[masraum]

using some sort of metal (copper painted black) would be better in #1 if possible. I think you'd get a lot better heat transfer.

I'd love to use geothermal heating and cooling in my house.
http://www.reddawn.com/featart11-98.html

The application of geothermal heating/cooling, also known as ground source heat pumps, has been named "the most energy-efficient and environmentally sensitive of all space conditioning systems", by the Environmental Protection Agency. The system's basic concept takes advantage of the earth's constant temperature, approximately 55 degrees, to heat and cool a building. By tapping this steady flow of heat from the earth in the winter, and displacing heat in the earth in the summer, a geothermal heat pump can save homeowners 40 to 70 percent in heating costs and 30 to 50 percent in cooling costs compared to conventional systems.

I think the efficient use of air circulation is another. Like a fan to keep the temp of the air in the attic down in the summer.

I also think that if I added some sort of hot air vent system for my garage it would help in the summer. When we pull our cars into the garage in the evenings, that heat stays in the garage.

When I live off base in Japan, we had a tankless hot water heater. I'm also not sure if it would be considered green, but I would think so.