Peter, I was wondering if you have heard of geothermal energy? Tidal, wave, wind, and solar power are currently in use, and with wave power having the potential to provide ½ of the earth's required energy, and all four combined providing the entire earth's energy forever, geothermal energy trumps them all. Geothermal energy uses a technology called heat mining through a simple process using water. An MIT study in 2006 concluded that 13,000 ZJ's (zedo joules) are currently available, and 2000 ZJ's tappable with improved technology. An interesting technology considering that the total energy consumption of the earth is about 0.5 ZJ's a year, therefore approximately 4000 years available using this technology alone. The energy is also limitless, because the earth is constantly providing heat.
