The Iowa Utilities Board on Tuesday approved MidAmerican Energy Co.’s $922 million Wind XII project, which will add 591 megawatts and get the company closer to its goal of 100 percent renewable energy. The ratemaking case calls for a cost cap of $1.56 million per megawatt, including allowance for funds used during construction. The return on common equity will be 11 percent. MidAmerican has said that in 2021 it will generate enough renewable energy to cover all of its customers’ annual energy needs.