I wonder if, instead of dumping excess power into heaters (http://ludens.cl/Electron/picelc/picelc.html), it would ever make sense for this system to pump water back to the top of the system. It might not, if the water flow is short-term stable and only varies across seasons.
It doesn't look like they have much storage at the top of the system. An easier approach might be to dynamically control the flow through the system, restricting it during low load and returning more water to the stream as 'environmental flows' to keep it healthy.
