Black Hole

May contain traces of nut
This is just a casual enquiry in case anybody knows, and I am too busy/lazy to research it myself right now and by the time I am at a loose end I will have forgotten (this topic will remind me if necessary).

Air conditioners dehumidify - my car leaves an alarming puddle if I have been running the aircon, and at the beginning of the season I am likely to have a panic where it came from until I remember the aircon.

However, it seems to me that if dehumidification is required without necessarily chilling the air, it will be more energy efficient if the drawn-in air passes through a heat exchanger with the exhaust air. This would decrease the temperature of the inlet air on its way through the heat exchanger (possibly condensing some of the moisture on the way) reducing the work the chiller has to do, and warm up the exhaust air to almost the same as the ambient on the way out - all for free. To make it work as a chiller, have a control flap which bypasses the heat exchanger.

Do commercially available dehumidifiers actually do this?
The efficiency question is interesting. I leave my car A/C on auto all the time, even when the heating is on. It is the most effective way to demist.

I have no idea whether the system uses any sort of heat recovery but I have never been able to detect any consistent difference in fuel consumption with it on or off. Even modest differences in driving style make a far bigger difference. Perhaps the persistent belief that A/C is a fuel guzzler is just a legacy of days when systems were grossly inefficient and car engines much less powerful.

As most portable dehumidifiers are one box units operating within a single space ( ie the room ) they would always result in a net gain in room temperature anyway. But presumably these are not the type BH is thinking of.
No, a car aircon doesn't use heat scavenging, all it does is divert the air intake through or to bypass the chiller matrix before it gets to the heater matrix / bypass mixer.

Yes, I am thinking of the self-contained dehumidifiers, and where the waste heat goes is an interesting point.

I have also thought about a new design for a shower tray, where the waste outlet is plumbed into a heat exchanger to warm up the supply feed.
Presumably the only place the waste heat can go is back into the room, whether the output air is ducted through it or not? From my dodgy recollection of physics some of the energy is absorbed in the state change ( ie from water vapour to liquid water ) without a change in temperature. All I can recall from running one is that the back of the unit got warm but I don't think there was a significant warming in the room, though I appreciate perception of warmth is influenced inter alia by humidity.

I like the shower idea - and what about a heat scavenging WC, to pre-arm the seat? Maybe a step too far.
The all in one ACs that I have seen have an exhaust hose which you can take to the outside with a hole in the wall or out through an open window and air condition the nearby countryside. I suppose the net heat gain that must be present if you vent the exhaust into the same room is effectively nullified by the current of cold air being circulated.
Condensing water vapour into liquid water releases heat, which I think is the process which drives thunderstorms. The extraction of moisture, with the dry air returned to the room, ought (if I am correct) result in a net increase in temperature once the air masses are evenly mixed AND the condenser coils are being cooled by the air in the room - however, if your aim is to dehumidify rather than chill, the temperature increase is not a concern (eg when a flooded property is being dried out), and all you need to vent is the accumulation of water.

This does not affect the original premise: using the exhaust air to chill the inlet air through a heat exchanger ought to make the process more efficient.
But surely the exhaust air will be hotter than the air that you are trying to dehumidify?

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Not at all. To remove moisture from the air (ignoring chemical absorbents) you have to use a fridge to reduce the air temperature to dew point. Energy is released in the process, but that just means the fridge has to work harder. The direct output is therefore chilled air. Pass that through a heat exchanger, and even at 100% heat exchanger efficiency the exhaust can only be as warm as the inlet air, and the inlet air (once through the heat exchanger) can only be as cold as the output from the chiller.

Where does the heat go? Same as a fridge, to the condenser coils around the back.
That says to me that the exhaust is being used to cool the condenser. This is the kind of info I am interested in, thanks.
In the situation of drying out a flooded property, increasing the ambient temperature would be a distinct advantage as the warmer air has greater capacity to soak up the damp.