Chile has the advantage of being one of the sunniest places on Earth and the power itself will be generated at a 100-megawatt solar plant on the edge of the Atacama Desert, about 400 miles away from Santiago. Assuming the city hits its goal, by next year the sun will provide 60 percent of the energy needed to move the system's 2.5 million daily passengers. A nearby wind farm will fill in another 18 percent of the necessary power.

The solar plant is being built by California company SunPower using low-cost modular panels that cut down on setup and maintenance time. One key difference that sets Santiago's system apart from other cities is a direct connection that will transmit power from the solar facility straight to the subway system instead of tapping into the city's main power grid. And because desert dust has the potential to gunk up solar panels, SunPower is also using specially designed robots to keep the panels clean and running efficiently. In the United States, SunPower has actually partnered with Ford to provide renewable energy for owners of electric vehicles, but CEO Tom Werner thinks the Santiago plan could be the start of "a widespread trend" for cities with power-hungry transit systems.