You're correct. I shouldn't have said "free". I should've said "better than free", or "profitable". Something that generates more money than the costs of operation could also be described as "a cash cow".
From Google –
Yes, on average, a wind turbine generates more money than it costs to operate, meaning it is considered profitable; most wind turbines can pay for themselves within a few years through the electricity they produce, with the initial investment recouped over time after the construction phase.