Portland Just Installed Water Pipes That Generate Electricity!

[muriel_volestrangler]

12. The articles don't explain well; it's about using excess pressure when the system has it

The total power produced is linked to flow of water and the
excess head pressure, or the vertical distance through which
the water has fallen. The quantity of the water flowing per
second, or the flow rate, and the excess head pressure is
computed in order to determine the amount of available
energy that can be captured.
...
Pressure Reducing Valve (PRV) - PRVs are commonly used
in water systems to reduce the pressure of water flowing
between zones of the water system, and to reduce pressure to
a level appropriate for use by water system customers. Tens
of thousands of PRV valve applications are estimated to be
operating in the United States. PRVs are an excellent indicator
of significant pressure in a system that can benefit from the
LucidPipe Power System. We want to deploy our systems in
a PRV zone, but upstream of the PRV so to convert the excess
pressure to energy before it gets dissipated by the PRV. This
will also reduce wear and tear on expensive PRVs in the water
system.

http://www.lucidenergy.com/wp-content/uploads/2013/08/Glossary_july2013.pdf

I suppose water systems have to be designed to handle peak conditions, and that can mean that at other times there can be excess pressure. And that excess pressure must come from the physical height of a reservoir feeding the system, rather than a pump, for this to be worth it. It may be that they have to pump water into the reservoir at times; but this enables them to recover some of the energy they'd otherwise have to dissipate.