>> some way to combine a cistern and passive solar...
Maybe a very small windmill or hand-pump to move cold water up from the cistern into 55 gallon drums stacked along the North wall. Then a valve that slows down the water trickling back down through a radiator serving to absorb heat from sunlight.
Mostly rely on "heat rises" to transfer warmth up from the water to the plants at night.
However, it is a fairly huge engineering project! And you would wnat the drums and all piping to be clean enough that you can pump the water to the plants during the dry season.
I saw another greenhouse design where the floor was concrete poured around 6" or larger PVC pipes laid in a serpentine or mesh pattern. .They would hold a moderate amount of water (and heat energy), but also served as a circulating fluid through heat-absorbing or -releasing radiators. I think that also relied on an intermittent pump.
