Generating Electricity from Water in Pipes

On the surface, this idea has merit, and it’s been considered here in the U.S. for at least the last 20 years.  It’s never gone anywhere, because of:

The extreme cost of replacing existing pipe, and

The fact that it’s limited to water that flows downhill because of gravity.  (Obviously, if the water is pumped and you’re trying to extract some of its kinetic energy, you’re simply making the pumps work harder.)