In Muriate Thermal Mass I did some basic evaluation of the reversible hydration of the dihydrate of muriate of lime to the hexahydrate as a way to store up heat.
I concluded that it was a very attractive alternative to the miraculous salt of Glauber for domestic space heating applications, because although it needs to be heated to 45.5° to recharge it, by my calculations it holds 408 kJ/kg of heat energy and can produce temperature rises of well over 100°; and it’s considerably more flexible in when it releases that heat, instead of trying to release it all the time and needing to be somewhat restrained by thick thermal insulation.
However, I now conclude that I had only begun to scratch the surface of the amazing possibilities of such systems; the Cromer cycle is just the beginning.
It may be that all of this is irrelevant now that photovoltaic panels are becoming so cheap, with module prices down to 15¢ per peak watt, that collecting the large amounts of solar heat that make these things attractive is actually more expensive than just storing energy in batteries. I suspect not, though; more exploration of this theme below.
In Muriate Thermal Mass I described the use of muriate of lime a desiccant for heating in the following process, which turns out to be well-known:
By itself this process can produce only a limited range of temperatures — with muriate of lime my simplified calculation in Muriate Thermal Mass estimates a temperature rise of just under 200°, but other desiccants may not perform so spectacularly, and I suspect that even muriate of lime would require high pressure in step #3 to achieve this, since otherwise the water will volatilize.
However, a cascaded heating cycle can achieve higher temperatures, as follows:
If there were no heat losses and the absorption reactions were equally exothermic at all temperatures, this would give you unlimited temperatures, but neither of those is true.
But heating is just the beginning.
Refrigeration is monumentally important, historically. The introduction of refrigerated cadaver ships is what made Argentina the richest country in the world for a shining few decades, since suddenly we could export our virtually unlimited supply of cow corpses to Europe. Lee Kuan Yew credits refrigeration, specifically air conditioning, for making it possible for Singapore to develop economically:
Question: Anything else besides multicultural tolerance that enabled Singapore’s success?
Answer: Air conditioning. Air conditioning was a most important invention for us, perhaps one of the signal inventions of history. It changed the nature of civilization by making development possible in the tropics.
Without air conditioning you can work only in the cool early-morning hours or at dusk. The first thing I did upon becoming prime minister was to install air conditioners in buildings where the civil service worked. This was key to public efficiency.
A simple air conditioning cycle is as follows:
Optionally, after step 3, you can cool the dried, warm air by running it through a heat exchanger with outdoor air. This improves the efficiency of the system. As with the heating cycle, you can improve the system’s temperature range by cascading the reaction, cooling the air (and dry desiccant) through two or more stages.
Several different kinds of mass-exchange contact between the desiccant and the air are feasible: liquid desiccant can be sprayed into an air column or a fountain, or pumped over pads of excelsior, as in a traditional swamp cooler, or air can be bubbled through it; a solid desiccant can be held in granular or foam form in a rotating wheel, or in a packed bed, or stuck to the surface of many flat plates in a sealed box or boxes, which may or may not double as solar thermal collectors.
This is a perfectly orthodox desiccant refrigeration system. It can be simplified to an outdoor evaporator and a pair of exposed fountains in an indoor space, one of water and one of desiccant, though this may be more advisable for oil of lime than for, say, oil of vitriol. In general all the options for contact with liquid desiccant are also options for contact with liquid water.
It can be used as well for refrigeration and even freezing of food or water, and water ice may be a denser and cheaper way to store cold until it is needed than as dry desiccant; water ice is 333.55 kJ/kg. Depending on the efficiencies of the cycle, though, it’s entirely plausible that storage of energy in the form of some desiccants might be an even denser way to prepare for the need for cooling than water ice, and it certainly has a better shelf life and more flexibility.
Interestingly, brine of muriate of lime is commonly used in industrial refrigeration as a coolant rather than a desiccant — by virtue of remaining liquid down to -50° it permits the transport of a whole lot of cold in a very small pipe.
In addition to the above-mentioned heating and cooling applications, stored dry desiccant can of course be used to dehumidify indoor air simply by passing the air over it. And of course if you can heat up water, as explained above under “Heating”, you can humidify as well by passing air over the heated water.
Dehydration sounds like an extremely niche use (raw-food vegans and a few other people have 500-watt home dehydrators), but I think it isn’t; it just hasn’t been available at a low enough price previously. Consider the Earthship’s list of six basic human needs satisfied by architecture: energy, garbage, sewage, shelter, water, and food. Dehydration is directly applicable to three of those basic human needs: garbage, sewage, and food.
