Environmental groups wanted Iowa regulators to order a study of MidAmerican’s coal fleet before approving its latest wind investment.

A decision by Iowa utility regulators Tuesday to approve a 591-megawatt wind farm will increase MidAmerican Energy’s wind capacity by an amount roughly similar to the capacity of the typical coal-fired plant.

MidAmerican, however, has no plans to close any of its coal plants, something clean energy advocates had hoped would be addressed as a condition of approving the company’s wind expansion.

“If the fossil fuel fleet is not running less and/or retired, there will not be a reduction in emissions and the benefits that come from that,” attorney Josh Mandelbaum wrote in testimony for the Environmental Law & Policy Center.

The Wind XII project approved by regulators gets MidAmerican closer to its voluntary pledge of generating renewable energy equal to 100 percent of its load. The Iowa Utility Board’s decision came on the same day that Minneapolis-based Xcel Energy announced it will transition to 100 percent carbon-free electricity by 2050, seen as a more ambitious goal by climate groups.

Several critics expressed concern that MidAmerican may be imposing unnecessary costs on customers by continuing to operate its fleet of coal plants, which is among the 20 largest in the nation. Representatives for the Sierra Club and the Iowa Environmental Council urged regulators to require MidAmerican to analyze the economics of the coal plants to ensure they are not wasting customers’ money. The board declined to do so, and said that issue is beyond the scope of the wind farm proposal.

The cost of wind energy has fallen so much, according to Kerri Johannsen, who directs the energy program for the Iowa Environmental Council, that the total cost of wind energy is less than even just the typical operating costs of most coal plants.

“We want to see where MidAmerican’s [coal-fired] plants fall in that spectrum,” she said.

MidAmerican spokeswoman Tina Hoffman wrote in an email that, “While we believe our commitment to renewable energy is beyond question, we are also committed to transitioning our generation fleet to renewable sources in a way that maintains low prices and high reliability for our customers. MidAmerican’s desire to balance all of these interests for our customers means that we will continue to utilize a diverse generation mix — including coal-fueled production.”

Johannsen and other clean energy advocates commend MidAmerican for its enormous wind capacity. MidAmerican has been in the vanguard in Iowa, and nationwide, in developing wind energy. With the addition of Wind XII’s 591 MW, the utility will have a total wind capacity of nearly 5,000 MW. But the company continues to burn coal to produce power that it sells into the wholesale market run by the Midcontinent Independent System Operators.

“If customers are paying more for polluting energy that they don’t want,” Mandelbaum said, “we should address that.”

Regulators did not require MidAmerican to do an analysis of the costs and benefits of coal, but said that could be part of the company’s next rate case.

That’s cold comfort to Elizabeth Katt Reinders, who runs the Sierra Club’s Beyond Coal campaign in Iowa.

“We know full well it will be 12 years before there’s another rate case,” she said.

Rather than wait, Johannsen said MidAmerican should meet with a wide range of interested parties and should share its data and models to develop the most accurate possible picture of the costs and benefits of the company’s coal plants. “It’s only going to be impactful and applicable if everybody is at the table,” she said.

While state regulators and MidAmerican rejected the recommendations made by renewables supporters, Johannsen said, “This is just the beginning of figuring out what we have to do to move MidAmerican to a 100 percent clean energy system.”