Here is a web site that has lots of info about energy storage systems - it has a section on pumped storage. It also covers some large battery systems and flywheel energy storage.
I knew about Dinorwic in Wales since I visited it in 1985. They were very proud of their ability to startup quickly (I think they said 12 seconds) to meet the peaks in demand.
This is just an energy storage system - it buys cheap off-peak power from the grid and then sells it back to them at peak times. I guess if it rains into the reservoir they get a little bonus. Because of this model of operation the efficiency with which they pump water up and generate from the water coming back is they key to their profitability.
The peaks in the UK grid demand are more clearly defined in the US. For one thing, there is only one time zone - everybody does everything at more or less the same time. When I visited, there were only 3 channels of TV and a large majority watched the one commercial channel. The most popular show was a soap called Coronation Street. When the first add break of the show came a huge number of people went into their kitchens and turned on their electric kettles for a cup of tea. I can't remember the figures, but it was by far the biggest peak they ever recorded. They could judge the viewing figures for the show by the size of the peak.
Even with its huge size Dinorwic is smaller than the Guangzhu pumped storage system (China). At 2400 MW it doesnt qualify as a microhydro project at all.
It would be interesting to see what the most cost efficient solution would be at much smaller scales. If you have to construct reservoirs (rather than having access to two natural ones) then it may be more cost effective to go with batteries - Alternatively, you may put the value of not using batteries above the cost of the pumped hydro system.
Bear in mind that you wont be able to completely drain the top reservoir. There will be mud, grit and possibly fish. What kind of size was being proposed for this? A lake or a pond or a tank?
One thought. Even if you don't live in the mountains, a disused mine shaft may give you enough head for this kind of system.
And then a follow on thought. If using a mine shaft it would be more convenient to have the generator at the top. Is this possible? Could a turbine work with the water sucking from behind, rather than being driven into it?
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