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Aggregate changes require way more energy than temperature changes. I vaguely remember that it takes about the same engergy to get water from 5°C to 80°C as getting ice to 1°C warm water.

Hence my question: Is a heat pump more/less efficient when there is fog compared to the same temperature delta (inside/outside), but with lower outside humidity?

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  • $\begingroup$ Consider thermal mass flow rate. Does fog heated flow away faster? Does chilled fog flow away slower? $\endgroup$
    – Abel
    Commented Feb 28 at 10:15
  • $\begingroup$ The efficiency of the heat pump itself would, I believe, consider heat-flow vs power-used -- and that ratio as a function of temperature difference across which the heat flows. Whereas the effects of the environment are at the next system level, where the heat-flow out of the pump then produces a change in temperature. So the impact to the full system would be on the power consumption to maintain some temperature or temperature-ramp-rate, to a given thermal load, and again as a function of temperatures. The fog changes the thermal load I suppose. $\endgroup$
    – Pete W
    Commented Feb 28 at 14:07

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A more humid environment outside will result in more use of defrost cycles for the outside evaporator, reducing efficiency. I supposed that fog would be the upper limit for humidity.

Heat transfer to/from air is not really dependent on the air's moisture level.

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  • $\begingroup$ Ok, so if the temperature causes defrost that makes sense. What if its warm enough that this does not happen? $\endgroup$
    – Martin Thoma
    Commented Feb 28 at 19:19
  • $\begingroup$ @MartinThoma if no freezing I don't think there would be any difference. $\endgroup$
    – Tiger Guy
    Commented Feb 28 at 20:15
  • $\begingroup$ If for some reason there was fog and you were trying to dump heat into it you may get some evaporation, which would help. Otherwise -- it's a wash, or you're trying to cool, you ice up your evaporator, and it needs a defrost cycle. $\endgroup$
    – TimWescott
    Commented Mar 3 at 0:13
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The volumetric heat capacity of water vapor is approximately 1000 times that of dry air. The water droplets in air have about 3 times more heat capacity, and the phase change between vapour and liquid liberates or absorbs huge amounts of energy.

This means that the external heat exchanger is significantly more efficient in misting air.

That may or may not have an effect on the efficiency of the total system.

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