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The afternoon sun hits the solar-cell covered front wall of the University of Kentucky's solar-powered home. It's one of 20 houses put up up on the National Mall that are competing in the Solar Decathlon, a Department of Energy-run event to showcase the potential of solar energy and energy-efficiency in buildings. If you can't make it to Washington before October 13, these photos will give you a taste of the architecture and technology used inside and outside these net zero-energy homes.

The University of Arizona team made their home a laboratory for cutting edge, green building technology and design. The intent was to show that green homes can be attractive and that the buildings can be mass produced.

The wall on the left, designed by one student for the event, is a "water wall," which aids heating and cooling. The water serves as a thermal mass that can absorb the sun's heat during the day and release it at night. Behind the wall, which is made of plastic, is a layer of air that circulates around the structure's ceiling for cooling. Shades can be hooked up to the building automation system to keep heat from coming in.

The homes in the competition cannot have a footprint over 800 square feet. Working with limited space, many designers focused on modularity and space-saving tricks. The Team Arizona house seen here is built around four segments, which are attached together. More modules can be added as the homeowner's space needs grow.

Many of the homes in the Solar Decathlon used relatively new green technologies, including the one from Team Pennsylvania. It used unique solar panels from California start-up Solyndra, seen on the roof and awning. These panels are a series of tubes made from thin-film solar cells. The solar cell material--a combination of copper, indium, gallium, and selenide (CIGS)--and the shape allows the collectors to capture light from different angles and produce electricity in diffuse light. Under the panels is a green roof of plantings, which aid building insulation and reduce heat to help panel production. On the front wall are flat solar thermal panels (in black) for making hot water.

Some designs were made specifically for the event, like that of Team Spain. This house uses building-integrated photovoltaics with solar cells embedded in the glass walls. The roof is a one-of-a-kind solar collector, which tracks the sun over the course of the day. The roof generates electricity and heats water, which is stored in large tanks and then circulated under the floor to heat the inside. Rainwater is also drained from the roof and saved for plantings.