alt Hacker NewsOne alternative to a freezing pool of water that I'm intrigued by is using thermochemical heat batteries to transport heat energy from summer to winter.
For example, evaporating a 40,000 L solution of calcium chloride brine can store 4.6 megawatt-hours(*) of thermal energy and release it in the winter by passing water vapour over the solution to dilute it. This can heat geothermal temperatures from 10°C to 27°C which can be used in in-floor heating.
But the big asterisk is that only 11% of that 4.6 megawatt-hours of that thermal energy being "stored" is held in the enthalpy of dilution, and 89% of that heat is coming from the enthalpy of vaporization, and acquiring water vapour in winter isn't so easy.
One would probably need a source of water and use geothermal heat to raise dry cold air temperature to the needed 10°C, while absorbing water vapour from a (relatively small) tank of water. The net result is more of a thermochemical heat pump rather than a thermochemical battery per se. But the point is that, other than some fans, this is not electrically powered.
Then there is the issue of storing such tanks safely underground, and more problems. Still, there might be some innovative solutions lying in this direction. For example, one can replace this open-loop system with a closed-loop vacuum system to boil water, but at the cost of far more complex and dangerous engineering.
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Heating up a pile of dirt to a few hundred degrees sounds easier and more energy efficient.
Oh, such tactics are still applicable to dehumidify outside air in an air source system (and possibly further extract enthalpy by freezing off the excess water captured in the "wet scrubber").
In general, using heavily salted water (not sodium chloride that's quite ineffective) to dehumidify humid cold air during heating and reject heat with a wet interface without having to source potentially scarce water to evaporate, is quite an intriguing alternative to classic fan-laden AC "condenser" coils.
Incidentally, KOH is also interesting as it dehumidifies down to quite low humidity (at least indoor-facing) before it saturates (and precipitates out), and as a side-gig scrubs CO2 that can be precipitated out with CaO/Ca(OH)2 as CaCO3 (limestone).
This can be thermally recovered at around 800~900 C to essentially "boil" the CO2 off.
IMHO a chemically appropriate wet heat exchanger based on (cross flow: easy) or (counter-flow: harer to prevent droplet formation) with just a simple corrugated structured packing should be able to operate more hygienically than a theoretically dry, practically humidity-condensing classic mini-split AC coil. And allow most dust to be washed off fully automatically from normal operation. It'd "just" need a regenerator to dry out excess unwanted humidity, but I'd assume there's a reasonable way to boil off the excess humidity from the "salty water". Maybe the common reverse-osmosis filter materials can resist these chemicals; in that case it'd sound like a question of pressurization energy recovery; or it could be put under vacuum and a small heat pump could transfer the enthalpy of vaporization from the condenser [distilled water plus dissolved gasses, vacuum] (that isn't fully flooded due to presence of dissolved gasses in the feed) to the evaporator [salty water, same vacuum]. If the ratio of dissolved gasses relative to the extracted condensate is small enough, I'd guess a water jet "ejector pump" using that very condensate (after it's cooled down enough to not boil in the ejector) would actually work to provide the vacuum for continuously pumping the dissolved gasses that boil out along the humidity... A small centrifugal pump for that can easily be made completely sealed using magnets to transmit the torque to the impeller and a long life ceramic bearing, at least if there's a long enough pipe down to the pump to build minimum inlet pressure (against cavitation).
Also: For vacuum btw. remember the convenience of using gravity to pull a (nearly) 10m column of water down. Dissolved gasses are likely the dominant complication.