Oh, sorry, solar powered pump.
I'm not completely familiar with thermal motion and fluid mechanics, but heat tends to rise, and having the heat source up in the air wouldn't be that effective. So I think it would require a small pump to circulate the water. I think the air system might actually be more effective. You could also have thin, flat, black containers near the roof that collect solar heat, then run it through a compost that sits outside the greenhouse, and then into the ground, all using a small pump powered by a solar panel. You don't want to compost inside the greenhouse, because the accumulating gases would really not be healthy for you.
I'm not completely familiar with thermal motion and fluid mechanics, but heat tends to rise, and having the heat source up in the air wouldn't be that effective. So I think it would require a small pump to circulate the water. I think the air system might actually be more effective. You could also have thin, flat, black containers near the roof that collect solar heat, then run it through a compost that sits outside the greenhouse, and then into the ground, all using a small pump powered by a solar panel. You don't want to compost inside the greenhouse, because the accumulating gases would really not be healthy for you.