[Bikers4Bush]

It's not perpetual motion, you have maintenance that needs to be done a pump that needs to be kept running while in operation etc.

Would you consider a large dam to be perpetual motion?

[eraser2005]

A large dam does not include the pumping of water back up to the level above the dam. Dams produce energy by capturing the potential from falling water. There is NO pumping it back up.

[PA Engineer]

"They line the shafts with smaller turbines and then run water down the sides generating power from top to bottom using gravity. They then use some of the electricity they create to pump the water back up continuing the process."

This is a perpetual motion machine description. I assumed you first description was that of a peak shaving plant that evens out the load during morning and evening hours.

[-YYZ-]

"Would you consider a large dam to be perpetual motion?"

Normal hydroelectric plants don't involve pumping water back up into the resevoir (although this can be done as a means of storing excess electrical power from other sources for a later time). The water does make its way back up into the resevoir eventually, thanks to the input of solar energy that drives the precipitation cycle (you know, solar heat causes evaporation which turns into rain in the mountains that ends up in the resevoir again). Technically I guess you could call hydroelectric solar power, but then almost every form of energy on earth does originally derive from solar energy, if the "fossil" fuel model is correct. Nuclear and geothermal energy are exceptions.

[Myrddin]

Would you consider a large dam to be perpetual motion?

The energy for a large dam comes from sunlight evaporating water. Clouds rain down precipitation at a higher elevation (gravitational potential). The water flows downhill or down though a water turbine/electric generator facility. It's not perpertual motion. Just a transformation of solar energy.

[SengirV]

A large dam generally doesn't have to supply it's own water to dam.

A large damn can't operate in a closed system. What you described soundeds like it could easily be made into a closed system and supply more energy than it consumed.

[ModelBreaker]

Would you consider a large dam to be perpetual motion?

No. The sun has evaporated the water from the oceans and thereby raised it to a higher level. The sun is the external source of energy that gets stored as gravitational potential energy in the water at a high elevation.

When the water falls (once) thru the turbines, the energy the sun stored in the water is released and we have a working hydroelectric dam. So hydroelectric energy is really indirect solar energy.