Energy Secretary Steven Chu has announced that he will leave his post by the end of February or when a successor is approved and in office.

Chu, who announced his decision on Feb. 1, has led the Dept. of Energy since early 2009,  a period during which several regions of the United States experienced the start of a natural gas development boom.

At DOE, Chu, a Nobel Prize-winning physicist, oversaw the growth of the Advanced Research Projects Agency—Energy (ARPA-E) which supports high-risk, high-reward technology development, as well as energy-efficiency block grants.

Chu also oversaw the administration of the troubled federal loan guarantee program for clean energy, which came under fire when the one of the program’s beneficiaries—Solyndra, a solar company in Fremont, Calif.—went into bankruptcy.  In a letter to DOE employees announcing his resignation, Chu defended the loan program. He said: “While critics try hard to discredit the program, the truth is that only 1% of the companies we funded went bankrupt. That 1% has gotten more attention than the 99% that have not.”

Environmental groups credit Chu with pushing the renewable-energy sector forward. Gene Karpinski, League of Conservation Voters president, said in a statement, “Secretary Chu has led the Energy Dept. at a time when our nation made the single largest investment ever in clean energy and doubled our use of renewables.” Karpinski added, “He has proven himself to be one of the world’s greatest scientists and an ally in the fight against climate change.”

Chu said in his letter, “In the last four years, the production of clean, renewable energy from wind and solar has doubled—driven in part by our administration’s investments in the development and deployment of the latest technologies.” According to the American Wind Energy Association, 42% of new energy capacity in the U.S. was from wind.

Chu said DOE made “historic progress” in cleaning up nuclear contamination from the Cold War, reducing the total footprint by nearly 75% and permanently cleaning up 690 sq miles of contaminated land. But he also acknowledged that "the environmental cleanup projects still have considerable technical and project management challenges."