U.S. Army to build 500 MW solar power plant

* Project's first phase to be 500 megawatts, could be 1 GW

* ACCIONA Solar, Clark Energy to develop plant over 13 yrs

* Developers to fund project in exchange for land lease

LOS ANGELES, Oct 15 (Reuters) - The U.S. military is tackling a new mission in the field of alternative energy, moving to power up a 500-megawatt solar facility at Fort Irwin's sprawling desert complex in California.

The U.S. Army Corps of Engineers tapped ACCIONA Solar Power, a unit of Spain's Acciona SA (ANA.MC), and Clark Energy Group to develop the project, which launched its first phase on Thursday.

The project, located at the Army's largest training range in California's Mojave Desert, could grow as large as 1 gigawatt in the future.

The companies will finance and build the plant in exchange for leasing of the military land. The project, planned for five sites over 13 years, could cost $2 billion.

The solar power plant is part of the Army's mission to meet a federal mandate that calls for it to cut its energy use by nearly a third by 2015 and get a quarter of its energy needs from renewable resources by 2025.

The facility at Fort Irwin will surpass the 14-MW solar plant at Nellis Air Force Base in Nevada as the U.S. Department of Defense's largest solar power plant.

The new plant will have both photovoltaic solar panels and mirrors and turbines for concentrated solar power.

That approach takes advantage of solar thermal's low-cost and solar panels' fast installation, Laurence Greene, who directs development at ACCIONA Solar Power, told a conference call.

"We can tailor individual site development to the needs of the marketplace," Greene added.

Greene said they will be working with other companies on the project, but had not chosen any yet.

At most the military complex uses 28 MW of electricity, said Jerry Hansen, the Army's senior energy executive, leaving nearly 475 MW for developers to sell to regional utilities.

(Reporting by Laura Isensee; Editing by Richard Chang)