Christine Stuart file photo
DEEP Commissioner Daniel C. Esty (Christine Stuart file photo)

Department of Energy and Environmental Protection Commissioner Daniel C. Esty announced Wednesday that he will resign Feb. 3 and return to teaching at Yale University.

Esty, one of Gov. Dannel P. Malloy’s first appointments, was the first person to lead the blended environmental and energy regulatory agency. His sabbatical leave from the university was scheduled to expire this year.

“He’s been an unbelievable gift to the state from Yale. To release one of their most quoted faculty members to us for the period of time that they shared him with us was really quite fantastic,” Malloy said Wednesday at an unrelated event.

Malloy said he wrote to the university, which agreed to allow Esty to stay on as commissioner for a longer period than what had originally been agreed upon.

“They were kind enough to extend his leave without terminating his relationship there. It’s time for him to go back. I can assure you I was not unprepared,” he said.

Malloy said he had a replacement in mind and would announce that person before Esty departed.

A well-regarded author and academic before taking the state job, Esty’s main belief was that controlling pollution and managing environmental resources were critical to marketplace success. As a commissioner he got to implement those policies — sometimes to the dismay of environmentalists, who initially championed his appointment.

From changing the state’s mix of renewable energy sources to expanding natural gas lines, both Malloy and Esty believe the state is in a better position then it was when it comes to how it handles energy policy.

In the process, Esty and Malloy engaged in some heated debates with environmentalists and other New England governors who largely opposed how they changed the state’s renewable portfolio standard.

The bill changing the renewable standards passed the House 112-33 and the Senate 26-6. Debate on the bill in the House was delayed after Esty’s conference call with UBS investors, which included a dialogue about the bill.

Concerned that Esty gave away information lawmakers didn’t even have at that point, the House delayed debate and environmentalists attempted to change the bill to exclude large-scale Canadian hydropower. Those attempts failed.

Esty apologized in April 2013 for agreeing to take part in the call with investors who hold an interest in Northeast Utilities.

Malloy defended Esty in the wake of that dust up.

“I read the entirety of the transcript,” Malloy said in April. “Everything he said there he’s said elsewhere. Almost everything he said I’ve said publicly time and time and time again.”

In his two-page resignation letter, Esty cited the state’s approach to energy as one of his accomplishments: “Delivering the state’s first Comprehensive Energy Strategy, putting Connecticut on a path toward a cheaper, cleaner, and more reliable energy future.”

Esty’s departure is not a surprise since his sabbatical leave was expected to end this summer. But the early departure signals that his wife, U.S. Rep. Elizabeth Esty, is getting ready to campaign for a second term.

Rep. Esty has been criticized by her opponents for raising money from companies her husband regulates. Throughout the 2012 campaign, Rep. Esty said the donations from Northeast Utility executives had nothing to do with her husband.

“I’m not going to say I’m not going to accept support from people who’ve known me for a long time and support me and my policies,” she said during an interview with WNPR’s John Dankosky. “I’m not my husband. I make my own decisions. I always have. Welcome to the 21st century.”

The National Republican Congressional Committee called on Esty to give back the money. Esty did. She gave back about $3,500 in campaign donations from NU executives this past April after her husband’s call with UBS investors.

Hugh McQuaid contributed to this report.