I want to post the graphic that I showed at Maker Faire, that explains my working concept.
On the right-hand part of the picture, the image shows a full reservoir below the two blue parallel containment walls (heheh...sounds 'nuclear'!)  with the leaf spring stack (yellow) arranged in a circular form.  These springs are sheets of fiberglass.  I am thinking, that perhaps steel (being so cheap with a worldwide glut) might be a significant add-in as a leaf spring.

As the hydraulic fluid (red) is pumped into the bladder (not visible, too thin for this drawing) by a hydraulic pump (powered by electricity from renewable sources) the bladder assumes the least possible geometric shape, which is a circle, and pushes the containment walls into a circle.  Because the containment walls are attached at their uppermost end to the leaf spring stack, the deformation of the containment walls into a circle draws down the leaf spring stack into a flat oval (left image).  Energy is stored in the deformed spring stack, rather than by any elastomeric substance (e.g. rubber) or compressing a gas (which is typically the case in most hydraulic accumulators).

To release energy stored in the spring stack deformation, hydraulic fluid is released from the bladder, and is directed through a hydraulic motor/440v AC generator combination, and back into the no-pressure reservoir.