The outlines of a massive solar thermal power plant—the largest ever—are starting to appear in the wilderness outside of Las Vegas. The $2.2 billion project, which is being built by Oakland, California-based BrightSource, stretches over 3,600 acres near Ivanpah, California. When it’s finished, it will generate 370 megawatts of electricity on sunny days.

The project has been a long time coming. BrightSource first filed an application for the project in the summer of 2007. Approval took three years. Construction was temporarily slowed to accommodate the care and relocation of desert tortoises—a threatened species—found in larger numbers than expected. The project, which will generate electricity by using mirrors to concentrate sunlight to heat up water and drive steam turbines, is now expected to be finished next year.

Even as the project nears completion, the future of solar thermal power plants is in doubt. That’s in large part because prices for solar panels—which convert sunlight to electricity directly—have dropped quickly in the last few years, causing at least one company to abandon plans to build solar thermal plants in favor of making ones that use solar panels.

Yet solar thermal has at least one great strength compared to many other types of solar power: the heat it produces is easy to store, so electricity can be generated even after the sun goes down, and power can be dispatched to the grid whenever it’s most needed.