Renewable Energy Idea, page-19

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Your idea is basically one of the underlying principles of the Snowy Mts. Hydro Scheme.

The difference is that base load coal powergen elsewhere provides the off peak power to pump water back up to higher dams.

The Scheme generally provides powergen for morning and evening peaks loads.

In Sydney, I supoose a new long, low dam could be constructed below Warragamba Dam to collect water after hydro powergen, to be pumped back up using off peak day time solar/wind from elsewhere in the state.

Cheers
Dex