Entropic energy storage

Energy can be stored not only in bond energy but also in a systems internal order.
That is so long the system is not near 0K. As storage capacity is linearly proportional to temperature.

Simplified image

Charging

Here's a simplified mental image:
One can imagine a bunch of loose chaotic chain molecules with ends attached to two opposing coplanar plates.
This represents an empty uncharged battery.
Now stretching the bundle of chain molecules by pulling apart the two plates
leaves the chain molecules less room and options to wobble and wiggle around.
They become more parallel and more orderly.

The degrees of the freedom of the atoms on the chains get reduced.
The equipartitioning theorem says that (as first approximation) every degree of freedome gets 1/2 kT of energy.
Now the energy from the removed degrees of freedom ads to the remaining ones leacing one with more than 1/2 kT per DOF.
That means to get bac to 1/2kT per DOF the pemperature needs to rise.

Very loosly visually speaking it's kind of like squeezing energy out of the bundle of chain molecules by stretching it out.
Aking to squeezing water from a towel.
This is not heat from friction. In principle (if the heat is kept from dissipating) the stretching work done is fully recuperable.
Otherwise it is not fully but still partially recuperable.

Entropic elastic restoring force

There is an entropic force trying to pull the chain molecule bundle back to shorter lengths and more disorder.
(More DOFs, more microstates, arrow of time, …)

Decharging

Reversely recuperating energy from the entropic foces
letting the etretched bundle of chain molecules unstretch and perform useful work
more DOFs free up and temperature drops.
Like an endoergic chemical reaction. Just that it performs useful work too here.
Again visually speaking: Decharging the entropic battery soaks in heat energy cooling it down.
The towel soaking in water and unwinding in that wild visual analogy.
Though for forces the analogy breaks down.

(wiki-TODO: add a bit of math)

Non chain like storages

(De)Mixing enthalpy …
(wiki-TODO: elaborate)

PROs / advantages

Potentially higher inherent safety.
When the energy storage is damaged by accident, disaster, or malicious intent
rather than exploding, catching fire, or getting very hot
the energy storage does the opposite and gets cold.

CONs / disadvantages

The maximal energy storage density is likely more limited
compared to chemical energy storage solutions.

– Energy capacity is dependent on ambient temerature.
– Single molecular chains might be a good choice but are limited in thermal resillience (and radiadion hardness)

Related

• Entropomechanical converter
• Entropic energy

External links

• Entropic force
• Entropic elasticity of an ideal chain
  math on chain stretching