text: Two, actually- One was a simple water heater for a cabin of mine, using a rooftop black insulated tub with a large clear cover-the sun heated the water, and I was able to take a warm shower at night- The other was a triple paned greenhouse made out of 3mm plastic- It was an experiment I did to see how long I could keep a greenhouse going in the winter here in Manitoba.  The cost of either was negligible.