Garbage and sewage are mostly problems because they rot and stink and carry pathogens.
(Garbage is a more complex problem with many aspects, though: used motor oil, radium paint, nickel-cadmium batteries, demolition debris, linseed-oil-soaked rags, and so on; but food waste in particular, like these chicken bones I have here, is mostly a problem for those three reasons. And the physical volume of non-construction non-food garbage can be kept pretty minimal, like, cubic meters per person per year, or less. So I’m focusing here on food waste as the central core of the garbage problem.)
At Burning Man we deal with food waste first of all by dehydrating it, after which we can burn it or just store it in its inert dry state until it’s time to carry it away. There, it’s easy to dehydrate things: you put them in one of those plastic netting bags they sell oranges in and hang them out in the sun and wind.
Abundant desiccant regeneration capabiity makes it possible to dehydrate food waste thus even in humid climates.
And similarly for sewage. At Burning Man, we just copped out and used chemical toilets, and at a local ecovillage here they used to use potash. Piss they would dilute with water and use as fertilizer, but shit they would pickle with wood ash from the cooking fires. Eventually, they switched to composting toilets, and getting those to work with aerobic mesophilic rotting instead of the usual noxious kind is a matter of partial dehydration: you cover your shit in the bucket with dry leaves, which also cut down on the relative nitrogen content, which gives you better rotting. Civilized people don’t have dry leaves, so instead they use sawdust or coir or something in their composting toilets.
But, if you have ample desiccant capacity, you can use that to arrest decay completely, and then you can either burn the remains or you can bake them to kill all the pathogens and then use them as safe manure.
In the food category, there are a lot of foods that can be preserved for longer periods of time by dehydration than by refrigeration, although the change in flavor may be agreeable or disagreeable. Outside of the former Tawantinsuyu, food dehydration is usually done hot, which also changes the flavor of the food; in particular, dehydrated eggs are made by a desugaring and flash-spray-drying process that requires significant amounts of equipment and chemistry to replicate. Freeze-dried food in, for example, the US, is merely a novelty: Astronaut Ice Cream, etc. The tradition of chuño from Tawantinsuyu is unknown. And freeze-drying also changes the flavor of food, mostly through changes to mouthfeel, though chuño also owes its flavor to a fungus that grows during the process.
I think that food preservation by desiccant drying should be feasible at low temperatures, and might offer possibilities for food preservation with much less impact on flavor.
Thus a stored desiccant is a sort of all-purpose indoor climate control resource, capable of blowing hot or cold, like Aesop’s traveller in the satyr’s cave.
But wait! Don’t touch that dial! There’s more!
Water supplies are a major concern for the humans’ survival in many places, to say nothing of their ability to farm. But if you can dehumidify by sucking water out of the air into a desiccant — and the equilibrium relative humidity for some of these desiccants is very low indeed — then you can recondense that water when you regenerate the desiccant, particularly if you can chill a sort of cold trap to help the water recondense.
This allows you, in theory, to harvest an amount of water limited only by the available low-grade heat energy (solar or otherwise) and the amount of humidity in the air you can lure into the moisture vaporators of your moisture farm.
Although in the above I chiefly referred to the properties of the muriate of lime, many other possible desiccants exist and could be thus applied, including alabaster, amorphous mesoporous magnesite, zeolites, silica hydrogel, ferric chloride, polyacrylate of sodium, pearl-ash, salt, sugar, silica aerogel, chloride of zinc, oil of vitriol, activated charcoal, soda-ash, lye, the bromide or muriate or nitrate of lithia, quicklime, oxide of phosphorus, nylon 6, carnallite, chloride or sulfate or perchlorate of magnesia, waterglass, infusorial earth, cellulose-rich waste plant material such as sawdust and straw, porous dehydrogenated hydroxide of alumium, fired clay ceramic, unfired clays, other desiccants, and mixtures of the above.
Different desiccants have different tradeoffs, and some may not be well suited to some uses; for example, perchlorate of magnesia must be regenerated under vacuum, quicklime must be regenerated at the inconveniently high temperature of 512°, and the hydration of alabaster only produces a temperature rise of some 60° or less. It’s unlikely that muriate of lime is the optimal choice, but I haven’t investigated the tradeoffs thoroughly.
