Wind and solar don't seem to offer all that much in the current scheme of things due to acquisition/maintenance costs but I've certainly wondered a lot about tidal energy generation.

With the vast stores of natural gas we have here in the US it would seem this should be a reasonable source for use in cars. It's relatively safe, burns damned clean and engines actually last far longer runnin on it.

I 1st came across use of it in studyin HP adders some years back. Seems it was used as both a Hi-Po fuel and as an octane booster/add fuel for gas engines, but the technology just never gained overall market acceptance