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In this case the working fluid is air rather than water.

In its case the working fluid is water itself, but, like Hydrostor’s system, the pressure head is created by putting the storage vessels underwater—in this case, 100 metres down in Lake Constance, a depth that creates an excess pressure of ten atmospheres.

At that time I was experimenting with a new type of refrigeration system that would use water as a working fluid instead of ozone-destroying CFCs.

The difference is that instead of starting at room temperature the working fluid ends up there, and starts instead at -196°C, the boiling point of nitrogen.

Using the carbon dioxide as a so-called working fluid - used to make the turbine function - it would pass through the system in a loop, to be recycled and used again.