Muriate of lime can work as an aqueous solution, avoiding the massive inconvenience of your lovely pebble bed deliquescing into a sticky, solid, impermeable lump, but I suspect that a porous solid mass of alabaster may have even better heat transfer properties than the aqueous solution of muriate of lime. And alabaster doesn’t deliquesce; at worst it may crack a bit. As explained in Plaster Foam, I got a nice porous alabaster biscuit by mixing calcined powdered alabaster with baking powder and baking it in a tin in an ordinary oven.
One of the chief figures of merit here is the price — either per joule or per kg of water absorbed. Other relevant quantitative information includes the minimal relative humidity the desiccant can reach, the temperature needed to regenerate it, the maximum temperature rise it can produce, its water capacity, and its energy capacity. Relevant qualitative information includes whether it is solid or liquid, its viscosity if liquid, its tendency to clump if solid (fixable in some cases with larnite or similar substances), and its hazards if spilled or inhaled.
But let’s look at prices first.
Muriate of lime, or oil of lime, has been one of the chief desiccants used for centuries, and it is relatively cheap — US$1.60/kg here in Argentina, for example, as I noted in Dercuano. A few other candidates approach or excel this price. Slaked lime sells for US$0.12/kg at retail here, and alabaster as the hemihydrate for about US$0.40/kg, and the USGS gives its wholesale price as about US$8/tonne, thus US$0.008/kg, while giving the various potassa products including pearl-ash as closer to US$800/tonne (US$0.80/kg) as fertilizer, and soda-ash as US$150/tonne (US$0.15/kg). Lime is calcined from limestone, which the USGS lumps with crushed stone in general at US$12/tonne (US$0.012/kg), but the calcining and slaking process is a significant cost by comparison. Raw natural zeolites have their wholesale price given as US$50–300/tonne (US$0.05-US$0.30/kg), various clays as US$10–140/tonne (US$0.01–0.14/kg), and infusorial earth US$310/tonne (US$0.34/kg). The USGS report on magnesia gives no explicit price but it seems to be about US$0.70/kg. For rock salt they say US$60/tonne (US$0.06/kg).
Waste plant material is often free or of negative cost, but often must be treated to stop fire and rot.
These bulk minerals, except for pearl-ash and magnesia, have wholesale prices in the two orders of magnitude of US$0.005-US$0.5/kg. Probably most industrially-produced materials are unable to approach that range, though perhaps a few, such as lye, oil of vitriol, and muriate of lime might make it.
L29Ah was kind enough to point out that the random Russian website opt6.ru offers a tonne of 99.2%-pure muriate of lime for 21000 rubles; a ruble is presently US$0.01294 reportedly, so that’s US$272/tonne or US$0.272/kg. If this price is correct, it’s toward the high end of the price per kg of the cheap desiccants, and 34× the price of calcined alabaster, but it’s still kind of within the range.
Using the 408 kJ/kg number from [Muriate of Lime], this price works out to 1.5 MJ/$, 666 nanodollars per joule.
Alabaster is especially tempting due to its 34 times lower price per mass, and also because it doesn’t glom together into a sticky mass when you regenerate it, though it can when you hydrate it.
Alabaster’s molar mass is 136.14 g/mol anhydrous, 145.15 g/mol as hemihydrate, and 172.172 g/mol as dihydrate. Converting the dihydrate back to the hemihydrate is more difficult than with muriate of lime, requiring 100°–150°, and conversion back to the anhydrous form requires 180°. Upon hydration it can reportedly reach 60°. The key datum I lack here for comparison is the enthalpy of formation of the different hydrated forms.
Quicklime is notorious for producing enough heat to boil water when rehydrated, and it’s very nearly as cheap as alabaster. However, regenerating it requires inconveniently high temperatures, and it’s lethally caustic.
Farulla et al. characterize “thermochemical thermal energy storage” systems like these as storing 120–250 kWh/t, or 430–900 kJ/kg in SI units, much higher than sensible-heat thermal energy storage systems at 10–50 kWh/t (36–180 kJ/kg); but it claims TCTESs also cost €8–100/kWh (2500–32000 nanodollars per joule), far more than the €0.1-10/kWh (32–3200 n$/J) of sensible TES, identifying these high capital costs as a key reason for TCTES’s non-adoption.
