Reply #16: All fluids are compressible, liquids and gases.

[NNadir]

Both liquids and gases are considered fluid states.

Solids are also compressible. The volume of a condensed state of matter changes with pressure according to a more or less constant value known as the bulk modulus. The bulk modulus of most materials is very much a function of their composition. This is different than is the case for gases, where generalized "ideal" states can be visualized as a first approximation. (In fact gases vary considerably under certain circumstances from "ideal behavior" and many state functions for gases have been used, depending on the accuracy required and the conditions applicable, all of which contain constants that are functions of composition. These composition dependent functions are analogous to a "bulk modulus" for gases.)

As a condensed state of matter, liquids and solids resist compression more than gases.

That said, condensed states require considerably more work to compress than do gases. I have a lot of trouble visualizing how this thing might work. In essence though, the behavior of fluids under compression is essentially what underlies the operation of all hydraulic devices. My problem has to do with the mechanical conversion of pressure energy of a condensed state (liquid) to rotary motion like that in wheels. I'm sure though that there are mechanical engineers who understand such things far better than I do.

There is a point for all substances, known as the critical point, at which gases and liquids are indistinguishable. Many modern power plants operate above the critical point of water, which is only present at temperatures above 373C and pressures 22,000 kPa, 220 atmospheres. Thus some "steam" turbines are not really "steam" turbines. They are supercritical water devices. Some critical fluids are accessible at temperatures close to room temperature. Supercritical carbon dioxide, which already enjoys wide technological use, with many other such uses contemplated (including dry cleaning), is accessible at 31C and 73 atmospheres.

I think compressed air - which is already widely used to store (or distribute) energy for powering tools like impact wrenches in auto repair shops and air brakes on trucks - has many properties that recommend it as an energy storage device. One possible technological side product for the wide use of compressed air in large plants is the production of water - albeit in small amounts and, through the use of gas separation membranes, the removal of carbon dioxide. It also has possible application to heating and cooling. Whether compressed air is a wise choice for automobiles, I doubt, but I haven't looked into the problem in any detail.