Secretary of Energy Steven Chu announced Friday that he was resigning pending the confirmation of a successor. (Chip Somodevilla/GETTY IMAGES)

Energy Secretary Steven Chu resigned Friday after a four-year tenure during which he handed out tens of billions of dollars of grants and loans to foster renewable energy technologies — and ended up fostering controversy over whether the money was well spent.

The Nobel Prize-winning physicist, who was brought to Washington by President Obama because of Chu’s deep concern about climate change, found himself embroiled in controversy over a half-billion-dollar loan to solar-panel-maker Solyndra, which went bankrupt.

But the president stood by Chu, who shared his belief that the administration needed to turn more of the nation’s investment toward renewable energy and to set tougher efficiency standards for appliances. Under Chu, the department also issued a loan guarantee for the construction of a nuclear power plant and a controversial permit for a company to export liquefied natural gas.

Chu’s resignation intensified speculation over his replacement. Obama might pick another scientist such as Ernie Moniz, a physics professor at the Massachusetts Institute of Technology who served as undersecretary of energy under President Bill Clinton and who more recently served as a member of Chu’s blue ribbon commission on nuclear power and waste disposal.

The president is also said to be weighing politicians with some experience on energy issues, such as former governors Bill Ritter of Colorado, Jennifer Granholm of Michigan or Chris Gregoire of Washington, said sources who are close to the administration but are not authorized to discuss the deliberations. Deputy Energy Secretary Daniel Poneman is another possibility, one said.

The president’s decision might be swayed by the political need for diversity in the Cabinet.

The Energy Department job is unusual: Most of the department’s roughly $25 billion budget is devoted to maintaining the nation’s nuclear weapons stockpile and cleaning up environmental problems at government nuclear sites. The 2009 economic recovery act gave Chu one-time resources, grants and loan guarantees worth about $36 billion, according to his resignation statement.

Some energy experts say Chu was a bystander in the biggest upheaval in energy — the sharp increase in domestic natural gas and oil production resulting from the combination of hydraulic fracturing and horizontal drilling techniques.

“My sense is that he was technically brilliant but naive in terms of energy politics and energy markets,” said J. Robinson West, chairman of PFC Energy, a Washington-based consulting firm. “The other thing is, in the last four years we’ve been going through this extraordinary transformation in energy and he’s had nothing to do with it through no fault of his own.”

While Chu assembled a commission to study hydraulic fracturing, his main focus was elsewhere. Susan Tierney, an energy consultant at the Analysis Group, praised Chu’s ability to set advanced research centers on energy known by their acronym, ­ARPA-E, and research “hubs” in areas such as biofuels, batteries and solar power.

“His attention to the integrity of the science and technology missions were really utmost to what he was working on,” she said.

In his resignation letter to department employees Friday, Chu cited Martin Luther King Jr. and paraphrased him, saying that in “the scientific world, people are judged by the content of their ideas.”

“I believe we should be judged not by the money we direct to a particular state or district, company, university or national lab, but by the character of our decisions,” he added. “The Department of Energy serves the country as a Department of Science, a Department of Innovation, and a Department of Nuclear Security.”

Chu plans to remain in office at least until the end of the month.