A common phase-change material used for thermal mass in household climate-control applications is the miraculous salt of Glauber, issuing from his production of muriatic acid. Its melting at 32.38° enables it to store comparatively immense amounts of latent heat to be released upon its resolidification.
Consider, though, muriate of lime, notorious for its deliquescence throwing off heat. This, too, is a process that can be reversed by the application of gentle heat, driving out water from the material — nearly the reverse of the process with the salt of Glauber. Burns are said to have resulted from injudicious ingestion of crystals of this muriate. Yet it is very cheap and much more widely available than the salt of Glauber.
Unlike the situation with the salt of Glauber, the heat-absorbing reaction requires the reversible separation of two components, the muriate and the water. This is a virtue in that it can delay both the absorption of the heat until a higher temperature is reached, by virtue of holding the water vapor in by pressure, and, more importantly, the evolution of the heat at lower temperatures, by preventing moisture from entering the mixture again until the heat is desired. So sealing only, not thermal insulation, is required.
Perhaps this exothermic disssolution can supply an alternative form of “phase-change thermal mass” for household use. Not only can it be used to store heat for heating the house, but also it can provide stored heat to drive a desiccant-refrigeration cycle of the well-known type, if the desiccant can be regenerated at a suitable temperature. One such suitable desiccant is muriate of lime itself, in higher states of hydration than those used to store the heat.
In this way it is possible to store up the heat of the sun during the day to power an air-conditioning system during the night.
It supports monohydrate, dihydrate, tetrahydrate, and hexahydrate solid states, as well as a liquid solution and an anhydrous solid state. The anhydrous state weighs 2.15 g/cc, melts at 772°, and boils at 1935°; the monohydrate weighs 2.24 g/cc and dehydrates at 260°; the dihydrate weighs 1.85 g/cc and dehydrates at 175°; the tetrahydrate weighs 1.83 g/cc and dehydrates at 45.5°; and the hexahydrate weighs 1.71 g/c and dehydrates at 30°. Thus gentle heating can get us to the dihydrate, but fiercer heating would be required to obtain the ferociously hygroscopic anhydrous salt.
The anhydrous salt’s standard enthalpy of formation is -795.42 kJ/mol,
compared to -1403.98 kJ/mol for the dihydrate and -2608.01 kJ/mol for
the hexahydrate. However, presumably much of that is already in the
water, whose standard enthalpy of formation is -285.83 kJ/mol; six
times that is -1715 kJ/mol, leaving only -97 kJ/mol from adding water
to the anhydrous form (- 2608.01 795.42 (* 6 285.83)) = 97; from the
hexahydrate to the dihydrate we have (- 2608.01 1403.98 (* 4 285.83))
= 60 kJ/mol. The molar mass of the anhydrous muriate is 110.98 g/mol,
and of the water is 18.015 g/mol, so this is 147.01 g/mol for the
dihydrate or 219.07 g/mol for the hexahydrate; in theory, then, we can
produce 408 kJ per kg of the dihydrate by hydrating it to the
hexahydrate, whose heat capacity is 300.7 J/mol/K; this works out to a
temperature rise, theoretically, of 199.5°. (The heat of dissolution
of the hexahydrate I do not know.) This 408 kJ/kg compares very
favorably to 251.2 kJ/kg for the salt of
Glauber.
(The water-solubility of the muriate does increase with temperature, so it is also possible to absorb heat in this fashion, as with the salt of Glauber. I am not entirely clear on how the humidity of air and temperature interact with the degree of hydration of the salt.)
Short-circuiting the process a bit, you could cool the house at night with the sun’s daytime heat by dehydrating the muriate with sun, then let it cool, eliminating its sensible heat. Then, when cooling is desired, we can first dry some air by passing it over the muriate, then cool the air back to the outdoor temperature by passing it through a recuperator or regenerator, then cool the air further by evaporating water into it.
To cycle the desiccant between liquid and solid states, it would be useful for the solid state to be granular; otherwise it will tend to agglomerate into a solid mass impermeable to air. Spray drying is a potentially useful way to do this; to avoid getting grains that are too small, perhaps an updraft can be used to select only drops in a certain size range, before passing the remainder into a continuous updraft that keeps them at a stable height until they are dry.
Alternatively, the liquid could be held in very carefully leveled, very shallow pans, drying into a thin crust in the bottoms of these pans when 50° air is passed over them. This way the solid salt will have a large surface area without being granulated. The pans can be made of thin plastic, for example PET, polystyrene, or PMMA, and surrounded with high walls on all sides, with the airflow coming in from and departing from above, so that an error in tilt will not spill the solution.
