Re: [Phys-L] Earth as a "heat engine"

[Francois Primeau <fprimeau@uci.edu>]

On Wed, Oct 9, 2019 at 11:48 AM Folkerts, Timothy J <FolkertsT@bartonccc.edu>
wrote:

> Francois says:
> > For the atmosphere there is a net input of radiative energy in the
> tropics and a net loss at high latitudes.
> My conceptual objection here is that this is simply convection.  If there
> is a hot object at one side of a room (eg a radiator or wood stove) and a
> cool object at the other side (eg a window) then I will set up exactly the
> same sort of convective circulation, with a net source of thermal energy in
> one place and a net sink of thermal energy in another place.  Is this empty
> room a "heat engine"?

My point was that to an oceanographer, the atmosphere does look like a heat
engine in the sense that the atmosphere takes in low entropy short-wave
radiation, exports high entropy long-wave radiation and in the process does
work on the ocean via the wind stress.  So the atmosphere does more than
just move heat from the tropics to the poles.  The heat engine analogy
seems somewhat useful to me. Especially when it is used by way of contrast
with the energy fluxes and transformations in the ocean.


> John says:
> > However, in a very practical sense, you can stick a windmill into the
> atmosphere ...
> I would say you have made a fundamental change here.
> * If water just flows down a river, it is not an 'engine'.  If I add a
> waterwheel to extract energy, then I have an 'engine'.
> * If I have a hot object touching a cold object, that is not an 'engine'.
> If I add a thermopile between the objects to extract energy, then it is a
> 'heat engine'.
> * The atmosphere is not an engine.  When I add a wind turbine to extract
> energy, THEN it is an engine.
>
> John says:
> > but it is not entirely crazy.
> I agree.  People who know about thermodynamics can navigate these ideas
> easily and get to the core of the issue.  (Much like people who understand
> thermodynamics can deal with people using "heat" when they are discussing
> "internal energy".)
>
> I have also heard people call the atmosphere a "heat pump" because it
> pumps heat from the tropics toward the pole.  But the atmosphere is
> certainly now a 'heat pump" in the sense HVAC people/engineers/physicists
> would understand.
>
> It is probably just another case of letting go in colloquial discussion
> and educating in non-colloquial discussion.
>
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