I have done several different experiments in my greenhouse trying to develop additional heating. I think I might have been working too hard and making it too complicated. This fella has just gone direct from panels to household water heater elements into a container of sand as a solar battery. A 30-gallon drum of sand and two elements might be worthy of a trial run. Might even consider using a steel trough planting bed and the elements below the plant root level? Affixing a water heater element temp switch to the side of the container or some other temperature switch in the planting bed to control the temp. Embedding the elements into a rocket stove heater mass?
http://www.youtube.com/watch?v=X6KOWGN6C28