It seems plausible that one of the materials described above could deliver low capital costs, in the range of sensible TES costs or even lower. Farulla et al. are not unaware of these materials, and they even survey a number of published results from prototypes using them, as well as results designed for both heating and cooling. However, it seems that a great deal of research in the field has been focused on somewhat more exotic and therefore costly materials such as bromide of strontia, synthetic zeolites, muriate of lithia or baryta, and so on. I need to finish reading their paper.
Wholesale photovoltaic modules at 15¢ per peak watt at 20% capacity factor cost US$0.75 per average watt, which is 86.4 kJ per day. At a 6% annual discount rate an average watt amounts to 30.6 net present MJ in the first year, 59.3 in the first two years, 135 in the first five years, 235 MJ in the first ten years, 362 MJ in the first 20, 487 MJ in the first 50, asymptoting to 509.7 MJ at infinite time. (That is, although it produces 30.6 MJ per year, the 1580 MJ it produces in its first 50 years are only worth 487 present MJ to us at a 6% discount rate.) So photovoltaic modules work out to 680 MJ per US$ (at 6% APY).
If your thermochemical energy storage system can store 10 MJ per US$, which Farulla et al. say that current TCES systems don’t come close to, how does that compare? How about the 2500–32000 nanodollars per joule (0.03–0.4 MJ/$, US$2.50–32/MJ) reported by Farulla et al.? How do you measure energy storage against energy generation?
Well, they aren’t really commensurable. No amount of photovoltaic modules on your roof will allow you to run the air conditioner at night, and no amount of calcium chloride will heat or cool your house if it’s all fully hydrated; the TCES as such trades off against batteries, not solar panels. And it doesn’t trade off against all uses of batteries. And it also trades off somewhat against other climate control systems like vapor-compression refrigerators.
But when I was looking at balcony batteries a couple of years ago in Dercuano, lead-acid batteries cost US$23-73/MJ, which I don’t think has changed much (though possibly lithium-ion will surpass them in a year or two). In crude terms this is about 1 to 700 times more expensive than a TCES, depending on whether you rely on Farulla et al.’s reports on existing prototype systems or my optimistic projections from possibly impractical but very cheap desiccant materials. But that doesn’t include the cost of the vapor-compression air conditioning system itself.
Energy storage is strongly complementary, in the economic sense, to solar energy, and this is responsible for much of the interest in thermal energy storage systems in recent years. The cheaper TES is, the more valuable solar modules become; the cheaper solar modules become, the more valuable TES is. TES can’t fulfill all of the energy storage needs for intermittent solar and wind energy, because it has very poor round-trip efficiency for mechanical energy, light, and so on. So batteries will still be needed.
(Still, for small low-power things like clocks and cellphones, TES might be a useful backup power source, perhaps using a thermoelectric generator or a Stirling engine.)
However, it’s very likely that you can get more solar energy for your TES by gathering solar thermal energy than by gathering electrical energy with solar cells with an efficiency of 16% or 21%. And you can do it with collectors that are cheaper than photovoltaic modules, which still cost US$0.15 per peak watt. For example, you can use 1 m × 1 m × 19 mm boxes made of thin styrofoam, open at the top (one of the large faces), painted black on the inside, with plastic wrap wrapped around them to let light in, and smeared with a “chemical sunscreen” to slow UV damage. The airspace within permits air to be blown through there, using a couple of holes in the back of the box, to harvest the heat. I think these will be about 30% efficient. The material would cost about US$20 for a 4’×8’ sheet (3.0 m² in non-medieval units), so that’s about 1000W peak for US$20, US$0.02 per peak watt.
(Rather than plastic wrap, you might be able to use UV-blocking polyester film intended for outdoor use.)
So solar collectors for a TES can probably be about a factor of 5 or 10 cheaper than photovoltaic modules.
One of the great advantages of thermochemical energy storage is that you don’t need to insulate it. This, in turn, means that you can scale it down from building-sized systems to very small systems, and the stored energy has a shelf life of potentially years; “self-heating cans” have used muriate of lime for many years, for example.
You could thus scale these systems down to a wearable size, providing personal climate control.
