Activist hedge fund Clinton Group Inc. sent a letter to Boston-based Atlantic Power Corp.'s (AT) - Get Report board of directors on Thursday, calling for the company to restart its sale process, which it had pulled last month.

The news sent the company's stock price soaring: Atlantic Power closed up 22.39% on Thursday at $2.46 per share.

Clinton Group's letter said, "The Board of Directors should immediately charge Goldman and Greenhill to re-engage with the potential buyers and solicit the best available deal. Given the strength of the auction as we understand it, we believe there is a deal to be achieved at prices above $4.00 per share, or over a 100% premium to yesterday's stock price."

An industry source said it is his understanding that management is not willing to sell the company for $4 per share, as it believes the company is valued at a minimum of $7 or $8 per share. The management team is not realistic about its stock price, as it would like to see the stock trading at $13 to $14 per share, this person said.

The source said he would have expected to see some sort of asset sale announced at this point. "It is a highly leveraged company ... and it has a complicated portfolio with some assets not as appealing as others." The company has $2.1 billion in debt obligations.

The industry source said the renewable portfolio is highly attractive and of great interest to many of the yieldcos, which mimic the economics of an MLP.

"A sale of the wind assets would probably soothe many of the investors as the company would get a good valuation for them and it could use the proceeds from the sale to help pay down the debt," that person said.

There is indeed a great deal of interest in the renewables piece, according to the source. "Atlantic Power's phone has been ringing off the hook for these assets."

Atlantic Power's renewable portfolio represents 20% to 25% of its Ebitda guidance for this year, which ranges from $280 million to $305 million, an industry analyst previously said.

NRG Yield Inc. (NYLD) and Brookfield Renewable Energy Partners LP (BEP) - Get Report were previously cited as potential acquirers.

An industry analyst said there is also interest in some of the company's U.S. natural gas assets.

On Sept. 16, when the company said it would remain independent, it also said it was still retaining Goldman, Sachs & Co. and Greenhill & Co., to continue to advise it on possible asset sales or joint ventures. At that point, an industry source told The Deal, an activist might make an entrance, noting that Atlantic Power was "enormously underpriced." Its stock price had fallen hard — 34% — over the past three months. The Deal subsequently put Atlantic Power in the No. 1 slot on its top 10 list of potential activist targets.

The company owns and operates a fleet of power generation assets in the U.S. and Canada, and has a 1,117 megawatt renewable generation portfolio. It has 2,945 megawatts of generation capacity with aggregate ownership interest in about 2,024 megawatts. Its power generation projects span 11 states and two provinces.

Atlantic Power could not be immediately reached for comment.