A worker walks down a tunnel to the Arizona sector elevator [Getty Images]

UNITED STATES - JUNE 03: A worker walks down a tunnel to the Arizona sector elevator of Hoover Dam hydroelectric power plant near Boulder City, Nevada, U.S., on Wednesday, June 3, 2009. There are 17 generators at the dam that that convert mechanical energy into electrical energy. Investors poured $155 billion into renewable power projects worldwide last year, exceeding for the first time new spending on fossil fuel-based technologies like coal-fired electricity plants, the United Nations said. (Photo by Ronda Churchill/Bloomberg via Getty Images)

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