Solvation Energy Generator

[Aggie]

Just had a little flash of inspiration (might have just been gas) in regards to Stirling engines.

IIUC, what's needed to run a Stirling engine is essentially a temperature gradient - a hot area and a cold area.  I was wondering whether it would be possible to drive the temperature gradient using two separate solvation chambers (one exothermic, one endothermic).  For example, one could use NH₄NO₃ and NaOH solids, and dissolve them in water.   Provided that a minimum of water was used, one could regenerate the 'fuel' by evaporation, which should be generally possible at ambient temperatures or using passive solar heating.

The biggest barrier that I see is that enthalpies of solution are much less than enthalpies of combustion (also the molar mass of the compounds involved could easily be larger than most HC fuels), so a heck of a lot of solute would be necessary (it's probably not suitable for vehicle use, but could work for energy generation).  One may need to engineer nontoxic compounds showing very large (+ and -) enthalpies of solution - not an easy task.  If that barrier could be overcome, you'd actually have an engine that could 'run on water' and regenerate via evaporation.


One could stand in any significantly exo/endothermic and easily reversible chemical reactions - not sure what else would work.

[Swatopluk]

The NaOH solution method has already been investigated on a large scale. In WW1 there were steam engines for submarines in development that used that principle (Natron-Dampf-Uboot). While on the surface a conventional boiler would up-concentrate the NaOH solution through evaporation. When the submarine dived the water would be fed back and the dilution heat used to produce steam for the engine.
The process was dropped in the end for a number of reasons. The efficiency was not very good, the extinction of the firebox took too long (even in WW1 diving preparations of 15 min were not tolerable) and the whole system produced so much waste heat that the conditions were simply intolerable for the crew.

[Aggie]

Given that we've seen a few technological advances in insulating materials etc., and that this would drive the engine directly rather than be used to convert to steam, it might be worth re-examining (for land-based stationary applications - remote power generation for example).   Glad to hear that it's actually been tested to some degree.

As much as I do like having some NaOH around the house for cleaning and clearing, a less corrosive alternative would be nice.  NH₄NO₃ is somewhat less scary to the terrestrial environment (bad in groundwater and surface water), although can be explosive.

BTW, will adding excess water to a saturated NaOH solution produce heat as well?  I was (probably mistakenly) assuming that one would need to bring it back to solids - conc. sol'ns would be much better.

[Swatopluk]

The original idea

1885
American JOSIAH H. L. TUCK demonstrated "Peacemaker" – powered by a chemical (fireless) boiler; 1500 pounds of caustic soda provided five hours endurance. Tuck's inventing days ended when relatives – noting that he had squandered most of a significant fortune – had him committed to an asylum for the insane.

worked with solid caustic soda, the German test in WW1 with concentrated solutions.

Why didn't they use sulphuric acid? That is really effective (as any first grade chemistry student knows through experience).

[Aggie]

True.   

I don't have access (well, couldn't find online and doubt the public library will suffice) to a decent table of enthalpy of solution to look for other option.

Also not sure whether there are water-soluble organics that would work on the order that the inorganic compounds do.  If so, then it's probably possible to engineer even better ones.

[Swatopluk]

I think that is a definite No (organic polysulphates are thermically unstable, aromatic sulphates are in equilibrium with the pure aromate and the free sulphuric acid).

[Aggie]

How about pure caffeine?

[Swatopluk]

The mad squirrels would use too much oxygen in the sealed container

[Aggie]

Reconsidering this, if one could incorporate a solar-powered concentrator/dehydrator to the design such that the solution would be concentrated during the daytime, and the removed water could be fed back in during the night, one may be able to have a single unit that continually runs and regenerates daily (assumes a constant strong solar source in the day).   May be trickier in the endothermic chamber, as the point is to keep it cold (wouldn't want direct heat applied to boil it off).   Would also need a vastly larger volume of conc sol'n than is actually in the chamber so that the solar regeneration was being carried out on a large scale. 

But I suppose this would essentially leave us with a lot of chemicals to accomplish nothing more than energy storage - direct solar influence on the hot side of the Stirling may be more efficient than solar regeneration.  May have some merit more WRT using an endothermic rxn to cool the cold side, but again perhaps there is a more efficient way of cooling.

[Griffin NoName]

 

......and the whole system produced so much waste heat that the conditions were simply intolerable for the crew.

We could sell the waste heat. Just need a delivery mechanism to the places it's needed.

[Sibling Chatty]

but again perhaps there is a more efficient way of cooling.

Would geothermal cooling work in this instance?

Guess it depends on location and size...

[Aggie]

Yes, I think so.  Depends on the temperature of the aquifer (if using groundwater), but most around these parts are 4*C or thereabouts.  Ammonium nitrate can go significantly colder, though, I think.

[Swatopluk]

But that would withdraw necessary raw materials from improvised opportunity device building

[Aggie]

No ANFO for you, Swato!

Ammonium nitrate is available quite cheap in large quantities I think.

[Swatopluk]

It think "was" is the right term here. Since it has been prominently used for homemade bombs (militias, WTC I), its trade has been somewhat restrivted, i.e. if you are not a farmer or a certified pyrotechnician, trying to buy large quantities will draw the attention of the authorities.

[Aggie]

It think "was" is the right term here. Since it has been prominently used for homemade bombs (militias, WTC I), its trade has been somewhat restrivted, i.e. if you are not a farmer or a certified pyrotechnician, trying to buy large quantities will draw the attention of the authorities.

In this case, that may not be a problem (would anticipate that one could apply for proper use permits), and in any case I know enough farmers...   

It used to be available in 50-lb sacks quite readily - we used it as a de-icing compound as it's better for the surrounding vegetation than plain salt.  That was pre-911, though.

[Swatopluk]

The old ambiguity of Rasensprengen*

*which usually means sprinkling the meadow but can also be translated as blowing the meadow up