The panel is filled with cans painted black to soak up heat from the sun and get hot.
The project is pretty extensive, and you need a total 240 aluminum cans.
For a complete guide check out the link below.
Update: Looks like we’ve featured asimilar heating solutionbefore.
Sorry for the repost.
Building a solar heater with recycled cans| *faircompanies viaAdafruit
