Set your cold water bucket (source) up higher than your warm water bucket (return) and the whole thing will gravity siphon. Due to the design of this collector (both ports return to the same location on the panel) it will not thermo siphon. For that to happen you would need to cut the long return pipe and have it exit at the top of the panel.
A word of warning, this panel works VERY WELL on a very sunny day and within seconds the water coming out of the panel was hot enough TO SCALD. Burned fingers. This very hot water is only formed when the water inside the panel is allowed to sit for about a minute without moving. If the water is moving (do to the gravity siphon) the water exiting the return pipe is about 110 degrees, and while hot, will not burn you.
The water does not flow through the panel very fast (as the pipes are very small) but that is sort of a good thing as it allows the water to heat up a lot on its journey through the collector. It does take a while to heat up a 5 gallon bucket of water, you ended up building an insulated return bucket that was all black and sealed on the top except for the port where the water tube enters. This kept the returned hot water hot long enough to be of use.
Run for a couple of hours one hot sunny day and it heat up a five gallon bucket of cold water (measured at 70 degrees F) to over 110 degrees F. The temp that day was about 76 degrees F. If the water is allowed to sit in the panel for several minutes and then forced out (by blowing in one of the hoses) the water was measure at 170 degrees F. All in all we are much happier with the performance (and cost) of this panel. It performs much better than the previous one.
Our next modifications to this design will be to alter the return port so that it will thermo siphon, in this way the return hose can be fed into the source bucket and the water will continually circulate in the panel getting hotter and hotter. There is talking about adding mirrors to the panel to concentrate more heat. The goal is to boil water. This entire project cost less than five dollars, as there are already the screws, and the duct tape. The only thing to purchase was the air hose, which cost $3.76. Now you can enjoy hot water.
Source: www.agua-luna.com. Visit it for more info.