Answer
Okay, I don't think anyone's taken this seriously yet, so... How about, hydro-electric, geothermal steam, wind turbines, augmentative passive solar, such as water heating and daylighting (somwhat different then solar lighting, or skylighting, although skylights count.) Also let's not forget rain harvesting, and gray-water usage.these save water, and energy from a treatment, billing, delivery standpoint. Now that I asnwered the name other part: challenges, what happens when there is no sun/wind/water-flowing, if you have all of these, it won't happen that often, but when it does, are batteries a practical solution? Won't they eventually wind up in land-fill off-gasing? If you only use one or two of the 3 majors, what about when any of those aren't available? Availability is still a big concern, hopefully that will be the next stock-market bubble and drive a massive influx of green-products. The common challenges with non-renewables, rather than starting from an environmental standpoint, how about we start with what they're called. NON-RENEWABLE, eventually we will run out. It's theorized that we've found all oil on earth, and will start a downward turn on production by somtime in 2008 or 2009. Aside from that, there's the obvious global warming issues. O hope this helps some.