Teams of college students converged on the nation’s capital this week to exhibit their ideas for saving the world, or at least sustaining it.

At the National Sustainable Design Expo, a U.S. Environmental Protection Agency event on the National Mall, 50 groups that emerged from a nationwide competition offered their “green” designs for cleaner water, cleaner air, renewable fuels and environmentally friendly construction. The teams already had won $10,000 to expand their concepts and contraptions; the expo offered an opportunity to compete for another $75,000.