Some are state-of-the art units controlled by microprocessors, while others are based on 17th-century technology. Most are made in the United States, though a few come from Europe and one from even farther away. Yet despite their differences, they share a common goal: to lead the next generation of affordable, cleaner-burning woodstoves for residential heating.

Popular Mechanics is a partner and judge in this year's Wood Stove Design Challenge, a competition that looks to bring more innovation to the world's most popular and widely used renewable energy device.