Posted: 28/10/2013 at 14:20
Even if there was such as thing as solar powered heating, for 3-4 months of the year daylight duration, angle and strength would render it useless. The coldest period is 3am-7am in winter. On Dec 21st it can be daylight only from 9am-4pm, so solar will never work.
If you mean some sort of heat exchange using water warmed by the sun, then again the above with the added complication that the water will freeze to some extent.
An electric fan heated with a thermostat set to 7C is probably the best value given that paraffin produces so much moisture and you can kill more with botrytis than cold.