What Happens to Latent Heat?

Most recent answer: 01/09/2016

Q:
A previous answer to a question on why does salt lower the freezing point of water said in part, "The amount of energy released is 80 calories per gram of water when it freezes". Is that energy actually released, as in it gets away, or is that just another way of saying that the energy is no longer present due to the fact the molecules are locked up? If the energy is actually released into the surroundings, could that energy be captured? All to set this scene for this question: If a desalinization plant were set up in the Arctic using ocean water to produce pure water, could that "released energy" be harnessed as an energy source since there would be no additional energy input to cool the water, thus producing both desalinated water and energy at the same time?Thank you for considering to answer my question. Bob
- Bob (age 56)
Peoria, IL USA
A:

As the water freezes, that "latent heat" energy just goes to heating up the water and the things it's in contact with.

The arctic desalinization scheme you ask about isn't entirely clear, but we can be sure of one thing. There is no way that cold salty water, approximately in thermal equilibrium, can be turned to pure water (frozen or not) plus saltier brine without an input of free energy. Getting an output of free energy would violate the second law of thermodynamics, which implies that the free energy in equilibrium is already minimized. 

You notice that I discussed "free energy" rather than "energy". The reason is that energy itself is conserved. It's free energy, which you can roughly think of as energy not accompanied by entropy, that tends to get lost and that we need for machines, etc. More generally, what we really need is negative entropy, compared to what the entropy would be in equilibrium. That makes it particularly clear that there's no way to extract what we need from an equilibrium situation.

That said, it's true that if you do put the work in to desalinate seawater at say -2°C, the pure water out should freeze. There should be a clever way to use that to slightly lower the work input required to desalinate the water.  If, for example, you're using reverse-osmosis to desalinate the water at -2°C, you should be able to use that the ultimate output is ice, which has slightly lower free energy at that temperature than does liquid water.

The bottom line is still that you need to pay for work to desalinate the water.

Mike W.


(published on 01/09/2016)