updated - JUST REALIZED MY POST DOESN'T INVOLVE ALTERNATIVE ENERGY... sorry... maybe it will help someone else though...
Once we did a project about popcorn. You would need just regular popcorn kernels (not microwave popcorn) and baggies. It was an experiment to see if popcorn popped better if cold (refrigerated) vs room temp. We might have also put some in the freezer, I can't recall. If you google "science fair popcorn refrigerator" you should be able to get the information to do the project.
Thought you might already have these items at the house. Hope that helps.