Build Solar Water Heater

I am helping my daughter with her science fair project and she has decided to go with alternative energy. From that she decided she wanted to build a model solar home or car and show how using solar energy would help decrease pollution. Her science teacher has told her to make sure she uses more than one variable..Now we are lost..What variables can be used with this type of project? Any help will be greatly appreciated…

Will only renewable energy and no fossil fuels be enough to run the things that we label ‘essential’ for life?

I need to know how much money it would cost to supply the entire united states with kinetic, wind, solar, and hydro-electric energy THANKS!!!!