Energy efficient heating for an unheated porch?

I live in a cold climate. I have a three season porch that I have heated with a space heater for several years. I am looking for a more efficient way of keeping it above freezing -- it doesn't have to be 70 degrees-- during cold season. In-flooring heating is not possible. I hate infrared and I don't want to burn wood, pellets, etc. I'm hoping someone reading this will be able to suggest something else that might work. Again, I don't need it house warm, but I do want it to stay above 32 degrees.