Desiccants for Thermal Storage

I mentioned the following in another post, but it seems this deserves a thread of its own as it's interesting. Desiccants like silica gel or zeolite can and are being used for heating (and cooling) applications. When these substances adsorb water vapor, then the water condenses onto the internal porous structure of the material to release latent heat. This increases the temperature of the desiccant. In one pilot application the system sent humidified air through a large bed of zeolite. This heated the zeolite and the air, and this high temperature air was used for space heating (indirectly using a heat exchanger). Interestingly, the temperature of the air leaving the bed was well over 200F.

(Brief discussion) http://www.techthefuture.com/energy/zeolite-stores-thermal-energy-for-unlimited-amount-of-time/
(A more thorough discussion, and an existing product) http://www.vaillant-export.com/homeowners/renewable-energy/development/

The desiccant is recharged by heating. Once dry, the desiccant will effectively store energy indefinitely, and the desiccant can tolerate an indefinite number of cycles that is virtually unlimited. Solar heat has been used to regenerate the desiccant. For example, in one test silica gel was shown to be efficiently regenerated with solar heated air between 160F-180F. Solar seems an ideal heat source for its intermittent nature. Another application can be to harvest waste heat from micro scale power generation to regenerate the desiccant. In that case, the steam exhausted from the desiccant during regeneration can be captured in water heating. For example (just to consider one possibility), a small heat engine might be operated for a few hours daily for battery charging and with the heat from the engine used for desiccant regeneration. The heat exhausted from the desiccant is used for water heating, and the desiccant is drawn upon for space heating (or space cooling). It seems this could be an efficient means to make use of a limited fuel source (one example might be wood used in a wood gas engine system).

SPACE COOLING: These desiccants can also be used in cooling applications by drying air sufficiently to allow evaporative cooling to be effective. They are currently used in systems called "desiccant wheels" or "enthalpy wheels".

I would investigate silica gel before incorporating it into any permies project.
I understand that some of it has been banned by the European Union as a known carcinogen.
I don't know if the ban is just limited to use in food storage, or if it is a general ban.
I do know that a major seed bank in Europe has had to replace all of theirs with other, safer products.

The culprit is the cobalt chloride, used as an indicator.
If your silica gel turns from blue to pink when hydrated, it is the banned type.
Heating it may exaggerate the problem.

See: http://www.seedcontainers.net/a_guide_to_long-term_seed_preservation.html

The colour usually shown by silica gel is due to an indicator, added to see directly when it is dehydrated and when it has absorbed moisture. For many years, cobalt chloride (Cl2Co) has been used. This substance gives the dehydrated gel a strong blue colour and a pale pink colour to the gel having absorbed moisture. Recently, the European Union banned its use because of considering it carcinogen through inhalation. A search for new alternatives led to some iron salts, where the change in colour can be poorly distinguished. At present, the most advisable alternative is methyl violet, which gives the dehydrated gel an orange colour and a green colour to the hydrated gel.

EDITED to add attached picture of the two types of silica gel.

I use Activated Alumina as a gravel media for ring culture mainly for its water retention purposes but can be used for frost protection on seedlings with an early morning mist activating the heating of the alumina.

An interesting use for this principle has been done in the past where steam engines were operated in a system often called "fireless". In that system a strong desiccant material such as caustic soda is used (sodium hydroxide, or soda lye), although other materials have been used including calcium chloride. In these systems a hot strong solution is supplied with steam exhausted from a steam engine. The solution absorbs the steam to generate heat at a higher temperature through absorption. The steam boiler is contained within the solution, and thereby generates steam to power the engine. The engine will continue to operate as long as the solution is sufficiently strong. Once diluted, then the system is recharged with heat. Note that the vessel containing the solution is not pressurized. Rather, only the tubing that contains the steam generated is pressurized, and this tubing is submerged in the solution. In fact, the vessel containing the solution may be under vacuum and vented to atmosphere via check valve, or pressure relief - so no positive pressure is used there. Copper tubing has been used in actual systems. Now, I don't consider this system to have a practical application, but it is interesting nonetheless.

Just for interest, consider a configuration to power an organic rankine cycle. A benefit is that the temperatures are not high enough to stress some lubricating oils, so the oil can circulate with the working fluid. The condenser in this case is connected to the vessel, and the engine exhaust transfers its latent heat to a store of water via a heat exchanger. The water will vaporize at a low temperature under the vacuum, and the steam will be directed into the vessel for absorption and thereby generate the heat necessary to vaporize the refrigerant working fluid contained in a heat exchanger within the vessel. It's also possible in principle to recharge the system with a solar concentrator, but it seems biomass is preferable. When regenerated, the system will emit superheated steam at atmospheric pressure, and this heat can be put to use in heating applications.

A small and slow moving compounded piston steam engine is probably the best way to go about using this if ever. I can only speculate how admitting lubricating oil into the solution with the steam exhaust would affect things, but perhaps it won't be a problem. A good compounded piston steam engine should achieve 7-8% thermal efficiency under these conditions. It should be possible to regenerate the system while the engine is running.

Ok, I've speculated enough on this.

