Painting the jugs themselves black would make the water warm up the fastest during the day. Clear jugs will pick up heat from the air as the greenhouse warms during the day, but not very much from the sunlight directly. Try two of them side-by-side and you will see.
Also, if you are using electric/gas heat, the wall of water can actually increase the time that your heat has to run in the morning. A lot depends on the exact conditions and cloudiness, but a "heat sink" should be more appropriately termed an "energy sink." If your goal is to increase temperatures, and that water is cold, it's going to work against your efforts to raise temperatures.
|