Solar Heater Idea for Small Greenhouse.

I have a small 10x10 shed I am turning into a greenhouse. This is a homemade Solar heater using a garden hose and a rain barrel. The idea is to heat up the water in the barrel with the sun during the day and have the water hold its heat through the night. We have pretty mild winters. My goal is mostly just to keep the temperature above freezing and to not have to use electric heat.