Thoughts on a greenhouse below ground level. This uses ground heating to help winter lows. He has not done the Earth Battery but is still seeing real winter gain on heating.
With the addition of an Earth Battery, like I posted several days ago, his highes would be reduced, and his lows would be raised significantly. Adding the Earth battery beneath the manufactured berm, would not require added excavation!
Making year round harvesting possible; further north, and not just cold tolerant crops either.