Dreams or visions about a windmill tell you that your heart is filled with life force, energy and dynamics. You want to make progress and you make personal progress. Another explanation is, that you can ask yourself whether you have focused your energy, life force on the right project, wish or idea and that you are not going too fast.
The mill reflects the work you do to attract abundance into your life, how this is experienced depends on your situation and the energy you put into something. It also reflects your heart, your home, your source, your basis and how you feel and / or how you live life.
The question, however, is how hard (heart) do you work to make situations, relationships, work and so on successful. It's really about the balance question and the awareness of the fact that. An alternative explanation is that you can learn to sway with your natural energy rhythm, the wind, the outside influences.
If the windmill in your dream or vision is not turning fast or is it standing still, it tells you that your heart, your true self is without life force, energy and dynamics. You are listless, you have no capacity to do anything, you cannot get your true heart active anymore. This is really about the balance question and the realization that a quiet period is not a bad idea.
A modern wind turbine obviously has the same explanation as an old-fashioned windmill.