Solar Power - An active Google+ Community focussed on all things solar power
This community has just under 18,000 members and is regularly updated. The nice thing with a Community (vs my Collection where I post and others just comment) is that any member can also post themselves. So the content can be quite dynamic.
See the Community at https://plus.google.com/u/0/communities/115977234520990967948?cfem=1