Using Free energy sounds good to me. This DIY solar heater by Hemmings Auto Blog, takes simple common household items and combines them to make something useful. I have the same problem in my workshop. In the winter it is just to cold without some sort of heater. The concept is simple create a box of wood with a Plexiglas top, add some metal to collect the heat, attach a inlet and outlet and align it to the sun. The results were a 15 degree differential between the inlet and the outlet.