I am thinking the north wall would be 55 gallon drums filled with water 2-3 high I can get plenty free and water has a better thermal mass then earth or rock. And earth bag walls on the east west and south sides.the cold sink is a must. if you need extra heat i am sure a co2 generator could handle that task.
All these sound like great ideas, especially once running a CO² burner is factored in- with one exception, the drums full of water. I understand their thermal heating quotient, but I don't see the value if the wall behind it can be used instead. What's got more thermal mass than the earth itself? If you feel the need, finish the wall with a material that maximizes the solar gain, maybe something as simple as black made from wetted charcoal. I would consider a similar approach with the ends of the facility as well.
I would also angle the east and west walls out to catch more sunlight. The floor of the greenhouse can't be too far below the lip of the hole on the south side, lest the low angles of winter sun defeat the whole purpose. I'm envisioning a more gentle slope or terrace of only 30 degrees from horizontal, and a flat floor set further back. This makes for a wider structure from north to south, but it would maximize winter heat gain. Summer lighting need not be compromised with such a floorplan, and in summer the sloped/terraced area could be effectively used in production as well.
Add a light depo cover, high intensity indoor lighting, a modern integrated climate control system and automated grow controls and this might be the well dressed grow op of the future...
