Hi
I think it can be put in two ways:
Efficiency (no losses) must be nearly 100% because the losses in this case are from the main tank to the outside.
Effectiveness (how much heat you can transmit) is greater as the Temperature diference rises. You won't be able to equal the two temperatures- the coil and the tank, as the system will not work when the 2 temperatures get near. There will always be a difference. Of course a copper coil will exchange more energy than an iron one.
I think this system is good in this case:
A solar panel made of iron, like those water house heating radiators you can find in junk, filled with oil circulating through the tank.
Advantages: no rust in water, no water freezing in the panel.
I don´t know if oil if good for termosifoning...
That picture is very nice.
Paulo