Last week I discussed flow temperature sensing as a way to balance the flow in multi-bank arrays. Another concept that has been used to great effect is parallel reverse return piping.
You need to balance the flow in multi-bank arrays because water will choose the path of least resistance. What that means is if you have two banks of collectors supplied by a single pump the water will tend to flow through the piping path that provides the least resistance. In multi-bank arrays this means that water will flow through the shortest path. To insure that each bank has similar flow path you should use parallel-reverse return piping. Parallel-reverse return piping insures that each bank has the same overall flow length (and therefore piping resistance). Among the different principles for balancing flow this method generally involves the most cost (in copper and additional installation) but it does provide a robust solution that is not subject to a system owner changing the balance by adjusting ball valves or balancing valves.