[peppercorn (OP)]

Storing potential energy by compressing a gas is easy to visualise, but obviously most liquids and some solids have some amount of squashiness...

How do the three states of material act differently?

For each state:

• Is there a maximum rate compression can occur and conversely a 'springing-back' rate?
• What governs whether a material heats up when compressed & are there materials that don't?

Also, could a specially created solid potentially store more energy under compression than a gas?

Thanks.

[lyner]

Stored energy can be calculated in terms of pressure times volume change. With gases there is a huge volume change so they are best suited to energy storage.
For example - a diver's air bottle, when full of air is a real potential 'bomb'. The air inside it has been compressed by a compressor of, perhaps 2kW, running for half an hour to bring it to 100 Atmospheres. If it were to burst, it would take out the back of your car with all that stored energy.
Fill it with water (to pressure test it) and a single stroke of the handle of the (water) pump on the pressure testing apparatus will bring the pressure to 200Ats because the water is relatively 'incompressible'. If it burst under these conditions there would just be a cracking noise and the excess pressure would instantly fall   to  zero and it would be a non-event because of the small amount of energy stored.
If you want to store energy using a solid, you don't compress it, you bend / twist it as in a spring. You can then get a large force times distance (=work = energy). A clock spring isn't a bad way of storing energy but , for a  vehicle, a bit limited on quantity of energy stored..

[graham.d]

To be really, really pedantic, a solid has the maximum potential energy because it has the greatest density and E=mc2. OK, OK, I know that's not what you were getting at :-)

[peppercorn (OP)]

If you want to store energy using a solid, you don't compress it, you bend/twist it as in a spring. You can then get a large force times distance (=work = energy).

Could a solid be 'designed' (or a material chosen) so that under compression, say in one plane (as with a crystal) energy could be stored by compressing the lattice?

Some materials DO have very strong bonds in one plane and week ones in others - eg. graphite.

[syhprum]

Although gases because if their inherent compressability are a useful way of storing a small amount of energy the number of joules per Kg is very restricted by the mass of the container.
A solid body in the form of a flywheel is very much more efficient and now that F1 cars will be required to have some form of kinetic energy recovery system next year that is the preferred option.
When one stores energy in a spring by bending one is in effect partly compressing one side and stretching the other, this is simply a convenient way of applying these distortions and is no different in principle from compressing a gas. 

[shmengie]

Amazing how energy can be stored and used.

http://en.wikipedia.org/wiki/Air_car [nofollow]
http://www.popularmechanics.com/automotive/new_cars/4251491.html [nofollow]

[lyner]

When one stores energy in a spring by bending one is in effect partly compressing one side and stretching the other,

But the process does not alter the volume of material; it distorts it but it takes up the same volume by flowing, elastically. This isn't the same as when a gas is compressed.
Air struts are a useful application of a compressed gas storing energy. About the only one I can think of. By having the gas 'pre-compressed' you can get any  stiffness you want from the same basic strut design you would need a range of steel springs to achieve the same thing.

[peppercorn (OP)]

Could a solid be 'designed' (or a material chosen) so that under compression, say in one plane (as with a crystal) energy could be stored by compressing the lattice?

[lyner]

I have a feeling that using a solid to store what is sometimes referred to as 'strain' energy in a solid is inherently inefficient because of the large number of bonds which are involved. Effectively , you have a lot of internal friction as the bulk material moves and this will result in energy loss as the solid 'flows'. For instance, rubber is often used in vehicle suspensions because of its damping  (energy absorbing) properties.
I also think that a solid has far too high a modulus of compression (it needs huge force / pressure to make its volume change). If you want to squeeze it you need an even stronger material for the machine it is squeezed with.
There may be some fancy materials with a much lower modulus than normal solids but they will, I'm sure, need to be based on polymers and not a simple (crystal) structure. Most chemical bonds operate over very short distances and, when stretched  too far, they just break. That results in either a fracture or inelastic deformation.

[peppercorn (OP)]

Effectively, you have a lot of internal friction as the bulk material moves and this will result in energy loss as the solid 'flows'. For instance, rubber is often used in vehicle suspensions because of its damping  (energy absorbing) properties.

Thanks.
Does that mean that your car's rubber dampers will heat up if you wizz round a lot of S-bends?

[syhprum]

Yet paradoxically the most common form of energy storage device that has been in use for at least 400 years the watch spring uses the distortion of a solid materiel.

[lyner]

Does that mean that your car's rubber dampers will heat up if you wizz round a lot of S-bends?

If you did 10miles on bumpy roads they might get a bit warm.