This Brazilian Eco Designer developed a low cost and intelligent method of reducing energy costs and preserving the environment by reducing waste through the heating of water by passive methods.
By using PET bottles and and some PVC pipe, according to his calculations this is an extremely low cost and safe project that you can do yourself at home.
This project has become popular on the web and has been adapted to homes and even schools. The State of Panama in Brazil created a Do It Yourself downloadable PDF manual (in Portuguese) that many consumers have followed and built. It looks simple enough... notice the metal pipes run through the bottles creating a "greenhouse effect" so to speak to heat the pipes via the suns energy. The more hot water you need, the more piping you add.