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

[Francois Primeau <fprimeau@uci.edu>]

I think the concept of heat engine is more useful if we look at the
atmosphere alone as opposed to the whole climate system.
For the atmosphere there is a net input of radiative energy in the tropics
and a net loss at high latitudes.  The coupled ocean-atmosphere system must
therefore transport energy polewards. Because the atmosphere is heated from
below,  the heating in the tropics causes air to expand and therefore acts
to lift columns of air, thus doing work against gravity. The winds in the
atmosphere are maintained by the transfer of this potential energy into
kinetic energy. I think this is the sense  in which the atmosphere is
thought of as a heat engine. In contrast the ocean is heated and cooled at
the surface, which is roughly a surface of constant gravitational potential
energy, and therefore does little to directly drive ocean currents.
Rather, the ocean currents are driven mechanically by the wind stress at
the surface. In this sense the ocean is not thought of as being a heat
engine.     I have glossed over some subtle  issues. For example, the
dissipation of tides in the ocean is associated with turbulence that acts
to mix heat downwards in low latitudes.  This downward mixing of heat  does
result in a store of available potential energy for driving ocean
currents.

(I'm not sure if I used the correct pedagogical terminology. I hope this
forum will correct any sloppy usage.)


On Tue, Oct 8, 2019 at 2:34 PM Folkerts, Timothy J <FolkertsT@bartonccc.edu>
wrote:

> I occasionally hear earth's atmosphere and/or oceans described as a "heat
> engine" pumping air and water and energy around.
>
> I think this is highly questionable nomenclature.  I'd like a second (or
> third or tenth) opinion.
>
> A heat engine takes an amount of thermal energy from a hot reservoir,
> Q(h), converts some of it into work that is extracted from the engine, W,
> and then gives remaining energy to the cold reservoir, Q(c).
> Q(h) - Q(c) = W.
>
> No matter how I look at it, there is no work done by the atmosphere and/or
> oceans.  On the largest scale, the hot reservoir is the sun and the cold
> reservoir is space.  These two (approximately) balance.  And the imbalance
> is due to global warming/cooling, not to any work being done.   Sure, the
> temperature differences drive winds and ocean currents, but these are
> *internal* to the 'device' and no net work is extracted.  Whatever
> subsystem I consider, I can still see no work being extracted.
>
> I suppose you could call it a heat engine with efficiency = 0, but then
> you would have to call any system with convection a 'heat engine'.  Heck,
> even conduction or radiation from hot to cold would be a 'heat engine'.
>
> I am fine in colloquial settings letting people use the term 'heat engine'
> as a short hand for "it moves because of heat", but there is no 'heat
> engine' in the technical physics/engineering sense.
>
> Thoughts?
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