Early in my quest for a DIY A/C system that might actually work in our (often) humid summers I came across a couple of youtube videos produced by Tech Ingredients that led me down an interesting path.
The first one, link here, introduced me to the idea of liquid desiccants. It used liquid desiccant (LD for short) to pre-dry air that is cooled by flowing through an evaporative cooler. It was fairly complex, using a second evaporative cooler to cool down the hot and regenerated liquid desiccant (more on this later in my post). The second one, link here, is a system they built that was (hopefully) sized for a real-world application but didn't work all that well, possibly due to poor efficiency of their chilling tower and desiccant-solution tower. I think that their spray head scheme didn't work too well -- it's likely that most of the spray quickly wound up flowing down the inner walls of the tube. The laminar flow of the counter-flowing air then formed a "dead layer" that prevented good contact between the bulk of the air and the water or desiccant. There are devices called "turbulators" that break up laminar flow into more-turbulent flow that might improve the performance of those towers.
So, what is liquid desiccant (LD) and why is it particularly useful for drying air for A/C purposes?
Folks should be familiar with one-shot desiccants like the silica gel packets found in prepackaged food, vitamins and other food supplements, or products like "Dry-Z-Air", used to capture moisture in locations like RVs, closets etc. In the latter case, it actually uses the same chemical that is often used in LD applications -- calcium chloride. I should add that all these desiccants can be regenerated by getting them hot enough to release the water they have absorbed. I have purchased silica gel beads that actually have an indicator in them to show when they are exhausted and need to be baked so they can be re-used. And I've seen at least one blog post where someone did something similar with calcium chloride, but it was a pretty dangerous process -- it's necessary to get CaCl pretty hot, and at that temperature it is very corrosive.
There are other solid desiccants like zeolites, some types of clay, molecular sieves etc. They HAVE been used to perform continuous dehumidification by putting them in a rotating wheel or drum configuration. One side of the drum is heated and air is passed through it. The high temperature plus air flow pull the water out of the desiccant. Then the wheel rotates out of the hot zone into a cool zone, so the desiccant can again absorb moisture. Then inside air is passed through the wheel and dried.
Systems like this have been used in industrial applications where other process machinery generates high temperatures, so the heat is re-used. Since the desiccant wheel would need to be heated anyway, this equals a savings in money. They aren't used for private houses because houses typically don't have that kind of high-quality waste heat available; and they also are pretty large so there's enough capacity in the system to significantly dry the air.
In contrast, LD solutions -- typically they use something among the following: lithium chloride, calcicum chloride, potassium formate or potassium acetate -- don't require really high temperatures to be regenerated. In fact, they can be regenerated with systems that are very similar to (good) solar hot water heaters. This is very attractive because typical demand coincides with lots of sunlight around. Once your solar LD heater is built, the energy is "free". Not quite because it has to be pumped through some other apparatus, but that doesn't take much energy to accomplish.
Most research in the field has found that lithium chloride is the most efficient LD. It also is the most expensive so it's automatically eliminated from my consideration. Among the rest, calcium chloride probably is the most efficient but it has some problems. The first is that the solution, which is about 35-40% CaCl, is very corrosive so the pipes, pumps and heat exchangers used to heat and cool it have to be either plastic, stainless steel or ceramic. This jacks up the price, at least for heat exchangers and pumps. Of course, its corrosive nature is worse at elevated temperatures so a good design approach is to place our expensive pumps in the loop where the LD is at its lowest temperature. This would right in front of the regenerator, which heats the LD up in order to shed the water it absorbed. Another problem is that concentrated CaCl solutions have a very high freezing point, 40F and higher so it's necessary to keep the solution warm enough so it doesn't freeze and stop the system from working. The other problem also is related to CaCl's corrosive nature, and that is "carryover". Since the dehumidifier designs have to put interior air and CaCl solution in intimate contact, there is the possibility of CaCl solution droplets being carried into the interior space, where they can corrode metal and degrade fiber -- like rugs, furniture, clothing....so the design of the absorber portion of the system is very important. This, by the way, is another problem with the Tech Ingredients approach because they deliberately try to atomize their LD solution. They are depending on some kind of post-absorber filtration setup, one way or another, to prevent that. Absorbers that use air flowing at relatively high speeds are particularly susceptible to this problem.
Other LD solutions like potassium formate and potassium acetate are more benign in this regard, but they (1) aren't as efficient, (2) are more expensive; and (3) in the case of potassium acetate, its solution is reported to be very viscous so it is hard to pump it through the dehumidifier system.
It appears that the best way to prevent carryover is to use either packed-bed absorbers or so-called falling-film absorbers. Unfortunately, the best media for packed beds is pretty expensive -- I calculated that a 1 cubic-meter absorber would require over $2,000 worth of media (basically specially-designed plastic whiffle balls). So some kind of falling-film scheme looks best.
For developing different types of absorbers I'm planning on sampling the exit air with a high-voltage arc to ionize any calcium ions that are present, to be analyzed with (naturally, a home-made) visible-light spectrometer. That will quickly reveal if the design has any carryover or not.
The Tech Ingredients' second design is meant to use the same LD solution to simultaneously cool the air and dehumidify it, in contrast to their first design which just dehumidifies the air entering an evaporative chiller. However, their second design depends on an unassisted evaporative chiller to cool the LD solution -- not viable for a region that has high humidity, since the ability to cool the LD solution is limited. The problem with their first design is sort of related, because they're using an unassisted evaporative cooler to chill the LD solution. There are two alternatives that could improve the situation. First, build an oversized chiller using an air pre-cooler to sorta-kinda replicate a Maisotsenko-cycle system; and use the chilled water to both cool the house and operate an LD dehumidifier's absorber in a separate system to control the house's interior humidity level. The second is a kind of bootstrap system where the chiller is fed by an outside "feed" air flow that has been dehumidified by an LD system -- which in turn uses the same chiller water. It's bootstrapped because as the chiller operates the dehumidifier front end, the dehumidifier becomes more and more effective -- it's helping to decrease the wet-bulb temperature because the feed air's RH is reduced by the dehumidifier, so the chiller water temperature goes down and further reduces the RH of the input air. And so on. I haven't found any papers that describe a system like this so at this point it is a wild guess on whether or not it is a real improvement or not.