If this is such an advantageous technology, why hasn’t it been adopted previously? The humans have used fire to warm themselves for at least a million years. The calcining of alabaster goes back at least to Old Kingdom Egypt, the calcining of lime even further, to the Neolithic, before Çatal Höyük. Tubes of dried clay for guiding air date back, I think, at least to the beginning of iron smelting in the Hittite empire 4000 years ago. Evaporative cooling via the qanat goes back 3000 years in Iran. Texts purporting to be from 1200 years ago, by Jabir ibn Hayyan (“Geber”), described the “spirits of salt”, and undoubtedly observed their action on chalk, producing bubbling and oil of lime. Émilie du Châtelet’s discovery of energy was published in 1756, after Leibniz’s pioneering efforts in the 1670s and 1680s. Thermodynamics was well-developed in the 19th century. Solvay began mass production of soda ash, with a byproduct of muriate of lime, in 1864. Gibbs described his “available energy” in 1873. Refrigeration and air conditioning was developed in the late 1800s, and the hazards of leaks of toxic and caustic refrigerants were such a major issue that Einstein and Szílárd patented their ammonia-absorption refrigerator without moving parts in 1930, a variant of the 1922 Munters–von Platen design, and Electrolux immediately put it into production; the same year, Midgley famously snuffed a candle with a breath of dichlorodifluoromethane, which he’d developed for the same humanitarian reasons, and which became the most popular refrigerant for decades. Harold Ellingham published his “Ellingham diagram” in 1944.
So the materials needed for thermochemical energy storage systems have been not only available but abundant for centuries, if not millennia; the theory necessary to design them for a century and a half; and they fulfill needs that have been universal human experiences for a hundred times longer than civilizations have existed. So, if these systems are so advantageous, why have they not been applied widely?
In the particular case of sewage, given the depth of mind-crippling tabus on the subject, I don’t think we need much reason for slow diffusion of shit-handling innovations; the US still hasn’t adopted bidets, for example. Squat toilets like the traditional Turkish and Japanese designs help greatly with constipation. The US has a huge constipation problem. Nevertheless, diffusion of squat toilets is actually negative, because aping the less-functional English design is more prestigious than using a design that works better anatomically. Garbage suffers from similar mind-crippling tabus, but they are less severe, and indeed garbage-handling practices have changed dramatically and rapidly in past decades.
So, in the case of garbage, but especially in the case of food preparation, air conditioning, and heating, I think we need a better explanation. There are some commercial installations using desiccant air conditioners, dating back to the 1980s in some cases, but it is not a widely adopted technology. There are even a few cases of using thermochemical energy storage for both heating and cooling in this way, though I haven’t seen previous suggestions of using a single thermochemical energy store for so many different purposes: space heating, heat for cooking, air conditioning, air dehumidification, food refrigeration, food dehydration, garbage dehydration, and sewage dehydration.
Moreover, many of the deployed and research systems use expensive desiccants such as lithium bromide; I can’t find any trustworthy sources on its cost, but I doubt it approaches the US$0.27/kg price of muriate of lime or US$0.008/kg of alabaster. Lithium carbonate costs US$13/kg and is 18.8% lithium, making the lithium cost US$69/kg. Bromine costs US$2.19/kg. Lithium bromide is 8% lithium and 92% bromine. This suggests a cost of US$5.50 for the lithium and US$2 for the bromine, per kg of lithium bromide, thus US$7.50.
I tentatively suggest that perhaps what I am proposing in this note has not been tried, though I cannot imagine why not.
Compressed air has been a widely used temporary storage form for energy for over half a century; air-powered tools are common in auto shops all over the world, and the non-electric Amish in particular have developed quite an economy of compressing air with windmills, shipping compressed air around in tanker cars, storing it in enormous underground tanks, and so on, with the objective of easing their work without becoming dependent on the “English” for electricity.
One of the disadvantages of compressed air energy storage is that, when air is adiabatically compressed, much of the compression energy is lost as heat rather than being stored in the compressed gas. Another is that when room-temperature compressed air expands, it cools, and this cooling can condense water out of it, which tends to cause various kinds of problems in compressed-air-powered and compressed-air-handling machinery.
The solution, in theory, is isothermal compressed-air energy storage, where the air is cooled to maintain a constant temperature as it is being compressed, and heated back up as it is being decompressed.
Doing this on a small scale is difficult, because doing it the normal way requires access to some kind of “heat absorbing and releasing structure” connected to a huge heat reservoir, such as a lake or the ocean, to keep its temperature change minimal. But phase-change and thermochemical energy-storage systems have the possibility of absorbing and later releasing massive amounts of heat without changing their temperatures; thermochemical systems additionally have the possibility of releasing the heat at a lower temperature than it was initially provided. This reheating not only improves the efficiency of the energy storage device; it also avoids condensation.