What about having a weight permanently built into the system that lifts an empty bucket? When the bucket reaches the top, it activates a valve that fills the bucket with water. When the weight of the water overcomes the permanent weight it starts falling slowly, powering the fan(s). When it reaches the “bottom”, the water is dumped into a reservoir that benefits the plants. A capture device would need to be designed for both top and bottom stations to allow for complete filling and emptying.
