The Ultimate Dehumidifier

[NiHaoMike]

The catch is that the calcium chloride gets "consumed", meaning you'll either have to keep replacing it or regenerating it by putting the container in sunlight on a hot day.

[MEMPHIS91]

Gotcha, I guess I was thinking that somehow only the water went down the drain. But the salt is absorbing the moisture and becoming a solution and flowing out.

OK so I really don't mind the whole manual labor of taking the container out and letting the sun bake it. My question is how many lbs of salt would I need in order to suck about 5 gallons a day of water out of the air? I could do about 5 containers that way they were more spread out over the house. I could build 10 containers And just swap rack one out for a new one everyday.
Or build a waterfall closed loop system. Hhhhmmmm.....

EDIT: I found this on a web site for a 4lb tub. "DampRid's natural crystals effectively control musty odors caused by excess moisture for up to 60 days in areas up to 1000-square feet, and up to 6 months in a 250-square foot area, depending on temperature and conditions." So 4 lbs will do 1,000 square feet. I have 2,300 square feet so I should need 12 lbs just to be safe. Right? I don't see whats so hard in swapping this stuff out for a regenerated one? Seems too good to be true.

[MEMPHIS91]

http://www.amazon.com/dp/B002Q152VI/...um-chloride%2F

[jeff5may]

Go buy some at your favorite local big box store. This amazingly common product is called damprid. Super deliquescent anhydrous calcium chloride plus essence of smell. Pop the top and it starts grabbing water. Works well down to about 30 percent rh... By the pound, ice melter is cheaper. I believe someone broke this down before ad to the yield vs water-grabbing power.

[jeff5may]

Oops, it helps to update my browser before commenting...

Main idea is the solid salt dehydrates the air faster than a solution. A strong solution works faster than a weak one. More relative humidity is easier to remove than less.

[MEMPHIS91]

I am interested in the waterfall idea, but I have a solar cooker, I'm thinking that if I put as much as I can in a super large black pot, let it soak up moisture and then throw it on the cooker, it will dry it out good for me.

SO many thoughts, I really want to get the humidity down to 40%ish and right now without the big ac coming on the lower my current dehumidifier will get it 47-50%.

I waterfall loop would be awesome, but I don't want to spend the time and money on a high maintenance system, and without some more examples of really well working systems it is hard to pull the trigger. IF I did build one, I'm thinking something that I can put in my window that way when winter comes I don't have to worry with freezing.

[jeff5may]

The calcium chloride solution is good down to like -40 degC before it freezes. Check out this chart:


In one of the previously cited threads, another similar chart was posted that showed the properties of the two forms of the intermediate-temperature crystals that form upon cooling. As long as there is some solid salt in the solution, it is saturated, and will closely follow the chart. On highways and roads, it is a better de-icer than rock salt.

[DEnd]

Ok so I am far from an expert in HVAC and dehumidification so take what I say with a grain of salt (pun intended).

Your initial idea of running a GSHP with the goal of reducing the latent heat load on your A/C unit is basically a good one. You get a rise in Coefficient Of Performance (COP) by lowering the temperature of where you are pumping heat to. This is the same effect but in reverse as you saw with switching the heat source (where you are pumping heat from) of your heat pump water heater. You were also planning on increasing system efficiency by keeping the heat out of the house (by placing the compressor and condenser outside). This is also a very good idea as a stand alone dehumidifier is basically a 110% efficient (or higher) electric heater.

Now let us take a look at what happens with your system design if it is working in such a way that it removes all the water it possibly can from the air that enters it for a given evaporator temperature, let's say that temperature is 33°F. So what happens? The air enters the system at say 75°F and 60% Relative Humidity (RH). When it leaves the system it will be 33°F and 100% RH. This is because RH is relative, the percentage is the percentage of the amount of water the air can hold at that temperature.

This is not a very efficient way to operate a dehumidifier. Why? Because we are removing sensible heat also, not just latent heat. We we want to use a dehumidifier to dehumidify the air, not to cool it. We want to remove latent heat only... So how can we reduce the amount of sensible heat that is lost to the dehumidifier? Well if we remember that heat flows from high to low we see there is a temperature difference between the air flows. Ideally we want the temp of the air coming out of the dehumidifier at 75°F, not 33°F. Since the air entering is at 75°F already we can use that energy to warm up the air leaving right? There are multiple ways to do this, air-air heat exchanger, heat pump, heat pipes, etc...

