Le Châtelier Cell

[svetz]

Le Châtelier's Principle states that the “favorability” of a reversible reaction is based on things like temperature. That is at some temperature a reversible reaction will proceed backwards (all rechargeable batteries use reversible reactions).

So, why not two cells, one hot and one cold? In one cell the reaction proceeds “forwards”, in the other cell it proceeds “backwards”. When they reach equilibrium for their temperatures (e.g., no more power available), you rotate the ambient cell into the hot area and vice-versa. Now the reaction will proceed again providing more power.

That is you never “charge” the battery, just switch which is "hot".

Ideally, the hot side could be solar supplied and the cold side ambient. Not sure if it’s possible, but ideally the “hot” cell reaction is exothermic – that is once you get the cell heated up you don’t have to put more energy into it.

Never heard of anything like this, but thought I’d toss it out there for discussion.

 

[svetz]

Found this [ref]:

When we increase the temperature, the reaction will favour whichever reaction is endothermic to take in heat and reduce the temperature. On the other hand, if we decrease the temperature, the exothermic reaction will be favoured, as this will give out heat and increase the temperature.

So, that's probably why we don't see this. Still, might be a novel way to use low-quality heat (aka low-temperature waste heat, e.g., car exhaust) if you could find a suitable chemistry.

Update: The quote above is probably a generality and not a rule... just occurred to me that run-away reactions take place because the reaction proceeds more favorably at a higher (or low) temperature.

 

[svetz]

PV Panels are 20% efficient. Solar thermal systems are 60 to 70% efficient. Even if the Le Châtelier Cell was only 50% efficient, that would be a net efficiency of 30%. Plus you could store excess heat as a power source to the "battery". Have to do some more research on this, someone must have thought of this already.

 

[danielfp248]

Nothing forbids you thermodynamically from charging an electrochemical system with heat and then discharging it by putting it in cold temperature and reversing the directionality of the reaction.

This requires a reaction that has a positive enthalpy change and a positive entropy change such that the change of free energy of the reaction will be negative at high temperature and positive at a low temperature (so that the reverse reaction has a negative change of free energy at low temperature). The challenge you have would be finding a reaction that has these characteristics, can be put into an electrochemical system and has an entropy change that allows this flip to happen somewhere in the 50-100C range.

As far as I can see, nothing in the laws of nature prevents such a system from existing, but finding one that fits the bill would be the key challenge.

 

[svetz]

Ran across Thermogalvanic Cells, but it's not the same thing. They don't swap the cells and are making use of a very small voltage difference in the electrodes from delta T. As such, they have very poor efficiencies.

Thermogalvanic cells (or thermocells) are being increasingly investigated due to their ability to directly convert modest temperature gradients into electricity, using redox chemistry.

Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest - Nature Communications

Achieving high thermopower in liquid-state thermogalvanic cells is vital to realize a low-cost technology solution for thermal-to-electrical energy conversion. Here, the authors present aqueous thermogalvanic cells based on modified electrolyte with enhanced Seebeck coefficient and thermopower.

www.nature.com

Sci-Hub | Thermogalvanic cells: A side-by-side comparison of measurement methods. Journal of Electroanalytical Chemistry, 872, 114280 | 10.1016/j.jelechem.2020.114280

Using electrochemical reactions as an electrochemical battery does should yield higher power and better efficiency (if you can find the right reactants). So, still looking, someone must have researched these.

 

[cass3825]

Peltier (sp?) junctions may be of interest…

To elaborate, applying a temperature differential to a peltier cooling plate will generate a voltage on the “input” wiring.

 

[svetz]

Peltier (sp?) junctions may be of interest…

It's not something I'm trying to do per se, it just occurred to me the technique could be used to generate electricity and it seemed like it should be pretty efficient with the right highly reversible reaction at the optimum temperature. But when I went looking for it couldn't find it. I doubt I stumbled across anything new, seems like an obvious mechanism. Most likely the first problem is I don't know the real name for it and trying to search for it I'm stymied by the number of false hits. The second problem is there probably isn't much data as finding the "right" highly reversible electrochemical reaction for the temperature range is probably hard.

 

[svetz]

Le Châtelier Heat Pump?​

If the reaction is electrochemical and reversible, then it should also be possible to put energy into it to make one side hot (exothermic) and the other side cold (endothermic).

Yup... just a bit of googling I found "chemical heat pumps". These are like ammonia refrigerant cycles though, using heat rather than electricity as the driver. Still, if the Le Châtelier cell works, then you should be able to make a heat pump out of it too. Probably thought of this triggered by @cass3825's mention of Peltier.

 

[svetz]

Found a couple of interesting side ideas while searching for this:

• Regenerating a battery from waste heat
• Using thermogalvanic effect to get more power out of a battery than electrical power input