Many of these designs may be too large or too expensive for your Make A Difference Project, but you might be able to make some smaller models using materials you can find for free and test out different versions.
Solar Heaters
This site contains lots of reviews and information about solar heaters using aluminum cans.
7 Reviews on Solar Heaters
Rain Barrels
Rain Barrel DIY
Rain Barrel Designs
More Rain Barrels
Wind Turbine
Video: Mini Wind Turbine
Video: Vertical Axis Windmill Idea
Kid Wind - Ideas for wind turbine designs - I don't expect you to buy these kits, but this might provide some ideas for how to create your own from other materials.
Mini-Greenhouse
Small greenhouse ideas
