My impression from the video was that the cans were filled only with air: basically large tubes with thin walls.
Thus it had zero heat capacity; it was only a warm air generator, and only generated warm air while the sun shone directly on it.
Maybe a similar device could be built, replacing the large tin cans with small diameter, black-coated copper tubing. Push cold water through the copper tubing slowly and it would come out warm or even hot if it trickled slowly enough.
You could send the warmed water to a 55 gallon drum inside the greenhouse. Maybe insulate it during the day and remove the insulation at night?
Say the "warm" drum sat inside the greenhouse at floor level or slightly sunken.
Each morning you could pump the cool water UP to another drum, elevated, either inside or outside the greenhouse, but hopefully in the sun. Then let it trickle by gravity through the "solar heater" and back into the "warm" drum inside the greenhouse, all day. Then the pump only needs to run briefly each day, and gravity runs all day for free.
The air got so hot because it has such a low heat capacity, and probably flowed slowly through the cans.
It took hardly any sunlight to warm it from 30 F to 180 F.
Cp Air = = = 0.24 BTU per pound-mass x degrees F
Cp Water = 1.0 BTU per pound-mass x degrees F
So water has four times the thermal capacity PER POUND compared to air.
Water is 784 times as dense as water (at sea level and 15'C).
Therefor, water has 3,267 times the heat capacity of an equal volume of air.
For heat STORAGE, use water or rocks, not air.
