On 10/10/2015 8:45 PM, Muggles wrote:

We had a solar water heater for our above ground pool, but it didn't
work that well when the temps got cooler at night. Maybe you're talking
about something different than what we had for our pool.

ISTM that you're looking for ways to *retain* heat moreso than
*gain* heat. Your solar pool heater suffered from heat *loss*
(evaporative and radiated to the blackness of space). An insulated
pool *cover* would have benefitted you for the overnite hours.

I.e., you want to keep things at a relatively constant temperature
despite large (?) variations in the surrounding *air*.

You wouldn't, for example, want to heat the water to ~130F during
the daytime lest you kill the fishies and plants (?)

So, you want to put lots of energy into something that will store
it and slowly re-radiate it when needed. Water and earth are
two good candidates: water can be moved relatively easily
(so you can bring heat from point A to point B); earth is
cheap to amass.

I still think a ground sourced heat pump of some sort. Though not
necessarily a COTS solution as your plants don't need HEPA filtration,
heat exchangers to isolate the "outdoor" air, etc.