I'm thinking as redneck as a car radiator with a small fan pushing air across it. Small pump pushing water into the hx with a gravity return. Tie the barrels together somehow... Supply from one end of the train, return to the other
Threw this together so we would be on the same page...
Several questions. Where's the best place to put the pump? What flow rate? The most drums I could get into the intended space would be 8. Along with the piping, radiator, about 450 gallons. Cycle that in 8-12 hours? So, about .5 - 1.5 gpm? What horsepower/wattage would the pump motor have to be rated at?
Tying the barrels together is a big somehow. Not certain the bungs are threaded with anything available in PVC. Same for any small car radiator. Would need at least a couple of venting/priming valves.
When the air temp is warmer than the water you'll be storing heat in the water. When lower harvesting into your space.
This might be the easiest part.
I bet a small pump and some kind of radiator or heat exchanger would increase your efficiency a ton. Over just passive barrels
And this is the predominate question. How *much* would it increase? And is that increase worth the hassle and cost?