Sunfish, that's a good take on the system with the solar, with the pea gravel. The only thing that seems like it would make it better is more of a underground pattern. Maybe inground water pipes with a solar water heater would be a good option for off the grid too-
A Real 'Green' House: No Heating Bill for 25 Years | LiveScience
Just saw on one site that they insulate the ground with hard insulation before they put the floor down in the greenhouse, never would have thought of it.