(AXcess News) – Steven Chu announced Friday he will resign his position at the Department of Energy,after serving four contentious years as Energy secretary.

The Nobel-Prize-winning physicist made renewables a centerpiece of his tenure. But while surges in wind and solar installations garnered praise from Democrats,Republicans excoriated the Obama administration when clean-energy investments backfired.

In a lengthy resignation memo to his staff,Mr. Chu ticked off what he counts as the department’s accomplishments under his lead: increased investment in high-risk,high-reward energy technology development; a doubling in production of clean,renewable energy from wind and solar; and more than $90 billion in Recovery Act loans to green energy companies.

Energy Secretary Steven Chu testifies on Capitol Hill in Washington. Mr. Chu will earn the title of longest-serving Energy secretary by the time he officially steps down.

When Solyndra,a solar panel manufacturer backed by $535 million in federal loan guarantees,filed for bankruptcy in 2011,critics of investments in private energy firms pounced. Chu downplayed the incident,saying that only 1 percent of the more than 1,300 companies funded by DOE Recovery Act funds went bankrupt.

“The test for America’s policy makers will be whether they are willing to accept a few failures in exchange for many successes,” Chu wrote in his letter Friday. “America’s entrepreneurs and innovators who are leaders in [the] global clean energy race understand that not every risk can – or should – be avoided.”

Chu said he will remain in the post at least through February and until a new secretary is selected.

When then President-elect Obama nominated Chu in 2008,he bucked a trend of appointing politicians or industry leaders to the post. The choice sparked optimism among academics who felt they had one of their own as the nation’s top energy administrator.

“His appointment should send a signal to all that my administration will value science,” Mr. Obama said at the time. “We will make decisions based on facts,and we understand that the facts demand bold action.”

Indeed Chu,formerly the director of the Lawrence Berkeley National Laboratory,won praise from many corners for the implementation of smaller,focused programs like energy efficiency block grants,the weatherization assistance program,the advanced research projects agency – energy.

But some say Chu’s lack of experience inside the beltway overshadowed his wealth of experience outside it.

“Obama tapped the Nobel physicist to lead his fight to stop global warming and transform the nation’s energy economy but the broad climate bill the president hoped would become part of his legacy never made it through Congress,” the National Journal reported Friday,”and Chu later found himself embroiled in the political controversy over the bankrupt solar company Solyndra.”

Chu,who will earn the title of longest-serving Energy secretary by the time he officially steps down,takes the long view.

“Just as today’s boom in shale gas production was made possible by Department of Energy research from 1978 to 1991,some of the most significant work may not be known for decades,” he wrote in his resignation letter. “What matters is that our country will reap the benefits of what we have started.”

The president has not yet named a successor,but Politico lists former North Dakota Sen. Byron Dorgan,former Colorado Gov. Bill Ritter,former Washington Gov. Chris Gregoire,and former Michigan Gov. Jennifer Granholm as potential candidates. All of them are onetime politicians.