If you have a system that has some fluid that does not freeze
easliy this will be better. I have had solar collectors that
freeze (water expands with enough force to burst copper pipes)
even when the temp is a bit ABOVE freezing, as the heat
dissapates (radiates) to the night sky making temps drop
inside the collector.

some units have a small low voltage pump (grundfos brand?)
to circulate the fluid, and some have a heat exchanger that