keeping the price of solar panels high is a lack of demand. A solar panel is mostly silicon. Nothing too exotic about it. It's expensive because solar panels aren't produced in large enough numbers for the production costs to come down. That's because relatively few people are buying them, because they are so expensive. That will change, but it will take time.
I agree about the hydrodgen fuel cells though. I just can't see that ever being practical. Aside from the platinum, you're going to use more energy making the hydrogen fuel than you're going to get out of it. Sure, it's cleaner than petroleum, but I'd call it more of a storage medium than an actual source of energy. Ethanol has similar problems. Other than fewer emissions, there's no net gain.
Biodiesel is another one people aren't really thinking through. Sure, you can make it at home out of old cooking grease and pay less than you would for regular diesel, assuming you make enough to buy your chemicals in bulk, and your time isn't worth anything to you. People always forget to include their time in the cost of things. Should be a clue that nobody other than hobbyists seems to be making biodiesel. You'd have a hard time making any money producing it commercially.
For the most part, I support alternative energy. It's one of the few areas where the environmentalist movement is trying to move forward instead of backward. But as with most things, it's a matter of separating the good from the bad.