Google will join BrightSource Energy Inc. and NRG Solar LLC as an equity investor in the Ivanpah solar electric generating system by making a $168 million investment.

Ivanpah is the world‘s largest solar project under construction in the Mojave Desert in California and when completed in 2013 the companies expect the project to nearly double the amount of solar thermal electricity produced in the U.S. today.

The system will generate 392 gross MW of clean, solar energy. That’s the equivalent of taking more than 90,000 cars off the road over the lifetime of the plant, projected to be more than 25 years, Google said.

This is our largest investment to date, and we’ve now invested over $250 million in the clean energy sector, Google said in a blog post.

The Ivanpah Power Tower will be about 450 feet tall and will use 173,000 heliostats, each with two mirrors. The project is being constructed by Bechtel, a major engineering firm. Construction began October 2010, and is projected to finish in 2013.

Power tower technology works by using a field of mirrors, called heliostats, to concentrate the sun’s rays onto a solar receiver on top of a tower. The solar receiver generates steam, which then spins a traditional turbine and generator to make electricity. The mirrors focus a tremendous amount of solar energy onto a small area to produce steam at high pressure and temperature.

As part of the financing, BrightSource finalized $1.6 billion in loans guaranteed by the U.S. Department of Energy‘s Loan Programs Office.

In October 2010, NRG Solar LLC, an NRG Energy unit, announced its commitment to invest up to $300 million to become the lead investor in the project.

We‘re excited to be making our largest clean energy investment to date. With this investment, we‘re helping to deploy the first commercial plant of a potentially transformative solar technology able to deliver clean energy at scale, said Rick Needham, Director of Green Business Operations at Google. Ivanpah will be the largest solar power tower project in the world, able to produce clean electricity at the highest efficiency of any solar thermal plant. We hope it can serve as a proof point and spur further investment in this exciting technology.