. At it's most simple, all you have to do is mount your barrels higher than the collector and thermosiphon will take care of moving the heated water. Depending on location, size of greenhouse, amount of water, &c, that may be all you need.
This is why you get the big bucks. Great idea.
. Am I not about due for a raise? I'd settle for a 10% increase.
Sure, I'll use my blackmail photos of Eric to take care of it right away. Your next check will reflect the increase (less taxes, fees, surcharges and my commission, of course).
Use a small pond pump to circulate the heated water through tubing run into the greenhouse and back to the collector. You'll get the best heat exchange from copper tubing.
Use a DC pump, and run the pump off a small solar panel. No sun, no pump. Nice hot sun, nice hot water, pump runs.....
Very smart idea.