In a 21st Century take-off on the mobile-home park, a San-Francisco-based company is rolling out a line of pre-fabricated modules that promise to provide zero-net energy housing. The company, ZETA, revealed a dwelling constructed of the modules last month in Oakland. The modules are big boxy things that can be stacked like Legos. They don’t look much like mobile homes. According to the company, they are meant for urban infill, student housing, apartments, and duplexes. None are scaled for single-family homes. The concept is to use land more intensely to prevent suburban areas from spreading into farmland and forests and damaging the environment. The modules, fabricated in San Leandro, use what the company calls “premium” building materials. Unlike trailer pre-fabs, and much new home building, the use of the material alone improves indoor air quality. Manufacturing in bulk on a production line also lessens environmental impacts that would otherwise occur in building on-site--for instance, less waste has to be sent to landfills. ZETA claims the boxes use 40 to 60 percent less energy than traditional new buildings. With the addition of solar panels, they could reach zero-net energy. They are designed for passive solar use, have efficient windows, and include a built-in control system for managing internal systems. Given the distribution plans, if not the design, it seems unlikely they will face hits from marauding tornadoes anytime soon.