The wind-energy industry said an opinion filed by Minnesota pollution-control regulators defining wind-turbine noise will stifle its growth.

The Minnesota Pollution Control Agency (MPCA) said the state's limit for wind-farm noise applies not only to sounds from turbines but also should include background noise such as road traffic, said the filing with the Minnesota Public Utilities Commission (PUC).

The filing is part of a contentious permitting case for a proposed $300 million project southeast of Albert Lea, Minn., called the Freeborn Wind Farm, which would include 42 turbines in Freeborn County and another 82 turbines across the state border in Worth County, Iowa.

But both critics and proponents of Freeborn Wind say the MPCA's declaration would apply to the entire wind industry in Minnesota, which ranks seventh in the U.S. in wind-power generation.

The MPCA acknowledged in its filing that the PUC — and the Minnesota Department of Commerce — may have indeed interpreted the rule to mean only turbine noise.

"Nevertheless, the MPCA has historically, and consistently, interpreted and applied … noise standards for total sound," the filing said.

The wind industry has argued that the noise standard applies to turbine noise only, and it said that's the way the PUC has interpreted the rule.

"It's not logical, and there is no scientific basis for [the MPCA's opinion]. How can a developer control the noise of trees, the breeze and traffic?" said Beth Soholt, head of St. Paul-based Wind on the Wires, a group that represents wind-power developers and renewable energy advocates.

But Carol Overland, an attorney representing opponents of Freeborn Wind, questioned why it has taken so long for the MPCA rule to come into play.

"This has a really big effect, and it's an indication of a systemic problem," she said. "It weighs very heavily on rule-making."

While the PUC has been permitting wind farms since 2005, it has only been during the past few years that complaints about turbine noise have reached a level requiring the commission's intervention. The PUC is scheduled to vote on the Freeborn project next week.

Some Freeborn County residents fear the wind farm would cause both noise and visual pollution as well as other possible problems. Their opposition has sparked a "contested case" before the PUC, a rarity for wind-farm proposals in Minnesota.

Freeborn Wind is being developed by Chicago-based Invenergy, a large U.S. wind-power developer, and it would supply electricity to Xcel Energy. Dan Litchfield, an Invenergy senior manager, said the MPCA's opinion contradicts the company's understanding of state noise rules.

"We looked very closely at the case history before investing [in Freeborn Wind], and we never saw it done this way," he said.

In May, a state administrative law judge, LauraSue Schlatter, recommended that the PUC deny a permit to Freeborn Wind, saying the project failed to meet state noise standards. Administrative law judges are appointed to examine contested cases; their recommendations are nonbinding but can carry significant weight with the PUC.

Schlatter concluded that the noise standard applies to background noise combined with wind-turbine noise, citing the MPCA.

Two wind-industry trade groups, several wind-farm developers and four clean- energy and environmental groups all filed objections with the PUC over Schlatter's recommendation.

The Commerce Department, which represents the public before the PUC, has tried to stake out what it's called a "middle ground" on the issue, which the MPCA said it supports. In a June PUC filing, the Commerce Department proposed allowing a 1-decibel sound increase at night due to turbine-only noise, an amount that it called "insignificant" to the human ear.