I have to ask again, Why are we trying to heat water in barrels, that has no other use than to try and store heat, when we could be heating the AP water directly and storing our heat in the hundreds or thousands of gallons of water in our AP system????
DB, I am just not sure what you would gain, by burying a bunch of barrels then heating the water up in them?? I understand since you are planning to use electric heaters and electric water fountain coolers, efficiency is not your main goal, but I am still not sure why you would not simply forget about all the barrels and heat up the water in your AP system, or cool it, depending on the season? What is to be gained by heating water in barrels then trying to transfer that heat to the water in your AP system?? If you have the ability to heat water in barrels, you should have the ability to heat water in your AP system. Efficiency wise it only makes sense that it would be more costly to heat more water ( barrel water, plus AP water) than it would be to heat less water, just AP water??
I think people are missing the fact that the reason they used big black barrels with water, as a heat sink in green houses in the past, and still today, is because they lacked the AP system, that has a large water mass and acts as a heat sink all by itself.