The Solar Decathlon on the National Mall in 2009.Photo: NREL Solar DecathlonThe National Mall has long served as the nation's front yard, a place where citizens can gather and display what's important to them -- whether it's a protest to end wars, a rally to restore sanity, or even a celebration of mid-Atlantic maritime communities. Every other year since 2002, the National Mall has played host to the Solar Decathlon, a building competition sponsored by the Department of Energy that puts 20 solar-powered homes designed by college students from around the globe on display for a two week long design …