There are a lot of alternative energy forums, however many of them are inactive. Here are the top three most active forums and their current subject matter:
Permaculture Forums- Current topics include two liter air conditioning, experimenting with Pyramids, and best methods for solar energy generation.
http://altenergyshift.com - More corporate, news oriented. Current topics cover the Seattle Seahawks use of alternative energy and an interview with the FuelCell Energy CEO.
element14's Alternative Energy Forum -Topics include lightning strike harvesting, fusion reactor to create mini sun on Earth, and solutions to store bad CO2 underground.