Just found this link on "The Caustic Soda Locomotives" (enjoy!): http://www.aqpl43.dsl.pipex.com/MUSEUM/LOCOLOCO/soda/soda.htm

Yet another application of this principle (i.e. absorption): http://knowledgeableideas.blogspot.com/2013/05/a-solar-ammonia-absorption-icemaker.html

Calcium chloride absorbs ammonia vapor to drive an intermittent absorption refrigeration cycle. The system is used to make ice. The solution is regenerated with a solar concentrator. Again, not terribly practical (at least not in the "developed" world). Yet, I like the idea for its low tech approach (it's rather elegant, don't you think?). A chest freezer using a thermal mass with a low enough freezing point (such as a salt water) could use this system. Actually, a modest freezer with super insulation might be practical here. Imagine being able to recharge the system with just a few pounds of wood each day. A chest freezer that consumes one KWh of electricity each day sees a cooling load of roughly 5000 btu over this period. This would require about a gallon of ammonia, and yes... burning just a few pounds of wood each day could regenerate it.

I just considered an interesting configuration for the "caustic soda steam engine". Heating the solution during engine operation can provide a system with the benefits of a fire tube boiler (high energy storage capacity), but without the disadvantages of a large pressure vessel. Rather, the pressure vessel is a monotube steam generator contained within. So, the system can be heated at a low rate, but provide a store of energy much like a fire tube boiler to take the engine to higher power levels as required. The quantity of solution required would be much smaller, and the concentration can be maintained high for higher steam temperatures and pressures. Also, the superheated steam vented can be used to preheat boiler feed water for higher efficiency. Add a recuperator that uses furnace exhaust gases to preheat combustion air for even better efficiency. Since the furnace output can be a constant low output that corresponds to the average engine output, then the recuperator heat exchanger can be fairly small. I like the idea of a compounded piston steam engine with a steam reheat heat exchanger placed in the vessel as this will add efficiency also, and it's not much more trouble to do this. Still, something like this won't get more than about 6-7% net thermal efficiency. I like the idea of using this on a small steam boat fueled by stick wood and wood chucks. It might be practical in a stationary setting only if the heat can be put to full use, and an advantage there is that the battery system can be very small and possibly even eliminated.

Marcos Buenijo wrote:A chest freezer that consumes one KWh of electricity each day sees a cooling load of roughly 5000 btu over this period. This would require about a gallon of ammonia.

I just caught this error. Actually, 5000 btu cooling with ammonia would require more like 1.5 gallons of liquid ammonia (about 7.7 pounds). Also, additional ammonia would be required since the ammonia will not be fully evaporated from the absorbent (calcium chloride in this case) during regeneration. However, a high temperature heat source like a small furnace would do a lot better than a crude solar concentrator. Figure on 2 gallons of liquid ammonia for a 24 hour intermittent freezer to achieve the same cooling capacity as a typical 10 cf chest freezer (assuming similar insulation is used - super insulation can significantly increase capacity beyond this).

Well I'm glad there is a topic on desiccants here, too bad its this old though.

zeolite test aparatus

This is an interesting demonstration piece, and it would be simple to copy the basic design principal.

With a modest budget, aluminum tube, vacuum pump, valves, and a sumbmersible heater, someone could make a small rechargable zeolite absorbtion reactor.

I wonder if a sterling engine/generator could be powered by the combined warming and cooling effect that the absorbtion process provides?

Recharging the zeolite requires it to be dried @ 150C, well within the range of a decent parabolic collector, or the exhaust gasses of a rocket mass heater.
Perhaps zeolite could be rapidly heated by using a finer grain size, and running the powder through a jet of dry heated air.

Another use might be to embed pockets of zeolite inside the mass of a rocket stove, being aware of the need to leave some type of vent to the outside. The idea is that as it heats up, the zeolite desorbs trapped water in the form of steam... When the heater is off, the zeolite cools, pulling in water from the air, which causes the zeolite to heat. This would regulate the tempurature of the mass between heating cycles.

Jason, I've tried to generate some discussion here on permies on desiccants with no participation. The subject does get complicated. My interest is as Energy Recovery Ventilators on the market that use them to recover moisture in a heat exchanger, and also in the envelope of a building to promote enthalpy or latent heat of evaporation for humidly buffering and to generate renewable heat/cooling. A quick glimpse at zeolite looks very interesting but where do we source it?

BTW: The link in your post is broken.

This link works better ; )

Sorry, couldn't figure out how to edit that other post.

Fascinating these absorbent materials.

Zeolite is abundant, cheap, mostly non-toxic but not all zeolites are the same. I found out there's a mine in my region.

A google search turns up a great many different types of zeolites, some apparently intended to adsorb ammonia out of fish tank water.

Which types of zeolite are appropriate for use in a Desiccant Thermal Storage system?

I don't know if all natural zeolites have the same affinity to pure water. UN FAO website

http://www.solaripedia.com/13/389/5545/solar_zeolite_ice_maker_diagram.html

http://m.rimg.geoscienceworld.org/content/45/1/589.short

A solar recharging zeolite system should be able to provide hot water and ice.
With heat exchangers that should handle heating, cooling, and refrigeration.
The temperature difference should be more than enough to run a sterling pump for the heat exchange.

I'm thinking it'd be fun to build a small scale system to try this out. Maybe just big enough to use for a crock pot and chest cooler?