Electronics > Power/Renewable Energy/EV's

Caustic soda chemical heat storage.



First of all, the basis of this idea or not electronic related. I know there are plenty of forum about renewable energy but I find there are way too many people in them who don't have a clue about thermodynamics or conservation of matter/energy.

The idea is that sodium hydroxide, when mixed with water releases heat, a substantial amount of it. In theory, you can evaporate the water and combine them again an infinite amount of time.

That has been used in what was called soda locomotives. Where two tanks, one containing water heated up to steam and the other containing caustic soda were thermally coupled. The steam, after doing some work was condensed and added to the caustic soda to release heat, keeping the steam coming.

Though that part is relatively well documented, I cannot find anything about how they were regenerating the sodium hydroxide.

I have been playing with the idea of applying this to heat storage. From summer to winter of even from day to night.

What I picture is a closed loop system, similar to an ammonia fridge, with one tank containing the hydroxyde solution and the other containing the water, when heat is available (solar heat, excess solar electricity, air conditioner's heat exhaust, wood furnace) the water would be evaporated from the solution and condensed on the other side.

And then, I see two practical options (or a combination of both):

The "simplest" would be that when heat is needed, water would be pumped from the water tank to the solution tank as needed while the heat is pumped out.

The more complicated, much harder to manage would be to transfer the concentrated solution to a water boiler to generate some steam.

Does anyone has any experience or literature about anything related to this?

Mainly the regeneration process, how to stop the sodium hydroxide from producing aerosols and contaminate the water tank?

What material are suitable for a hot and caustic environment?

What are the options in case of catastrophic failure of the system? You can obviously not douse it with water.

Are there any legal issues regarding the storage of sodium hydroxide in relatively big quantities? In Australia, just in case you wondered.
(say 1 to 5 tonnes, which would represent a capacity of storing up to, respectively, 0.3 to 1.5 MWh in chemical energy if I didn't stuff up my math, that's not accounting for thermal energy storage)

The way they regenerated that was simply to heat up the now cooler sodium hydroxide till the water boils off, and then heat the now dry NaOH till all the water in the crystal structure is driven off as well, making for some very dry NaOH powder that you can then use again. Energy efficiency is about zero, or close to it, but was used in places like coal mines before electric motors were available in a sparkproof form, or hydraulic systems were available, or compressed air was something you could supply from the surface, to drive a technology the Victorians were familiar with, low pressure steam locomotives, that were used to replace pit ponies in mines.

Yes you get steam at low temperature out, and very nasty corrosive wet steam as well, which pitted every part quite fast, and which meant all parts were malleable cast iron or thick steel, and nothing in there was made of a copper containing alloy at all. Only used because they could not have a coal fired steam engine because of the explosion danger, and very quickly replaced with DC battery technology and sealed control systems with flameproof DC motors which had a very short operational life, but which still were a lot easier to service than the soda locomotives.

For driving the water out one does not only need a heat source, but also a heat sink to condense the water back to liquid form. The heat sink can be a little smaller, but there is still quite some heat get rid off. There is a certain temperature difference needed - so the waste heat from AC is not an option at all. The boiling point of the concentrated soda solution is quite a bit higher than that of pure water. The system pressure can to a certain degree set the temperature.

Stopping aerosols should be relatively easy, just like in normal distillation. If the water is used in liquid form to get the heat out (not just as vapor), the system might also tolerate quite some contamination. So I don't think this would be a real issue.

As long as the soda is separated from to much water, there is not much catastrophic failure. I would guess the main issue would be to much steam pressure. So it needs a save way to release the steam with possible contamination with caustic aerosols.

Heat, yes, but not a lot of heat, it's hardly capable of boiling water...

Melting the stuff would be much more effective, and has the bonus of not needing to fumble with mixed phases and crystal growth.

(AFAIK, nitrate blends tend to be used for thermal storage.  I don't know the heat capacities of all the salts in consideration.)



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