An interesting thing happens when we do that. The Relative Humidity of the air going to the evaporator rises with the temperature drop. Ideally the temperature would drop to 33°F raising the RH to 100% before it gets to the evaporator now the heat pump in the dehumidifier is only moving Latent heat. Now this is impossible to achieve, but the closer we can get the air going into the evaporator to 100% humidity the more efficient it becomes at removing water from the air.

Now let us look at this from another angle. The refrigerant in the system has to be compressed in order to raise its temperature, it needs to be compressed enough that its temperature becomes higher than where the condenser is. The more we have to compress it to reach that temperature the more work the compressor has to do, thus using more energy. What we really want is the temperature difference between the evaporator and the condenser to be as low as possible. We can't affect the temperature of where we are pumping heat to and thus we can't control the ideal condenser temperature. We can control the temperature of the evaporator however, this is because we know that a dehumidifier is more efficient the closer to 100% RH the air entering the evaporator is. If we can raise the RH to 100% (no matter what the temperature is) then the evaporator can be at any temp.

So with our system instead of moving heat from the air going into the evaporator to the air coming out of the evaporator, what we really want to do is move moisture from the air coming out of the evaporator to air coming into the evaporator. Fortunately there are ways to do this, an energy recovery air to air exchanger is one way this is basically a material that can absorb water yet block air. Alternatively we can move a desiccant (a material that uses sorption to collect water from a substance, in this case air) between the air flows. When a desiccant becomes saturated it will give off a portion of its water to an air flow if the RH of that air flow is less than 100%. The common ways to do this are with a desiccant water fall or a desiccant wheel. Realistically the air going into the evaporator from the desiccant can never be at 100% RH, just like we can never move all the heat from in incoming air stream to the out going air stream.

Why not just use a desiccant and forget using any refrigeration equipment in the dehumidifying process? I mean we can move it outside and heat it up right? The issues with that are #1 controlling the dehumidification, and #2 we will need to dehumidify when there isn't solar energy to dry the desiccant.

Theoretically though you could use a large storage tank to hold a liquid desiccant and then do an indoor waterfall and an outdoor one in a solar oven. Given enough capacity of the tank you should be able to have a big enough buffer to cover the no solar times. The other issue is you would end up pumping heat into the house that way, likely more heat than you'd have with just using a dehumidifier.

[DEnd]

An interesting side note here:

Joe Lstriburek once used cellulose insulation as a desiccant. If my memory serves this was in Florida and he pumped air through the attic insulation (blown in cellulose) during the night, then during the day the heat from the attic dried out the cellulose. He air sealed the attic, then used a vapor open membrane at the ridge to vent the moisture out of the attic. He still needed mechanical dehumidification, and he (understandably) wasn't comfortable blowing air through the cellulose.

BSI-088: Venting Vapor | Building Science Corporation

[MEMPHIS91]

Jeff, yeah I saw the chart, I just don't trust liquids not freezing. Lol I'm not throwing any idea out the door yet. This dissucussion is awesome, and I'm trying to weigh all my options while sourcing parts for each build.

DEnd, nice write up, thanks for taking the time. I guess my thoughts are that I need both cold air and moisture removal and even though the dehumidifier is not removing as much water it is adding to my AC load by cause the 20+F heat gain. I probably could build a better designed dehumidifier with both the evap and condenser inside, I will add that to the list of things to look into. Right now my unit runs for about 10 hours and sucks about 2 gallons of water out keeping the house at 80-82F and 47-50% RH.

I do really like this salt idea, it is just way to cool to not put some serious time into testing. I have 50lbs coming in tomorrow and I'm going to source my large black pot today. (Yard sales).

I am going to build a small scale system just as soon as I source the parts. I can simply run it and the dehumidifier and see how much extra the salt absorbs.

All I really need is about 3 gallons of moisture removed without heating the house as much. I'm even giving thought to what jeff was saying about me using some of the HPWH water to recharge the salt. As cheap as my hot water is it would be a great idea, I'm just not sure how to build such a beast YET.

Thanks again, very very helpful post!