At the White House on Friday, President Obama publicly thanked Chu for his service.
"Steve has been a great friend a tremendous colleague over the past four years working on a whole range of energy issues but also designing a cap to plug a hole in the middle of the Gulf of Mexico when nobody else could figure it out and that's typical of the incredible contributions that he's made to this country," Obama said.
When Steven Chu came to Washington in 2009 from U.C. Berkeley he had a Nobel Prize in physics, but no real experience in Washington politics.
"He had said some very honest things about climate change and energy that he had to for political reasons walk back during his confirmation hearing," said U.C Energy Institute Dr. Severin Borenstein, Ph.D.
Things like, coal is my worst nightmare, and that country needs a much high gasoline tax.
"Most energy policy people agree with that but of course politically that's just off limits," Borenstein said.
And the head of U.C. Berkeley's Energy Institute says the political pressure ramped up.
After the Solyndra scandal broke, Chu was criticized for his handling of the $528 million federal loan to the solar panel maker that went bankrupt, laying off more than a thousand workers.
"They talk about that one failure but there were many other successes," Jay Keasling said.
Keasling worked with Chu at the Lawrence Berkeley National Lab. He says Chu's legacy goes well beyond stimulus spending on alternative energy.
"I think his biggest accomplishment is emphasizing renewable energy, emphasizing the science," Keasling said.
The energy experts I spoke with at Berkeley say Chu transformed the department from an agency that had been primarily concerned with nuclear weapons and nuclear waste, to becoming an energy research organization.
"I think it's been so great having a Nobel Prize scientist run a Department of Energy just in terms of attracting scientists to this important problem." Keasling said.
Chu says he would like to return to teaching and academic research but he will stay on at energy until the president finds a replacement.
