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If I understand correctly, both air conditioners and dehumidifiers conventionally exhaust waste heat through their exhaust air, though both also produce exhaust water. I'd like to know,

  1. Are air conditioners or dehumidifiers currently being produced that exhaust heat through their exhaust water?
  2. If not, are there engineering or physical reasons why this is not feasible?

Use case: I currently live where the weather is regularly both hot and humid, and my current dwelling is configured such that my only feasible options for reducing internal heat and humidity are:

  1. an energy-inefficient external air conditioner (located outside of, but connected to, my dwelling) that also seems inefficient at humidity reduction;
  2. an energy-efficient internal dehumidifier that exhausts
    • waste heat into the dwelling
    • waste water into an open reservoir (which I filter for drinking).

What I'd like would be an internal unit like my dehumidifier, but which would instead:

  1. use its wastewater for cooling;
  2. exhaust that warm water into a closed, insulated reservoir;
  3. {sound alarm, shutoff} when the reservoir filled (as it does now) or overheated.

I would then remove and empty the reservoir outside.

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  • $\begingroup$ Why do you say the external compressor is "energy-inefficient" whereas the internal dehumidifier is "energy-efficient?" Also, you beg the question of whether it's worth using small amounts of water as a passive heat sink. $\endgroup$ – feetwet Jun 3 '16 at 20:24
  • $\begingroup$ @feetwet: 1) Empirically, in this particular use case, the external AC (which is very old) consumes much more electricity in operation than comparable units for which I can also monitor electricity consumption. 2) I don't beg that question, I ask it: 'are there engineering or physical reasons why this is not feasible?' $\endgroup$ – TomRoche Jun 7 '16 at 23:01
  • $\begingroup$ "Feasible" is, in this case, a trivial question: Yes, it is feasible. $\endgroup$ – feetwet Jun 8 '16 at 1:48
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I can provide some information about a solution that you would have to build / modify yourself.

From what a quick research yielded I understand that you cannot use the wastewater for drinking directly. Feel free to look up greywater. As you pointed out, you want to filter it first. I strongly recommend to use a biomembrane-based filter and make sure that the filter will generate potable water. You might want to read this Q&A from grist.org about reusing water from a dehumidifier.

The second point I want to address is the use of your the heat generated and the cold water. Air to water heat transfer is not very feasible since the heat transfer is rather poor. So firstly I would see it's not very efficient to try and heat the condensed water with hot air. Which would make up my next point, you wouldn't see a great effect in using the condensed water for cooling anything. You shouldn't be able to significantly chill your rooms with it. What I could think of very quickly is that you could cool water with it, although the efficiency here wouldn't be astonishing either. Given a certain $\Delta T$ of 10–20 K you would need for heat transfer to take place and given the fact that you would operate with condensed water of a temperature around 5 °C you would be able to cool something down to 15 °C best case scenario. Maybe 10 °C. This is not significantly lower the the cold tap water I get around here so I guess it's not worth the trouble.

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    $\begingroup$ The hot air exhaust from the dehumidifier is the desired product; i.e. dry air. Venting it outside would defeat the purpose of dehumidifying it. $\endgroup$ – Carlton Jun 15 '15 at 13:18
  • $\begingroup$ Thanks for pointing that out! I was mainly focused on the temperature at this point and forgot about the dehumidifying. I will update my answer accordingly. $\endgroup$ – idkfa Jun 15 '15 at 13:26
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As a professional engineer, the first step is to say that you should not modify your dehumidifier at home. The refrigerant in use has a high global warming potential, and emissions could have serious disasters on the environment. Also, the high pressures in the cycle can be very dangerous. Not to mention if refrigerant does leak into your water tank you may not even be able to filter out the contamination. Finally if the machine is improperly designed, you could tear up your compressor or all sorts of things. So, if you are interested, get a professional to design this system for you.

With that said, if you were to say obtain a custom built, custom designed dehumidifier, it is possible. Looking at the typical refrigeration cycle (image linked from Wikipedia):

Look up refrigeration cycle on WIkipedia

Essentially, your dehumidifier has the fan in the opposite direction. It pulls air across the evaporator first, then across the warm condenser. It has a pretty high efficiency because that cool air can remove a lot of the excess heat from the condenser.

A custom built dehumidifier would not blow the cold air across the condenser, but immerse the condenser directly into your water storage tank. With that, the only limitation on the heat you could hold would be that the condenser would still need to turn the refrigerant from vapor to liquid.

To figure out how warm you can get it, select a refrigerant and find the boiling point at the compressor's operating pressure. R-134a, a very common refrigerant, typically runs at around 18 bar. This saturation table states that at 1800 kPa, the refrigerant boils at 62.9 Celsius. So, that would be the maximum temperature your tank could reach before the refrigerator stopped working. Due to thermodynamic inefficiency, heat exchanger inefficency, etc. you probably would get 10 - 20 C below that, but it would certainly be warmer than your air. Ideally, with a custom designed process, you could reach really warm temperatures - again talk to a design professional. Do not modify your home system to immerse the condenser.

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Are air conditioners or dehumidifiers currently being produced that exhaust heat through their exhaust water? If not, are there engineering or physical reasons why this is not feasible?

It is unlikely a mainstream consumer unit would do this. These units have to be designed to work in a variety of conditions, including hot and dry. In a case like that, there would be very little water available, for cooling, so the system would has to be designed to work with just air cooling anyway.

The extra structures to augment the cooling system with the condensed water, when present, would make the unit more complex, larger, and more expensive. It's not worth it.

Even in humid conditions, there just isn't a lot of water in the air relative to the amount of air moving around. Think of the many cubic feet of air that go thru a dehumidifier for just one drop of water to condense out. Do the math:

Lets compare the heat in one cubic foot of air to the amount one drop of water could absorb. I've watched a dehumidifier in a damp basement, and it certainly wasn't dripping one drop per cubic foot or air moving thru. However, 1 cubic foot is 28.3 liters. At 30°C and 1 atm, this cubic foot of air has a mass of about 33 grams. In chemistry class we usually figured there were 20 drops of water per ml, so one drop has a mass of 1/20 g, or 50 mg. The cubic foot of air is therefore 660 times more massive than the drop of water.

I'm going to stop here since it seems pointless to continue. I haven't looked up the relative heat capacity of liquid water versus gasseous nitrogen and oxygen. But even is that of water is several times more a few degrees increase of the water drop simply isn't going to amount to much of a decrease in the air. And of course you can't transfer the heat from the air to the water 100% efficiently.

So even with a high condensation rate, this little bit of extra cooling just isn't worth the extra complexity, size, and materials to make use of it.

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What you could do is place a heat exchanger in the (warm) exhaust air flow, then run your (cool) waste water through it before discharging the water outside. I'm envisioning a recycled radiator from a car, with a small pump to get the water up from the dehumidifier into the upper opening of the radiator. You definitely need to dump the water outside though, since it will slowly leak heat back into your house if you store it inside.

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