Winter here can be extremely cold (-35°F) with sunny days. So I designed a collection system with a salvaged flat-plate water heater on a loop charged with 50% glycol antifreeze. The small PV panel drives a DC pump to circulate the loop.
There' a 400 gal. heatsink under the floor, with coils of PEX tubing carrying the heated glycol solution through plain water. An AC pump circulates the warm water through the floor, which is the main thermal mass.
Besides the dark pavers and black pots, there are four 75 gal. plastic tanks on the south-facing wall, passive heatsinks.
I took apart electric quartz radiant patio heaters and mounted four elements on the ceiling, wired to a hanging thermostat. They warm the foliage even when the air temp is quite cool (often around 40°F).
When it gets down to -20°F or so, I add a small milkhouse heater, with a small propane catalytic heater as backup, in case the power goes off.
The foundation and floor are insulated. The roof is 6-wall poly (R 3.8) and the sides are 3-wall. When it dropped to -40°F, I lost a few plants to cold air leaks, but have grown tomatoes year-round since 2009.