I was pleased to discover Evaluation of heat available from calcium chloride desiccant hydration reaction for domestic heating in San Francisco, CA, Michael Giglio’s 2017 mechanical engineering master’s thesis at Santa Clara University. He calls out as advantages of this “thermochemical energy storage” approach its ability to function without insulation and the higher storage capacity available for dehumidification and cooling, and reports a “best-case scenario” of 19 kWh/m³ [68 MJ/m³] of storage capacity for heating, using “a dilution reaction between 100% concentrated [muriate of lime] and water to a 20% solution”. While there are surely conveniences to using a purely liquid system, I think higher densities can be achieved using solids. Giglio’s final design used a closed system rather than exchanging moisture with ambient air.
It’s unfortunately very poorly written, with a great deal of repetition, occasional contradictions, and much ambiguity (What would a “100% concentrated” solution be? The anhydrous muriate?), but I managed to slog through it.
He reports (p. 7, 17/71):
A solar driven liquid desiccant cooling system has been developed in Singapore by L-DCS and shown to provide a storage capacity of 183 kWh/m³, while a similar system has been developed by ZAE Bayern that utilizes a lithium chloride solution and district heating to provide 12 kW of cooling and a storage density of 150 kWh/m³ [11].
These densities work out to 660 kJ/ℓ and 540 kJ/ℓ respectively. His ref. 11 is “Hublitz, A., 2008, ‘Efficient Energy Storage in Liquid Desiccant Cooling Systems,’ Dissertation, Technical University of Munich.”
He also reports that muriate of lime costs US$0.10–$1.00/kg, but I’ve found it at retail in Argentina to cost more like US$1.60–$4/kg. He also reports (p. 44, 54/71) that he bought it for US$100/kg from Sigma-Aldrich.
He reports that the enthalpy of dissolution of the compound should be “-740kJ/kg of desiccant”, but I don’t know whether that’s dissolution of the anhydrous salt or what.
On p. 27 (37/71) he reports that he got a 63° temperature rise by adding water to muriate of lime; a bit later he explains that this was solid, but it’s unclear whether it was the anhydrous form or one of the solid hydrates. On p. 37 (47/71) he claims to have gotten 288 kJ per kg of muriate of lime.
Giglio concludes that his prototype system would cost too much to be economical if scaled up, estimating its cost at US$13000 with 8 square meters of solar collector costing US$8000, 285 kg of muriate of lime costing US$2850, and two copper-tubing heat exchangers costing US$775 each, and consequently providing a levelized cost of energy of US$0.86/kWh with a 30-year lifespan, competing with US$0.04/kWh for natural gas, 21½ times too expensive; by his calculations, US$600 would be the price point at which such a system would be cost-competitive.
I think US$500 is a more reasonable cost estimate for this quantity of desiccant, and using copper is a terrible idea — not only is it expensive, but it’s vulnerable to attack from the chlorine, and it cannot be made thin. Polyethylene, polypropylene, or polytetrafluoroethylene would be much better materials. Most of the necessary heat exchange can probably be provided by bubbling air through a liquid desiccant or blowing it over a solid one. And thermal solar collectors using air can be as simple as thin transparent glass or plastic, which costs about US$20/m², not US$1000/m², over a styrofoam box painted black on the inside. (See Solar Netting for how to prevent cheap glass from being broken by hailstones, using chicken wire.)
As noted in Desiccant Climate Control, there seem to be wholesale vendors that will sell you a tonne of muriate of lime for apparently US$272. This would bring the cost of the desiccant in Giglio’s system down from the US$28’500 he was paying Sigma, or the US$2850 he estimates, to US$78. This 36× reduction has a significant impact on the economic plausibility of such systems! Also, it’s likely that the wholesale vendor is selling the anhydrous salt, since that’s the most useful form for use as a desiccant, while what Giglio bought from Sigma is likely the hexahydrate, since that’s the form most stable when exposed to air at room temperature.
(The heat transfer rate of a bubbler is limited by the size of the bubbles, but a saturated solution of muriate of lime gets quite frothy, from which I conclude that either the muriate itself reduces the surface tension substantially, or my sample is contaminated with a surfactant. Either way, if a heat and/or vapor exchange is effected through direct contact, as in a droplet-column or bubbler exchanger, cyclonic separators or something might be necessary to filter the desiccant out of the air.)
Moreover, as mentioned above, this thermal collector and desiccant can serve not only to provide house heating, but also house cooling and, when necessary, dehumidification. This means it’s competing against not just natural gas but also the electricity to run an air conditioner and, perhaps, a refrigerator. Desiccant Climate Control goes into more detail here. It also notes some other desiccants that are more than an order of magnitude cheaper than muriate of lime.