[NiHaoMike]

Use the solar to power a heat pump. Compressors are cheaper than solar panels. It will give you 2-3 times the heat for the same amount of solar panels. With a closed loop inverter, you can use a 100-200w solar panel to power a refrigerator compressor (about 150w nominal, but can be less or more with an inverter) and get about 200-600w (resistive equivalent) of heat.

It's still less efficient than solar thermal, however. But if you have some way to use the cooling, it can be worth it.

Note that copper is very resistant to corrosion, especially if the working fluid contains chemicals to prevent such corrosion. (Propylene glycol solution used for solar thermal often has it.)