Hi, Aranon,

First off, I'm not sure there is any advantage in operating the panels at low speed. I think you would have to do some engineering calculations to decide how you get the most bang for your buck. My thought would be that on a really solar intense day, hi speed would be best and then, maybe on a cloudy to partly cloudy day it may be more efficient on low. That would be noticeably above my ability to calculate that but the slide rule dudes can do it.

Now, to your question. The short answer is that no matter how high the column of water is, the energy required to push up that water is equally offset by the column of water pulling down and returning to the pool on the outflow. In essence, you work against gravity going up but then gravity returns the favor by pulling down with an equal force on the descending water. That said, there can be virtually no air in the lines for that to work.

As to why that will not work for you I'm not sure. There is, of course, a little more friction from the increased length of PVC pipe (and, possibly lack of capacity) but the end result would only be a diminished but stable flow. Perhaps others will see this post and have experienced the same problem.